Isotopic variants of the H + H₂ reaction. 1. Total reaction cross sections of the H + D₂ and H + HD reactions as a function of relative energy

The total cross sections for the reactions H + D₂ = HD + D and H + HD = H₂ + D have been measured as a function of relative initial translational energy in the ranges 0.87 to 2.70 and 0.82 to 1.86 eV, respectively. The technique for measuring the rate constant exploits the fact that the atomic product and reactant have very similar absorption wavelengths and that nearly monoenergetic hydrogen atoms can be generated by photodissociation of particular small molecules. Detection of the hydrogen atoms was carried out by laser-induced fluorescence at such short times that single collision conditions prevailed. The measured cross sections are in excellent agreement with the classical trajectory calculations of Biais and Truhlar and Schechter and Levine. As a byproduct of these experiments it was shown by Doppler spectroscopy that at the wavelengths 248 and 193 nm the transition dipole moment for H₂S is perpendicular to the molecular plane and that almost all of the available energy in this photodissociation is released into translation.