Experimental determination of the reaction time for hydrogen isotope exchange H2+D2→2HD on Ni(110)

The mean reaction time of the hydrogen isotope exchange reaction H-2 + D-2 --> 2HD on the Ni(110) surface has been measured in a high-resolution time-of-flight (TOF) scattering experiment. The reactants were supplied by a pulsed supersonic D-2 nozzle beam with incident energy E-i less than or equal to 120 meV and by a room temperature effusive H-2 source. The TOF spectra of the product HD molecules measured with a chopper located between the incident D-2 beam and the sample are a convolution of the reaction time ton the surface and the velocity distribution of the desorbing HD products. The latter distribution is then measured with a chopper placed between the sample and the detector. The reaction time tau as determined from a deconvolution of the two spectra decreases with increasing surface temperature T-s from tau similar to 3000 mu s at T-s = 450 K to tau similar to 10 mu s at T-s = 700 K. An Arrhenius plot of the reaction time reveals that the reaction energy is E-r = 440 meV at surface temperatures below about T-s = 600 K and E-r = 790 meV for temperatures T-s > 700 K. Possible interpretations for the two reaction channels are proposed. (C) 1999 American Institute of Physics. [S0021-9606(99)00213-5].