VOLUME 76, NUMBER 11 PHYSICAL REVIEW LETTERS 11 MARCH 1996 Surface Enhanced Photodissociation of Physisorbed Molecules L. ˘ Siller, 1,2 S. L. Bennett, 3,4 M. A. MacDonald, 3 R. A. Bennett, 1,2 R. E. Palmer, 1 and J. S. Foord 4 1 Nanoscale Physics Research Laboratory, School of Physics and Space Research, University of Birmingham, Birmingham B15 2TT, United Kingdom 2 Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom 3 CLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom 4 Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom (Received 10 November 1995) We have investigated the photon stimulated desorption of ions from physisorbed layers of CO and O 2 on graphite using VUV synchrotron radiation in the range of 13 to 35 eV. In both systems we find a significant enhancement in the monolayer regime of the desorption of O 2 ions via resonant states which correspond to direct molecular photoabsorption. This enhancement, which is accompanied by suppression of positive ion desorption, is attributed to surface electron attachment following molecular photoexcitation. PACS numbers: 82.50.Fv, 68.45.Da, 79.20.Kz, 82.30.Fi One of the principal results emerging from the new experimental studies of surface photodynamics is the possibility of “substrate-mediated” photodissociation or photodesorption — electrons (or holes) photoexcited in the substrate drive the desorption process [1–4]. The experi- mental interest in these new desorption channels is mir- rored by the emergence of a distinct theoretical effort to describe desorption via electron transfer [5,6]. One im- portant mechanism is the dissociative attachment of pho- toemitted electrons to adsorbed molecules [7]; work with low energy electron beams shows that tuning the electron energy to a specific molecular negative ion resonance state results in efficient bond rupture [8,9] or vibrational excitation [10,11]. However, from the viewpoint of ap- plications, e.g., chemically selective surface modification, direct molecular dissociation, or desorption is more promising than a substrate-mediated process; in the case of direct excitation, the wavelength dependence of the process is resonant (and hence chemically specific) be- cause it reflects a specific intramolecular photoexcitation event, as in the free molecule. In this Letter we report a study of photon stimulated desorption from physisorbed layers of CO and O 2 on graphite, using VUV synchrotron radiation in the range 13 to 35 eV, which finds that reso- nant photodissociation of both molecules is enhanced in the monolayer regime. This enhancement, evident in the negative ion yields (O – ), is associated with a suppression of the corresponding positive ion desorption, and leads us to propose a new mechanism based on surface electron attachment following direct molecular photoexcitation. The experimental results were obtained at beam line 3.1 of the Daresbury Synchrotron Radiation Source. Syn- chrotron light was dispersed in the energy range 13 – 35 eV with a 1 m Seya-Namioka monochromator incorporating a 2400 lines mm –1 ruled grating. The bandpass of the monochromator optics was set at 6 Å in order to enhance the photon flux. Below 20 eV this corresponds to an energy resolution of better than 0.2 eV. From 20 to 35 eV the resolution is still below 0.6 eV. Desorbed positive and negative ions were detected by a pulse counting quadrupole mass spectrometer, de- scribed previously [12]. The background signal, given by the signal at mass 15, was subtracted from the data. The highly oriented pyrolytic graphite (HOPG) substrates were mounted on a liquid helium cryostat in an ultrahigh vacuum chamber and cleaned by electron bombardment heating. The calibrated base temperature of the sample, measured with a four-wire rhodium-iron resistance ther- mometer, was 29 K. Gas dosing was accomplished by filling the chamber with ,2 3 10 28 mbar of CO or O 2 . All experiments were conducted at a residual gas pressure below 5 3 10 210 mbar. The ion yields were measured as a function of both photon energy and coverage (i.e., exposure). Figure 1 shows the yield of O – ions photodesorbed from a submonolayer coverage of CO physisorbed on graphite at 29 K. A well-defined resonance is observed, centered at 22 eV. This resonance feature has been FIG. 1. Photon stimulated desorption yield of O – ions from submonolayer (3 L) CO physisorbed on graphite as a function of photon energy. 1960 0031-9007y 96y 76(11) y1960(4)$10.00 © 1996 The American Physical Society