Momentum Distribution of Molecules Desorbed by Vibrational Excitation with Laser Infrared
Desorption from ionic crystal and metal surfaces can be stimulated by excitation of adsorbate internal vibrations with resonant infrared laser radiation. By measuring the dependence of the desorption yield upon laser frequency and comparing with the linear infrared spectra of the adsorbate, it was shown that SF6  and CH3F [2–5] molecules adsorbed on NaCl surfaces at low temperature can be desorbed by resonant excitation of the adsorbate internal vibration, v 3-SF6 and v 3-CH3F stretching vibration, respectively. The rates of the fast desorption processes were determined from yield vs. laser fluence plots [2,4,5]. Resonant desorption of pyridine from KCl  and Ag [7,8] surfaces was observed, and from the relation between the desorption yield and the polarization of the incident radiation, as detected for silver surfaces, it was inferred that internal vibrational excitation is the primary activation step. Resonant desorption is always in competition with relaxation causing resonant adsorbate heating, which could induce thermal desorption. Relating the measured desorption yield for a series of laser fluences with the absorption cross section, the quantum yield for the resonant desorption of CH3F from NaCl surfaces, induced by laser infrared, was found to be rather high  as compared to photodesorption processes in the visible and ultraviolet, but it turned out that only a minor part of the energy absorbed is used to increase the potential energy of the molecule on the surface necessary for desorption.
KeywordsVibrational Excitation Resonant Excitation Desorption Yield Registration Device Quadrupole Mass Spectrome
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