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Back | Show only these items: | Untangling the formation of the cyclic carbon trioxide isomer in low temperature carbon dioxide ices

1
Untangling the formation of the cyclic carbon trioxide isomer in low temperature carbon dioxide ices

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

The formation of the cyclic carbon trioxide isomer, CO₃(X ¹A₁), in carbon-dioxide-rich extraterrestrial ices and in the atmospheres of Earth and Mars were investigated experimentally and theoretically.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Carbon dioxide ices were deposited at 10 K onto a silver (111) single crystal and irradiated with 5 keV electrons.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

Upon completion of the electron bombardment, the samples were kept at 10 K and were then annealed to 293 K to release the reactants and newly formed molecules into the gas phase.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

The experiment was monitored via a Fourier transform infrared spectrometer in absorption-reflection-absorption (solid state) and through a quadruple mass spectrometer (gas phase) on-line and in situ.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

Our investigations indicate that the interaction of an electron with a carbon dioxide molecule is dictated by a carbon–oxygen bond cleavage to form electronically excited (¹D) and/or ground state (³P) oxygen atoms plus a carbon monoxide molecule.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

About 2% of the oxygen atoms react with carbon dioxide molecules to form the C_2v symmetric, cyclic CO₃ structure via addition to the carbon–oxygen double bond of the carbon dioxide species; neither the C_s nor the D_3h symmetric isomers of carbon trioxide were detected.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Since the addition of O(¹D) involves a barrier of a 4–8 kJ mol⁻¹ and the reaction of O(³P) with carbon dioxide to form the carbon trioxide molecule via triplet-singlet intersystem crossing is endoergic by 2 kJ mol⁻¹, the oxygen reactant(s) must have excess kinetic energy (suprathermal oxygen atoms which are not in thermal equilibrium with the surrounding 10 K matrix).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

A second reaction pathway of the oxygen atoms involves the formation of ozone via molecular oxygen.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

After the irradiation, the carbon dioxide matrix still stores ground state oxygen atoms; these species diffuse even at 10 K and form additional ozone molecules.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Summarized, our investigations show that the cyclic carbon trioxide isomer, CO₃(X ¹A₁), can be formed in low temperature carbon dioxide matrix via addition of suprathermal oxygen atoms to carbon dioxide.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

In the solid state, CO₃(X ¹A₁) is being stabilized by phonon interactions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

In the gas phase, however, the initially formed C_2v structure is rovibrationally excited and can ring-open to the D_3h isomer which in turn rearranges back to the C_2v structure and then loses an oxygen atom to ‘recycle’ carbon dioxide.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

This process might be of fundamental importance to account for an ¹⁸O enrichment in carbon dioxide in the atmospheres of Earth and Mars.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

Introduction

Ever since the first tentative characterization of the carbon trioxide molecule in photolyzed ozone–carbon dioxide ices at 77 K,¹ the CO₃ species has been a subject of various spectroscopic and theoretical studies.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Moll et al¹. and Jacox et al². assigned four fundamentals at 2045 cm⁻¹ (CO stretch), 1073 cm⁻¹ (O–O stretch), 972 cm⁻¹ (C–O stretch), 593 cm⁻¹ (C–O stretch), and 568 cm⁻¹ (O–CO stretch) in low temperature carbon dioxide matrices; in argon matrices, these absorptions were shifted to 2053 cm⁻¹, 1070 cm⁻¹, 975 cm⁻¹, and 564 cm⁻¹;³ no feature around 593 cm⁻¹ was identified in solid argon.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Absorptions at 1894 cm⁻¹ (argon matrix) and 1880 cm⁻¹ (carbon dioxide matrix) were tentatively assigned as a Fermi resonance of the 2045 cm⁻¹ band with an overtone of the 972 cm⁻¹ fundamental.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Jacox et al. conducted also a normal coordinate analysis and suggested a C_2v bridged structure (Fig. 1(1)); in strong contrast, LaBonville et al. allocated a C_s symmetric structure of the carbon trioxide molecule (Fig. 1(2)).⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The interest in the carbon trioxide molecule has been also fueled by the complex reaction mechanisms of carbon oxides (carbon monoxide and carbon dioxide) with atomic oxygen in the Martian atmosphere.^5–8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

Carbon dioxide, CO₂(X ¹Σ+g), presents the major constituent (95.3% by volume); nitrogen (2.7%), argon (1.6%), carbon monoxide (0.7%), molecular oxygen (0.13%), water (150–200 ppm), and ozone (0.03 ppm) make up the rest.⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

It has been suggested that the photodissciation of carbon dioxide by solar photons (λ < 2050 Å) produces carbon monoxide and atomic oxygen.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

Near the threshold, only ground state O(³P) atoms are produced; shorter wavelengths supply also O(¹D).⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

The primary fate of electronically excited oxygen atoms is thought to be quenching to form O(³P); the detailed process is not known and has been postulated to proceed via a carbon trioxide molecule.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

Once O(³P) and carbon monoxide has been formed, it is difficult to restore carbon dioxide, since the reversed reaction is spin forbidden.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

Detailed photochemical models suggest that the oxygen atoms rather react to molecular oxygen and ultimately to ozone.⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

The CO₃ molecule has been also implied as an important intermediate in the ¹⁸O isotope enrichment of carbon dioxide in the atmospheres of Earth and Mars.^10,11

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

Computations and laboratory experiments indicate that the ¹⁸O enrichment in ozone might be transferable to carbon dioxide,^12,13 possibly via a CO₃ intermediate.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

On Earth, photolysis of stratospheric ozone generates O(¹D), which in turn might react with carbon dioxide to form a carbon trioxide molecule.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

In the gas phase, the latter was postulated to fragment to carbon dioxide and atomic oxygen, possibly inducing an isotopic enrichment in carbon dioxide via isotopic scrambling.¹⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

However, the explicit structure of the CO₃ intermediate has not been unraveled yet.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

Various kinetic measurements have been carried out to determine the temperature-dependent rate constants of the reaction of electronically excited oxygen atoms, O(¹D), with carbon dioxide.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

At room temperature, rate constants of a few 10⁻¹⁰ cm³ s⁻¹ have been derived.^15–17

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

This order of magnitude suggest that the reaction has no or only little activation energy, proceeds with almost unit efficiency, and most likely involves a reaction intermediate.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

However, neither reaction products nor the nature of the intermediate were determined.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

On the other hand, a CO(X ¹Σ⁺) + O₂(X ³Σ−g) exit channel was found to have an activation energy between 15 and 28 kJ mol⁻¹ in the range of 300–2500 K.¹⁸

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

This finding correlates also with a theoretical investigation of the singlet and triplet potential energy surfaces of the CO₃ system.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

Froese and Goddard suggested that the barrier-less, spin-forbidden quenching pathway to form ground state oxygen atoms and carbon dioxide (Δ_RG = −190.0 kJ mol⁻¹) dominates over the formation of CO(X ¹Σ⁺) plus O₂(X ³Σ−g) (Δ_RG = −157.5 kJ mol⁻¹) and CO(X ¹Σ⁺) plus O₂(a ¹Δ_g) (Δ_RG = −63.3 kJ mol⁻¹).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Further theoretical calculations indicated that the CO₃ isomer identified in the carbon dioxide and argon matrices might be the C_2v symmetric bridged molecule.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

A D_3h structure was identified as a local minimum, too, but lies 16.8 kJ mol⁻¹ higher in energy than the cyclic isomer (Fig. 1, (3)); according to calculations, both structures are connected via a transition state located 36 kJ mol⁻¹ above the cyclic molecule.²⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

At temperatures higher than 100 K, the cyclic CO₃ isomer was predicted to decay to carbon dioxide and ground state oxygen atoms via singlet-triplet transitions.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

Despite this information on the reaction of carbon dioxide with atomic oxygen, an incorporation of these data into homogeneous gas phase models still fails to reproduce the observed abundances of carbon dioxide, carbon monoxide, oxygen, and ozone in the Martian atmosphere quantitatively.⁹

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Atreya et al. pointed out the necessity to include heterogeneous reactions on aerosols or carbon dioxide ice particles in the Martian air.^21–23

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

However, these processes have not been investigated in the laboratory so far.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Also, the explicit structure and the actual formation mechanism of the CO₃ isomer and its role in the ¹⁸O isotopic enrichment in stratospheric carbon dioxide remain to be solved.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

In this paper, we present a detailed experimental and theoretical investigation on the formation mechanism of carbon trioxide in low temperature carbon dioxide ices and the implications for gas phase chemistry.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Reactive oxygen atoms are generated via electronic energy loss of high energy electrons to the carbon dioxide molecule in the solid sample.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met2

Our first goal is to identify the infrared absorption features of the carbon trioxide molecule, to resolve the true nature of the 1880 cm⁻¹ absorption of the carbon trioxide molecule unambiguously, and to assign the structure of the newly formed species.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

Secondly, reaction mechanisms to synthesize the carbon trioxide molecule together with other newly formed species will be derived combining our experimental data with electronic structure calculations.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

Finally, important implications of these results to planetary and atmospheric chemistry are addressed.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

Note that these studies also have important implications to planetary, cometary, and interstellar chemistry since carbon dioxide has been identified as a major component of ices on Mars, in comets such as Halley,²⁴ and of low temperature grain mantles in cold molecular clouds like the Taurus Molecular Cloud (TMC-1)²⁵.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

Theoretical calculations

The geometries of various local minima and transition states on three potential energy surfaces (PESs) of carbon trioxide, CO₃, including the lowest triplet and two lowest singlet electronic states, have been optimized using the multireference complete active space self-consistent field (CASSCF) method^26,27 with the 6-311G(d) basis set.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The active space in the CASSCF calculations included 16 electrons distributed over 13 orbitals, i.e., this was the full-valence active space excluding 2s lone pairs on three oxygen atoms.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Vibrational frequencies and infrared (IR) intensities have been also computed at the CASSCF(16,13)/6-311G(d) level of theory.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

Single-point energies for various species have been subsequently refined employing internally-contracted multireference configuration interaction MRCI method^28,29 with the same (16,13) active space and the larger 6-311+G(3df) basis set.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

All calculations were carried out using the MOLPRO 2002³⁰ and DALTON³¹ programs.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

Experimental

The experiments were carried out in a contamination-free ultrahigh vacuum (UHV) chamber; the top view of this machine is shown in Fig. 2.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

This setup consists of a 15 l cylindrical stainless steel chamber of 250 mm diameter and 300 mm height which can be evacuated down to 2 × 10⁻¹⁰ torr by a magnetically suspended turbopump backed by an oil-free scroll pump.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

A two stage closed cycle helium refrigerator-interfaced to a differentially pumped rotary feedthrough is attached to the lid of the machine and holds a polished silver (111) single crystal.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

This crystal is cooled to 10.4 ± 0.3 K, serves as a substrate for the ice condensate, and conducts the heat generated from the impinging electrons to the cold head.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

To minimize the radiative heat transfer from the chamber walls to the target, a 40 K aluminum radiation shield is connected to the second stage of the cold head and surrounds the crystal.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The ice condensation is assisted by a precision leak valve.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

During the actual gas condensation, the deposition system can be moved 5 mm in front of the silver target.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

This setup guarantees a reproducible thickness and composition of the frosts.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

To allow a selection of the target temperature, a temperature sensor, cartridge heater, and a programmable controller are interfaced to the target.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The carbon dioxide ices were prepared at 10 K by depositing carbon dioxide gas onto the cooled silver crystal.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Blank checks of the pure gas (BOC Gases, 99.999%) via a quadrupole mass spectrometer and of the frosts via a Fourier transform infrared spectrometer were also carried out.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

Fig. 3 depicts a typical infrared spectrum of the frost; the absorptions are compiled in Table 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

To determine the ice composition quantitatively, we integrated numerous absorption features and calculated the column density, i.e. the numbers of absorbing molecules per cm², n, via the Lambert–Beer relationship (1) and eqns. (2)–(3).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

The integrated absorption features, the corresponding integral absorption coefficients, and the column densities are summarized in Table 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

These data suggest a column density of (1.1 ± 0.3) × 10¹⁸ molecules cm⁻².

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

Considering a density of 1.7 g cm⁻³ at 10 K,³² this translates into an averaged target thickness of 0.48 ± 0.11 μm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

We would like to stress that the integrated absorption coefficients have been taken in transmission experiments,⁴⁷ but the experiment has been carried out in an absorption–reflection–absorption mode.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

This probably causes the large variations in the film thicknesses estimated from different absorption features.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

I() = I₀()e^−ε()nwith the intensity of the IR beam after, I(), and before absorption, I₀(), at a wavenumber , the wavenumber dependent absorption coefficient ε() in units of cm⁻² and the number of absorbing species per cm², n.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

Reformulating eqn. (1) with A() = lg(I₀()/I()) gives A() = ε()n/ln 10.Integrating from ₁ to ₂ yields with the integrated absorption _₁∫^₂A()d in cm⁻¹ and the integral absorption coefficient A_exp = _₁∫^₂ε()d in cm.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

The factor cos(75°) accounts for angle between the surface normal of the silver wafer and the infrared beam, whereas the division by two corrects for the ingoing and outgoing IR beams.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

These ices were irradiated isothermally at 10 K with electrons of 5 keV kinetic energy generated in an electron gun at beam currents of 100 nA (60 min) by scanning the electron beam over an area of 3.0 ± 0.4 cm².

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Accounting for the extraction efficiency of 78.8% and the irradiation time, this exposes the target to 1.77 × 10¹⁵ electrons.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Higher beam currents, which increase the temperature of the frost surface, should be avoided.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

After the actual irradiation, the sample was kept isothermally at 10 K and heated then by 0.5 K min⁻¹ to 293 K.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

To guarantee an identification of the reaction products in the ices and those subliming into the gas phase on line and in situ, two detection schemes are incorporated: (i) a Fourier transform infrared spectrometer (FTIR), and (ii) a quadrupole mass spectrometer (QMS).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The chemical modification of the ice targets is monitored during the experiments to extract time-dependent concentration profiles and hence production rates of newly formed molecules and radicals in the solid state.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

The latter is sampled via a Nicolet 510 DX FTIR spectrometer (6000–500 cm⁻¹) operating in an absorption–reflection–absorption mode (reflection angle α = 75°; Fig. 2); spectra were accumulated for 2.5 min at a resolution of 2 cm⁻¹.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

The infrared beam is coupled via a mirror flipper outside the spectrometer, passes through a differentially pumped potassium bromide (KBr) window, is attenuated in the ice sample prior and after reflection at a polished silver waver, and exits the main chamber through a second differentially pumped KBr window before being monitored via a liquid nitrogen cooled detector (MCTB).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

The gas phase is monitored by a quadrupole mass spectrometer (Balzer QMG 420) with electron impact ionization at 90 eV electron energy of the neutral molecules in the residual gas analyzer mode.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

The raw data, i.e. the temporal development of the ion currents of distinct mass-to-charge ratios, are processed via matrix interval algebra to compute absolute partial pressures of the gas phase molecules.³³

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Since, for example, carbon dioxide can fragment to molecular oxygen and also to carbon monoxide in the ionizer of the quadrupole mass spectrometer, different molecular species add to one mass to charge ratio (m/z) of, e.g. 32 (O₂).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Therefore, we must perform the raw data processing via matrix interval algebra to calculate the actual partial pressures of the molecules in the gas phase.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Briefly, m/z ratios are chosen to result in an inhomogeneous system of linear equations including the measured ion current (right hand vector), partial pressures (unknown quantity), and calibration factors of fragments of individual gaseous species determined in separate experiments.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Since all quantities are provided with experimental errors, matrix interval arithmetic, i.e. an IBM high accuracy arithmetic subroutine defining experimental uncertainties as intervals, is incorporated in the computations to extract individual, calibrated components of gas mixtures.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Results

Computational results

Our calculations suggest that two minima exist on the lowest singlet CO₃ potential energy surface (PES): a C_2v-symmetric three-member cyclic structure s1 and a D_3h-symmetric isomer s2 (Fig. 4). s1 and s2 have similar energies and reside 197.5 and 197.1 kJ mol⁻¹ lower than the O(¹D) + CO₂(X ¹Σ+g) asymptote, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

These two singlet isomers can rearrange to each other by ring opening/ring closure and are separated by a low barrier of 18.4 kJ mol⁻¹ with respect to s1 occurring at transition state s-TS1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

The cyclic structure s1 can be produced in a reaction between O(¹D) and CO₂(X ¹Σ+g).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

The calculations suggest that the reactants first form a weakly bound complex (∼2.9 kJ mol⁻¹) s5, which then rearranges to s1 with a barrier vias-TS2 of 5.6–7.6 kJ mol⁻¹ relative to O(¹D) + CO₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

In the triplet electronic state, separated O(³P) and CO₂ have the lowest energy, while the C_2v isomer t1 (³B₂) resides 96.3 kJ mol⁻¹ higher. t1 has a structure rather similar to that of s2, except that the three C–O bond lengths are not equal; there are one double (1.201 Å) and two single (1.343 Å) bonds.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Isomer t1 can decompose to O(³P) + CO₂ overcoming a barrier 51.5 kJ mol⁻¹ at transition state t-TS1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

In the reverse direction, the barrier for the O(³P) + CO₂(X ¹Σ+g) → t1 reaction is calculated to be as high as 147.7 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

Interestingly, we were able to locate a minimal energy crossing point (MSX) between the lowest triplet and singlet electronic states in a close vicinity of t-TS1, both the geometry and energy of MSX are similar to those for the triplet transition state.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

Another possible isomer of triplet CO₃, C_s-symmetric OCOO t2 (³A″) is much less favorable and lies 260.7 kJ mol⁻¹ higher in energy than O(³P) + CO₂(X ¹Σ+g).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

100

In addition, we found two local minima on PES of the first excited singlet electronic state, which have lower energies than O(¹D) + CO₂(X ¹Σ+g).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

101

For instance, s3 (C_2v, ¹A₂) is a complex of O(¹D) with carbon dioxide and stabilized by 27.6 kJ mol⁻¹ relative to the separated species.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

102

The structure of s4 (C_s, ¹A″) is similar to that of s2, however, all three C–O bonds and OCO angles are slightly unequal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

103

Transition state s-TS2 separating s3 and s4 lies 46.4 and 159.8 kJ mol⁻¹ above O(¹D) + CO₂(X ¹Σ+g) and s4, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

104

Table 3 summarizes the infrared absorptions of s1, s2, and t2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

Infrared spectroscopy

105

The FTIR spectra are analyzed in three steps.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

106

First, we investigate the new absorptions qualitatively and assign their carriers.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

107

Hereafter, the temporal developments of these absorptions upon electron irradiation are investigated quantitatively as outlined in Section 3.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

108

Finally, these data are fitted to calculate production rates of synthesized molecules in units of molecules cm⁻² (column density), molecules per impinging electron, and absorbed electron volt (eV) per target molecule (dose).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

109

The integration routine of the absorption features is accurate to ±10%.³⁴

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

Qualitative analysis

110

The effects of the electron irradiation of the carbon dioxide target are displayed in Figs. 5–11.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

111

A comparison of the pristine sample (Fig. 3) with the irradiated ice at 10 K clearly shows new absorption features of carbon monoxide at 2139 cm⁻¹ (ν₁(CO stretching); Fig. 5; Table 4)) and the corresponding isotopic pattern of ¹³CO at 2092 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

112

These data are in close agreement to matrix isolation studies of the carbon monoxide molecule.³⁵

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

113

We were also able to detect four fundamentals of the C_2v symmetric, cyclic CO₃ structure at 2045 cm⁻¹ (ν₁), 1068 cm⁻¹ (ν₂), 973 cm⁻¹ (ν₅), and 565 cm⁻¹ (ν₆) (Fig. 6).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

114

The position of all peaks is in excellent agreement with earlier matrix isolation studies (Section 1) and with our calculated, scaled frequencies (Table 3).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

115

Note that although the unobserved ν₃ and ν₄ modes have larger absorption coefficients than the detected ν₆ transition, the ν₄ absorption overlaps with the broad ν₂ band of the carbon dioxide reactant; the ν₃ band of carbon trioxide is too close to the cut-off of the MCTB detector to be observable.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

116

Finally, we detected also a transition at 1879 cm⁻¹, which was assigned tentatively as a Fermi resonance of the 2044 cm⁻¹ band with an overtone of the 973 cm⁻¹ fundamental.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

117

The calculated symmetry of the carbon trioxide modes (Table 3) confirm this tentative assignment.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

118

Since the ν₅ at 973 cm⁻¹ has b₁ symmetry, the overtone (2ν₅) holds an a₁ symmetry (b₁ ⊗ b₁ = a₁), the latter has the same symmetry as the ν₁ fundamental; this can give rise to the Fermi resonance as observed at 1879 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

119

These data make it exceptionally clear that the observed carbon trioxide molecule has a cyclic, C_2v symmetric structure.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

120

Neither the C_s nor the D_3h symmetric structures of carbon trioxide were observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

121

Besides the carbon monoxide and the carbon trioxide molecules, we were also able to identify the ozone molecule (Fig. 7).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

122

Two absorptions at 1042 cm⁻¹ (ν₃, anti symmetric stretch) and the weaker bending mode at 704 cm⁻¹ (ν₂) were identified.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

123

These data agree very well with previous assignments³⁶.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

Quantitative analysis

124

Figs. 8–11 compile the temporal development of the column densities of the carbon dioxide reactant and of the products (carbon monoxide, carbon trioxide, and ozone) during the irradiation at 10 K, the consecutive equilibration period at 10 K, and the heating phase.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

125

During the irradiation of the carbon dioxide ice, the column density of the CO₂ molecules decreases only slightly from 1.188 × 10¹⁸ cm⁻² to 1.158 × 10¹⁸ cm⁻² (Fig. 8); note that these data are afflicted with an error of ±25% (Table 2).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

126

This means that only 3.01 × 10¹⁶ cm⁻², i.e. 2.5%, of the carbon dioxide molecules are destroyed at the end of the irradiation (Table 5).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

127

Accounting for the target surface and the electron beam current, we can conclude that each implanted electron destroys 17 ± 4 CO₂ cm⁻², i.e. 51 ± 13 carbon dioxide molecules.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

128

As expected, the carbon dioxide column density stays constant during the isothermal phase.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

129

With increasing temperature the CO₂ molecules sublime; as the temperature is raised from 10 K to 20 K, strengths of all carbon dioxide absorptions start to diminish; at 94 K, no solid carbon dioxide is left on the silver waver.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

130

With decreasing carbon dioxide column density, new absorptions arise from carbon monoxide (Fig. 9) and carbon trioxide (Fig. 10).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

131

The carbon monoxide column density rises almost linearly with increasing irradiation time to 3.5 ± 0.4 × 10¹⁶ cm⁻², i.e. an average production rate of 60 ± 6 carbon monoxide molecules per implant, i.e. 20 ± 2 CO cm⁻²; the integral absorption coefficient for the 2139 cm⁻¹ band of 1.1 × 10⁻¹⁷ molecules cm⁻¹ is accurate to ±10%.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

132

On the other hand, the carbon trioxide column density rises quickly but starts to saturate toward the end of the experiment at 1.8 × 10¹⁵ cm⁻² (2044 cm⁻¹), 1.5 × 10¹⁵ cm⁻² (1067 cm⁻¹), and 1.1 × 10¹⁵ cm⁻² (972 cm⁻¹).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

133

These data translate to a synthesis of 2.5 ± 0.5 carbon trioxide molecules per electron (0.8 ± 0.2 CO₃ cm⁻² per electron; production rates are averaged over those obtained from three CO₃ fundamentals).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

134

Based on these information, we can now investigate the carbon balance of the target.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

135

Based on our integration routine, 3.0 ± 0.8 × 10¹⁶ CO₂ cm⁻² lead to the formation of 3.5 ± 0.4 × 10¹⁶ CO cm⁻² and 1.5 ± 0.3 × 10¹⁵ CO₃ cm⁻², i.e. destruction of 3.0 ± 0.8 × 10¹⁶ cm⁻²versus formation of 3.7 ± 0.4 × 10¹⁶ cm⁻²; within the error limits, we can conclude that the carbon budget is conserved in the experiment.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

136

This strongly correlates with our experimental findings that carbon dioxide and carbon trioxide are the only newly formed carbon-bearing species in our experiment.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

137

Note that whereas the carbon monoxide column density stays constant during the isothermal phase at 10 K, the carbon trioxide column density seems to decrease slightly by 5%.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

138

However, since the integration is accurate only to 10%, the drop of the 2044 cm⁻¹ absorption might be within the experimental error limits: similar the increasing carbon monoxide column density upon warming the matrix to 20 K; alternatively we might conclude that the carbon trioxide molecule starts to decompose even at 10 K. Upon heating the target, both the carbon monoxide and carbon trioxide column densities decrease.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

139

Note, however, that whereas the carbon dioxide absorptions disappear at 94 K, no carbon monoxide and carbon trioxide bands were observed at temperatures higher than 91 K. At 91 K, a column density of 5.6 ± 1.2 × 10¹⁷ CO₂ cm⁻² remains (0.21 ± 0.04 μm CO₂ ice).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

140

Since the carbon dioxide matrix sublimes in layers, those layers exposed to the vacuum sublime first.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

141

Considering an initial column density of 1.188 × 10¹⁸ cm⁻² CO₂ cm⁻² (0.48 ± 0.11 μm CO₂ ice), we can conclude that the newly synthesized molecules are formed within the first 0.28 ± 0.09 μm of the sample, i.e. those layers which are subliming first into the vacuum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

142

Once these layers have been released, the remaining carbon dioxide ice of 0.20 ± 0.04 μm does not contain any newly formed molecules.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

143

This in turn indicates that the 5 keV electrons are absorbed and induce radiation damage within the first 0.28 ± 0.09 μm of the carbon dioxide sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

144

Compared to the carbon oxides, the temporal development of ozone depicts striking differences (Fig. 11).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

145

The ozone column density increases after 60 min irradiation time to 9.3 ± 1.3 × 10¹⁵ cm⁻²; data has been calculated with an integral absorption coefficient of 1.4 ± 0.2 × 10⁻¹⁷ molecules cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

146

This requires a destruction of 2.8 ± 0.4 × 10¹⁶ CO₂ cm⁻².

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

147

Statistically, each electron generates 15 ± 3 O₃ molecules in the sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

148

Since the formation of a single ozone molecule requires the destruction of three carbon dioxide molecules to initiate three oxygen atoms, 45 ± 9 carbon dioxide molecules have to be destroyed to account for the experimentally derived ozone production rate.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

149

To account for the total oxygen balance after the irradiation, we have to include the newly synthesized carbon trioxide molecules (1.5 ± 0.3 × 10¹⁵ cm⁻²), too.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

150

Hence, the total oxygen column density of the freshly formed molecules calculates as the sum of the carbon trioxide column density plus three times the ozone column density; based on these considerations, a column density of 2.9 ± 0.4 × 10¹⁶ cm⁻² has to be generated in the carbon dioxide ice by the electrons.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

151

On the other hand, the decay profile of the carbon dioxide absorptions suggest that 3.0 ± 0.8 × 10¹⁶ CO₂ cm⁻² have been destroyed after the irradiation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

152

Since each carbon dioxide molecule fragments to carbon monoxide and atomic oxygen upon interaction with an electron, we would expect an identical number of oxygen atoms to be incorporated inside the newly formed molecules, i.e. ozone and carbon trioxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

153

Within the error limits, the oxygen balance seems to hold.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

154

During the isothermal phase, the ozone column density slightly increases from 9.3 ± 1.3 × 10¹⁵ cm⁻² to 1.0 ± 0.1 × 10¹⁶ cm⁻² during the 10 K equilibration period; this suggest that oxygen atoms, which are mobile at 10 K, are present in the carbon dioxide matrix and may react with molecular oxygen to form additional ozone.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

155

We have to keep in mind that this analysis only comprises the oxygen balance of the infrared active molecules, but not of infrared inactive species such as molecular and atomic oxygen.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

156

In the solid state, the O₂ absorptions at 1591 cm⁻¹ and 1617 cm⁻¹ hold absorption coefficients of about 10⁻²¹ cm molecule⁻¹ (^{ref. 37}).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

157

It is not surprising that we were unable to detect these transitions in our experiments.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

158

Therefore, we can conclude that molecular oxygen and oxygen atoms reside inside the carbon dioxide matrix as well.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

159

This effect is even more pronounced while the sample is heated to 60 K. Here, the ozone column density rises significantly by about 30% reaching a maximum at 1.3 ± 0.2 × 10¹⁶ cm⁻² before the column density drops sharply due to the subliming carbon dioxide matrix.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

160

This suggests that at least 1.1 ± 0.3 × 10¹⁶ cm⁻² of the oxygen atoms exist in the form of molecular or atomic oxygen.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

161

Including the enhanced ozone column density in the oxygen balance gives a column density of generated oxygen atoms of 4.0 ± 0.6 × 10¹⁶ cm⁻²versus destruction of the carbon dioxide molecules of 3.0 ± 0.8 × 10¹⁶ cm⁻².

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

162

At 92 K, the ozone absorption disappears completely.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

163

It is interesting to compare this temperature with those where the bands of carbon monoxide 91 K, carbon trioxide 91 K, and carbon dioxide 94 K vanish.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

164

As indicated in the previous section, carbon monoxide, carbon trioxide, and the oxygen atoms are formed initially in the first 0.28 ± 0.09 μm of the carbon dioxide sample.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

165

As the temperature rises, oxygen atoms diffuse and could penetrate also those regions of the carbon dioxide ice which has not been penetrated by the electrons; here, the oxygen atoms could recombine to ozone.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

166

This could explain the presence of ozone absorptions at the temperature of 92 K where absorptions of carbon monoxide and carbon trioxide are absent due to their sublimation with the outer layers of the carbon dioxide matrix into the vacuum.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

Mass spectrometry

167

It is interesting to correlate the infrared observations with a mass spectrometric analysis of the gas phase.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

168

During the annealing phase of the sample, the carbon dioxide matrix and the newly formed molecules (CO₃, CO, O₃) sublime into the gas phase.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

169

Upon heating the sample to 25 K (180 min experimental time), the carbon dioxide matrix start to sublime (Fig. 8); similarly, the embedded carbon monoxide molecules are being released into the gas phase (Fig. 9).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

170

Note that the ozone column density increases upon warming the sample due to reactions of atomic oxygen with molecular oxygen (section 4.2); at 60 K (240 min experimental time), the ozone column density decreases, too.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

171

However, the mass spectrometric results do not correlate entirely with the infrared data.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

172

Although the partial pressure of carbon monoxide begins to increase at 25 K (as expected from the infrared data), the temporal development of the partial pressure of ozone shows two distinct peaks: a small hump starting at 240 min experimental time (60 K), and a second intense peak at 291 min experimental time (Fig. 12).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

173

A similar pattern has been observed for the carbon dioxide molecule, too.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

174

To interpret the discrepancy between the infrared and mass spectrometric data, we have to keep in mind that the silver target (first stage) is annealed while the cold head is still in operation; this means that the outer aluminum cold shield, which is mounted to the second stage of the closed cycle helium refrigerator, is still cooled down.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

175

Therefore, the molecules subliming during the annealing phase of the silver target (first ozone peak) can actually re-condense onto the aluminum cold shield.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

176

Once the heat load from the cartridge heater increase also the temperature of the second cold head stage, those molecules condensed on the aluminum shield can sublime, too (second ozone peak).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

177

Note that two peaks were observed only for ozone, carbon dioxide, and oxygen, but not for carbon monoxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

178

Here, carbon monoxide does not re-condense at the 50 K aluminum cold shield since carbon monoxide ice is unstable at temperatures higher than 30 K.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

Discussion

179

Our investigations indicate that the response of the carbon dioxide system upon the keV electron bombardment is governed by an initial formation of carbon monoxide and atomic oxygen, eqns. (3)–(4).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

180

Depending on the energy transfer form the impinging electron to the carbon dioxide molecule, the oxygen atom can be generated either in its electronic ground (³P) via inter system crossing to the triplet manifold and/or excited state (¹D) on the singlet surface.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

181

CO₂(X ¹Σ+g) → CO(X ¹Σ⁺) + O(³P),CO₂(X ¹Σ+g) → CO(X ¹Σ⁺) + O(¹D).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

182

These reactions are endoergic by 532 kJ mol⁻¹ (5.51 eV) and 732 kJ mol⁻¹ (7.59 eV), respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

183

Our experiments indicate that each electron initiates 60 ± 6 CO molecules within the carbon dioxide ice (Table 5); this translates to 331 ± 33 eV and 455 ± 46 eV to form O(³P) and O(¹D), respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

184

Considering the energy of the electron of 5 keV, about 10% of its kinetic energy is utilized to cleave the carbon–oxygen bonds of the carbon dioxide molecules; note that this calculation assumes all the carbon monoxide molecules are formed in their vibrational ground states; also, the oxygen atoms have no excess translational energy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

185

However, to escape the matrix cage, the oxygen atom must have at least 0.5 eV excess kinetic energy; if its kinetic energy is less than the lattice bonding energy, at least O(¹D) can react back without an entrance barrier to recycle the carbon dioxide molecule.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

186

To fit the experimentally obtained profile of the carbon dioxide and carbon monoxide column densities, we employed the following model.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

187

The carbon monoxide molecule was assumed to ‘decay’ first order upon electron bombardment similar to a radioactive decay, i.e. it follows the velocity law (5) (the square brackets indicate the column density in cm⁻²; I_e = 4.92 × 10¹¹ s⁻¹ presents the electron current in electrons s⁻¹): −d[CO₂]/dt = k₁I_e[CO₂] = k′1[CO₂],d[CO]/dt = k₂I_e[CO₂] = k′2[CO₂].This translates to the following temporal evolutions of the column density for carbon dioxide (7) and carbon monoxide (8) with the constant a: [CO₂](t) = [CO₂](t = 0)e^k′1t,[CO](t) = a(1 − e^−k′₂t).The best fits of the carbon dioxide and carbon monoxide profiles are shown in Figs. 13 and 14, respectively, with [CO₂](t = 0) = 1.19 ± 0.30 × 10¹⁸ cm⁻², k′1 = 7.35 ± 0.20 × 10⁻⁶ s⁻¹, a = 7.1 ± 0.7 × 10¹⁶ cm⁻², and k′2 = 1.93 ± 0.1 × 10⁻⁴ s⁻¹.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

188

Accounting for the electron current, this yields k₁ = 1.5 ± 0.2 × 10⁻¹⁷ and k₂ = 3.9 ± 0.2 × 10⁻¹⁶.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

189

We now investigate the fate of the generated oxygen atoms quantitatively.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

190

Our data indicate that the carbon trioxide molecule is formed via reaction of atomic oxygen with carbon dioxide.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

191

We were able to fit the temporal development of the column density assuming that a carbon dioxide dimer decomposed to carbon dioxide, atomic oxygen, and carbon monoxide; within the matrix cage, the generated oxygen atom reacts with the non-reacted carbon dioxide to form carbon trioxide via eqns. (9) and (10): −d[(CO₂)₂]/dt = k₃I_e[(CO₂)₂] = k′3[(CO₂)₂],d[CO]/dt = k₄I_e[(CO₂)₂] = k′4[(CO₂)₂].This leads to the temporal evolution of the column density for carbon trioxide via eqn. (11):[CO₃](t) = b(1 − e^−k′4t).Fig. 15 depicts the best fit of the carbon trioxide profile respectively, with b = 1.5 ± 0.3 × 10¹⁵ cm⁻², and k′4 = 1.1 ± 0.1 × 10⁻³ s⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

192

Accounting for the electron current, this yields k₄ = 2.2 ± 0.2 × 10⁻¹⁵.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

193

Extrapolating eqns. (8) and (11) for t → ∞ and extracting the ratio of [CO](t = ∞)/[CO₃](t = ∞) calculates the fraction of released oxygen atoms reacting with carbon dioxide to 47 ± 14.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

194

This means that only 2.1 ± 0.5% of the generated oxygen atoms react to carbon trioxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

195

What might be the reason for the low fraction of oxygen atoms reacting to carbon dioxide?

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

196

The potential energy surface (Fig. 4) helps to understand this scenario.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

197

First, the experiments clearly indicated the formation of the cyclic s1 isomer; neither s2 nor t2 have been detected.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

198

Our calculations show that in the gas phase, only the addition of O(¹D) can form s1vias5 and s-TS3.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

199

This is a clear indication that the interaction of energetic electrons with the carbon dioxide molecules generates reactive oxygen atoms.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

200

However, this reaction has to pass s-TS3 which is located 5.6–7.6 kJ mol⁻¹ above the separated reactants.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

201

This requires that the O(¹D) reactant has at least 5.6–7.6 kJ mol⁻¹ excess kinetic energy to overcome the barrier.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

202

Once s1 has been formed in the solid state, the surrounding matrix can divert the internal energy (197.5 kJ mol⁻¹) of the carbon trioxide species; this stabilizes the latter and prevents an isomerization vias-TS1 to s2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

203

Non-reactive O(¹D) can be quenched in the matrix easily via intersystem crossing to the ³P ground state.³⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res42

204

On the triplet surface, ground state oxygen atoms can react solely to yield t1; followed by intersystem crossing, the latter can form s2 in the carbon dioxide matrix.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res43

205

However, since only the cyclic carbon trioxide isomer was detected, this pathway can be likely ruled out.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

206

Similarly, the electronically excited ¹A″ surface can be excluded to contribute to the formation of the cyclic carbon trioxide isomer since only s4 can be formed vias3 and s-TS2.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

207

Instead, we located the seam of crossing (MSX) which connects the triplet to the singlet surface.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

208

In the gas phase, this crossing is located close to t-TS1, and O(³P) with a sufficient amount of kinetic energy can surpass this barrier to form also s1.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

209

In the solid matrix, however, the energy of MSX is likely lowered; the exact barrier height is currently under investigation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

210

Actually, the calculated equilibrium constants for the isomerization s2 ↔ s1 correlate with the failed observation of the D_3h symmetric CO₃ isomer in the ice matrix.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

211

Here, K(10 K) = 0.002176482 means that at 10 K the concentration of s1 should be 500 times higher than that of s2, if they are in equilibrium.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

212

The spectroscopic data suggest that the remaining oxygen atoms rather react via molecular oxygen to form ozone via eqns. (12)–(13).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

213

Note that only ground state reactants have been considered; this is certainly true for the reaction during the equilibrating phase at 10 K and the annealing program to 60 K. However, during the electron bombardment, O(¹D) atoms could react, too.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con18

214

Both reactions are exoergic by 498.5 kJ mol⁻¹ and 106.5 kJ mol⁻¹, respectively and involve no entrance barrier except the diffusion energy of the oxygen atoms to migrate to the reaction site.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

215

The ability of the oxygen atoms to diffuse even at 10 K and in particular at elevated temperatures has been established previously (section 4.2. and Fig. 11).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

216

The detailed formation mechanism of ozone via eqn. (13) and/or electronically excited species is currently under investigation and might involve a short-lived cyclic ozone molecule.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con19

217

So far, we were not able to fit the temporal evolution of the ozone column density; this fit requires knowledge of the diffusion coefficient of the oxygen atom which is currently under study.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con19

218

A simple exponential fit fails as expected since the ozone is clearly a higher-order reaction product.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con19

219

O(³P) + O(³P) → O₂(X ³Σ−g),O₂(X ³Σ−g) + O(³P) → O₃(X ¹A₁).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

220

Finally, we would like to comment on the possibility to generate reactive carbon atoms via the interaction of energetic electrons with carbon dioxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

221

Recall that the slight increase of the carbon monoxide column density in the equilibration phase (Fig. 9) could indicate a recombination of diffusive carbon atoms with mobile oxygen atoms, eqn. (14), a process similar to the formation of molecular oxygen according to eqn. (12).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

222

C(³P) + O(³P) → CO(X ¹Σ⁺).Detailed electronic structure calculations depicted that a release of carbon atoms from a linear carbon dioxide molecule does not occur.³⁹

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

223

Instead, the linear carbon dioxide species has to isomerize first to a cyclic structure which lies 582 kJ mol⁻¹ higher in energy than the linear structure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

224

The cyclic isomer ring opens and forms a linear COO(X ¹Σ⁺) molecule which then loses a carbon atom in its excited ¹D state.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

225

However, our experiments identify neither the cyclic carbon dioxide nor the linear COO(X ¹Σ⁺) molecule as a reactive intermediate in our matrix.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

226

Since the carbon budget is conserved in our experiment (section 4.2.2) we suggest that the contribution of reactive carbon atoms, if any, is only minor.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

Astrophysical and planetary implications

227

Carbon dioxide ice presents also a major constituent of ices as condensed on sub-micrometre sized grain particles in cold interstellar clouds.