Threshold Electron–secondary Ion Coincidence Research Articles

Electronic state dependence of the ion–molecule reaction CH3CN+ + CH3CN → CH4CN+ + CH2CN: threshold electron–secondary ion coincidence (TESICO) and direct ab initio molecular dynamics study

The ion-molecule reaction, CH(3)CN(+) + CH(3)CN → CH(3)CNH(+) + CH(2)CN, has been investigated using the threshold electron-secondary ion coincidence (TESICO) technique. Relative reaction cross sections for two microscopic reaction mechanisms, i.e., proton transfer (PT) from the acetonitrile ion CH(3)CN(+) to neutral acetonitrile CH(3)CN and hydrogen atom abstraction (HA) by CH(3)CN(+) from CH(3)CN, have been determined for two low-lying electronic states, (2)E and (2)A(1) of the CH(3)CN(+) primary ion. The cross section for PT of the (2)A(1) state was smaller than that of the (2)E state, whereas that of HA are almost the same in the two states. Ab initio calculations showed that the dissociation of the C-H(+) bond of CH(3)CN(+) is easier in the (2)E state than that in the (2)A(1) state. The direct ab initio molecular dynamics (MD) calculations showed that two mechanisms, direct proton transfer and complex formation, contribute the reaction dynamics.

State-Selected Ion-Molecule Reactions Studied by the Threshold Electron-Secondary Ion Coincidence (TESICO) Technique, Ar+(2P3/2, 2P1/2) + HCl

Abstract The reactions of Ar+ with HCl were studied for the two spin-orbit states of Ar+, 2P3/2 and 2P1/2, by the threshold electron-secondary ion coincidence (TESICO) technique. State selected relative reaction cross sections of the two product channels, hydrogen abstraction (HA) to form ArH+, and charge transfer (CT) to form HCl+, were determined at five collision energies in the range of 0.71–3.5 eV. At low collision energies, the cross sections of both channels show no substantial difference between the two spin-orbit states. At high collision energies, the cross sections for the HA channel also show no difference between the two states, whereas those for the CT channel show significant dependence on the spin-orbit state, the cross sections for Ar+(2P1/2) being always larger than those for Ar+(2P3/2). Combining these results with the coincidence time-of-flight (TOF) spectra, a direct reaction mechanism is suggested for both channels. The importance of the nonadiabatic transitions between the surfaces correlating to the Ar+ + HCl and Ar + HCl+(A2Σ) states in leading the reaction is proposed. Proton transfer follows the nonadiabatic transition for the HA channel and radiative or nonradiative transition follows for the CT channel.

State-selected charge transfer reaction of CO + (X, v; A, v′) with Ar

State-selected charge transfer reactions of CO + (X, v; A, v′) with Ar have been studied for v = 0–9 and v′ = 0–2 using the threshold electron secondary ion coincidence (TESICO) technique. In the endoergic CO + (X) + Ar → Ar + + CO reaction, the cross-section has been found to be greatly enhanced by vibrational excitation to v ⩾ 7. The cross-sections of the exoergic CO + (A) + Ar → Ar + + CO reaction have been found to be generally large, having values of the order of 10 −15 cm 2, and to show particular enhancement at v = 1. The experimental results have been discussed in conjuction with the potential energy surfaces calculated by G. Parlant, P. Archirel and E.A. Gislason (J. Chem. Phys., 92 (1990) 1211).

Mechanisms and rate constants of the collisional deactivation of CO+(A 2Π,v) by He and Ar

Collisional deactivation processes of CO+(A 2Π,v) by He and Ar are investigated in detail using both a time-resolved laser induced fluorescence and a threshold electron-secondary ion coincidence technique. Three deactivation mechanisms, pure vibrational relaxation, internal conversion, and charge transfer reaction, are identified. The time-resolved fluorescence from both the initially photoexcited and vibrationally relaxed levels of CO+ A 2Π makes clear the two distinct vibrational relaxation mechanisms: indirect two-step population of the lower levels in the CO+ (A)+He system and direct transition to the lower levels in the CO+(A)–Ar system. Threshold electron-secondary ion coincidence (TESICO) experiments demonstrate that the charge transfer reactions CO+ (A,v)+Ar→Ar++CO contribute significantly to the overall fluorescence quenching by Ar.

The study of the internal and collision energy dependence of the two microscopic reaction mechanisms in the ion–molecule reactions MH++MH→MH+2+M (MH=CH3F, CH3Cl, CH4)

The ion–molecule reactions of the type MH++MH→MH+2 +M (MH=CH3F, CH3Cl, CH4) have been investigated using the threshold electron secondary ion coincidence (TESICO) technique. Two peaks observed in the time-of-flight coincidence spectra of mass-selected product ions MH+2 have been interpreted as arising from the two microscopic reaction mechanisms of the MH+2 formation, i.e., the hydrogen atom abstraction by the ion (HA) and the proton transfer from the ion (PT). Relative reaction cross sections (σHA and σPT) have been determined for each of the two mechanisms as a function of both the internal energy of the reactant ion and the center-of-mass collision energy. The results indicate that these reaction systems are classified into two types according to the relative importance of the two mechanisms and its energy dependence: Type (I) in which the ratio σPT/σHA is almost unity and independent of the total energy (CH3Cl system) and Type (II) in which PT dominates over HA at all energies with the ratio σPT/σHA increasing with the total energy (CH3F and CH4 systems). A reaction model which regards the HA mechanism as the electron transfer process followed by proton transfer has been proposed to interpret the experimental results. In this model, the difference in the behavior of the ratio σPT/σHA between the two types is ascribed to the difference in the ease with which the electron hopping takes place in the entrance channel of the reaction. In addition, the upper limit of the barrier height for this electron hopping has been calculated using an ab initio method for the CH3Cl++CH3Cl and CH3F++CH3F systems, with the results which support the above model.

State selected ion–molecule reactions by the TESICO technique. XIII. Vibrational state dependence of the cross sections in the reaction C2D+2(ν2)+H2

Vibrational-state selected reaction cross sections have been measured for three product channels of the reaction C2D+2(ν2)+H2, by use of the threshold electron–secondary ion coincidence (TESICO) technique. The ν2 vibrational states of the C2D+2 ion were selected up to v=2 and the collision energies were changed from 0.1 to 2.0 eV. At low collision energies up to 0.2 eV, considerable enhancement of the cross sections for the channel producing C2D2H+ was observed when the vibrational quantum number was increased successively. As the collision energy was increased, the extent of this enhancement diminished gradually and the cross section became almost independent of v at 2.0 eV. The cross section for the H/D exchange channel, on the other hand, was found to decrease with increasing vibrational quantum number at low collision energies. The latter cross section also became independent of v at high collision energies. These results have been explained in terms of the formation of an intermediate complex and the statistical decomposition of this complex to products.

State selected ion–molecule reactions by a TESICO technique. X. O+2(v)+CH4

Vibrational state selected (relative) reaction cross sections have been determined for v=0–3 of the O+2 ion, for each of the three product channels of the reaction O+2(v)+CH4, viz. O+2(v)+CH4→CH3O+2+H (1) →CH+3+HO2 (2) →CH+4+O2 , (3) using the TESICO (threshold electron–secondary ion coincidence) technique. At a fixed collision energy of 0.27 eV, it has been found that the cross section of exoergic channel (1) increases most prominently with increasing vibrational quantum number v in the range v=0–2, but decreasees sharply in going from v=2 to v=3. The cross sections of endoergic channels (2) and (3) also increase with increasing v but their rates of increase are much smaller than that of channel (1) in the range v=0–2. When v is increased to 3, however, charge transfer channel (3) is enhanced dramatically and the CH+4 ion becomes the most abundant product ion. The cross section of channel (2) also increases more sharply in going from v=2 to v=3 than in the range v=0–2, but the CH+3 ion still remains the least abundant of the three product ions. As a result of these variations in the individual cross sections, the overall cross section for the O+2+CH4 reaction increases monotonically with increasing v throughout the range studied (v=0–3). The results are compared with that of the collision energy dependence as obtained in drift and flow-drift experiments and the implications are discussed in conjunction with the structure of the CH3O+2 ion and the relevant potential energy surfaces.

State selected ion–molecule reactions by a TESICO technique. IX. Vibrational state dependence of the cross sections in the reaction C2H+2(ν2)+D2(H2)

Vibrational-state selected reaction cross sections have been measured for four product channels of the reaction C2H+2+D2, by use of the threshold electron–secondary ion coincidence (TESICO) technique. The ν2 vibrational states of the C2H+2 ion were selected up to v=2 and the collision energies were changed from 0.2 to 3.7 eV. At low collision energies close to 0.2 eV, considerable enhancement of the cross sections for the channels producing C2H2D+ and C2HD+2 was observed when the vibrational quantum number was increased successively. As the collision energy was increased, the extent of this enhancement diminished gradually and the cross sections finally became almost independent of v at about 2 eV. The cross section for the H/D exchange channel, on the other hand, was found to decrease with increasing vibrational quantum number at low collision energies. However, the v=0 and v=1 cross sections of this channel, especially the former, decayed very quickly as the collision energy was increased, reaching essentially zero at ∼2 eV, while the cross section for v=2 remained substantial even at this and higher collision energies. These results have been discussed in terms of two concurrent reaction mechanisms, i.e., a complex mechanism predominating at low collision energies and a direct one predominating at high collision energies. The ν2 state selected cross sections for the reaction C2H+2+H2 →C2H+3+H have also been studied in a similar collision energy range for comparison.

State selected charge transfer reactions in the BC++Ar⇄BC+Ar+ systems: Comparison among BC=H2, O2, and NO

The relative state selected cross sections for the low-energy charge transfer (CT) reactions in the systems H+2(X2 Σ+g,v)+Ar, Ar+(2PJ)+H2, O+2(a 4Πu,v) +Ar, Ar+(2PJ)+O2, NO+(a 3Σ+,v)+Ar, and Ar+(2PJ)+NO have been determined using the threshold electron-secondary ion coincidence (TESICO) technique. In the (H2+Ar)+ system, the cross sections for both the forward and backward reactions were found to show a characteristic internal-state dependence which can be consistently interpreted in terms of a single model based on energy defects (ΔE) and Franck–Condon (FC) factors. In contrast, in the (O2+Ar)+ and (NO+Ar)+ systems, the strong dependence of the cross sections on the selected internal states were observed only for the forward (starting from the diatomic ions) reaction, and not for the backward (starting from the Ar+ ion) reaction. The results for the forward reactions were again interpreted, at least partially, by the energy defects and FC factors between the reactant and product states. These features of the internal-state dependence of cross sections have been discussed in conjunction with the characteristics of the relevant potential energy surfaces of each system. The discrepancy between the (H2+Ar)+ system and the (O2+Ar)+ and (NO+Ar)+ systems in the behavior of the forward and backward cross sections was ascribed to the difference in the number of potential surfaces involved; in the former system only two surfaces are involved in both forward and backward reactions allowing exactly the same mechanism for both reactions, whereas in the latter two systems, the occurrence of more than two surfaces causes different mechanisms for the forward and backward reactions. In the (O2+Ar)+ system, the doublet and quartet surfaces participate in the backward reaction, leading to two different states (X 2Πg and a 4Πu) of the O+2 product ion, while only quartet surfaces are involved in the forward reaction. In the (NO+Ar)+ system, different reaction paths arise between the [NO+(a 3Σ+)+Ar] and (Ar++NO) states due to the anisotropy in these interactions. This allows the different behavior of the forward and backward reactions, in spite of the fact that the product state of the backward reaction is predominantly NO+(a 3Σ+)+Ar. An ab initio calculation of partial potential energy surfaces for the triplet states of the (NO+Ar)+ system supported this view. From these considerations, low-energy charge transfer reactions in the (BC+Ar)+ systems have been classified into two groups according to the possible types of nonadiabatic transitions. These groups are considered to correspond respectively to the phenomenological ‘‘direct’’ and ‘‘intimate’’ reaction mechanisms.

State selected ion–molecule reactions by a TESICO technique. VIII. Vibronic-state dependence of the cross sections in the reaction NO+(a 3Σ+, v ; b 3Π, v)+Ar → NO+Ar+

Charge transfer reactions NO++Ar → Ar++NO (1) have been studied by selecting the vibronic states of NO+ using the threshold electron–secondary ion coincidence (TESICO) technique. The vibronic states selected were a 3Σ+, v=0–5 and b 3Π, v=0, for each of which the relative cross sections have been determined at two collision energies 1.4 and 5.8 eV. The cross section for the a 3Σ+ state has been found to show a resonancelike enhancement at v=2. This feature is reproduced fairly well by the simple two-state theory of Rapp and Francis combined with the consideration of the Franck–Condon factors. However, a closer comparison of the theoretical and experimental results over the entire range of v reveals that the reaction cross sections for the a 3Σ+ reactant ion state consist of two components, one in which the vibrational-state dependence is determined simply by the energy defects and Franck–Condon factors, and the other in which the vibrational-state dependence cannot be interpreted by those factors. The cross section for the b 3Π, v=0 state has been found to be much smaller than those for the exoergic reactions of the a 3Σ+ state (v=1–5). This has been attributed to the difference in the nature of the molecular orbitals that play primary roles in the a 3Σ+ and b 3Π reactions.

State selected ion–molecule reactions by a TESICO technique. VII. Isotope effect in the reactions O2+(X 2Πg,a 4Πu)+HD→O2 H+(O2D+)+D(H)

The kinetic isotope effect in the reactions O2++HD→O2H+(O2D+) +D(H) (ΔE=1.78 eV) has been studied by selecting the vibronic states of O2+ using the threshold electron-secondary ion coincidence (TESICO) technique. Relative cross sections for the individual vibronic states, v=19 and 20 of the X 2Πg state and v=0–4 of the a 4Πu state of O2+, have been determined at a collision energy of 0.4 eV. The results for the a 4Πu state show that, regardless of the vibrational state, the isotope effect is consistent with that expected from a direct reaction mechanism. A similar isotope effect has also been observed for v=19 and 20 of the X 2Πg state, suggesting that the above reaction with O2+(X 2Πg) proceeds via a direct mechanism even at collision energies below the translational threshold when a sufficient vibrational energy is supplied to the reactant ion.

State selected ion–molecule reactions by a TESICO technique. VI. Vibronic-state dependence of the cross sections in the reactions O+2(X 2Πg, v; a 4Πu, v)+H2→ O2H++H, H+2+O2

The threshold electron-secondary ion coincidence (TESICO) technique has been utilized to select the vibronic states of the O+2 reactant ions in the endoergic reactions O+2+H2→O2H++H(1) and O+2+H2→H+2+O2(2). The vibronic states selected were X 2Πg, v=19, 20 and a 4Πu, v=0–8, for each of which the relative reaction cross sections have been determined. It has been found that while reaction (1) indeed proceeds with the ground X 2Πg state ions by the vibrational excitation (at collision energies lower than the translational threshold), it is considerably (an order of magnitude) enhanced by the electronic excitation of the reactant ions to a 4Πu. When vibrational energy is added to the a 4Πu state, the cross section of reaction (1) further rises sharply (by a factor of 2) at v=2 and then decreases more slowly, reaching the original (v=0) value at v=4. After that, the cross section apparently decreases by about a factor of 2 at v=8. The cross section of reaction (2) behaves in a manner similar to that of reaction (1) as a function of vibrational quantum number. The magnitudes of the cross sections for both electronic states are, however, consistently smaller than those of reaction (1) by roughly a factor of 10.

State selected ion–molecule reactions by a TESICO technique. V. N2+(v)+Ar→N2+Ar+

Charge transfer reactions N2+(v)+Ar→Ar++N2 (1) have been studied by selecting the vibrational states of N2+ using the threshold electron–secondary ion coincidence (TESICO) technique. Relative cross sections σ(v) for the individual vibrational states v = 0–3 have been determined at three collision energies, 0.3, 1.5, and 11.8 eV. Results show that Reaction (1), which is endoergic for v = 0, is considerably enhanced by the vibrational excitation of N2+ at all collision energies. While excitation of one vibrational quantum enhances the cross section substantially, excitation of additional quanta further increases the cross section up to v = 3. The ratios σ(2)/σ(1) and σ(3)/σ(2) are, however, much smaller than σ(1)/σ(0) and are significantly larger at the highest collision energy than at the other two collision energies. These results are discussed in conjunction with the calculated results based on the simple two-state theory of Rapp and Francis and the Franck–Condon factors.

State selected ion–molecule reactions by a TESICO technique. IV. Relative importance of the two spin-orbit states of Ar+ in the charge transfer reactions with N2 and CO

Charge transfer reactions Ar+ (2PJ)+N2→N+2+Ar (1) and Ar+(2PJ)+CO→CO++Ar (2) have been studied for the two spin-orbit states (J = 3/2 and 1/2) separately using the threshold electron-secondary ion coincidence (TESICO) technique. Relative cross sections for the two states σ(3/2) and σ(1/2) have been determined at three collision energies 0.2, 1.4, and 5.8 eV. It has been found that in Reaction (1), σ(3/2) is larger than σ (1/2) with ratio σ(1/2)/σ(3/2) ranging from 0.5 to 0.8 depending on the collision energy, whereas in Reaction (2), σ(1/2) is larger than σ(3/2) with the ratio ranging from 1.2 to 1.6. The implications of these results are discussed with regard to the roles of energy resonance and Franck–Condon factors in charge transfer processes.

State selected ion–molecule reactions by a TESICO technique. III. H2+(v)+Ar→ArH++H, Ar++H2: Observation of enhanced charge-transfer cross sections at near resonance

Reactions of the H2+ ions with Ar, i.e., H2++Ar→ArH++H [proton transfer, Reaction (1)] and H2++Ar→Ar++H2 [charge transfer, Reaction (2)] have been studied for the individual vibrational states v (v = 0–4) of the reactant ion at collision energies of 0.48, 0.77, and 1.24 eV, utilizing the threshold electron–secondary ion coincidence (TESICO) technique. At 0.77 and 1.24 eV, the cross sections for Reaction (1) are almost independent of the vibrational energy or slightly increase with increasing vibrational energy, wheras those for Reaction (2) show a resonance-like enhancement at v = 2 . At the lowest collision energy studied (0.48 eV), however, the enhancement of the charge transfer cross section at v = 2 is accompanied by a concurrent decrease in the proton transfer cross section, indicating the competitive nature of the two channels at this collision energy. These results are consistent with the trajectory–surface hopping model of the ArH2+ system by Chapman and Preston. Reaction (2) has also been studied at a higher collision energy of 19 eV, where the endoergic reaction with v = 1 is found to be considerably enhanced and has a cross section close to that for the near-resonant reaction with v = 2.

State selected ion–molecule reactions by a TESICO technique. II. Separation of the reactant spin–orbit states in the reaction Ar+(2P3/2, 2P1/2)+H2(D2)→ArH+(ArD+)+H(D)

Cross sections for the atomic rearrangement reaction Ar++H2(D2)→ArH+(ArD+)+H(D) [Reaction (1)] and the charge transfer reaction Ar++H2(D2)→Ar+H+2(D+2) [Reaction (2)] have been determined for each of the Ar+ spin–orbit states (2P3/2 and 2P1/2) separately in the collision energy range from 0.05 to 0.5 eV (c.m.), utilizing the threshold electron–secondary ion coincidence (TESICO) technique which we developed recently. It has been found that the cross sections of Reaction (1) are larger for the 2P1/2 excited state than for the 2P3/2 ground state by a factor of 1.5 (with H2) and 1.3 (with D2), regardless of the collision energy in the range studied. The cross sections for both the H2 and D2 reactions show an approximately E−1/2.5 collision energy dependence which is somewhat milder than that expected from the simple Langevin theory. The cross sections of Reaction (2) have been found to be about seven times larger for the 2P1/2 state than for the 2P3/2 state with H2, whereas these are almost the same for both states with D2. The charge transfer cross sections are almost independent of the collision energy in the same energy range as above. A simple reaction model has been developed and a calculation of cross sections has been performed based on it, with a result which satisfactorily explains the essential features of the experimental results.

State-selected ion–molecule reactions by a threshold electron–secondary ion coincidence (TESICO) technique. I. Apparatus and the reaction H2++H2→H3++H

A technique which enables the direct observation of state-selected ion–molecule reactions through coincident measurements of secondary ions with threshold electrons has been developed. Using this technique, the reaction H2++H2→H3++H has been studied in detail at average collision energies between 0.11 and 0.93 eV. State-selected reaction cross sections for individual vibrational states v=0–3 of the H2+ ion have been determined. It has been found that while the cross section decreases with increasing vibrational energy at low collision energies, this decreasing tendency becomes less and less prominent as the collision energy increases and the cross section is almost independent of vibrational energy at the highest energy studied. This feature qualitatively agrees with the previous study by a different technique. A detailed description of the apparatus and its performance is also given.

Threshold electron–secondary ion coincidence technique for the study of internal energy dependence of ion–molecule reactions

Threshold electron–secondary ion coincidence technique for the study of internal energy dependence of ion–molecule reactions