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Back | Show only these items: | Photodissociation dynamics of pyrrole: Evidence for mode specific dynamics from conical intersections

1
Photodissociation dynamics of pyrrole: Evidence for mode specific dynamics from conical intersections

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

The H and D atom elimination mechanisms in the photodissociation of jet cooled pyrrole and pyrrole-d₁ have been studied by photofragment velocity map imaging.

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

The molecules were excited to the 1 ¹A₂ (πσ*) state at λ = 243 nm and to the 1 ¹B₂ (ππ*) state at λ = 217 nm.

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

H/D atoms were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI) at λ = 243 nm.

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

The analysis of the images and the resulting translational energy distributions from the 1 ¹A₂ state demonstrates the existence of two decay pathways, fast mode-specific cleavage of the NH bond in the excited state (channel A) and internal conversion (IC) to the electronic ground state (S₀) followed by unimolecular decomposition of the vibrationally hot S₀ molecules (channel B).

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

The angular distributions of the H/D atoms from the direct dissociation in the excited state are strongly anisotropic, whereas the decay of the S₀ molecules leads to spatially isotropic distributions.

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

The results at λ = 217 nm indicate that the 1 ¹B₂ state undergoes an ultrafast radiationless transition to 1 ¹A₂ followed by the abovementioned direct mode-specific NH bond fission on the 1 ¹A₂ potential energy surface (channel A′) or conversion to S₀ and subsequent unimolecular decomposition (channel B′).

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

The latter pathway may also be initiated by a direct relaxation from 1 ¹B₂ to S₀.

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

The anisotropy parameter of β ≈ –1 for the direct NH bond fission at λ = 217 nm is in accordance with the expectations for a perpendicular electronic excitation and a dissociation lifetime that is short compared to the rotational period of the molecules.

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

The fast decay dynamics of both excited electronic states can be rationalized with reference to the theoretically predicted conical intersections between the ππ*, πσ*, and S₀ potential energy surfaces and the antibonding nature of the πσ* potential energy surface with respect to the NH bond [A. L. Sobolewski, W. Domcke, C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys. 2002, 4, 1093].

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

Introduction

Recent excited state ab initio calculations for H atom photodetachment processes from NH-containing heteroaromatic compounds, aromatic amines, or phenols have suggested a new paradigm for the photochemical dynamics in these systems.¹

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

Accordingly, the fate of the photoexcited molecules is controlled by conical intersections between the usually strongly absorbing first excited ¹ππ* state, a neighboring optically dark ¹πσ* state, and the S₀ electronic ground state.

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

The key role in the ensuing dynamics is played by the πσ* state which becomes repulsive along the NH or OH stretching coordinates and intersects the S₀ state at long NH/OH bond lengths because the latter does not adiabatically correlate with the respective dissociation products in their electronic ground states.

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

Conical intersections between the ππ* and πσ* and the πσ* and S₀ potential energy hypersurfaces thus open efficient pathways for radiationless electronic transitions leading to an ultrafast relaxation of the excited molecules to the electronic ground state and/or to a fast NH/OH bond fission and corresponding dissociation of the molecules.

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

These mode specific channels have been postulated to have great importance for the photochemical properties of the nucleic acid bases and the biologically relevant aromatic amino acids,^2–6 for intramolecular H atom transfer reactions in excited electronic states (ESIHT),⁷ and for photo-induced intermolecular H atom transfer reactions in hydrogen-bonded molecular complexes.^6,8–10

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

The pyrrole molecule is an almost ideal model system for which the photochemical dynamics can be studied in detail.

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

In addition, it is a building block of many biologically important compounds which may show related mechanisms.^1,11

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

Last but not least, it plays a role in the synthesis of biologically active compounds, pesticides, organic polymers, and organometallic magnets.¹²

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

The UV absorption spectrum of pyrrole shows two strong bands, the first with an intensity maximum near λ = 210 nm, the second near 165 nm.

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

In addition, a very weak feature is observed with a maximum around 240 nm.^13–16

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

The spectrum between 190 and 260 nm is reproduced in Fig. 1.

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

As can be seen, the absorption bands are very broad and unstructured and this has hampered their analysis for a long time.

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

Further work has been performed by electron energy-loss spectroscopy,¹⁵ photoelectron spectroscopy,^17–19 and resonance enhanced multiphoton ionization (REMPI).²⁰

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

In addition, the experimental studies were complemented by theoretical electronic structure investigations.^15,21–23

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

The most recent quantum chemical calculations by Roos et al²³. showed that the bulk of the intensity of the 210 nm band arises from the intra-valence transition to the 1 ¹B₂ (ππ*) state.

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

A small contribution (≈15%) may also come from the neighboring 2 ¹A₁ (ππ*) state.

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

The 165 nm band has been attributed to the excitation to the 1 ¹B₁ and 2 ¹B₂ states which belong to the 3p Rydberg series.

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

The analysis of the weak 240 nm band has, however, been controversial.

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

The respective transition has traditionally been assigned to the 1 ¹A₂ (3s) Rydberg state.^22,23

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

The observation of this electronically forbidden band can be explained by a vibronic coupling with the 1 ¹B₂ (ππ*) state induced by out-of-plane vibrations (b₁ symmetry).

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

One earlier paper has also assigned the 240 nm band to the excited 2 ¹A₁ valence state.²⁴

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

However, Sobolewski and Domcke^1,11 recently demonstrated that the stretching of the NH bond in the 1 ¹A₂ state leads to a Rydberg-to-valence transformation by which that bond becomes anti-bonding, i.e., the electron configuration becomes πσ*.

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

The Rydberg character remains evident by the diffuseness of the σ* orbital, but the πσ* character was postulated to be responsible for a fast mode-specific N–H bond dissociation and, through a conical intersection with the S₀ state, for an efficient radiationless relaxation of the electronically excited molecules.

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

Moreover, a fast predissociation of the ππ* state by the πσ* state was suggested to explain the apparently extremely short lifetimes of the excited states of pyrrole and the resulting absence of observations of laser induced fluorescence.

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

Direct experimental evidence for these theoretically predicted processes had, however, been missing.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot4

In a recent communication, we reported on first results of an investigation of the formation of H atoms from pyrrole and, for comparison, N-methylpyrrole in their lowest excited electronic states by photofragment velocity map imaging.²⁵

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

Pyrrole was excited to the 1 ¹A₂ (πσ*) state at λ = 243 nm, and the nascent H atoms were detected at the same wavelength by 2 + 1 REMPI.

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

The observed H atom velocity maps showed the existence of two dissociation channels.

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

The first was found to produce very fast H atoms and appeared to be due to a rapid direct NH bond cleavage in the excited electronic state, as predicted by the theoretical studies.^1,11

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

The H atom kinetic energy distribution had a strong, narrow peak at high translational energies.

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

The less important second channel was observed to lead to much slower H atoms with a very broad kinetic energy distribution, consistent with a statistical formation via unimolecular decay reactions in the electronic ground state after internal conversion from the excited state.

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

This conclusion was supported by the results for N-methylpyrrole which showed only H atoms corresponding to the second channel.

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

The experimental results²⁵ thus corroborated the proposal by Sobolewski and Domcke^1,11 that the lowest excited electronic state of pyrrole has πσ* character and is anti-bonding with respect to the stretching of the N–H bond.

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

In this discussion paper, we present the results of an extended study of the photochemical dynamics of normal pyrrole (pyrrole-h₁) and selectively deuterated pyrrole (pyrrole-d₁) at two excitation wavelengths, λ = 243 nm and λ = 217 nm.

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

The reactions studied can be written as C₄H₄NH + hν → H + C₄H₄NandC₄H₄ND + hν → D + C₄H₄N → H + C₄H₃NDThe two excitation wavelengths (243 and 217 nm) will be indicated below by unprimed and primed labels, where appropriate.

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

In the first set of experiments (λ = 243 nm, one colour), we probed the H and D atom photodetachment from both isotopomers via the 1 ¹A₂ (πσ*) state.

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

The obtained new D atom images from pyrrole-d₁ (reaction (2d)) were seen to exhibit an efficient ultrafast mode-specific dissociation of the ND bond of the molecules on the πσ* potential energy surface, confirming the corresponding interpretation of our earlier experiments²⁵ and providing direct evidence for the theoretical predictions of Sobolewski and Domcke.^1,11

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

In the second set of experiments, we performed two-colour measurements, in which the molecules were excited to the 1 ¹B₂ (ππ*) state at λ = 217 nm and the H atoms were again probed at λ = 243 nm.

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

The results of these experiments corroborate the postulate that the 1 ¹B₂ (ππ*) state is predissociated by the repulsive πσ* state.

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

Experimental section

The photofragment velocity map imaging apparatus built in our laboratory will be described in some detail elsewhere.²⁶

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

The basic set-up resembles that of Eppink and Parker.²⁷

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

A gas mixture containing ≈0.1% of pyrrole in He was prepared by bubbling the seed gas through a sample of liquid pyrrole stored in a glass reservoir at –20 °C and 1 bar pressure.

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

The gases expanded into a differentially pumped stainless steel vacuum chamber through a solenoid actuated pulsed valve (General Valve #9) operated at 10 Hz repetition rate with ≈350 µs pulse^–1 opening times.

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

A molecular beam entered the second vacuum stage through a 1 mm diameter electroformed conical skimmer (Beam Dynamics).

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

Based on our experience with other molecules under similar expansion conditions,²⁸ the rotational temperature of the pyrrole in the molecular beam was estimated to be of the order of T_rot ≈ (20 ± 10) K.

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

The photolysis/probe laser beams intersected the molecular beam at right angle halfway between the repeller and extractor plates of the Wiley-McLaren ion imaging electrode assembly.

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

A liquid nitrogen cryo-pump surrounding the arrangement minimized the residual hydrocarbon background.

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

The laser radiation at λ = 243 nm (≈0.05–1.4 mJ) was generated by frequency-doubling the output of a Nd:YAG pumped dye laser (Quanta-Ray GCR-3/Lambda Physik FL-3002) in a BBO I crystal and focused into the molecular beam with an f = 1 m lens.

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

Light at λ = 217 nm (≈0.1–2.0 mJ) was generated by frequency-doubling the output of a XeCl excimer pumped dye laser (Lambda Physik EMG 201/Lambda Physik FL-3002) in a BBO II crystal.

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

To avoid a photolysis of the pyrrole with the λ = 243 nm probe light in the two-colour experiments, the probe power was lowered until the 243 nm one-colour signal in the absence of the 217 nm photolysis beam had virtually disappeared.

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

The probe beam was time-delayed by ≈15 ns with respect to the counterpropagating photolysis beam.

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

The laser polarizations were oriented with the electric field vectors perpendicular to the plane defined by the molecular beam and the laser beams using Wollaston polarizers and Fresnel double rhombs.

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

All imaging measurements were made under velocity mapping conditions.²⁷

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

The probe laser was periodically scanned by ±6 cm^–1 resp. ±4 cm^–1 around the line centers of the recoiling H and D atoms to cover their complete Doppler profiles.

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

The resulting H⁺/D⁺ ions were detected by a microchannel plate (MCP) detector coupled to a phosphorescence screen.

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

The obtained signals were recorded with a CCD camera (640 × 480 pixels, LaVision) and accumulated over up to 200 000 laser shots using single ion counting and centroiding²⁹ to improve the detection sensitivity and spatial resolution and discriminate against noise.

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

Background images were taken under the same experimental conditions with the molecular beam valve closed.

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

Mass selectivity was achieved using a fast transistor switch (Behlke) to gate the MCP voltage.

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

Normal pyrrole (Aldrich, 98%) was used after repeated fractionated distillation until the liquid was clear.

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

Catalytic reactions in the stainless steel supply pipe were suppressed by maintaining a slow flow of the pyrrole/carrier gas mixture through a 1 mm id teflon tube extending in the supply line from the sample reservoir all the way to the molecular beam nozzle.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

Pyrrole-d₁ was prepared from the normal isotopomer by repeated stirring with excess D₂O.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

The final product was dried over Na₂CO₃.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

The deuteration (>97%) was checked by NMR spectroscopy.

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

The gas supply lines of the apparatus were thoroughly purged overnight with D₂O vapour before the measurements with the deuterated sample to prevent isotope exchange with H₂O at the walls.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

Results

The two-dimensional (2D) velocity mapped H/D images provided by the experiments at λ = 243 nm and λ = 217 nm were inverse Abel transformed³⁰ to obtain the respective three-dimensional (3D) velocity distributions using the basis-set expansion method of Dribinski et al.³¹

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

The required calibrations were performed by measuring H/D photofragment images from HBr/DBr and HI/DI, for which the dissociation energies are precisely known (D00(HBr) = 30 210 cm^–1, D00(HI) = 24 630 cm^–1).^32–35

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

The results obtained at the two excitation wavelengths will be considered separately.

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

λ = 243 nm

H/D velocity maps

The H atom image from pyrrole-h₁ and the D and H atom images from pyrrole-d₁ recorded in the one-colour photodissociation experiments at λ = 243 nm are displayed in Fig. 2a–c in the upper row.

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

The respective 3D velocity distributions are given in the second row.

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

All images show a clearly visible, spatially anisotropic outer ring corresponding to very fast H/D atoms with a sharply peaked speed distribution and a diffuse, spatially isotropic inner region reflecting slower H/D atoms with a broad speed distribution.

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

With the data for the deuterated molecule, these distinctive dissociation channels can be identified with some confidence.

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

Reiterating first the conclusions for the case of pyrrole-h₁ (reaction (1), Fig. 2a),²⁵ the fast H atoms can be attributed to the direct NH dissociation in the πσ* state (referred to in our earlier paper and in the following as channel A) and the slower H atoms may be ascribed to unimolecular decay reactions after internal conversion (IC) to the S₀ state (referred to as channel B).

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

However, the evidence for the assignment of the fast H atoms to the NH group had so far only been circumstantial.²⁵

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

The present results provide the missing additional information: In particular, it can be seen from the image for reaction (2d) in Fig. 2b that the sharp ring of fast D atoms from pyrrole-d₁, for which the site of the D atom is unambiguous, is even more striking than that of the H atoms from pyrrole-h₁ (Fig. 2a).

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

At the same time, the slow D atom signal from pyrrole-d₁ is the by far weakest in the series.

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

These results demonstrate clearly that the fast D atoms originate from the N position and not from any of the four C positions of the parent molecule.

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

In contrast, pyrrole-d₁ shows the highest intensity of slow H atoms in the series of the three molecules (Fig. 2c, channel B), although there is also a small fraction of fast H atoms from pyrrole-d₁.

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

The latter observation may indicate the existence of a third pathway (referred to in the following as channel C, see discussion).

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

The relative yields of D and H from pyrrole-d₁ were determined by recording a REMPI spectrum (Fig. 3).

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

From the ratio of the integrated signals, the ND group accounts for ≈(74 ± 5)% of the observed atoms, while the four CH sites account for ≈(26 ± 5)%.

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

On a per bond basis, CH dissociation is therefore really only a minor process.

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

In the following, the attention will be focused mainly on the direct dissociation dynamics via channel A.

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

Translational energy distributions

The total center-of-mass (cm) translational energy distributions P(E_T) of the photofragments which follow by integrating the 3D velocity distributions (Fig. 2) over the angular directions and applying linear momentum conservation for the two fragments are given in Fig. 4a–c.

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

The areas under the experimental curves in the three panels were normalized with respect to each other to reflect the respective branching ratios.

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

The clearly bimodal distributions nicely confirm the existence of the aforementioned distinctive dissociation channels that were apparent from the images.

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

In order to determine quantitative data for a comparison of the three isotopomers and their dynamics at the two photoexcitation wavelengths, the P(E_T) profiles were fitted by using a sharply peaked Gaussian to represent the contributions of the fast atoms and a simple polynomial to account for the broad background of slow atoms.

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

As seen, the experimental profiles could be nicely described.

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

Alternatively, the fast atoms were also modeled using a three-parameter trial function of the form³⁶P(E_T) = E_avl[faT(1 – f_T)^b],where f_T is the fraction of the available excess energy (E_avl) channeled into fragment translation.

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

This flexible fitting function has the advantage that E_avl, which is given by the photon energy minus the dissociation energy of the molecules, can be determined by the fitting.

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

100

Apart from this point, however, the differences between using this expression or the Gaussian were negligible.

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

101

The parameters from the fits are compiled in Table 1.

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

102

In addition, the table lists the relative yields (branching ratios) y_i of the different dissociation channels given by the areas under the respective curves.

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

103

For reaction (2), the y_i values account for the branching ratio between (2d) and (2h) from Fig. 3 so that the total yield is ∑_i y_i = 1.

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

Energy balance and determination of the NH/ND bond dissociation energy

104

The energy balance for the dissociation according to channel A is given by E_hν = D00 + E_T + EPyVR – EPVR.E_hν is the photon energy, D00 the NH/ND bond dissociation energy, E_T the total translational energy of the two fragments, EPyVR the vibration–rotation energy of the pyrrolyl co-product, and EPVR the internal energy of the parent molecules.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

105

Considering the efficient cooling of the molecules in the molecular beam expansion, the latter term is small and may be neglected.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

106

Thus, the available energy partitioned among the translational and internal (vibration–rotation) degrees of freedom of the fragments is E_avl = E_hν – D00 = E_T + EPyVR.Since the fastest H/D atoms may be assumed to correspond to internally cold pyrrolyl radicals (EPyVR ≈ 0), E_avl can be determined from the sharp cut-off of the P(E_T) distribution at the allowed maximal value (EmaxT) of E_T , eqn. (I).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

107

The data are given in Table 1.

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

108

The corresponding values for the bond dissociation energy according to eqn. (III) are

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

109

D00(NH) = (31 000 ± 500) cm^–1,

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

110

D00(ND) = (31 500 ± 500) cm^–1.

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

111

Furthermore, it can be seen from Table 1 that on average about 67% of the available energy is channeled into relative fragment translation, 33% is deposited as internal energy in the pyrrolyl radical.

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

Recoil anisotropies

112

The photofragment angular distributions P(θ) are obtained in principle by integrating the 3D velocity maps in Fig. 2 over the radii.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

113

The resulting distributions can then be fitted using the standard expression³⁷P(θ) = (4π)^–1[1 + βP₂(cos θ)].θ is the angle between the recoil direction and the laser polarization, P₂(cos θ) is the second-order Legendre polynomial, and β is the anisotropy parameter.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

114

In the limiting case of a single dissociation channel resulting from a pure parallel or perpendicular electronic transition, β is known to take the values +2 or –1, respectively, as long as the dissociation is fast compared to the rotation of the molecules.

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

115

In the present case, we have an anisotropic (β_A < 0) distribution (channel A) above an isotropic (β_B = 0) background (channel B).

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

116

In order to separate both contributions as well as possible, we integrated the 3D velocity maps by considering only the anisotropic outer shells with E_T in the range 5000 cm^–1 ≤ E_T ≤ 8000 cm^–1.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

117

The resulting P(θ) distributions from reactions (1) and (2d) are presented in Fig. 5.

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

118

The values for the effective asymmetry parameters (β^eff) which were fitted to these data are given in Table 1.

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

119

The anisotropy parameters for channel A (β_A) listed in Table 1 in the last column were then found by correcting β^eff for the relative fractions of the two channels in the integration range.

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

120

The data confirm the assumption of a preferentially perpendicular distribution.

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

λ = 217 nm

H/D velocity maps

121

The H/D velocity maps obtained after excitation of the 1 ¹B₂ (ππ*) state at λ = 217 nm in the two-colour experiments and the resulting 3D velocity distributions are depicted in Fig. 6a–c.

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

122

In principle, the images look similar to those seen before at λ = 243 nm.

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

123

In particular, they exhibit again a sharp, anisotropic outer ring reflecting fast H/D atoms with a narrow speed distribution and a broad, structureless inner feature corresponding to much slower atoms.

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

124

The sharp rings in the H atom image from reaction (1′) in Fig. 6a and the D atom image from reaction (2d′) in Fig. 6b are again attributed to a fast dissociation of the NH/ND bond (referred to in the following as channel A′).

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

125

Since the excited 1 ¹B₂ (ππ*) state does not correlate directly with H/D + pyrrolyl, this dissociation is assumed to proceed via the lower-lying πσ* state, in accordance with the theoretical predictions.^1,11

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

126

The structureless inner signal is again ascribed to unimolecular decay processes in the S₀ state (channel B′).

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

127

In addition, pyrrole-d₁ also leads to H atoms (Fig. 6c, channels B′ and C′).

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

128

A REMPI scan (Fig. 7) gave relative yields of ≈(40 ± 5)% D and ≈(60 ± 5)% H (see discussion).

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

Translational energy distributions

129

The total translational energy distributions P(E_T) at λ = 217 nm are given in Fig. 8.

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

130

The data were fitted in the same way as above; the results are collected in Table 2.

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

131

Considered on a per bond basis, the direct fission of the NH/ND bond (channel A′) can be seen to remain a major dissociation pathway.

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

132

However, compared to the results at λ = 243 nm, the data at λ = 217 nm show significantly higher yields for channel B′.

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

133

In line with the higher photon energy, the P(E_T) distributions for both channels A′ and B′ extend to higher translational energies (i.e., higher EmaxT).

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

134

At the same time, the distributions are significantly broader.

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

135

However, the cut-off of channel A′ at EmaxT is not as well defined at λ = 217 nm than at 243 nm.

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

136

Indeed, the increase of the apparent EmaxT is smaller than the increase in the photon energies (ΔE_hν = 4900 cm^–1).

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

137

Furthermore, the distributions at the two wavelengths reach their peaks at practically the same energies (EpeakT).

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

138

These findings are reflected by a correspondingly smaller fraction of the available energy funneled into fragment translation (43% at λ = 217 nm compared to 67% at λ = 243 nm).

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

139

Thus, in summary, a higher fraction of the excess energy appears to be deposited in internal degrees of freedom of the pyrrolyl radical.

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

Recoil anisotropies

140

Despite the greater contributions of channel B′, the images at 217 nm show stronger spatial anisotropy than those at 243 nm.

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

141

The angular distributions P(θ) from reactions (1′) and (2d′) are presented in Fig. 9.

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

142

The effective asymmetry parameters β^eff and the resulting β_A′ values are listed in Table 2.

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

143

The average value of β_A′ = –0.95 ± 0.05 is practically equal to the perpendicular limit of β = –1.

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

Discussion

144

The photodissociation of pyrrole has first been studied by Blank et al. using photofragment translational spectroscopy (PTS).³⁸

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

145

The authors performed experiments at two fixed wavelengths, λ = 248 nm and 193 nm.

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

146

The reported results for the formation of H atoms at both wavelengths are comparable to those obtained in the present work, although the underlying mechanisms could not be elucidated in detail at the time because of the lack of information on the excited electronic states of pyrrole.

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

147

However, the PTS results are of interest for this discussion because of the information on the branching ratios of the ensuing channels.

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

148

At λ = 248 nm, H + pyrrolyl were the only observed products.

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

149

At 193 nm, the authors also observed substantial amounts (≈50%) of products from ring opening reactions.

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

150

The experimental results presented in this paper demonstrate clearly that the photochemical mechanisms of pyrrole in the near UV can only be rationalized by strong interactions between two excited electronic states in the region, 1 ¹A₂ and 1 ¹B₂, and the electronic ground state S₀.