Laser surface chemistry

Laser surface chemistry has been used as a basis for many new methods in surface processing, for example, photochemical deposition of metals and photochemical etching of solid substrates, which are potentially useful techniques for the microelectronics industry (1-3). However, molecular dynamical studies of UV photodissociation of adsorbates on solid surfaces have been very scarce (4-7). We have studied UV laser photodissociation of small molecules on solid surfaces using photofragment spectroscopy. 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

There is a large volume of contemporary literature dealing with the structure and chemical properties of species adsorbed at the solid-solution interface, making use of various spectroscopic and laser excitation techniques. Much of it is phenomenologically oriented and does not contribute in any clear way to the surface chemistry of the system included are many studies aimed at the eventual achievement of solar energy conversion. What follows here is a summary of a small fraction of this literature, consisting of references which are representative and which also yield some specific information about the adsorbed state. 

In a related approach, arrays with different types of surface chemistries such as hydrophobic, hydrophilic, anionic, and affinity are used to absorb certain protein groups from biological or patient samples. The chip-absorbed proteins are then directly detected by surface-enhanced laser desorption/ionization time-of-flight MS (SELDl-TOF MS) (Issaq et al. 2002). The resulting protein masses can be used in pattern analysis and thereby provide a useful diagnostic tool. 

Two techniques are principally responsible for the experimental development of dynamics in surface chemistry. These are the application of molecular beams and laser state-to-state techniques to gas-surface interactions. This roughly parallels their application to gas phase chemistry, although there are certainly some different technical requirements. More detailed discussion of some of these experimental techniques are in Refs. [104] and [105]. 

R. D. Levine As emphasized by the speakers on femtosecond pumping schemes, an important point is that the initial excitation is localized within the Franck-Condon regime. The question is whether the sheer localization can be used to advantage to induce laser-selective chemistry (K. L. Kompa and R. D. Levine, Acc. Chem. Res. 27, 91 (1994)]. As we understand better the topography of potential-energy surfaces for polyatomic molecules, it may be possible to launch the system with such initial conditions that it will, of its own accord, proceed to cross a particular transition state and so exit toward a particular set of products. 

An alternative for the low detection efficiencies of the emitted particles is to ionize them with a UV laser beam, either in a resonant or non-resonant manner [46]. In this way, the ionization efficiency is increased about 1000-fold and the attractive perspective of performing SNMS under static conditions at sensitivities comparable to those of SIMS comes into the reckoning. As yet, however, we are not aware of any applications in the fields of catalyst characterization or in catalytic surface chemistry. 

A new technique for measuring equilibrium adsorption/desorption kinetics and surface diffusion of fluorescent-labelled solute molecules at surfaces was developed by Thompson et al.74). The technique combines total internal reflection fluorescence with either fluorescence photobleaching recovery or fluorescence correlation spectroscopy with lasers. For example, fluorescent labelled protein was studied in regard to the surface chemistry of blood 75). 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Mass-spectrometry principles and techniques have been employed in other kinds of surface studies in which sample atoms are sputtered by interaction with a laser beam or by RF glow discharges. These approaches are more highly specialized, but it should be clear that mass spectrometry is an important tool in surface chemistry. The student should compare SIMS and ISS with other surface analytical techniques such as ESCA, Auger spectroscopy, electron microprobe, and low-energy electron diffraction (see Chaps. 14 and 15). 

Such a sharp increase in surface free energy after a slight laser treatment may arise for three different reasons efficient cleaning of the composite surface modification of the surface chemistry as the result of the initial ablation process [20] or the growing influence of fiber reinforcement [21]. 

We would like to thank the S.E.R.C. and Unilever Research for support for one of us (DNS) Unilever Research also provided access to Si NMR and laser Raman spectrometers. We thank EKA AB, Surte, Sweden, for financial support for another of us (KRA) and for help with the ultrafiltration experiments. We are also very grateful to Mr. Kenneth Rosenquist of the Swedish Institute for Surface Chemistry, Stockholm, for his assistarce in applying the dynamic light scattering technique. Finally we are grateful to the PQ Corporation for making it possible for this paper to be presented at New York. 

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