Measurement of relative state-to-state rate constants for the reaction D+H2(v, j)→HD(v′, j′)+H

Abstract

We have measured state-to-state integral rate constants for the reaction D+H2(v,j) →HD(v′=0,1,2;j′)+H, in which the H2 reagent was either in the ground state, H2(v=0,j), or prepared in the first excited vibrational state, H2(v=1, j=1), by stimulated Raman pumping. Translationally hot D atoms were produced via UV photolysis of DI, generating two center-of-mass collision energies corresponding to the two I atom spin–orbit states. Resonance-enhanced multiphoton ionization and time-of-flight mass spectrometry were employed to detect the nascent HD product in a quantum-state-specific manner. Two experimental geometries were used: (1) a probe-laser-induced geometry, in which the same laser both initiated the reaction, by photolysis of DI, and detected the HD and (2) an independent-photolysis-source geometry, in which photolysis of DI was carried out by an independent laser. We find that vibrational excitation of the H2 reagent results in substantial HD rotational excitation for each product vibrational state, a shift in the vibrational product state distribution such that the rates for the reaction D+H2(v=1, j=1) into HD(v′=0) and HD(v′=1) are comparable, and somewhat surprisingly, almost no change in the total rate into HD(v′=0,1,2;j′). The experimental results are consistent with a model in which internal energy is conserved, i.e., internal energy of the reagents appears as internal energy of the products, while relative translational energy of the reagents appears primarily as translation of the products. Good to excellent agreement is found between the experiment and recent quantum-mechanical scattering calculations of Neuhauser, Judson, and Kouri. Minor discrepancies persist, however, between theory and experiment for some product rotational distributions.