Unenhanced surface Raman spectroscopy has been shown to be an exceptionally powerful probe of the structures and reactions of molecules on a wide variety of solid surfaces which include metals, semiconductors and dielectrics. Its advantages include high spectral and spatial resolution, a wide spectral range and the important ability to probe through nonvacuum ambient phases. The last characteristic is especially relevant to the characterization of thin films under actual growth conditions. We review here the basic elements of the surface Raman experiment, including experimental conditions and selection rules and illustrate its capabilities as an in situ diagnostic tool with examples from electrochemistry and polymer thin film synthesis.

Examples from this laboratory illustrating laser probing of gaseous products formed in chemical reactions on surfaces are taken from the areas of catalytic reactions, silicon etching, and surface photochemistry. The emphasis will be on the unique advantages of laser diagnostics in these dynamical and mechanistic studies in which the chemical reaction product species undergo no collisions after leaving the surface before detection by the laser. UHV experiments on the catalytic oxidation of H2, D2 and NH3 by N02 and H2 by NO2 used the laser-induced fluorescence (LIF) technique to measure the apparent desorption energies of OH radicals and the rotational energy accommodation of the desorbing OH, OD and NO reaction products. The observed different degrees of "rotational cooling" in the different products will be discussed. In multiphoton ionization mass spectrometry experiments (MPI/MS) spectroscopies of three different MPI processes for the SiF2 radical were studied and two of these were applied to the study of silicon etching by XeF2, F2, and NF3. The only observed gaseous products were SiF4 and the SiF2 radical and their apparent production energies correlated with the case of surface fluorine atom production. In the area of surface photochemistry, an excimer laser was used to photolyze iodobenzene, trimethylgallium, and copper acetylacetonate adsorbed on a cold surface. The time-of-flight (TOF) spectra of the fragments or reaction products were measured by using a mass spectrometer equipped with multiphoton (MPI) or electron (EI) ionization.

Two laser spectroscopy techniques for adsorbate vibrational spectroscopy are discussed: electroreflectance vibrational spectroscopy (EVS) and polarization-modulated IR reflection absorption spectroscopy (PS). Both techniques were used to obtain quantitative spectra of CO on Ni(100). The sensitivity of EVS is greater, but the two techniques provide complementary information. Chemical applications are given.

Optical surface second-harmonic generation (SHG) has been used to investigate the nucleation and growth of Ag layers and 3-D Ag nanoclusters (<10 monolayers thick) on a clean Cu(110) single-crystal surface. The surface SH intensity undergoes large changes below coverages corresponding to one Ag(111) monolayer that were highly temperature dependent. At surface temperatures above 300 K, the SH intensity increased three-fold due to the nucleation and growth of three-dimensional Ag nanoclusters on a Ag(111) monolayer thick template. Comparisons of the surface SH intensity and polarization dependences with other surface analysis techniques indicate that SHG provides a strong fingerprint for both changes in the interfacial electronic valence band structure of the surface and the nucleation and growth of Ag nanoclusters. These results are discussed in terms of a simple model relating changes in the SH intensity to changes in surface morphology by separating the coverage dependence of the nonlinear susceptibility of the interface into regions dominated by changes in either the interfacial electronic valence band structure, or structurally enhanced local-field contributions to SHG.

This paper deals with the application of plasmon surface polaritions (PSP) for the characterization of thin films, e.g. Langmuir-Blodgett multilayer assemblies. Examples are given for total internal diffraction of PSP by dielectric phase gratings, interferometry at an index step and, finally, recent developments in the new field of PSP imaging and microscopy are presented.

Rapid-scan Fourier transform infrared (FTIR) spectroscopy is becoming a useful method for monitoring reaction kinetics. The excimer laser has recently appeared as a source of intense UV radiation for laser-assisted photochemistry. These two tools are combined in this study of the polymerization rates of commercially useful transparent thin films. Liquid mixtures made up of 3 parts epoxy oligomer and 2 parts di- or tri-acrylate monomer, with 3-5% of various photoinitiators and 3% amine, were spread on NaCl plates in 7 μm layers. Transmission FTIR spectra were taken with the sample in an N2 atmosphere. Excimer laser pulses at either 308 or 351 nm and spanning a range of fluences from 10 -4 to 10 -2 J/cm 2 (below the damage level) illuminated the sample. The photoinitiator absorbed the UV light and initiated a chain reaction that led to full polymerization of the coating. FTIR scans of 4 cm -1 resolution were taken every 80 msec during the reaction. Extent of reaction was monitored by the disappearance of the acrylate C=C bond at 810 cm -1, with reference to the unchanging peak at 830 cm -1. Typical experiments at high fluence involved a pulse or fast series of pulses (sufficient to lead to complete polymerization) followed by rapid FTIR scanning as the reaction proceeded. At low fluence, many series of pulses were required to complete the reaction, and each series was followed by an FTIR scan. Kinetics data were taken with several fluences, wavelengths, and sample compositions. These and other simpler kinetics diagnostics measurements are discussed in the context of a search for an industrially useful laser-cured polymer coating process.

Direct picosecond time-resolved measurements of vibrational energy relaxation for a molecular adsorbate at a bulk metal surface were made. Vibrational energy relaxation from ν=1 to ν==0 for C-H stretching modes of the terminal methyl group in a Cd stearate Langmuir-Blodgett monolayer on an evaporated silver film was measured using infrared-visible sum spectroscopy to dynamically probe vibrational level populations. Multicomponent decay processes with lifetimes of 3 ps to >1 ns indicate complex intramolecular vibrational energy transfer processes in these ordered monolayer films, which may be different in related polymer and molecular liquid systems.

The surface-specific process of optical second-harmonic generation has been applied to investigate adsorbed molecular monolayers exhibiting a periodic modulation across the surface. In addition to the usual reflected second-harmonic signals, these spatially modulated monolayers are found to give rise to several orders of diffracted second-harmonic radiation. An analysis is presented relating the characteristics of the second-harmonic diffraction pattern to the spatial properties of the modulated adlayer. In this study, gratings in the adsorbate density of dye molecules adsorbed on insulating substrates were formed by photo-desorption in the field of two interfering laser beams. From the second-harmonic diffraction data, adsorbate density profiles have been inferred. These results can be explained by a model for the formation of the molecular gratings based on a thermal desorption mechanism.

Laser-based nonlinear optical probes can be used to study the dynamics of surface reactions and phase transitions on timescales as short as a few femtoseconds. Here we report the results of an experiment in which second harmonic generation is used to follow the change in symmetry of crystalline silicon as it evolves, after the silicon has been electronically excited with a 100 femtosecond optical pulse. One finds that the electronic structure in the top 75 - 130 Å of the Si surface loses cubic order only 150 fsec after excitation. This suggests that the atomic disorder is induced directly by electronic excitation and occurs before the material becomes vibrationally excited. In contrast, the electronic properties of the equilibrium molten phase are not obtained for several hundreds of fsec.

Second Harmonic Generation (SHG) has many advantages as a diagnostic tool for use in studying processes which occur at surfaces. It can be used to study surface properties of the substrate itself, or those processes which involve adsorbed molecular layers. These advantages include: 1) fast time resolution, since the SHG occurs only in the presence of the light, and laser pulses as short as the femtosecond time scale can be used for SHG. 2) frequency resolution. Electronic and vibrational transitions can be resolved through resonantly enhanced SHG with 0.1 meV resolution using tunable lasers from the IR to UV region. 3) surface sensitivity, since, in the dipole approximation, SHG is only generated at the surface or interfacial region of a centro-symmetric material. It has been shown that the surface SHG is sensitive to submonolayer adsorbate coverages and to surface structural changes.2 4) nonintrusiveness. The light intensity required for SHG is usually far below the damage threshold of the surface and is low enough to avoid significant heating of the adsorbate/substrate system. The frequency of the light can be chosen to avoid any resonant absorption by either the adsorbate or the substrate. Thus no specific excitations will perturb the system under study. 5) in situ diagnosis. A surface process can be probed directly on the surface by SHG as it proceeds. The system can be a clean surface, or a surface with submonolayer or multilayer adsorbates.

Optical experiments on free-standing liquid crystal films which address the issues of structural ordering, fluctuations in two dimensions and enhanced surface ordering will be reviewed. Also, results on a novel electrohydrodynamic mode will be presented.

Optical second harmonic generation can be used to probe a number of important properties of liquid crystals, including polar ordering, orientation of molecules in monolayers, and the in-plane symmetry of the molecular arrangement at various interfaces. We have used second harmonic generation to investigate the effects of liquid crystal/substrate interactions on the alignment of bulk liquid crystal.

Brillouin spectroscopy has been used to characterize the elastic properties of Langmuir-Blodgett (LB) multilayer assemblies. First, some of the data obtained with cadmium arachidate layers are reviewed. These show highly anisotropic elastic constants reminiscent of smectic liquid-crystalline materials with a very small ratio of shear to compressional moduli. Then, recent results obtained with polymeric LB films are presented. For a polyglutamate system the complete set of elastic constants could be determined.

Molecular assemblies prepared by the Langmuir-Blodgett technique provide useful structures to probe structure-property relationships for nonlinear optical processes. In this paper we present selected results of a comprehensive research program in which we study the Langmuir-Blodgett films of optically nonlinear organic structures. The films are carefully characterized by a variety of surface and spectroscopic techniques. Both second and third order processes have been investigated. A detailed study of third-order resonant nonlinearity in a Si-phthalocyanine using femtosecond degenerate four wave mixing is presented. Using a new approach of simultaneously monitoring signals corresponding to first and second order diffractions and their power dependence, important information on excitation dynamics is derived. The theoretical analysis shows that both first order and bimolecular exciton-exciton annihilation processes in the studied Si-phthalocyanine contribute to the decay of resonant third-order optical nonlinearity.

Metallic silver is characterized in the near ultra-violet by a dielectric constant of small imaginary and small negative real components. This allows for intense, collective, conduction electron resonances at plane silver surfaces and for microscopic surface features. The consequent absorption and localization of electro-magnetic energy in such features is used to explain the giant enhancement observed for Raman scattering, second harmonic generation and other optical processes on roughened surfaces. For instance silver which has been vapour deposited onto low temperature substrates, so that there is little thermal annealing of the ballistically condensing metal, can show a 10 6 -fold enhancement in the Raman scattering of adsorbed molecules. We are interested in the photoemission of electrons from these metal films. Unfortunately the optical resonances occur at lower energies than that of the bulk plasmon at 3.8 eV, while clean silver has a photoelectric threshold above 4.1 eV; its precise value depending on the crystal face exposed. However the threshold is sensitive to adsorption and may be lowered a few tenths of an eV even by physisorbed gases. In this proceeding we report on photoemission at 3.7 eV from rough silver films. Others have studied electron emission into vacuum for plasmon region energies with the photoelectron threshold lowered by pyridine adsorption [1], by cesiation [1],[2], and also emission into electrolytic solution [3]. We have recently used two-photon non-linear photoemission with photon energies down to 2.1 eV as a probe for clean, high threshold surfaces [4].

Continuous UV (λ ≥ 250 nm) irradiation of monolayer methyl halides adsorbed on Pt(111) leads to carbon-halogen bond cleavage. Contrary to what is found for gas phase and liquid phase photolysis, adsorbed methyl iodide is significantly more difficult to dissociate than methyl bromide. In coadsorption experiments, methyl bromide can be photolyzed without dissociating methyl chloride. Evidence for these processes is provided by post-irradiation analysis of products retained on the surface using vibrational, photoelectron, and thermal desorption spectroscopies.

A phenomenological model is adopted to explore possible novel photochemical phenomena for molecules in the vicinity of a corrugated thin metal film, with detailed results worked out for the photoabsorption cross section for molecules in the vicinity of a grating film. A mechanism is proposed by which enhanced selective photoabsorption may be achieved based on the different nature of the coupling of the molecular dipole and the incident laser light to the surface plasmon modes of the thin films.

Two different photolytic mechanisms were identified for CH3C1 on Ni(111). Direct photolysis and electron-induced fragmentation was observed in time of flight (TOF) studies using 193 nm radiation. At 248 nm only the electron induced channel was observed. Both mechanisms were strongly inhibited in the first 2 monolayers, due to electron transfer occurring on a 1 to 5 femtosecond time scale. The electron induced process was surface specific, occurring within a limited adsorbate thickness determined by the penetration depth of the excited substrate electrons into the adsorbate. Cross sections for the total photolytic removal of parent molecules are reported as a function of coverage.

A number of mechanisms have been identified for explaining recent experimental results of photon-induced chemistry of molecules adsorbed on surfaces of metals, semiconductors, and insulators. The initial step of these mechanisms invariably involves excitation of either the adsorbate or the substrate. Two illustrative systems, NO and Mo(C0)6 on Si(111)7x7, will be discussed. The desorption and dissociation of NO are found to occur via interaction of photogenerated holes with the NO - substrate complex. In contrast, photoelectronic excitation of Mo(CO)6 leads to the detachment of the CO ligands. However, a new photochemical pathway involving electron dissociative attachment is opened when Mo(C0)6 is coadsorbed with K on Si.

Some recent progress in the utilization of light-induced heating (or photothermal) effects for material testing and processing is described. Material testing applications include moisture adsorption and interfacial thermal contact-resistance measurements, while processing studies include the ablation of thin polymer films using picoseconds ultraviolet laser pulses.

Electronic absorption and laser-induced fluorescence spectroscopy were used to examine pyrene adsorbed on Al203(1120) in UHV as a function of surface coverage and adlayer order. The absorption maximum of the So->S2 transition shifted from λ = 337 nm at θ = 0.13 ML to λ = 349 nm at θ ≥ 3.0 ML. The observed frequency shifts were explained in terms of dispersion interactions that stabilize the energy of the excited state due to the polarization of the surrounding dielectric. The fluorescence spectrum at 21 K exhibited both monomer and excimer emission. The relative amount of excimer emission increased as the coverage was increased from θ = 0.30 ML to θ = 4.5 ML. Likewise, the fluorescence spectrum was temperature-dependent between 21 and 150 K. As the temperature was increased, the monomer fluorescence intensity decreased while the excimer fluorescence intensity increased. This temperature dependence, together with xenon overlayer studies, suggested that the excimer formation was dependent on pyrene surface mobility. Absorption and fluorescence measurements also indicated that the pyrene adlayer crystallized on the Al203(1120) surface when the adlayer was annealed above 230 K.

Homo- and hetero-epitaxial gallium arsenide (GaAs) films have been deposited on single crystalline GaAs substrates of (100) orientation and on silicon (Si) substrates of 30 off the (100) orientation by ArF excimer laser (193 nm) induced metalorganic chemical vapor deposition (MOCVD). Homoepitaxial GaAs films of good structural perfection have been deposited at 425° - 500°C, and their single crystallinity has been confirmed by transmission electron microcopy. The carrier concentration decreases with increasing AsH3/(CH3)3Ga molar ratio and with decreasing substrate temperature. The use of lower growth rate during the initial stage of deposition is necessary to obtain heteroepitaxial GaAs films on. Si substrates with good structural perfection. The transmission electron microscopic examination of GaAs films of 0.15-0.2 μm thickness deposited on Si substrates at 500°C has shown that the stacking faults were present in the GaAs films; however, there are no apparent threading dislocations in the surface region of the GaAs film. The GaAs films deposited on Si were also evaluated by the electroreflectance and Raman spectra. The electroreflectance spectra indicated that the carrier concentration distribution on the surface is uniform. The Raman spectra at different regions of the film are reproducible indicating the good quality of the deposited GaAs films.

The reactions of fluorocarbon free radicals on silicon and silicon oxide surfaces have been studied by producing the reactive species using laser photodissociation, and examining the resulting surface compositions using in situ electron spectroscopy. The reactivities of these and other fluorinating species can be explained by means of a simple thermochemical model which takes account only of chemical bonds made and broken at the surface of the material. The photochemical processes which take place in surface films directly exposed to laser irradiation remain to be established.

Photochemical reactions of free radicals and ions in cryogenic films have been investigated using infrared absorption spectroscopy. The amorphous films are formed by vapor deposition of one or more reagents onto an optical window at 10-77 K. Reactive intermediates are generated by pulsed UV laser photolysis, and photochemical quantum yields for product formation are determined spectroscopically. Comparisons of our results with those in the gas or crystalline phases are useful for discerning reaction dynamics in amorphous solids. In this report, we discuss work in progress to investigate acid-catalyzed photoinitiation, oligomerization and polymerization of amorphous formaldehyde.

Time-of-flight mass spectra are recorded from the laser-initiated plasmas from solid YBa2Cu307_8 using combined 1064 and 532 nm laser light pulses of 10 ns duration. The effect of changing the laser power density was studied for both low masses and also for cluster ions ranging up to 20,000 amu. The cluster ions grow in intensity with increasing laser power densities until they abruptly vanish at plasma energies (temperatures of ~10 4 K) associated with a white plume visible with the unaided eye reaching ~1-2 cm above the target. Fewer molecules are found below 400 amu with increasing laser power densities, but some molecules still persist at the laser's highest intensity of 4.3 J/cm2.

The high-temperature superconducting material, YBa2Cu307.x, and other related materials have a complex chemical bonding structure that makes the electronic structure difficult to understand. In addition, the oxygen stoichiometry, which is variable, is critical to the conductivity and superconductivity of the material. Using spectroscopic ellipsometry, we have studied the chemical dependence of the optical properties and found that these properties can be separated into contributions of the local chemical structures. This includes the identification of a strong absorption feature at 4.1 eV with linear 0-Cul+-0 complexes that form as a result of oxygen vacancies. Therefore we can use this feature to indicate oxygen loss at the surface. We have found that the formation of interfaces with Al or In causes such oxygen loss, but interfaces with Ag and Au do not. Room temperature vacuum exposure does not cause measurable oxygen loss at the surface.

Thin superconducting (10 to 500 nm) films of Y-Ba-Cu-O with transition temperatures (Tc) near 90 K and transition widths of less than 1 K were deposited using a excimer laser; films were superconducting down to 5 nm, but with lower Tc's. Films grown on (100) strontium titanate at temperatures above 700 K were nearly epitaxial with the Y-Ba-Cu-O c-axis normal to the substrate surface. The critical currents (Jc's) were also very high. A 100 nm film on strontium titanate had a Jc of 4x10E6 A/cm 2 at 81 K. A film deposited directly on Si showed a Tc of 50 K, however a Y-Ba-Cu-O film on epitaxial buffer layers of magnesium aluminate followed by barium titanate on Si gave a 87 K transition. Superconducting Y-Ba-Cu-O on fluorides directly showed for Mg-F a Tc of 83 K and 73 K for both Sr-F and Ba-F.

A number of photon-assisted techniques have recently been employed for processing of materials that include insulators, semi-conductors, metals and superconductors. The present article focuses on three techniques developed by the authors: (a) laser ablation processing for superconductors (b) laser induced forward transport of metals and (c) wavelength control laser induced chemical vapor deposition for magnetic alloys.

Thin films of Y-Ba-Cu-O and Y-Ba-Cu-O/Ag were deposited by pulsed Nd:YAG laser (1064 nm) ablation. Meissner flux exclusion and expulsion, magnetization hysteresis, and flux dynamics measurements were performed to demonstrate the utility of SQUID magnetometry in evaluating the superconducting properties of thin films.

When the number of monolayers sputtered per particle or per pulse is sufficiently small the emitted particles fly freely from the target surface. For yields comparable to 0.5 monolayer a limited number of gas-phase interactions occurs, leading to a Knudsen layer. As a result the particles develop moderate forward peaking ~cos4 θ) and begin to flow (Mach number ≈ 1). It is common, however, for yields to exceed 0.5 monolayer. We show that the resulting gas-phase interactions cause the Knudsen layer to evolve into an unsteady, adiabatic expansion which is formally like a gun which fires a finite charge into an infinite, one-dimensional barrel.