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) range of 0.05-10.00 eV, where (vvv) = (000), (100), and (020), and . Three product channels, HCO + OH, HOCO + H, and CO + HO, are identified. The measured σ(HCO) curve [σ(HCO) versus E 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 < 0.4 eV, but is enhanced at E > 0.4 eV by (100) vibrational excitation. The E 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 enhancement for σ(HOCO) at E ≈ 0.5-5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO at low E < 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 and the vibrational enhancement at high E observed for the σ(HOCO).