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Dissociative recombination of C₂H⁺ and C₂H₄⁺: Absolute cross sections and product branching ratios

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

Dissociative recombination (DR) of the hydrocarbon ions C₂H⁺ and C₂H₄⁺ has been examined at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm, Sweden).

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

Absolute DR cross sections were measured for center-of-mass collision energies between 1 meV and 0.1 eV for both ions, giving cross sections at 1 meV of 7.6 × 10⁻¹³ cm² for C₂H⁺ and 1.7 × 10⁻¹² cm² for C₂H₄⁺.

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

The dissociative recombination branching ratios were determined at minimal collision energy, showing that the carbon bond is broken in 57% of the dissociative recombination events for C₂H⁺, while this happens in only 7% of the events for C₂H₄⁺.

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

In the case of C₂H₄⁺, three-particle breakup into C₂H₂ and 2H is the dominant product channel with a branching ratio of 66%.

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

The present results are compared with previous DR measurements made at CRYRING for DR of C₂H₂⁺ and C₂H₃⁺.

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

Introduction

Dissociative recombination (DR), the process whereby a molecular ion captures a free electron and then stabilizes the capture by dissociating into neutral fragments, plays an important role in determining the chemical composition of plasmas cold enough to contain a molecular component.

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

Kinetic modeling of such plasma environments requires knowledge of both the energy dependent DR cross sections and the product branching ratios.

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

For most triatomic and larger systems, the rate constants for recombination are expected to be large (>10⁻⁷ cm³ s⁻¹ at 300 K).^1,2

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

Until recently, however, there has been little known about the product distributions that result from recombination.

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

Calculating the DR product branchings for polyatomic ions from first principles has proven to be exceedingly challenging, due to the difficulty of treating molecular dissociation of such a highly excited species along multidimensional potential surfaces.

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

The ability to study DR reactions experimentally has improved considerably with the advent of storage rings^3–5 where both energy dependent rate constants and product distributions can be measured.

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

We have examined the recombination of several small hydrocarbon ions at the CRYRING heavy ion storage ring.

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

In this paper we present absolute cross sections and product branching ratios for the dissociative recombination of two hydrocarbon ions, C₂H⁺ and C₂H₄⁺.

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

The results are compared to previously reported measurements of C₂H₂⁺ and C₂H₃⁺.^6,7

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

The significance of these results is discussed with respect to modeling the abundance of various molecular species in interstellar clouds⁸ and also to the modeling of plasma-enhanced combustion environments^9,10.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

Experiment

The experiments were carried out at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm, Sweden.

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

The experimental apparatus has been described previously.¹¹

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

Both ions were produced in a hot-cathode discharge ion source, using ethylene as the precursor compound, and extracted with a translational energy of 40 keV.

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

After mass selection, the ions were injected into the storage ring, which has a circumference of 51.6 m.

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

Using a radiofrequency accelerator system, the ions were further accelerated in the ring to 3.43 MeV (C₂H₄⁺) and 3.84 MeV (C₂H⁺), which is the maximum beam energy as limited by the magnetic rigidity of the ring.

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

The ring's vacuum system maintained the pressure below 10⁻¹¹ torr, minimizing beam losses from collisions with residual gas molecules.

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

The beam lifetime was in both cases measured to be 2.9 s.

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

After acceleration, the ions were merged with a monoenergetic, magnetically-confined electron beam over a length of 85 cm in one of the 12 straight sections of the ring.

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

The transversal temperature of the electrons was reduced to about 1–2 meV in this electron cooler section via adiabatic expansion of the electron beam.

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

Heat is transferred from the ion beam to the electron beam, resulting in a reduction of the momentum spread of the ions.

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

The 85 cm long interaction region also serves as the electron target during the recombination measurements.

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

The neutral products resulting from recombination were detected with ion implanted surface barrier detectors at a distance of approximately 3.4 m from the electron cooler.

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

At that distance, it is possible for the transversal spread of the fragments to be larger than the radius of the 900 mm² detector; therefore, a larger detector with an area of 3000 mm² was also used.

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

Although the larger detector has a higher collection efficiency, it has poorer energy resolution.

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

The detector's output signal is proportional to the energy of the incoming particle, and this signal was amplified and read by a multichannel analyzer (MCA) to give a pulse height spectrum.

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

The count rate as a function of storage time was measured by recording the amplified signal by means of a multichannel scaler (MCS) via a single channel analyzer (SCA).

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

Results

Cross section measurements

Cross section measurements were started 5 s after the ions were produced, which allowed time for the ions to relax into the ground vibrational state.

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

During this time, the electron energy was held at the cooling voltage, which implies zero center-of-mass collision energy.

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

The center-of-mass energy, E_cm, can be deduced from the electron energy, i.e. cathode voltage, E_e through where E_{e_cool} is the laboratory electron energy at cooling, which is at minimal collision energy due to the energy spread.

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

In order to measure the dissociative recombination cross section, the cathode voltage was systematically detuned to give an energy relative to the ions of 1 meV to 1 eV in the center-of-mass frame.

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

As shown in Fig. 1a, the cathode voltage was first increased rapidly from the cooling energy to a voltage corresponding to 1 eV center-of-mass collision energy.

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

It was then decreased linearly to a voltage below the cooling energy.

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

The output from the MCS, shown in Fig. 1b, represents a measure of the DR signal from the detector.

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

Since the cathode voltage was detuned linearly, the time scale corresponds to the cathode voltage.

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

At collision energies near 1 eV (times corresponding to 5 s and 6.5 s in Fig. 1), the DR cross section is very low.

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

Signals at those energies are considered to originate solely from background processes¹² and were used to determine the exponential decay of the background, which was subtracted from the signal.

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

During the cross section measurement, the ion current was intentionally kept low in order to prevent saturation of the surface barrier detectors.

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

Consequently, the ion current intensity could not be monitored directly during the measurement; instead, neutral fragments arising from collisions between the stored ion beam and the residual gas molecules were monitored with a scintillation detector, consisting of a BaF₂-window and a photomultiplier tube, in another straight section of the storage ring.

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

After the measurement with the surface barrier detectors, the ion current was increased and could then be measured directly with a current transformer, which essentially measures the weak magnetic field created by the stored ion beam, while the background was measured with the scintillation detector in the same way as during the previous measurement.

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

These two background measurements could then be related to each other to give an absolute value for the cross section.

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

The experimental DR rate coefficient in the interaction region at a given center-of-mass energy, 〈vσ〉, is determined by where C is the circumference of the storage ring, l the length of the electron cooler, and n_e the electron density.

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

N_DR and N_b are the signals resulting from DR processes and background processes, respectively.

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

The destruction rate per ion per unit time, R_b, is given bywhere v_i is the velocity of the ions, and I_i is the ion current.

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

The absolute DR cross sections are obtained by dividing the measured rate coefficient by the electron velocity.

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

The center-of-mass collision energy was corrected for the effect of electron space charge.¹³

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

In the two toroidal sections of the electron cooler, where the electrons are steered in and out of the beam path, the collision energy is different than in the straight section.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

Thus, some of the events measured at a specific center-of-mass energy actually result from processes occurring at other energies.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

Corrections have been made to account for this effect.¹⁴

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

The measured cross sections for C₂H⁺ and C₂H₄⁺ are both shown in Fig. 2.

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

Both ions exhibit large DR cross sections at low collision energy, with no resonances observed over the energy range from 1 to 100 meV.

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

At a collision energy of 1 meV, the cross sections are 7.6 × 10⁻¹³ cm² and 1.7 × 10⁻¹² cm² for C₂H⁺ and C₂H₄⁺, respectively.

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

The error bars indicate the statistical uncertainties.

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

There are also systematic errors arising from the uncertainty in the length of the electron cooler, the circumference of the storage ring, the ion beam current, and the electron density.

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

These combined errors are estimated to be 12%.

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

Integrating the energy dependent cross section, σ(E_cm), over an isotropic Maxwellian electron velocity distribution, gives the thermal rate coefficient α (in cm³ s⁻¹) as a function of the electron temperature T_e: The extracted thermal rate coefficients are shown in Fig. 3 for temperatures between 20 K and 1000 K. The 300 K rate coefficients for DR were found to be 2.7 × 10⁻⁷ cm³s⁻¹ for C₂H⁺ and 5.6 × 10⁻⁷ cm³s⁻¹ for C₂H₄⁺, with both ions exhibiting a DR rate dependence on T_e of −0.76.

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

The error bars given in Fig. 3 originate from the statistical error of the cross section.

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

Branching ratio measurements

In the dissociative recombination of C₂H⁺ the following channels are energetically accessible: where the indicated kinetic energy release is the maximum, which assumes all products are formed in their ground state.

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

For the C₂H₄⁺ ion, the following channels are energetically accessible:

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

In order to measure the branching ratios of the molecular ions as they dissociatively recombine with electrons into the various product channels shown above, a stainless steel grid with a transmission¹⁵ of 0.297 ± 0.015 was placed in front of the surface barrier detector.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

The signal from the detector was recorded with an MCA to give the energy spectra shown in Figs. 4a and 5a for C₂H⁺ and C₂H₄⁺, respectively.

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

The background signal, originating from collisions with the rest gas, is recorded with the electrons turned off.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

These spectra are shown in Figs. 4b and 5b.

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

Simultaneously with the two measurements for each ion, a background signal is recorded with the scintillation detector, making it possible to normalize the measurements and subtract the background.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

The background-subtracted DR signals for C₂H⁺ and C₂H₄⁺ are shown in Figs. 4c and 5c, respectively.

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

From these spectra, it is possible to determine the amount of each neutral fragment detected by fitting the spectra to Gaussian curves and determining the area.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

The energy scale is linear, and since the energy is proportional to the mass, each peak can be assigned to a particular fragment or sum of fragments.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

A set of linear equations is then used in order to assign the detected fragments to the different dissociation channels.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

Consider the relatively straightforward example of C₂H⁺ shown in eqn. (5).

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

The fragments resulting from dissociation events in channel α pass through the grid with probability T (transmission) and are detected as 2C + H, 2C, or H with probabilities T², T(1 − T), and T(1 − T), respectively.

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

The remaining channels are analyzed the same way, yielding the following probability matrix where N(2C + H) is the signal corresponding to detection of 2C + H fragments, and N(α) is the number of events occurring through channel α, etc. With the larger detector, the single hydrogen peak cannot be resolved, so that fragment is excluded from the analysis of the spectra.

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

As discussed above, not all the high-energy neutral fragments are collected with the smaller detector.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

In order to estimate the particle loss, a spectrum was recorded without the grid in front of the detector.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

All neutral fragments originating from the same DR event should then appear at an energy corresponding to the full ion beam energy (3.43 MeV and 3.84 MeV respectively).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

However, in the case of lost hydrogen atoms the fragments are detected at a slightly lower energy corresponding to the ion beam energy minus the energy of a hydrogen atom.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met15

This is shown in Fig. 6 for the DR of C₂H⁺.

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

A loss coefficient, c(H), representing the probability that a hydrogen atom is not detected, can then be determined.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met16

For C₂H⁺ the loss coefficient is given by where N(2C + H) is the number of counts in the 2C + H peak etc. This coefficient was determined to be 0.15 ± 0.02 for C₂H⁺ and 0.26 ± 0.02 for C₂H₄⁺.

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

For C₂H₄⁺ it is also possible that H₂-molecules from channel β miss the detector.

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

This was found to be the case in 4% of the DR events.

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

For C₂H⁺, only hydrogen from channel α can have the kinetic energy required to miss the detector; the probability for a hydrogen atom from that channel to be lost is given by where n(α) is the branching ratio for channel α.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met17

This loss factor could be implemented in the analysis through an iterative procedure by modifying the equation system in the following way

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

For C₂H₄⁺, H can be lost from both channel α and γ, while only the total number of lost fragments can be determined from the data.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

However, in the analysis it could be determined that there are undetected hydrogen atoms coming from both channels, since fragments coming only from channel α could not contribute in a sufficient amount to the number of lost fragments.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met18

The DR branching fraction for each channel is obtained after normalization, i.e.The product distributions resulting from the dissociative recombination of C₂H⁺ and C₂H₄⁺ at minimum collision energy are both shown in Table 1, together with previously reported results for C₂H₂⁺⁶ and C₂H₃⁺.⁷

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

The present measurements demonstrate that the carbon–carbon bond is broken in 57% of the DR events for C₂H⁺, while the carbon–carbon bond breaking channels account for only 7% of all products for C₂H₄⁺.

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

In the case of C₂H₄⁺, three-particle fragmentation, into C₂H₂ and two hydrogen atoms, dominates the DR process with a branching ratio of 66%.

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

In order to determine if the undetected fragments were correctly accounted for, the larger detector was used to complement these measurements.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met19

The branching fractions obtained with the small detector agreed with the data taken with the large detector.

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

Discussion

Comparing the present cross section measurements for C₂H⁺ and C₂H₄⁺ with that previously reported for C₂H₃⁺,⁷ demonstrates that at collision energies between 1 meV and 0.l eV the cross sections for C₂H₄⁺ and C₂H₃⁺ are approximately the same while that for C₂H⁺ is lower.

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

All three ions show similar energy dependences, −1.1, −1.4, and −1.2 for C₂H⁺, C₂H₃⁺, and C₂H₄⁺, respectively.

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

In the so-called direct mechanism of DR, which was first proposed by Bates,¹⁶ the electron is captured directly into a repulsive potential curve, which crosses the ionic potential curve near its minimum.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met20

When the direct process of DR dominates, the cross section is proportional to E−1cm at low collision energies due to the Coulomb attraction between the molecular ion and the electron.¹⁷

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met21

Bardsley introduced the indirect mechanism of DR in order to account for experimental rate coefficients that depart from the temperature dependence suggested by the direct mechanism.¹⁸

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met22

100

The indirect mechanism involves a sequence of two radiationless transitions, where the electron is first captured into a vibrationally excited Rydberg state, which is then subsequently predissociated by the same state that is reached in the direct process.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met22

101

The present measurements are in good agreement with the single pass merged beams experiment for C₂H⁺,^19,20 which yielded a cross section of 7 × 10⁻¹⁴ cm² at 10 meV collision energy with an energy dependence of E−1cm.

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

102

(Note that the results in ^{ref. 19} have been corrected in ^{ref. 20}, and thereby reduced by a factor of two.) The corresponding thermal rate measured in the single pass merged beams experiment was 2.7 × 10⁻⁷(T_e/300)^−0.5 cm³ s⁻¹.

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

103

Although the 300 K rate coefficients are in very good agreement, the two experiments exhibit slightly different temperature dependences, the present measurements showing the somewhat steeper temperature dependence.

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

104

No previous measurements have been made to determine the products that result from DR of C₂H⁺ and C₂H₄⁺.

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

105

As noted previously, the complexity of the DR process makes it difficult to develop a general theory capable of predicting branching ratios for DR of polyatomic molecular ions, and there have been relatively few theoretical attempts at understanding the fragmentation mechanism.

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

106

One often cited model developed by Bates²¹ argues that the most favored dissociative channels would be those necessitating the fewest rearrangements of valence bonds, implying the most likely product channel in the recombination of C₂H_n⁺ ions would be loss of a single hydrogen atom.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met23

107

Previously reported branching measurements for C₂H₂⁺ and C₂H₃⁺ have generally shown a higher degree of fragmentation than predicted by the Bates model,^6,7 and this appears to be the case for both C₂H⁺ and C₂H₄⁺ as well.

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

108

For C₂H₄⁺, a single channel does dominate the DR process, but it is the loss of two hydrogen atoms to form the very stable C₂H₂ product.

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

109

It is not known whether the mechanism involves a concerted loss of two hydrogen atoms or whether this is a two-step process in which sufficient energy remains in the initially formed DR products such that secondary decomposition occurs.

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

110

Similarly, it is not known whether the complete dissociation of the C₂H⁺ ion into 2C + H products results from a concerted or a sequential mechanism.

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

111

One use of the DR results presented here is inclusion into the chemical models used to predict abundances of complex molecular species in the dense interstellar clouds.

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

112

The abundances derived from various models are shown to depend on a number of variables including the choice of product channels and branching ratios for DR of complex ions.

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

113

For ions of the form XH_n⁺, two leading chemical models both assume dissociation product distributions in which loss of H and loss of 2H (or H₂) occur in equal probability for n > 2.

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

114

For species n≤2, the model of Millar and Nejad²² considers only loss of H while the model of Herbst and Leung²³ also allows for strong bond breaking channels, a difference that has implications for the growth of carbon chain species in molecular clouds, for example.²⁴

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

115

Experiments on XH₂⁺, i.e.n = 2 species, in storage rings have shown that complete fragmentation is the dominant pathway,²⁵ suggesting that the model of Herbst and Leung is more appropriate.

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

116

We have examined a series of small hydrocarbon molecular ions of the form C₂H_n⁺ from n = 1–4 and find that the smaller species (n = 1–2) do in fact undergo significant C–C bond breaking during recombination, while 90–95% of the DR process for the n = 3–4 species is attributed to loss of H or 2H (or H₂).

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

117

The results presented here are of importance not only to the modeling of reactions in interstellar molecular clouds,^8,24 but also to the modeling of plasma-enhanced combustion reactions.

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

118

This modeling shows that by introducing plasma into a hydrocarbon fuel reactor, the combustion is enhanced due to a reduction in ignition delay.^9,10