State-to-state dynamics of atom + polyatom abstraction reactions. I. The H+CD4→HD(v′,J ′)+CD3 reaction

Abstract

We report measurement of the HD product quantum state distributions and absolute cross section for the H+CD4→HD(v′,J′)+CD3 reaction at a collision energy of 1.5 eV. The total reaction cross section is small, 0.14±0.03 Å2, making the experimental measurements difficult. The HD quantum state distribution peaks at low J′ in both v′=0 and v′=1, the only vibrational states in which product is observed. Very little of the 1.5 eV available energy appears as internal excitation of the HD product molecule, 7% in HD product vibration and 9% in rotation. However, linear surprisal analysis shows that this limited internal energy disposal in the HD product in some ways exceeds that expected statistically, since two of the best-fit surprisal parameters (Θr=2.9±0.6 for v′=0, Θr=−1.9±0.5 for v′=1, λv=−2.2±0.6 ) are negative. The HD rovibrational state distribution shows an anomalous positive correlation of product vibrational and rotational excitation. Those molecules formed in the vibrationally excited state, v′=1, have significantly more rotational energy (〈Erot〉=0.17 eV) than those molecules formed in the vibrational ground state, v′=0 (〈Erot〉=0.13 eV). This behavior runs counter to the otherwise universal behavior for direct bimolecular reactions—a negative correlation of product vibrational and rotational excitation. We speculate as to the source of this anomalous energy disposal.