Threshold Electron–secondary Ion Coincidence Technique Research Articles

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 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. 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.

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