State-Selected and State-to-State Ion−Molecule Reaction Dynamics

Abstract

A unique triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion−molecule reactions. By employing this TQDO apparatus, we have recently examined the absolute total cross sections for a series of state-selected ion−molecule reactions involving Ar+(2P3/2,1/2), O+(4S, 2D, 2P), and organosulfur ions (CH3SH+, CH3CH2SH+, and CH3SCH3+) in their ground states. The cross section measurements, together with product ion kinetic energy analyses, have provided convincing evidence that the Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) reactions proceed via a charge-transfer predissociation mechanism. The comparison of absolute cross sections for product ions formed in the dissociative charge transfer of Ar+(2P3/2,1/2) + CO2 (CO, N2, O2) and those produced in photoionization of CO2 (CO, N2, O2) suggests that product ions formed by dissociative charge transfer are also produced by photoionization via a similar set of excited predissociative states of CO2+ (CO+, N2+, O2+). By preparing CH3SH+, CH3CH2SH+, and CH3SCH3+ in their ground states by photoionization of the corresponding neutrals, we have examined the dissociation of these ions via collision activation. Equipped with two radio frequency octopole ion guide reaction gas cells, the TQDO apparatus has allowed the identification of the isomeric structure of product ions by using the charge-transfer probing method. Strong preference is observed for C−S and C−C bond scissions, leading to the formation of CH3+ from CH3SH+, CH3CH2+, and CH2SH+ from CH3CH2SH+, and CH3S+ from CH3SCH3+ as compared to C−H and S−H bond breakages. The observation of these bond selective dissociation reactions is contrary to that found in photoionization of CH3SH, CH3CH2SH, and CH3SCH3 and is indicative of nonstatistical behavior for collision-induced dissociation of these organosulfur ions. An application of the radio frequency ion-guide for state-selection of O+(4S, 2D, 2P) prepared by the dissociative charge-transfer reactions of He+ (Ne+, Ar+) + O2 is described. The success of this method has made possible the absolute total cross section measurement of the state-selected ion−molecule reactions O+(4S, 2D, 2P) + N2 (O2, H2, D2, CO2, H2O), which are considered as the most important set of reactions in the Earth's ionosphere.