Proceedings Article | 25 September 1995
KEYWORDS: Molecules, Crystals, Temperature metrology, Vacuum ultraviolet, Carbon monoxide, Molecular lasers, Spectroscopy, Gas lasers, Thermal modeling, Laser beam diagnostics
Current understanding of the dynamics of photon stimulated desorption (PSD) of molecules from surfaces has been derived in large part from state- and energyresolved probes of the des orbed molecules in the gas-phase.1'2 Analogous to studies of gas-phase photodissociation, information concerning the electronic interactions and nuclear motions on the dissociative potential surface are inferred from the energy, internal state and angular distributions of the photoproducts. Of particular interest is the use of state-resolved methods to ellucidate the photodesorption mechanism which can involve substrate and/or absorbate excitations followed by facile energy transfer between the electronic and nuclear degrees of freedom. The primary concern of this work is desorption induced by photon energies well below the work function ( 1 —2 eV) which is nominally assumed to occur via a thermally activated process. From a dynamical standpoint, laser-induced surface heating results from the rapid thermalization of initially photoexcited electron-hole pairs which relax through inelastic e — e scattering and energy transfer to lattice modes of the substrate. Desorption results from random surface atom displacements which deposit vibrational energy in the absorbate—metal bond in excess of the binding energy. The desorption rate is highest at the maximum surface temperature induced by the laser pulse which can be determined with reasonable accuracy from a classical heat-diffusion modeL35 As a result, "thermally" desorbed molecules are expected to have internal and translational energy distributions characteristic of Tmax. State-resolved measurements performed by Buntin, et al.6 for NO/Pt(111) at a number of photon energies between 0.65 eV and 3.49 eV have identified two desorption channels, with the "slow" velocity component exhibiting near Boltzmann rotational and translational distributions. Although the measured angular distribution, photon energy dependence and translational energies of the slow channel are consistent with thermally activated desorption, the translational energies did not show a dependence on laser fluence as expected from the classical heat-diffusion model, i.e. T 4. [n addition, the rotational temperature (s100 K) was found to be significantly smaller than the expected surface temperature rise = 227 K). In a related study, Prybyla, et al. observed a Boltzmann rotational state distribution for NO desorbed from Pd(111) at 2.33 eV (532 nm), however, the derived rotational temperature was approximately half that of the surface temperature.7 Such "rotational cooling" has been attributed to strong coupling between rotation and translation induced by the molecule-surface potential and its anisotropy with respect to molecular orientation.7'8 In this work, we present state-resolved measurements for IR (1.17 eV, 1064 nm) photodesorption of CO physisorbed on a Ag(111) surface. In contrast to NO, there has been very little experimental work on CO photodesorption, partly due to the difficulties associated with multi.. photon or VtJV probes required to obtain state-resolved dynamics. Recent state-resolved measurements for CO/Pt (111) , CO/NiO (111) 10 and 211 have focussed on the observation of non-Boltzmann final state distributions induced by 1JV excimer radiation (308 nm, 248 nm and 193 nm) at laser fluences too low to induce substantial thermal desorption. The low desorption temperatures for both the monolayer (48 K) and multilayer (36 K) phases of CO/Ag(111), however, permit the study of photo-induced "thermal" processes at modest laser power densities. Furthermore, the low surfa.ce temperature and weak interaction between the CO molecule and the Ag(111) surface favors equilibration of the rovibronic and translational energies with State-resolved detection of desorbed CO is performed via (1 + 1') resonant multiphoton ionization (REMPI) using coherent VTJV radiation and time-.offiight mass spec trometry. The observed rotational and translational state distributions are well described by Maxwell-Boltzmann distributions with characteristic temperatures which indicate near equili.. bration of the rotational and translational degrees of freedom. These results are consistent with a photo-induced "thermal" desorption mechanism and are compared with the predictions of the classical heatdiffusion model.
