title: 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.
abstract: 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 (Ecm) 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 Ecmplot] 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 Ecm< 0.4 eV, but is enhanced at Ecm> 0.4 eV by (100) vibrational excitation. The Ecmonsets 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 Ecmenhancement for σ(HOCO+) at Ecm≈ 0.5-5.0 eV can be attributed to the direct collision mechanism, whereas the formation of HOCO+at low Ecm< 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 Ecmand the vibrational enhancement at high Ecmobserved for the σ(HOCO+).
authors.affiliation: Department of Chemistry, University of California, Davis, Davis, CA 95616, USA. cyng@ucdavis.edu.
authors.forename: Yuntao
authors.initials: Y
authors.lastname: Xu
authors.forename: Bo
authors.initials: B
authors.lastname: Xiong
authors.forename: Yih-Chung
authors.initials: YC
authors.lastname: Chang
authors.forename: Yi
authors.initials: Y
authors.lastname: Pan
authors.forename: Po Kam
authors.initials: PK
authors.lastname: Lo
authors.forename: Kai Chung
authors.initials: KC
authors.lastname: Lau
authors.forename: C Y
authors.initials: CY
authors.lastname: Ng