Photochemistry of Adsorbed Molecules. 19. Photodissociation of CH3I on LiF(001) and NaCl(001) at 248 nm: REMPI Probing of CH3

Laser Control of Desorption through Selective Surface Excitation

We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.

Photoinduced-Reaction Dynamics of Halogenated Alkanes on Iron Oxide Surfaces: CH3I on Fe3O4(111)−(2×2)

The adsorption, thermal chemistry, and photoreaction dynamics of methyl iodide on the (2x2) magnetite termination of natural single-crystal hematite have been investigated by time-of-flight quadrupole mass spectrometry (TOF-QMS), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). The methyl iodide thermal desorption spectra, taken after dosing the (2x2) surface at 100 K with CH(3)I, show a multiple-peak coverage-dependent behavior, consistent with the presence of several distinct adsorbed phases, along with defect-mediated dissociative chemisorption in the first monolayer. At >1 ML, methyl iodide forms a metastable physisorbed second layer, which desorbs at 148 K, but at higher coverage converts to a layer, which desorbs at 170 K. In the presence of low-fluence-pulse irradiation at 248 nm, angle-resolved TOF-QMS measurements show that 1.6 and 0.3 eV CH(3) fragments are ejected from the adsorbate surface; these fragments originate from direct photodissociation and dissociative photoinduced electron transfer, respectively. These energetic photoejected fragments have characteristic angular distributions peaked at approximately 0 degree with respect to the surface normal. These results and the coverage-dependent relative intensities suggest that the predominant orientation in the first monolayer of the adsorbed CH(3)I is normal to the crystal plane.

Laser control of product electronic state: Desorption from alkali halides

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.

Analysis of trans-resveratrol by laser desorption coupled with resonant ionization spectrometry. Application to trans-resveratrol content in vine leaves and grape skin

A method has been developed for nonvolatile compound analysis in fruits, namely, trans-resveratrol in grapes and vine leaves. This has been accomplished by the combination of laser desorption with resonance-enhanced multiphoton ionization coupled to time-of-flight mass spectrometry detection. After the optimization of the experimental conditions and the finding of the resonant wave-length of the substance, the full validation of the technique (i.e., linearity, repeatability, reproducibility, accuracy, detection limit, and quantification limit) was carried out with satisfactory results. Essential features of the method are as follows: (1) an enhanced desorption yield due to the mixing of the analyte with metal powder and (2) a high resolution and sensitivity and a low detection limit due to laser resonant ionization and mass spectrometric detection. Application to the analysis of trans-resveratrol in vine leaves and grape skin demonstrated the capabilities of the analytical method reaching detection limits of only few ppb.