<title>State-resolved dynamics of infrared photodesorption of CO from Ag(111)</title>

Abstract

1, INTRODUCTIONCurrent understanding of the dynamics of photon stimulated desorption (PSD) of molecules from surfaces has been derived in large part from state-and energy-resolved probes of the desorbed molecules in the gas-phase.'B2 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 (5 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 silrface atom displacements which deposit vibrational energy in the absorbatemetal bond in excess of the b i d i n g energy.The desorption rate is highest at the maximum surface temperature (Tmz) induced by the laser pulse which can be determined with reasonable accuracy from a classical heat-diffusion model.3 -' As a result, "thermally" desorbed molecules are expected to have internal and translational energy distributions characteristic of Tmoz.State-resolved measurements performed by Buntin, et aL6 for NO/Pt(lll) 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 energiei 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. Tmz cc Io. In addition, the rotational temperature (-100 K) was found to be significantly smaller than the expected surface temperature rise (Tmz = 227 K). In a related study, Prybyla, et al. observed a Boltzmann rotational state distribution for NO desorbed from Pd(ll1) at 2.33 eV (532 nm), however, the derived rotational temperature was approximately half that of the surface temperature? 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?j8In this work, we present state-resolved measurements for E t . (1.17 eV, 1064 nm) photodesorption of CO physisorbed on a Ag(ll1) surface. In contrast to NO, there-has been very little

Final State Distributions of O2 Photodesorbed from TiO2(110)

“…The surface photochemistry instrumentation used for these experiments has been described in detail elsewhere. , Briefly, the apparatus consists of a main surface science chamber with a base pressure of about 5 × 10 −10 Torr and a smaller chamber which serves as a windowless VUV source. The two chambers are connected via a doubly differentially pumped capillary light guide to allow passage of the VUV probe radiation while maintaining the UHV environment of the main chamber.…”