Laser spectroscopic excitation function and reaction threshold studies of the H + DCl --> HCl + D gas-phase isotope exchange reaction.

The dynamics of the gas-phase hydrogen atom exchange reaction H + DCl --> HCl + D were studied using the pulsed laser photolysis/laser induced fluorescence "pump-and-probe" method. Laser photolysis of H2S at 222 nm was used to generate nonequilibrium distributions of translationally excited hydrogen atoms at high dilution in a flowing moderator gas (Ar)/reagent (DCl) mixture. H and D atoms were detected with sub-Doppler resolution via Lyman-alpha laser induced fluorescence spectroscopy, which allowed the measurement of the line shapes of the moderated H atom Doppler profiles as well as the concentration of the D atoms produced in the H + DCl --> HCl + D reaction. From the measured H atom Doppler profiles, the time evolution of the initially generated nascent nonequilibrium H atom speed distribution toward its room-temperature thermal equilibrium form was determined. In this way, the excitation function and the reaction threshold (E0 = 0.65 +/- 0.13 eV) for the H + DCl --> HCl + D reaction could be determined from the measured nonequilibrium D atom formation rates and single collision absolute reaction cross-section values of 0.12 +/- 0.04 A2 and 0.45 +/- 0.11 A2 measured at reagent collision energies of 1.0 and 1.4 eV, respectively.