Detailed study of the dynamics of the O+(4S)+ HCN reaction. A case study of ion–molecule reactions in the spacecraft environment

Abstract

Guided-ion beam (GIB) cross-section and time-of-flight (TOF) measurements are presented for the O⁺(⁴S)+ HCN reaction from 0.2–25 eV centre-of-mass (cm) collision energies. The reaction exhibits a charge-transfer and three chemical reaction channels, NO⁺, CHO⁺ and CO⁺. Charge-transfer predominates above ca. 1 eV. The GIB measurements are augmented with selected ion flow tube (SIFT) rate constant measurements at 310 and 485 K. The total SIFT thermal reaction rate coefficients at 310 and 485 K are, 2.5 ± 0.6 × 10^–9 cm³ s^–1 and 2.3 ± 0.6 × 10^–9 cm³ s^–1. These values are compared to the 2.0 ± 0.5 × 10^–9 cm³ s^–1 reaction rate coefficient derived from the 0.2 eV GIB total cross-section.

The analysis of the TOF spectra with the osculating complex model reveals that product ion formation is mediated by long-lived intermediate complexes at low collision energies, while a direct mechanism dominates for the majority of the product ions at high energy. Contributions from both direct and complex-mediated mechanisms are observed at intermediate energies. The complex-mediated products show a near-statistical partitioning of reaction energy which is substantiated by a phase-space-theory based calculation. The chemical reaction product branching ratios are discussed in the context of recent ab initio[O—H—C—N]⁺ hypersurface calculations (A. Luna, A. Mebel and K. Morokuma, personal communication). The application of the measurements to an understanding of atmosphere–spacecraft interactions is also discussed.