The condensation kinetics of hydrogen on the (110), (111), and (100) planes of tungsten are examined as functions of crystal temperature, coverage, and population of the various binding states. Measurements were carried out simultaneously on all three crystals in the same ultrahigh-vacuum system in order to accurately compare sticking coefficients and amounts adsorbed. There are large variations in the initial sticking coefficients s0; at 78°K these vary between 0.26 on the (111) plane to 0.07 on the (110) plane. This implies that the electronic transition involved in bond formation depends sensitively on the geometric and electronic structures of the substrates. Also s0 is observed to be almost independent of substrate temperature for all planes, showing that phonon assisted processes do not determine the energy transfer. The coverage dependence of s shows considerable variation between states. In all cases a weakly adsorbed precursor state is involved, but the kinetics of conversion to the tightly bound states are quite different for different planes and states. Models are developed to explain condensation into multiple states for sequential and simultaneous population of the states on a plane, and these are compared with the experimental results.
Tamm et al. (Mon,) studied this question.