Photodesorption of physisorbed hydrogen molecules

We have determined rates of direct infrared photodesorption for H(2), HD, and D(2) physisorbed on low-index Cu surfaces. This process relies on the dipole activity of transitions from the ground state to continuum states of the physisorption well. In our calculations a simple functional form for the dipole moment is used, which we find to reproduce accurately the spectroscopically measured dipole intensities for transitions among bound states. The calculated rates of direct desorption, induced by blackbody radiation at different temperatures, agree remarkably well with the measured rates for H(2) adsorbed on the terraces of a Cu(510) surface. A calculated isotope effect on the desorption rate, with a desorption rate for D(2) that is about an order of magnitude smaller than for H(2), is also in agreement with observations. For HD, the theory predicts the possibility to induce resonant photodesorption by using an infrared laser. We have also found that the calculated rate of photodesorption induced by an indirect process, involving excitation of substrate phonons, is several orders of magnitude smaller than the direct rate.