INFRARED-INDUCED DESORPTION OF HD FROM LITHIUM-FLUORIDE

Low intensity ir radiation at 2-15 mum desorbs HD from 1.5-4.2 K lithium fluoride (LiF) via four distinct channels. The channel most studied here involves coverages of one monolayer or less and 9-15 mum radiation, causing HD to desorb with a translational energy T(B) of 21 K. This process, which is attributed to a single phonon ejecting HD from its lowest bound state, is precisely linear in the radiation intensity, and its efficiency increases with increasing wavelength, reaching a maximum of almost-equal-to 4 x 10(-5) molecules per photon at 15 mum. At multilayer coverages and a surface temperature of 4.2 K, at which HD has a significant vapor pressure, we observe a photoinduced thermal desorption signal (TB = 4.0-4.8 K), arising from a very small (almost-equal-to 0.005 K) perturbation of the crystal temperature. The evidence for thermal desorption of this channel strongly contrasts with the other three channels, which are nonthermal. A third channel involves radiation between 2.71 and 4.4 mum, which causes a very weak signal with TB almost-equal-to 20 K. While this region was chosen to include the HD vibrational transition at 2.75 mum, we present calculations which indicate that vibrational predesorption is not operative here. A fourth channel (T(B) almost-equal-to 34 K) is observed only for LiF containing bulk OH- impurities absorbing at 2.78 mum, and is attributed to phonons generated by the OH-excitation.