ROTATIONAL STATE DISTRIBUTION OF RECOMBINATIVELY DESORBING HYDROGEN FROM CLEAN AND S-COVERED PD(100)

The rotational state distribution of H-2 and D2 molecules desorbing from Pd(100) following atomic recombination has been determined by tunable vacuum ultraviolet laser ionisation spectroscopy. The surface temperature of the permeation source was varied between 325 and 740 K. A significant rotational cooling effect has been observed which leads to rotational temperatures of about 350 +/- 25 K at high T(s). No isotope effect is apparent. Preadsorption of S atoms changes the rotational temperature of the desorbing hydrogen molecules. The rotational cooling effect is discussed in the context of current theoretical models.