Measurements of weakly bound hydrogen on Pd(111) between 5 and 80 K

Abstract

The adsorption of hydrogen and deuterium onto a low-temperature surface of Pd(111) has been studied experimentally using electron-stimulated desorption (ESD) and temperature programmed desorption spectrometry (TPD). At a surface temperature TS=7.5±1 K, exposure to D2 leads to multilayer formation which is reflected in the desorption of deuterium ion clusters D+n by ESD (Ee−=100 eV). Cluster sizes up to n=21 have been observed which, except for n=2, consist exclusively of odd numbers of atoms. The temperature dependence of the ESD signal following exposure at TS=7.5±1 K suggests that by 9.5±1 K the multilayer desorbs leaving a diatomic molecular species which, in the high-coverage limit, is stable up to ∼20 K. Exposure to hydrogen at these low temperatures gives rise to two TPD peaks: the β (chemisorption) peak and a new TPD peak denoted γ (near 60 K at low coverage) which is assigned to desorption of a surface molecular hydrogen species. Mixed isotope experiments show that there is no mixing in the γ state consistent with this assignment. With increasing exposure, the γ peak grows in intensity and shifts rapidly towards lower temperature. TPD peak shape analysis gives a binding energy for the molecular state at low coverage of ∼0.020±0.010 eV and a corresponding preexponential of approximately 102 s−1.