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.
He(2 3S) Penning ionization of HCl and HBr leading to HCl+(A) and HBr+(A) has been investigated spectroscopically by using a low-pressure experimental apparatus coupled with a flowing afterglow source. The vibrational distribution of HCl+(A) agrees well with the result obtained by Penning ionization electron spectroscopy (PIES), which shows a Franck–Condon like distribution. In contrast, the vibrational distribution of HBr+(A) is more deexcited than the PIES one shifting to lower vibrational levels relative to Franck–Condon factors for ionization. These findings indicate that the collisional perturbation occurs only at the entrance channel for the He(2 3S)/HCl system, while at both entrance and exit ones for the He(2 3S)/HBr system. The rotational temperature of HCl+(A) decreases from 600±100 K for v′=0 to 300±100 K for v′=5, while that of HBr+(A) is 450±50 K for v′=0 and 400±50 K for v′=1.
The d 3Δi–a 3Πr, e 3Σ−–a 3Πr, and a′ 3Σ+–a 3Πr transitions of CO resulting from the dissociative recombination of CO2+(X̃ 2Πg:0,0,0) with electrons have been observed from the He afterglow reaction of CO2. The formation rate constants of CO(d), CO(e), and CO(a′) were estimated to be 1.6×10−7, 3.3×10−9, and 2.4×10−7 cm3 s−1, respectively. The vibrational and rotational distributions of CO(d:v′=0–6,e:v′=2,3,a′=3–11) were determined. Most of available excess energies (91%∼98%) were deposited into the vibrational energy of CO(d,e,a′) and the relative translational energies of the products, indicating that CO(d,e,a′) were produced by direct curve crossings between the entrance e−/CO2+(X̃ 2Πg:0,0,0) potential and repulsive CO(d,e,a′)+O(3P) potentials with linear geometries. The vibrational distributions of CO(d) and CO(a′) slightly shifted to lower states than those in photodissociation at a similar excitation energy. A simple statistical model was unable to explain the observed vibrational distributions obtained by dissociative recombination.
The dissociative electron–ion recombination processes of CO+2(X̃ 2Πg:0,0,0) has been studied by observing the CO(A 1Π–X 1Σ+) emission in the He and Ar afterglows. It was found that the CO(A:v′=0–2) states are formed in the dissociative recombination of CO+2(X̃:0,0,0) with electrons at thermal energy. The rovibrational distribution of CO(A) was N0:N1:N2=100:(T0=1000±100 K), 58±4(T1=700±50 K), and 9±2 (T2=400±100 K). The average fractions of total energy channeled into vibration and rotation of CO(A) and relative translation of the products were determined to be 〈fv〉=22%±2%, 〈fr〉=20%±2%, and 〈ft〉=58%±4%. The observed rovibrational distributions were in disagreement with statistical prior distributions, indicating that the reaction dynamics is not governed by the statistical theory. A comparison of the present results with the previous photodissociation data suggested that the CO(A:v′=0,1) states are formed through predissociation of near-resonant intermediate CO2** states coupled with a bent valence state, while the CO(A:v′=2) state is produced through predissociation of CO2** states just above the CO+2(X̃:0,0,0) state. The low CO(A:v′=2) population can be explained by the energetic constraint for thermal electrons plus CO+2(X̃:0,0,0) and/or a competition between predissociation and autoionization of CO2** states just above the CO+2(X̃:0,0,0) energy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.