VIBRATIONAL-ENERGY TRANSFER AND POPULATION-INVERSION IN CO OVERLAYERS ON NACL(100)

Strong population inversion in CO overlayers on NaCl(100) has been achieved by irradiating isotopically enriched samples with a Q-switched gas CO laser. Vibrational up-pumping, through dipole-dipole coupling, enabled the molecules to be populated up to the v = 30 level at 22 K. Measurements of the fluorescence decays of several states indicated that the vibrational energy can be stored within the thin films for times of milliseconds. Additionally, as opposed to previous studies, the samples were not fractured and remained transparent during the vibrational excitation. This is attributed to the epitaxial growth of high-quality single-crystal α-CO on the NaCl(100) surface which makes the overlayers an ideal lasing medium in the infrared region. The role that the substrate plays during the energy-transfer processes was explored by monitoring the vibrational population distribution and relaxation lifetime as functions of the number of overlayers on the surface. Fluorescence measurements revealed a similar population distribution from v = 8 to u = 30 between the samples of 390 layers and 40 layers. Moreover, lifetimes of the total fluorescence shortened only gradually with film thickness, from 2.9 ms for 120 layers to 1.2 ms for 5 layers. A simple kinetics scheme was proposed to account for this quenching effect by the substrate, and a molecule-surface relaxation rate of (2 ± 1) × 103 s-1 was estimated for the vibrationally excited CO monolayer on NaCl(100). These results verify our earlier report that vibrational motions of the adsorbed CO are poorly coupled to the NaCl(100) surface. © 1990 American Chemical Society.