Photon-stimulated charge transfer in condensedO2studied by negative-ion desorption

Abstract

Photon-stimulated desorption of negatively charged ions from condensed O2 is observed in the 17-42-eV photon energy range. Two classes of photon-stimulated charge-transfer processes, i.e., resonant processes via the formation of highly excited states of the condensed molecules and nonresonant processes involving energetic photoelectrons, are considered to account for these observations. PACS numbers: 31.70. Ks, 33.80.Eh, 68.45.Da, 79.20.Ds The charge-transfer process is of fundamental importance to many areas of biology and chemistry. For adsorbed molecules, photon-induced charge transfer may occur either directly by exciting molecular chargetransfer (ion-pair) states or indirectly by attaching a photoemitted electron (issued from the substrate or neighbor adsorbed species) onto the studied molecule. Photonstimulated charge transfer (PSCT) has been suggested by several authors [1,2] to play an important role in surface photochemistry although no direct evidence of PSCT has been obtained so far.The aim of this paper is to demonstrate, by using multilayers of condensed O2 as an example, that negative-ion desorption is a powerful and direct probe of PSCT in adsorbed molecules. The only previously reported observation of negative-ion desorption following photon excitation is that of negative H ~ ions from a cesiated W(100) surface by Hurych et al. [3]. However, in this case the absence of any variation of the desorption signal with photon energy prevented these authors from identifying particular desorption mechanisms. Negative-ion desorption from a number of adsorbed molecules was studied by electron impact [4][5][6]. Attention was focused on the dissociative attachment of slow electrons on physisorbed or chemisorbed molecules [5].In this Letter, we report the first study of photonstimulated desorption of negative ions from condensed gases involving intramolecular charge-transfer (ion-pair) states and intermolecular electron transfer. During the course of this work, observed for the first time photon-stimulated desorption of negative ions from condensed molecules produced by charge transfer from the substrate.Synchrotron radiation from Super-ACO at Orsay, dispersed by a grazing-incidence monochromator, is used as a photon source of variable energy in the 17-42-eV energy range with a 0.5-A bandpass. The experiments are performed in an UHV system reaching pressures below 10 _,° mbar. The molecular films are obtained by condensing O2 on a polycrystalline platinum substrate cooled at 15 K with a liquid-helium flow cryostat. The platinum substrate is thin enough (20 pm) to be resistively heated in order to be cleaned before any gas deposition. The film thickness of about five monolayers is estimated from photoabsorption measurements. Desorbed positive and negative ions are mass selected through a Riber SQX 156 quadrupole filter and counted as a function of the photon energy. Ion yields are normalized to the photon flux monitored by the photoemission of a gold mesh.The photodesorption yield of O ~ from condensed O2, r...

Surface photochemistry in the vacuum and extreme ultraviolet (VUV and XUV): high harmonic generation, H2O and O2

“…The VUV photochemistry of O 2 adsorbed on a polycrystalline Pt [166] or Si surface [13] was investigated by Dujardin et al Both O + and O − were observed to desorb from multilayers of O 2 physisorbed on Pt for irradiation with 17 eV photons, with a strong increase above 32 eV. O + 2 and O − 2 were also observed but with much lower probabilities.…”

Photon-stimulated desorption of F− ions from CF3Cl adsorbed on Si(111)-7×7

“…However, relatively little work has been undertaken to study photon-stimulated desorption of negative ions. [1][2][3][4][5] Several photodissociation/photodesorption mechanisms have been proposed. [6][7][8][9][10][11][12][13] For the desorption of negative ions, the incident photon may excite the adsorbed molecule AB to a neutral Rydberg state.…”