Experimental and theoretical total state‐selected and state‐to‐state absolute cross sections. II. The Ar+(2P3/2,1/2)+H2reaction

A quantum-rovibrational-state-selected study of the reaction in the collision energy range of 0.05-10.00 eV: translational, rotational, and vibrational energy effects

We report detailed absolute integral cross sections (σ's) for the quantum-rovibrational-state-selected ion-molecule reaction in the center-of-mass collision energy (E_cm) range of 0.05-10.00 eV, where (vvv) = (000), (100), and (020), and . Three product channels, HCO⁺ + OH, HOCO⁺ + H, and CO⁺ + H₂O, are identified. The measured σ(HCO⁺) curve [σ(HCO⁺) versus E_cm plot] supports the hypothesis that the formation of the HCO⁺ + OH channel follows an exothermic pathway with no potential energy barriers. Although the HOCO⁺ + H channel is the most exothermic, the σ(HOCO⁺) is found to be significantly lower than the σ(HCO⁺). The σ(HOCO⁺) curve is bimodal, indicating two distinct mechanisms for the formation of HOCO⁺. The σ(HOCO⁺) is strongly inhibited at E_cm < 0.4 eV, but is enhanced at E_cm > 0.4 eV by (100) vibrational excitation. The E_cm onsets of σ(CO⁺) determined for the (000) and (100) vibrational states are in excellent agreement with the known thermochemical thresholds. This observation, along with the comparison of the σ(CO⁺) curves for the (100) and (000) states, shows that kinetic and vibrational energies are equally effective in promoting the CO⁺ channel. We have also performed high-level ab initio quantum calculations on the potential energy surface, intermediates, and transition state structures for the titled reaction. The calculations reveal potential barriers of ≈0.5-0.6 eV for the formation of HOCO⁺, and thus account for the low σ(HOCO⁺) and its bimodal profile observed. The E_cm enhancement for σ(HOCO⁺) at E_cm ≈ 0.5-5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO⁺ at low E_cm < 0.4 eV may involve a complex mechanism, which is mediated by the formation of a loosely sticking complex between HCO⁺ and OH. The direct collision and complex mechanisms proposed also allow the rationalization of the vibrational inhibition at low E_cm and the vibrational enhancement at high E_cm observed for the σ(HOCO⁺).

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.

Time-dependent quantum wave packet study of the Ar+H2+→ArH(+)+H reaction on a new ab initio potential energy surface for the ground electronic state (1(2)A')

A new global potential energy surface for the ground electronic state (1(2)A') of the Ar+H2(+)→ArH(+)+H reaction has been constructed by multi-reference configuration interaction method with Davidson correction and a basis set of aug-cc-pVQZ. Using 6080 ab initio single-point energies of all the regions for the dynamics, a many-body expansion function form has been used to fit these points. The quantum reactive scattering dynamics calculations taking into account the Coriolis coupling (CC) were carried out on the new potential energy surface over a range of collision energies (0.03-1.0 eV). The reaction probabilities and integral cross sections for the title reaction were calculated. The significance of including the CC quantum scattering calculation has been revealed by the comparison between the CC and the centrifugal sudden approximation calculation. The calculated cross section is in agreement with the experimental result at collision energy 1.0 eV.

The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

This paper presents the methodology to generate beams of ions in single quantum states for bimolecular ion-molecule reaction dynamics studies using pulsed field ionization (PFI) of atoms or molecules in high-n Rydberg states produced by vacuum ultraviolet (VUV) synchrotron or laser photoexcitation. Employing the pseudocontinuum high-resolution VUV synchrotron radiation at the Advanced Light Source as the photoionization source, PFI photoions (PFI-PIs) in selected rovibrational states have been generated for ion-molecule reaction studies using a fast-ion gate to pass the PFI-PIs at a fixed delay with respect to the detection of the PFI photoelectrons (PFI-PEs). The fast ion gate provided by a novel interleaved comb wire gate lens is the key for achieving the optimal signal-to-noise ratio in state-selected ion-molecule collision studies using the VUV synchrotron based PFI-PE secondary ion coincidence (PFI-PESICO) method. The most recent development of the VUV laser PFI-PI scheme for state-selected ion-molecule collision studies is also described. Absolute integral cross sections for state-selected H2+ ions ranging from v+ = 0 to 17 in collisions with Ar, Ne, and He at controlled translational energies have been obtained by employing the VUV synchrotron based PFI-PESICO scheme. The comparison between PFI-PESICO cross sections for the H2+(HD+)+Ne and H2+(HD+)+He proton-transfer reactions and theoretical cross sections based on quasiclassical trajectory (QCT) calculations and three-dimensional quantum scattering calculations performed on the most recently available ab initio potential energy surfaces is highlighted. In both reaction systems, quantum scattering resonances enhance the integral cross sections significantly above QCT predictions at low translational and vibrational energies. At higher energies, the agreement between experiment and quasiclassical theory is very good. The profile and magnitude of the kinetic energy dependence of the absolute integral cross sections for the H2+(v+ = 0-2,N+ = 1)+He proton-transfer reaction unambiguously show that the inclusion of Coriolis coupling is important in quantum dynamics scattering calculations of ion-molecule collisions.