Laser spectroscopy of hydrogen desorption from Pd(100)

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Abstract
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 photonionization. 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 energiesE 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.

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