Resonant ir laser stimulation of the desorption of methanol adsorbed on Cu(110)

The reactive system ${\mathrm{CH}}_3$OH/Cu(110) has been characterized by thermal desorption and infrared reflection-absorption spectroscopies. This provides the knowledge of the reactive system properties necessary for the interpretation of the ir photodesorption experimental results. These properties are the sticking coefficient of methanol on Cu(110) at 90 K, the monolayer density, the ir absorbance, and the structure of the adsorbate. In particular, ir spectroscopy demonstrates the possibility of thermally activating the crystallization of the amorphous adsorbate obtained upon condensation on Cu(110) at 90 K. The experimental setup used for the measurements of the photodesorption yield (${\mathit{Y}}_{\mathrm{PD}}$) makes use of an optical parametric oscillator generating ir pulses about 15 ns long and frequencies tunable from 2800 to 3450 cm${.}^{\mathrm{-}1}$ This frequency range includes the ir bands corresponding to the asymmetric methyl and hydroxyl stretching modes. The ir ${\mathit{Y}}_{\mathrm{PD}}$ of amorphous and crystallized films have been measured as a function of the laser-beam frequency and fluence, of the substrate coverage, and of the temperature (from 90 to 120 K). The correspondence between the ${\mathit{Y}}_{\mathrm{PD}}$ spectra and the ir spectrum of liquid methanol indicates the occurrence of adsorbate melting during the photodesorption process. The saturable exponential evolution of the ${\mathit{Y}}_{\mathrm{PD}}$ as a function of the laser fluence reveals the thermal nature of the process. The ${\mathit{Y}}_{\mathrm{PD}}$ of the amorphous methanol thin films increases greatly upon their crystallization. This is tentatively explained as a consequence of a change in the film morphology. These experimental results are interpreted quantitatively in the framework of the thermal photodesorption mechanism due to the resonant substrate heating.