Dissociative Charge Transfer Reactions Research Articles

Multifold study of volume plasma chemistry in Ar/CF4 and Ar/CHF3 CCP discharges

Low-pressure RF plasma in fluorohydrocarbon gas mixtures is widely used in modern microelectronics, e.g. in the etching of materials with a low dielectric constant (low-k) materials). The multifold experimental and theoretical study of a radio frequency capacitively coupled plasma at 81 MHz in Ar/CF4/CHF3 has been carried out at 50 mTorr and 150 mTorr gas pressures. A wide set of experimental diagnostics together with hybrid PIC MC model calculations were applied to a detailed study of the plasmas. Measurements of the F atoms, HF molecules and CFx radicals, electron density, electronegativity and positive ion composition were performed. Absolutely calibrated VUV spectrometry was carried out to measure the VUV photon fluence towards the electrode. This combined experimental and model approach allowed us to establish the fundamental mechanisms of the charged and neutral species elementary reactions. Dissociative charge transfer reactions and fluoride transfer reactions influence the main ion (CF, CHF) composition in Ar/CF4/CHF3 plasma a lot. The mechanisms of heavy ion formation in Ar/CHF3 are also discussed. The important role of additional attachment mechanisms (besides dissociative attachment to the feedstock gases, CF4, CHF3) was analyzed. The catalytic chain mechanism, including the HF molecules, which defines the CFx kinetics in Ar/CHF3 plasma, was validated. This multifold approach enabled us to determine the complicated plasma chemical composition of the active species as well as the fluxes of VUV photons at the surface of the processed material, and is a result that is important for understanding low-k damage.

Ion imaging study of dissociative charge transfer in the N2+ + CH4 system

The velocity map ion imaging method is applied to the dissociative charge transfer reactions of N2(+) with CH4 studied in crossed beams. The velocity space images are collected at four collision energies between 0.5 and 1.5 eV, providing both product kinetic energy and angular distributions for the reaction products CH3(+) and CH2(+). The general shapes of the images are consistent with long range electron transfer from CH4 to N2(+) preceding dissociation, and product kinetic energy distributions are consistent with energy resonance in the initial electron transfer step. The branching ratio for CH3(+):CH2(+) is 85:15 over the full collision energy range, consistent with literature reports.

Low-Temperature Branching Ratios for the Reaction of State-Prepared N2+with Acetonitrile

In this work, the primary product branching ratio (BR) for the reaction of state-prepared nitrogen cation (N(2)(+)) with acetonitrile (CH(3)CN), a possible minor constituent of Titan's upper atmosphere, is reported. The ion-molecule reaction occurs in the collision region of the supersonic nozzle expansion that is characterized by a rotational temperature of 45 ± 5 K. A BR of 0.86 ± 0.01/0.14 ± 0.01 is obtained for the formation CH(2)CN(+) and the CH(3)CN(+) product ions, respectively. The reported BR overwhelmingly favors the formation of CH(2)CN(+) product channel and is consistent with a simple capture process that is accompanied by a nonresonant dissociative charge transfer reaction. The BRs are independent of the N(2) rotational levels excited. Apart from providing insights onto the dynamics of the title ion-molecule reaction, the reported BR represents the most accurate available low-temperature experimental measurement for the reaction useful to aid in the accurate modeling of Titan's nitrile chemistry.

Primary Branching Ratios for the Low-Temperature Reaction of State-Prepared N2+ with CH4, C2H2, and C2H4

Product branching ratios (BRs) are reported for ion-molecule reactions of state-prepared nitrogen cation (N(2)(+)) with methane (CH(4)), acetylene (C(2)H(2)). and ethylene (C(2)H(4)) at low temperature using a modified ion imaging apparatus. These reactions are performed in a supersonic nozzle expansion characterized by a rotational temperature of 40 ± 5K. For the N(2)(+) + CH(4) reaction, a BR of 0.83:0.17 is obtained for the dissociative charge-transfer (CT) reaction that gives rise to the formation of CH(3)(+) and CH(2)(+) product ions, respectively. The N(2)(+) + C(2)H(2) ion-molecule reaction proceeds through a nondissociative CT process that results in the sole formation of C(2)H(2)(+) product ions. The reaction of N(2)(+) with C(2)H(4) leads to the formation of C(2)H(3)(+) and C(2)H(2)(+) product ions with a BR of 0.74:0.26, respectively. The reported BR for the N(2)(+) + C(2)H(4) reaction is supportive of a nonresonant dissociative CT mechanism similar to the one that accompanies the N(2)(+) + CH(4) reaction. No dependence of the branching ratios on N(2)(+) rotational level was observed. In addition to providing direct insight into the dynamics of the state-prepared N(2)(+) ion-molecule reactions with the target neutral hydrocarbon molecules, the reported low-temperature BRs are also important for accurate modeling of the nitrogen-dominated upper atmosphere of Saturn's moon, Titan.

EUV Photochemical Production of Unsaturated Hydrocarbons: Implications to EUV Photochemistry in Titan and Jovian Planets

The EUV photochemistry of methane is one of the dominant chemical processes in the upper atmospheres of Titan and Jovian planets. The dilution of CH(4) with N(2) significantly changes the subsequent hydrocarbon chemistry initiated by EUV photoionization. At wavelengths below 80 nm, the presence of the dominant N(2) species in a N(2)/CH(4) gas mixture (=95/5) selectively enhances the formation of unsaturated hydrocarbons, such as benzene and toluene, while pure CH(4) gas leads to a wide mixture of saturated/unsaturated hydrocarbon species. This enhanced formation of unsaturated hydrocarbons is most likely initiated by the generation of CH(3)(+) via a dissociative charge-transfer reaction between N(2)(+) and CH(4). This mechanism was further confirmed with the dilution of CH(4) with Ar, which shows similarly enhanced formation of unsaturated species from an Ar/CH(4) (=95/5) gas mixture. In contrast, the depleted generation of unsaturated species from a H(2)/CH(4) gas mixture (=95/5) suggests that the CH(5)(+) ion generated via a proton-transfer reaction is not an important precursor in the production of complex unsaturated hydrocarbons. Therefore, it is the dissociative charge-transfer reaction of CH(4) that initiates the formation of unsaturated complex hydrocarbons through production of C(2)H(5)(+) with subsequent dissociative recombination. Implications regarding photochemistry in the upper atmospheres of Titan and the Jovian planets are discussed.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

Charge exchange in tandem mass spectrometry: dissociative single electron capture by doubly-charged toluene cations.

Single electron capture by doubly-charged toluene cations upon collision with various target gases has been investigated by sector tandem mass spectrometry. Both non-dissociative and dissociative charge transfer reactions leading to C(7)H(7)(+) + H and to C(5)H(5)(+) + [C(2),H(3)] are detected. Seven atomic or molecular target gases have been used with ionisation energies ranging from 8.8 eV to 14 eV. The branching ratios between the different non-dissociative and dissociative exit channels have been determined as well as the translational energy release on the dissociation products. The experimental data are compared to the predictions of a two-state semi-classical theoretical model that takes into account the non-adiabatic transition responsible for the charge transfer reaction. A wide reaction window shows up but the internal energies of the C(7)H(8)(+) cations produced by single electron capture are observed to be larger than expected. We assign this effect partly to the influence of the large density of vibrational states and to the multichannel nature of the process. Excited states of the dication are also most probably involved in the charge exchange reaction.

State-Selected and State-to-State Ion−Molecule Reaction Dynamics

A unique triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion−molecule reactions. By employing this TQDO apparatus, we have recently examined the absolute total cross sections for a series of state-selected ion−molecule reactions involving Ar+(2P3/2,1/2), O+(4S, 2D, 2P), and organosulfur ions (CH3SH+, CH3CH2SH+, and CH3SCH3+) in their ground states. The cross section measurements, together with product ion kinetic energy analyses, have provided convincing evidence that the Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) reactions proceed via a charge-transfer predissociation mechanism. The comparison of absolute cross sections for product ions formed in the dissociative charge transfer of Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) and those produced in photoionization of CO2 (CO, N2, O2) suggests that product ions formed by dissociative charge transfer are also produced by photoionization via a similar set of excited predissociative states of CO2+ (CO+, N2+, O2+). By preparing CH3SH+, CH3CH2SH+, and CH3SCH3+ in their ground states by photoionization of the corresponding neutrals, we have examined the dissociation of these ions via collision activation. Equipped with two radio frequency octopole ion guide reaction gas cells, the TQDO apparatus has allowed the identification of the isomeric structure of product ions by using the charge-transfer probing method. Strong preference is observed for C−S and C−C bond scissions, leading to the formation of CH3+ from CH3SH+, CH3CH2+, and CH2SH+ from CH3CH2SH+, and CH3S+ from CH3SCH3+ as compared to C−H and S−H bond breakages. The observation of these bond selective dissociation reactions is contrary to that found in photoionization of CH3SH, CH3CH2SH, and CH3SCH3 and is indicative of nonstatistical behavior for collision-induced dissociation of these organosulfur ions. An application of the radio frequency ion-guide for state-selection of O+(4S, 2D, 2P) prepared by the dissociative charge-transfer reactions of He+ (Ne+, Ar+) + O2 is described. The success of this method has made possible the absolute total cross section measurement of the state-selected ion−molecule reactions O+(4S, 2D, 2P) + N2 (O2, H2, D2, CO2, H2O), which are considered as the most important set of reactions in the Earth's ionosphere.

Dissociative Charge Transfer between Ground‐State He+and CO at Electron‐Volt Energies

The major mechanism for the destruction of CO in the ejecta of the supernova SN 1987A has been assumed to be the fast dissociative charge transfer reaction of He+(1s) + CO → products. However, no direct experimental data were available to confirm this hypothesis. In this measurement, we used the ion storage technique to measure the rate coefficient of this reaction within the temperature range of the ejecta. The measured rate coefficients are 1.33(±0.15) × 10-9, 1.48(±0.15) × 10-9, and 1.44(±0.12) × 10-9 cm3 s-1 at 2.9 × 103, 8.7 × 103, and 1.2 × 104 K, respectively. The measured rate coefficients do not appear to be temperature dependent within this temperature range. Our measured value is consistent with the value used in the chemical modeling of the supernova SN 1987A ejecta.

Resonant two-photon ionization of van der Waals adducts of 4-fluorostyrene with monomethylamine and monoethylamine: intracluster chemical reactions

Resonance-enhanced two-photon ionization of van der Waals complexes of 4-fluorostyrene clustered with some amines has been studied. The intracluster ionic chemistry leading to dissociative charge transfer and substitution reactions is discussed on the basis of experiments and structure calculations.

Resonant two-photon ionization of van der Waals adducts of 4-fluorostyrene with monomethylamine and monoethylamine: intracluster chemical reactions

Resonance-enhanced two-photon ionization of van der Waals complexes of 4-fluorostyrene clustered with some amines has been studied. The intracluster ionic chemistry leading to dissociative charge transfer and substitution reactions is discussed on the basis of experiments and structure calculations.

Transition states of charge-transfer reactions: Femtosecond dynamics and the concept of harpooning in the bimolecular reaction of benzene with iodine

We describe an approach for real-time studies of the transition-state dynamics of charge-transfer reactions. An application to the bimolecular reaction of benzene with iodine is reported. The measured 750±50 fs transient growth of the free I atom product elucidates the nature of the transition state and the mechanism for the dissociative charge-transfer reaction. The mechanism is formulated in relation to the impact geometry and the dative bonding, which are crucial to condense-phase and surface reactions, and is supported by molecular dynamics.

Dissociative electron capture in [formula omitted] collisions

Single and multiple electron capture processes are studied in slow (200–400 eV/amu) collisions between Ar 8+ and molecular nitrogen. The amount of dissociative charge transfer reactions is determined as well as kinetic energies of the ionic fragments of N 2 q+ ( q = 1–6) ions. Dissociation energies are obtained from the kinetic energies of the molecular fragments. The following results are obtained: 7% of single electron capture leads to dissociation of N 2 + into N + N + and nearly 90% of the double electron capture results in fragmentation into N + + N +. If more than two electrons are captured, all N 2 q+ ions dissociate into N r+ + N s+ . For even values of q we observed r = s, otherwise r = s + 1; i.e. the maximum difference between r and s is 1. A near symmetric charge distribution among the resulting atomic ions is preferred.

Isotope effects in the elimination of H and D atoms by dissociative charge transfer reactions in systems involving isotopic methanes

AbstractThe charge transfer reactions of H, CH, CH and N+ ions with CH4, CH3D, CH2D2 and CD4 were studied. The measurements were carried out using a tandem type mass spectrometer. The influence of the kinetic and internal energy of the incident ions was observed on the secondary mass spectra and also on the isotope effects expressed through πi(D/H), Ti(H) and Ti(D) factors. The dependence of the isotope effects on kinetic energy could indicate a conversion of kinetic energy into internal energy. Reactions in which momentum transfer resulted in charge transfer were also observed, probably indicating a ‘short‐range interaction’ reaction mechanism.

Zero kinetic energy, pulsed-field ionization spectroscopy of hydrogen iodide

The rotationally resolved, zero kinetic energy, pulsed-field ionization (ZEKE-PFI) spectrum of the HI+ X 2Π1/2, v+=0 level, obtained by double-resonance excitation via the HI F 1Δ2, v=0 level, is reported. The rotational and Λ-doubling constants for the HI+ X 2Π1/2, v+=0 level obtained from the experiment are close to those estimated theoretically by Mank et al. [J. Chem. Phys. 95, 1676 (1991)]. At higher pressures, the dissociative charge transfer reaction HI*+HI→HI++H+I− represents a potentially serious loss mechanism for the high Rydberg states that give rise to the ZEKE-PFI signal. This result is of more general applicability, because it provides evidence that collisions of the Rydberg electron with neighboring molecules can play a significant role in ZEKE-PFI experiments.

Ion-Molecule Reactions of ArN2+ with Butane and Isobutane at Thermal Energy

Abstract Product ion distributions and rate constants have been determined for thermal energy reactions of ArN2+ with n-C4H10 and i-C4H10 by using an ion-beam apparatus. C4H9+, C3Hn+ (n = 5—7), and C2Hn+ (n = 4,5) are produced from n-C4H10 with branching ratios of 6, 57, and 37%, while C4H9+ and C3Hn+ (n = 5—7) are formed from i-C4H10 with branching ratios of 13 and 87%, respectively. A comparison of the product ion distribution in the ArN2+/n-C4H10 reaction with that predicted from the fragmentation pattern of n-C4H10+ suggests that most of all fragment ions are formed through (pre)dissociation of precursor n-C4H10+ states at ca. 13.2 eV. Since this energy is close to the effective recombination energy of ArN2+ (ca. 13.5 eV), it is concluded that the ArN2+/n-C4H10 dissociative charge-transfer reaction proceeds through near-resonant n-C4H10+ states. The total rate constants are (6.9 ± 2.3) × 10−10 cm3 s−1 for n-C4H10 and (9.0 ± 2.6) × 10−10 cm3 s−1 for i-C4H10, which amount to 58 and 75% of the collision rate constants estimated from Langevin theory, respectively.

Qualitative and quantitative response characteristics of a capillary gas chromatograph/ion mobility spectrometer to halogenated compounds

The response of a capillary column gas chromatograph/ion mobility spectrometer (GC/IMS) system to chlorinated and brominated alkanes and alkenes in nitrogen, air and nitrogen/CO 2 mixtures was studied. Product ions were formed through dissociative electron capture processes or charge transfer reactions, yielding chloride or bromide ions. The quantitative response was shown to depend on the composition of the carrier gas in which atmospheric pressure ionization processes occurred as well as on the IMS cell temperature. Large variations in the quantitative response of the GC/IMS to different halogenated compounds were observed. The results were compared to those reported for the doped electron capture detector (ECD). Practical implications of the GC/IMS system as a monitor and detector for halogenated aliphatic compounds are discussed.

Dissociative charge-transfer reactions of Ar+ with simple aliphatic hydrocarbons at thermal energy

A flowing-afterglow apparatus coupled with a low pressure chamber has been used to measure product ion distributions and rate constants in the charge-transfer reactions of Ar+ with CH4, C2Hn(n=2,4,6), and C3Hn(n=6,8) at thermal energy. Only parent cation is formed for C2H2 due to energy restriction. Major product channels are dissociative charge transfer followed by cleavage of C–H bond(s) for CH4, C2H4, C2H6, and C3H6, while by cleavage of a C–C bond for C3H8. A comparison of the product ion distributions with the photoelectron–photoion coincidence data for CH4, C2H4, and C2H6 leads us to conclude that the mean energies of precursor (pre)dissociative states are 15.3–15.5 eV, which are 0.3–0.5 eV below the resonance states. Thus the fractions of available energy deposited into internal modes of precursor parent ions at the instant of charge transfer are estimated to be 85%–95%, indicating that most of the CT reactions occurs without significant momentum transfer. The total rate constants for CH4, C2Hn(n=4,6), and C3Hn(n=6,8) are (0.78–1.1)×10−9 cm3 s−1, corresponding to 60%–92% of the calculated values from the Langevin theory. The rate constant for C2H2, 4.2×10−10 cm3 s−1, amounts to 38% of the kcalcd value. The small kobsd/kcalcd ratio is attributed to the lack of ionic states with favorable Franck–Condon factors for ionization.

Dissociative charge-transfer reactions of Ar+ with fluoromethanes at thermal energy

A flowing-afterglow apparatus coupled with a low pressure chamber has been used to measure product ion distributions and rate constants in the charge-transfer reactions of Ar+ with CHnF4−n (n=1–3) at thermal energy. Near-resonant dissociative charge transfer followed by loss of H and/or F are major product channels observed. In the Ar+/CH3F reaction, CHF+ and CH2+ ions, which are either absent or very weak in He(i) photoionization, are also found as minor products. The product ion distributions suggest that the Ar+/CHnF4−n (n=1–3) reactions proceed dominantly through near-resonant charge transfer. The total rate constants are 1.7±0.6, 1.9±0.7, and 2.0±0.6×10−9 cm3 s−1 for CH3F, CH2F2, and CHF3, respectively. These values are in reasonable agreement with those predicted from the average dipole orientation (ADO) theory, being independent of the existence of ionic states with favorable Franck–Condon factors for ionization.

Dissociative charge-transfer reactions of Ar + with chloromethanes at thermal energy

Charge-transfer reactions of Ar + with CH n Cl 4− n ( n=1−3) at thermal energy have been studied by using a flowing-afterglow apparatus coupled with a low pressure chamber. Dissociative charge-transfer leading to CH + 3 (91 ± 5%), CH 2Cl + (95 ± 2%), and CCl + (69 ± 3%) are major product channels for CH 3Cl, CH 2Cl 2, and CHCl 3, respectively. Total rate constants are (1.3 ± 0.5) × 10 −9, (0.97 ± 0.42) × 10 −9, and (1.2 ± 0.6) × 10 −9 cm 3s −1 for CH 3Cl, CH 2Cl 2, and CHCl 3, respectively. These values amount to 57%–80% of the collisions rate constants estimated from either the Langevin or ADO theory.