Non-thermal photodesorption of N{sub 2} from Ag(111) Page: 1 of 11
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Non-thermal photodesorption of N2 from Ag (111)
R. M. Rao, R. J. Beuhler and M. G. White
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973
APR 0 8 1998
ABSTRACT O ST I
We have measured translational and rotational energy distributions of N2 molecules following desorption from a Ag(111)
surface by infrared (1064 nm) radiation. The observed desorption yields were large even at laser fluences far below that
required for laser-induced thermal desorption. State-resolved laser techniques using coherent VUV radiation showed that the
rotational and translational energy distributions of the desorbing N2 molecules are not consistent with the predictions of the
heat diffusion model governing laser-induced surface heating. These results suggest that physisorbed adsorbates can couple
directly to the nascent-phonon distribution or the nascent electron-hole pairs in the photoexcited substrate without heating of
the surface.
Keywords : photodesorption, state-selective detection, nitrogen, Ag (111)
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1. INTRODUCTION
Photon stimulated desorption (PSD) of physisorbed species can proceed via three different
mechanisms', depending on where the photon energy is deposited. The deposition of the photon energy
directly to the adsorbate-substrate bond can desorb the adsorbate by direct excitation from its vibrational
ground state to continuum translational states. 2-5 Resonant excitation of an internal molecular vibration
of the adsorbate by the photon can lead to transfer of energy to the surface bond and its subsequent
release from the surface.6-1 In general, for non-resonant excitations9, the photons are absorbed by the
solid, where substrate excitations couple to the surface bond and thus lead to desorption. In this study,
we shall present results of infrared-induced desorption of physisorbed N2 on Ag (111), where the photon
energy is far from threshold (no direct excitation of N2-Ag bond), and is not resonant with intra-
molecular vibrations. The expectation is that substrate excitations lead to desorption of N2 from the
silver surface.
In this work, we present the results of state-resolved measurements of infrared (IR) induced
desorption of physisorbed N2 on Ag (111). These studies are an extension of previous work on
CO/Ag (111) where we observed non-thermal desorption dynamics for photoexcitation with near-IR
(1064 nm) and visible (532 nm) laser light.9 Specifically, the translational and rotational "temperatures"
of the desorbed CO molecules were found to be well described by Boltzmann distributions, but they did
not exhibit any power dependence over a factor of 3200 in IR fluence. The lack of power dependence
over such a wide range in IR power is inconsistent with that expected for a laser-induced thermal
desorption process. Furthermore, the desorption yields were found to scale approximately linearly with
IR fluence instead of exponentially for thermally driven desorption following Arrhenius kinetics. These
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Rao, R. M.; Beuhler, R. J. & White, M. G. Non-thermal photodesorption of N{sub 2} from Ag(111), article, February 1998; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc703969/m1/1/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.