We have used monoenergetic nozzle beams of para- and n-hydrogen to determine the adsorption dynamics in the so called ‘precursor regime’ on a (1 × 2) reconstructed Pt(110) surface. At low nozzle temperatures clear differences in the sticking coefficients of the two hydrogen modifications can be observed. An analysis of the data yields rotational state resolved sticking coefficients for the J = 0, 1 and 2 states. There is a pronounced decrease of the sticking coefficients with increasing J at constant translational energy.

We have used seeded beam techniques to determine the influence of the rotational energy of hydrogen and deuterium molecules on the adsorption dynamics. A strong decrease of sticking with increasing rotational energy has been observed. In the low translational energy regime where the sticking coefficient decreases with beam energy the strongest effects are encountered. This result points to the importance of dynamical steering rather than to adsorption via a classical molecular precursor. Because rotational energy is quantized, clear isotope effects between hydrogen and deuterium can be observed.