Infrared-induced desorption of HD from lithium fluoride

Low intensity ir radiation at 2-15 μm 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 μm. radiation, causing HD to desorb with a translational energy TB 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 wave-length, reaching a maximum of ≈ 4 × 10 -5 molecules per photon at 15 μm. 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 (≈ 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 μm, which causes a very weak signal with TB ≈20 K. While this region was chosen to include the HD vibrational transition at 2.75 μ, we present calculations which indicate that vibrational predesorption is not operative here. A fourth channel (T B≈34 K) is observed only for LiF containing bulk OH- impurities absorbing at 2.78 μm, and is attributed to phonons generated by the OH- excitation. © 1993 American Institute of Physics.