C.-R. Wen

Photolysis of CF3Cl adsorbed on Si(111)(7×7) surface by monochromatic synchrotron radiation

We present an analysis of the photolysis of an adsorbed molecule by monochromatic synchrotron radiation. The system studied was CF3Cl adsorbed on Si(111)(7×7) at 30 K, and the techniques employed were photoemission spectroscopy (PES) and photon-stimulated desorption (PSD). The valence-level photoemission spectra, under various photon exposures, show that the photolysis effect induced by the incident monochromatic synchrotron radiation photons (hν=98 or 110 eV) occurs on this adsorbed gas–solid system. Variations of the F− and F+ PSD ion yields were measured as a function of monochromatic (110 eV) photon exposure at three adsorbate coverages (the lowest dose=0.3×1015, the medium dose=0.8×1015, and the highest dose=2.2×1015 mol/cm2). For the lowest CF3Cl-dosed surface, the photon-exposure dependencies of the F− and F+ yields show the characteristics: (a) at early stages of photolysis, the desorption of F− yields is mainly due to dissociative attachment (DA) and dipolar dissociation (DD) of the adsorbed CF3C...

Continuous-time photoelectron spectroscopy for monitoring monochromatic soft x-ray photodissociation of CF3Cl adsorbed on Si(111)−7×7

The continuous-time photoelectron spectroscopy was proposed to study the monochromatic soft x-ray photodissociation of CF3Cl molecules adsorbed on Si(111)−7×7. Evolution of adsorbed CF3Cl was monitored at two photon energies of 240 and 730eV to deduce the photolysis cross section as a function of energy. Dissociation of adsorbed CF3Cl by 240∕730eV photons is attributed to the C–Cl∕C–F bond scission following the excitation of Cl(2p)∕F(1s) core electron. Observation of time-dependent photodissociation also demonstrates the capability for “real-time” monitoring of the variations of electronic structure and chemical bonding of adsorbate in bond-selective photochemistry.

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

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F(-) ions from CF(3)Cl physisorbed on a Si(111)-7x7 surface at 30 K in the photon energy range 12-35 eV was studied. The F(-) ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF(3)Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF(3)Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.

Soft x-ray photoreactions of CF3Cl adsorbed on Si(111)-7×7 studied by continuous-time photon-stimulated desorption spectroscopy near F(1s) edge

The continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was employed to monitor the monochromatic soft x-ray-induced reactions of CF3Cl adsorbed on Si(111)-7x7 near the F(1s) edge (681-704 eV). Sequential F+ PSD spectra were measured as a function of photon exposure at the CF3Cl-covered surface (dose=0.3x10(15) molecules/cm2, approximately 0.75 ML). The F+ PSD and total electron yield (TEY) spectra of molecular solid CF3Cl near the F(1s) edge were also measured. Both F+ PSD and TEY spectra show two features at the energy positions of 690.2 and 692.6 eV, and are attributed to the excitations of F(1s) to 11a1[(C-Cl)*] and (8e+12a1)[(C-F)*] antibonding orbitals, respectively. Following Auger decay, two holes are created in the F(2p) lone pair and/or C-F bonding orbitals forming the 2h1e final state which leads to the F+ desorption. This PSD mechanism, which is responsible for the F+ PSD of solid CF3Cl, is employed to interpret the first F+ PSD spectrum in the sequential F+ PSD spectra. The variation of spectrum shapes in the sequential F+ PSD spectra indicates the dissipation of adsorbed CF3Cl molecules and the formation of surface SiF species as a function of photon exposure. From the sequential F+ PSD spectra the photolysis cross section of the adsorbed CF3Cl molecules by photons with varying energy (681-704 eV) is determined to be approximately 1.0x10(-17) cm2.

Adsorption and photon-stimulated desorption of CCl4 on an Al(111) surface investigated with synchrotron radiation

Adsorption and desorption of CCl4 molecules on an Al(111) surface at 90 K are characterized with photoemission spectroscopy (PES) and photon-stimulated ion desorption (PSID) techniques following valence-level and core-level excitations. Results of valence-level and Cl(2p) core-level PES spectra indicate that CCl4 dissociates partially upon adsorption on an Al(111) surface at submonolayer coverage and that molecular CCl4 adsorbs to form multilayers at large exposures. The dissociation upon adsorption of CCl4 on an Al surface at 90 K is likely mediated by the charge-transfer process. The Cl+ desorption threshold at ∼18.5 eV in valence-level PSID spectra may originate from the 5t2→7a1* (C–Cl antibonding orbital) transition consistent with the Menzel–Gomer–Redhead (MGR) mechanism. The total-electron yield (TEY) spectrum and the Cl+ PSID spectrum of solid CCl4 following the Cl L-edge excitation are clearly dissimilar. The enhanced desorption yield of Cl+ ions is detected at the Cl 2p→7a1* excitation, compared ...

Photon-exposure-dependent photon-stimulated desorption for obtaining photolysis cross section of molecules adsorbed on surface by monochromatic soft x-ray photons

Photon-exposure-dependent positive- and negative-ion photon-stimulated desorption (PSD) was proposed to study the photoreactions and obtain the photolysis cross sections of molecules adsorbed on a single-crystal surface by monochromatic soft x-ray photons with energy near the core level of adsorbate. The changes in the F(+) and F(-) PSD ion yields were measured from CF(3)Cl molecules adsorbed on Si(111)-7x7 at 30 K (CF(3)Cl dose=0.3x10(15) molecules/cm(2), approximately 0.75 monolayer) during irradiation of monochromatic soft x-ray photons near the F(1s) edge. The PSD ion yield data show the following characteristics: (a) The dissociation of adsorbed CF(3)Cl molecules is due to a combination of direct photodissociation via excitation of F(1s) core level and substrate-mediated dissociation [dissociative attachment and dipolar dissociation induced by the photoelectrons emitting from the silicon substrate]. (b) the F(+) ion desorption is associated with the bond breaking of the surface CF(3)Cl, CF(2)Cl, CFCl, and SiF species. (c) the F(-) yield is mainly due to DA and DD of the adsorbed CF(3)Cl molecules. (d) The surface SiF is formed by reaction of the surface Si atom with the neutral fluorine atom, F(+), or F(-) ion produced by scission of C-F bond of CF(3)Cl, CF(2)Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of this submonolayer CF(3)Cl-covered surface. Based on this model and the variation rates of the F(+)F(-) signals during fixed-energy monochromatic photon bombardment at 690.2 and 692.6 eV [near the F(1s) edge], the photolysis cross section was deduced as a function of energy.

Valence-level photoemission spectroscopy and photon-stimulated ion desorption studies of CH3Cl adsorbed on Al(111) surface using synchrotron radiation

Abstract The interaction of CH 3 Cl adsorbed on Al(111) surface at ∼90 K has been investigated by means of valence-level photoemission spectroscopy (PES), work function change and photon-stimulated ion desorption (PSID) using synchrotron radiation. The one-to-one correspondence between the gas-phase and the condensed-phase CH 3 Cl PES spectra suggests that the molecules are molecularly adsorbed without decomposition. Adsorption of CH 3 Cl leads to a decrease in work function of Al(111) ( Δo ∼ − 0.6 eV at a monolayer), indicating a dipole pointing away from the substrate and Cl is in direct contact with the metal. In the H + PSID spectra, three weak thresholds are observed, at ∼9.6, ∼13.9 and ∼16.5 eV, and a major threshold appears at ∼18.9 eV. CH + 3 PSID spectra show a weak threshold at ∼13.2 eV and two major thresholds near 9.0 and 16.7 eV. The desorption threshold below 18 eV may be due primarily to the single-hole excitation or Rydberg transition, whereas the ion desorption above 18 eV may result predominantly from excitations of C 2s electron correlation states.

Continuous-time photon-stimulated desorption spectroscopy studies on soft x-ray-induced reactions of CF3Br adsorbed on Si(111)-7×7

Continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was used to study the soft x-ray-induced reactions of CF(3)Br molecules adsorbed on Si(111)-7×7 near the Si(2p) edge (98-110 eV). The monochromatic synchrotron radiation was employed as a soft x-ray light source in the photon-induced reactions and also as a probe for investigating the produced fluorination states of the bonding surface Si atom in the positive-ion PSD spectroscopy. Several different surface coverages were investigated. The PSD spectra from the low-CF(3)Br-covered surfaces show the production of surface SiF species, while those from the high-CF(3)Br-covered surfaces depict the formation of surface SiF, SiF(2), and SiF(3) species. The photolysis cross section of the submonolayer CF(3)Br-covered surface is determined as ∼4.3×10(-18) cm(2). A comparison with the results on CF(3)Cl/Si(111)-7×7 surface is discussed.

Monochromatic soft-x-ray-induced reactions of CF2Cl2 adsorbed on Si(111)-7 × 7 studied by continuous-time photon-stimulated desorption spectroscopy near the F(1s) edge.

Continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was used to investigate the monochromatic soft x-ray photoreactions of CF(2)Cl(2) adsorbed on Si(111)-7 × 7 near the F(1s) edge (681-704 eV). Sequential F(+) PSD spectra were observed as a function of photon exposure at the CF(2)Cl(2)-covered surface (dose = 2.0 × 10(14) molecules cm(-2), ∼0.75 monolayer). The F(+) PSD and total electron yield (TEY) spectra of solid CF(2)Cl(2) near the F(1s) edge were also measured. Both F(+) PSD and TEY spectra depict three features in the energy range of 687-695 eV, and are assigned to the excitations of F(1s) to (13a(1) + 9b(2))[(C-Cl)(∗)], (7b(1) + 14a(1))[(C-F)∗] antibonding and 5p Rydberg orbitals, respectively. Following the Auger decay process, two holes are created in the C-F bonding orbitals producing the 2h1e final state which results in the F(+) desorption. This PSD mechanism, responsible for the F(+) PSD of solid CF(2)Cl(2), is used to explain the first F(+) PSD spectrum in the sequential F(+) PSD spectra. The variation of spectral shapes in the sequential F(+) PSD spectra shows the consumption of adsorbed CF(2)Cl(2) molecules and the production of surface SiF species as a function of photon exposure. The photolysis cross section of the adsorbed CF(2)Cl(2) molecules by photons with varying energy (681-704 eV) is deduced from the sequential F(+) PSD spectra and found to be ∼6.0 × 10(-18) cm(2).