Laser spectroscopy of hydrogen desorption from Pd(100)

The recombinative desorption flux of hydrogen molecules from a Pd{100} surface is investigated with rovibrational state selectivity by resonantly enhanced [vacuum ultraviolet (VUV)+ ultraviolet (UV)] two photon ionization. Using a permeation source for the hydrogen supply the surface temperature can be changed from 325 to 740 K. The population in the rotational states is significantly lower than expected for molecules in thermal equilibrium at the respective surface temperatures, whereas the vibrational population is much higher. This vibrational population increases exponentially with T_s. When interpreted as desorption rates into (v″=1) and plotted versus T⁻¹_s, activation energies E_a for vibrational excitation can be derived. For the three isotopes D₂, HD, and H₂ values for E_a of 234, 282, and 428 meV, respectively, are obtained, being in all three cases lower than the corresponding gas phase energies. The rotational cooling and the vibrational excitation are discussed in the context of current theoretical models.