EXCITATION-FUNCTION AND REACTION THRESHOLD STUDIES OF ISOTOPE-EXCHANGE REACTIONS - H-2-]D+HD AND H+D2O-]D+HOD(D)

Laser photolysis of H2S at 248 nm and at 193 nm was used to generate n onequilibrium distributions of translationally energetic hydrogen atom s at high dilution in a flowing moderator gas (N-2 or Ar). The pulsed laser photolysis/laser-induced fluorescence ''pump-and-probe'' method allowed the measurement of the line shapes of the moderated H atom Dop pler profiles as well as the concentrations of D atoms produced in the reactive collisions between the H atoms and Dr or D2O reagents. H and D atoms were detected with sub-Doppler resolution via (2p(2)P <-- 1s( 2)S) laser-induced fluorescence. The measured H atom Doppler profiles were used to describe the evolution of the initially generated nascent nonequilibrium H atom speed distribution toward its equilibrium Maxwe ll-Blotzmann form. In this way the excitation function and reaction th reshold for the reactions H + D-2 --> HD + D and H + D2O --> HOD + D c ould be determined for the first time from the measured nonequilibrium D atom formation rates and single-collision absolute reaction cross s ections measured at higher collision energies.