The energies of electronic states depend strongly on the size and dimension of the physical system. The high-resolution two-photon photoelectron spectroscopy of weakly-bound image- potential states can be used to monitor the surface morphology of ultrathin metal films. States on terraces of different layer height can be distinguished and the growth mode can be inferred from the presence or absence of several of these states. Lateral localization of the states on small islands leads to an increase of the energy which can be described by the simple model of an electron in a potential well. Distribution of island sizes can be derived on the nanometer scale and compare well with the results of nucleation theory. The high-resolution spectroscopy of image-potential states provides a new tool to study the growth of metal films without disturbing the surface.

Infrared-vision sum frequency generation (IR-vis SFG) spectroscopy and temperature programmed desorption (TPD) spectroscopy were used to investigate the systems prepared by the adsorption of formic acid onto MgO(001) and Pt(110) surfaces. Prominent vibrational bands observed on the SFG spectra were located at 2870 cm^-1 on the MgO surface and a 2960 cm^-1 on the platinum surface, which were assigned to the CH stretching band of formate ion (HCOO^-) produced by the dissociative adsorption. On the MgO surface, three different sites, produced by the repetition of adsorption-thermal desorption cycles, were identified as responsible for the adsorption. The tilt angle of the CH group was estimated from the polarization characteristics of SFG signals and was derived as 0 degree(s) +/- 30 degree(s) for the group giving dominant SFG peak. On the Pt(110) surface, at least six vibrational bands were identified on the SFG spectra, and the temperature dependence indicated that the orientation of formate molecules changed right before their desorption. Expressions of SFG tensor components of a diatomic molecule were derived.

The technique of optical second harmonic generation is used to study phase transformations at 2D surfaces and interfaces. Examples are given to illustrate that changes in surface symmetry, adsorption configuration, and electronic structure can be detected by this nonlinear optical technique. An oscillatory phase transformation of potassium adsorbed atoms on Al(111) probed by second harmonic generation is analyzed in detail.

Near-field scattering optical microscopy (NSOM) is used to characterize the emission output and to obtain photoconductivity maps of InGaAsP multiple quantum well lasers. The high spatial resolution of NSOM (approximately (lambda) /20) allows detailed imaging of the laser structure. Emission measurements not only provide direct visualization of the laser mode but also reveal unwanted emission due to InP electroluminescence. Near-field photoconductivity experiments yield high resolution measurement of carrier transport throughout the structure yielding valuable information on current leakage, defect formation, and the quality of p-n junctions.

In this work we present experimental data that show that the roughness of a metal surface strongly influences the metal induced optical transition frequency shift of alkali atoms that are adsorbed close (a few tens of Angstroms) to the metal. The metal induced changes of electronic lifetime depend on distance (alpha) d⁴, suggesting surface electron hole pair excitation to be the dominant relaxation mechanism for electronically excited Na atoms at distances between 24 and 32 angstroms from a rough Au surface. The nonlinear response of metal surfaces is also well known to be enhanced by surface roughness. It has been anticipated that this enhancement should be most pronounced for a third order nonlinear optical process. Here, we present data of strong enhancement of (chi) ⁽³)_eff for rough metal surfaces. The surfaces consist of large alkali metal clusters, adsorbed on dielectrics. By changing the cluster size distribution we are able to study the third order nonlinearity as a function of shape of all the alkali protrusions.

The reflectivity change induced by adsorbates on Cu(100) at (lambda) equals 632.8 nm is investigated. It is found that physisorbed molecules (water, methanol, acetone) induce no observable reflectivity change, while chemisorbed molecules (carbon monoxide, oxygen, acetylene) do. The magnitude of the reflectivity change is such that submonolayer sensitivity is obtained for chemisorbed species. Furthermore, the change is found to vary linearly with coverage. The results show that this technique is a versatile, inexpensive, and straightforward optical probe of the surface coverage, and as such is applicable to many different surface investigations.

Hot-electron thermalization by electron-electron scattering is measured by two-photon photoemission spectroscopy at clean Cu(110) and Cu(100) surfaces. Two-photon photoemission spectra are measured as a function crystal face, laser polarization, and photon energy to establish the excitation mechanism. Time-resolved measurements of hot-electrons dynamics using approximately 3.2 eV femtosecond laser excitation light show a two- component decay due to dephasing and population relaxation dynamics. Hot-electron lifetimes in the 1.3 - 3.3 eV energy range decrease from approximately 80 fs to approximately 15 fs as the energy is increased. Energy dependence of the population decay rate is approximately 6.5 times slower than predicted by the Fermi liquid theory and shows a modest dependence on the Cu crystal face. Observation of hot-electron coherence on <EQ 15 fs time scale opens the way for coherent control of hot-electron induced dynamics at metal interfaces by ultrafast lasers.

Photodesorption of nitric oxide from Pt(111) using femtosecond laser pulses at 620 nm and 310 nm is found to result in a superlinear dependence of desorption yield on absorbed laser fluence. The translational energy distributions of the desorbed molecules are found to be nearly Maxwell-Boltzmann. For both 620 nm and 310 nm pulses with adsorbed fluences greater than 2.5 mJ/cm², the average translational energy is found to increase linearly. At lower fluences, it is constant at 750 K, possibly suggesting a transition between DIMET and DIET desorption processes. In two pulse correlation measurements, it is found that the first pulse yield and the average translational energy have different widths as a function of delay between pulses.

The method of molecular dynamics with electronic frictions is employed to simulate femtosecond laser induced desorption of CO from Cu(100), at an initial surface temperature of 95 K. The effect of adsorbate coverage is assessed by comparing simulations at low coverage, and 0.5 ML coverage. For desorption due to a single 100 fs laser pulse, a significant enhancement of desorption yield due to overlayer coverage is obtained, particularly at the highest laser fluence considered (60 J m^-2). The origin of this coverage dependence is discussed. Additionally, the simulations are reported in which the exciting laser pulse is split into two components, separated by a variable time delay.

Recent experiments done in our laboratory provided a preliminary indication that morphology- dependent stimulated Raman scattering could be used to spatially image molecular structure and composition changes in regions located close to the surface of axisymmetric particles. We have just completed a new series of theoretical and experimental studies which confirm our initial results. The results of these studies are described here.

The surface chemical properties of ice and sulfuric acid in ultrahigh vacuum have been studied using temperature programmed desorption and Fourier transform infrared reflection absorption spectroscopy (FTIRAS). Ice and sulfuric acid were deposited on Pt(111) and W(100) as films between 10 and 100 monolayers thick. FTIRAS measurements imply that amorphous ice has a greater density of `dangling,' surface OH groups than crystalline ice. The crystalline and amorphous surfaces are also chemically different: adsorbate capable of forming hydrogen bonds of the type X(DOT)(DOT)(DOT)H-O exhibit a desorption state that is unique to amorphous ice. The adsorption of OCIO on ice was investigated in order to estimate the coverage of adsorbed OCIO on stratospheric ice particles. The sticking coefficient at 100 K is nearly unity. At low coverages, desorption is first order, with an activation energy of 23 +/- 1 kJ(DOT)mol^-1 and a frequency factor of 1.5 X 10^{9+/- 1} s^-1. The coverage of OCIO on stratospheric ice particles is probably so low that thermal- and photochemistry are unimportant in the atmosphere. For H₂SO₄, the dangling OH group cannot be spectroscopically observed, nor does HCl adsorb or absorb at 100 K.

The air/water interface of aqueous tryptophan and (t-butyloxycarbonyl)-tryptophan-tryptophan (Boc-Trp-Trp) has been investigated using the technique of interfacial second harmonic generation (SHG). The SHG response for both linearly and circularly polarized incident radiation has been recorded. For tryptophan no special response was observed for circularly polarized light but for Boc-Trp-Trp a wavelength dependent circularly differential harmonic generation (SHG-CD) and SHG optical rotation was observed.

Understanding the structure of the interface between ice and liquid water is essential to the study of molecular adsorption at this boundary. Despite great interest in the ice/water interface, experimental studies are sparse. In this work, the nonlinear optical laser technique, second harmonic generation (SHG), is used in a total internal reflection geometry to probe the single crystalline ice/water interface. SHG signals from the clean ice/water interface are observed and attributed to symmetry breaking at the boundary. We report observation of a linear adsorption isotherm when water is replaced by 0.2 to 7 (mu) M solutions of 2,2'- dihydroxy-1,1'-binaphthyl (BN). The coverage is most likely submonolayer; therefore, we observe only the beginning of the adsorption profile. We argue that BN adsorption is entropy driven. In a separate study, 0.02 to 1 mg/mL solutions of winter flounder antifreeze protein are contacted with the ice. The adsorption profile closely follows the freezing point depression activity curve of the protein.

The results of a fully quantum state resolved study of the laser-induced desorption of CO from an epitaxially grown film of Cr₂O₃(111)/Cr(110) at a surface temperature of 100 K are reported by using a (1+')REMPI (resonance enhanced multiphoton ionization) detection technique for the desorbing molecules via the B¹(Sigma) ⁺$IMPX¹(Sigma) ⁺ electronic state. In order to get an insight into excitation mechanism the wavelength dependence of the desorption process was studied at 3.5 eV, 5.0 eV, and 6.4 eV. For all three wavelengths we observed nonthermal desorption. The final state distributions did not show any significant changes for the three wavelengths except for the overall intensities. The velocity flux distributions were bimodal for v"equals1. the desorption cross sections measured vary between (5+/- 1)(DOT)10^-18 cm² for 3.5 eV and (3.5+/- 1)(DOT)10^-17 cm² for 6.4 eV. The behavior indicates that the origin of the desorption has to be an indirect process via electronic excitation of the substrate. Possible mechanisms are discussed.

The laser-induced desorption of ammonia from GaAs(100)-(4X6) and Cu(111) has been studied using (2+1)-photon resonance-enhanced multiphoton ionization as a state specific probe. A surprisingly marked isotope effect (NH₃/ND₃) in the photodesorption cross section of ammonia from GaAs(100) had indicated that internal degrees of freedom play a crucial role in the desorption dynamics. We find that NH₃ desorbs with a mean translational energy, (E_trans), of 300 K and 600 K for GaAs(100) and Cu(111), respectively. The average extent of vibrational excitation is about 1000 K. The rotational temperatures are 520 and 170 K. These results are interpreted in terms of a direct mechanism for NH₃ desorption in which a reaction path which initially follows the internal v₂ coordinate and then bends towards desorption is dominating. A marked unequal population of the inversion symmetry doublet states is observed.

Structural changes of chlorine-adsorbed silicon(111)-7X7 surfaces by laser light were studied with a scanning tunneling microscope, and desorption species (SiCo_x) from the surface were measured with a quadrupole mass spectrometer, avoiding strong thermal effects. For the laser stimulation with 4.7 eV in photon energy to chlorine saturated surfaces, we found that the remaining surfaces change to have a structure of the restatom order with a number of irregularities, and that SiCl₂ is primarily desorbed out. These results are ascribed to the stability of monochlorides and the instability of polychlorides for the electronic excitation by the light. We also found a drastic difference in the threshold laser fluence for the detection of desorbed SiCl₂ between the photon energy of 1.2 and that of 2.3 eV. One of the possible desorption processes is that photo-generated bulk carriers induces that bond breaking of polychlorides on the substrate.

The scattering of N₂ from Si(100) was studied using state-selective detection and molecular beam techniques. Both trapping-desorption and direct inelastic scattering channels were observed. For an incident N₂ energy of 0.08 eV, the trapping-desorption probability was found to be essentially unity for surface temperatures between 93 K and 573 K. The velocity and rotational state distributions of the desorbing molecules show that the nitrogen is fully accommodate with the surface. For an incident energy of 0.29 eV, a direct inelastic channel as well as the trapping-desorption channel was present. The rotational state distributions of both channels were studied as a function of surface temperature and incident energy.

Polarized ultraviolet light from an excimer laser (193 nm) was used to photodesorb and photodissociate N₂O and NH₃ adsorbed on a cold Pt(111) surface. The photodesorbed species and their time-of-flight (TOF) were monitored by Resonantly Enhanced Multiphoton Ionization spectroscopy. For N₂O, we have observed both the ejection of ballistic O atoms and the release of slow thermalized N₂ molecules. The ballistic oxygen atoms leave the surface either in the ground state O(³P) or in the first electronically excited state O(¹D). A lobular angular distribution pointing away from the surface normal was measured for the ballistic O(³P) in agreement with a binding geometry where the linear N₂O is tilted N-end down on the Pt(111) surface. Evidence for the production of N₂ photofragments thermalized by the surface includes both low (approximately 90 K) rotational and translational temperatures of the N₂ as well as a lack of correlation between rotational and translational energy. For NH₃ the irradiation of a submonolayer coverage largely favors the desorption rather than the dissociation of NH₃. For multilayer coverages however, a strong dissociation channel is activated and atomic H is seen to desorb from the surface. A bimodal distribution was found for the TOF of the H photofragments. The fastest channel (0.7 eV) corresponds to the ejection of undeflected ballistic H fragments produced near the surface. For bulk NH₃ photodissociated deep within the multilayer, the TOF of the H radicals shows an unexpected thermalized distribution.

The 248 nm photochemistry of methyl iodide thin films was studied using reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD) and time-of-flight quadrupole mass spectrometry (TOF-QMS). The formation of two major photoproducts, CH₂I₂ and CH₄, in the film was characterized by RAIRS and TPD. At all laser energies used in this study, CH₃, I, and CH₃I were ejected into the gas phase. The CH₃ TOF distribution showed the signature of gas phase CH₃I photodissociation dynamics--two sharp peaks corresponding to production of iodine atoms in the I (²P_3/2) and I^* (²P_1/2) states. The TOF distributions of I and CH₃I were fit to Maxwell-Boltzmann distributions corresponding to temperatures of 1400 K and 1170 K, respectively. The photochemistry of methyl iodide thin films can be understood in terms of photoprocess occurring in the film and at the film surface.

Photofragments produced following 257 nm irradiation of physiosorbed methyl iodide on MgO(100) and TiO₂(110) have been studied using resonantly enhanced multiphoton ionization coupled with time-of-flight mass spectrometry. The observed signal is associated with photofragment production from methyl iodide chromophores present in the near surface region. The dependence of the velocity distribution of methyl photofragments on ejection angle and polarization of the photolysis beam indicates that the multilayer film is composed primarily of an anti-parallel arrangement of methyl iodide molecules whose C-I bonds are aligned close to the surface normal. At submonolayer coverage on TiO₂ methyl iodide is predominantly in a parallel orientation with the iodine close to the surface and the methyl group pointing away from the surface.

Pulsed UV lasers such as the 4th harmonic emission of Nd:YAG laser at 266 nm and ArF excimer laser at 193 nm have been used for surface modification of polymer surfaces, nano- structure fabrications on polymers and silicon substrates, and diamond deposition using photoablated polymer plumes. We report research carried out in our laboratory on these three topics.

Unenhanced surface Raman spectroscopy has been used to study the chemistry of polymers adsorbed on solid surfaces and the chemical enhancement mechanism of surface-enhanced Raman scattering. The adsorption and reactions of the polyimide monomers pyromellitic dianhydride (PMDA) and oxydianiline on silver, copper and silicon surfaces under ultrahigh vacuum have been investigated. These include both nondissociative physisorption and dissociative chemisorption of the monomers, and the condensation polymerization to form adsorbed polyimide. The intermediate polyamic acid is detected for the first time in a surface experiment. PMDA adsorbed on Cu(111) shows chemical enhancement in the absence of electromagnetic enhancement. High resolution electron energy loss spectroscopy has revealed a strong charge transfer absorption near the Raman excitation frequency. This observation provides strong support for a proposed resonance Raman chemical enhancement mechanism.

We have used the planar laser induced fluorescence, PLIF, technique to study the OH concentration distributions outside a catalytic surface at different pressures and gas compositions. In the experiments a flow of hydrogen and oxygen at a total pressure of 0.2 - 1 Torr is directed towards a platinum foil, which is resistively heated to 1300 K. OH radicals are formed on the catalyst surface as an intermediate in the 2H₂ + O₂ yields 2H₂O reaction. A small fraction of the OH radicals desorb to the gas phase, instead of reacting with adsorbed H atoms to form H₂O. These desorbed radicals are detected with the PLIF technique in a region 1 - 6 mm from the surface, i.e., spatial distribution patterns are obtained. The recorded fluorescence patterns are corrected for laser beam intensity variations. The corrected patterns are used to determine the relative OH concentration distributions outside the surface at different pressures and gas compositions. The recorded fluorescence patterns from different transitions are used to determine rotational temperature distributions. The measurements are compared with results from a computer model that simulates the laminar stagnation point flow of hydrogen and oxygen on the catalyst. The model includes both gas phase reactions and surface reactions. The relative amount of OH radicals as function of the distance from the platinum foil and gas temperatures with the simulated temperature profiles we found that the gas phase temperature can not be calculated assuming continuity in the temperature over the gas-surface boundary.

Current understanding of the dynamics of photon stimulated desorption (PSD) of molecules from surfaces has been derived in large part from state- and energyresolved probes of the des orbed molecules in the gas-phase.1'2 Analogous to studies of gas-phase photodissociation, information concerning the electronic interactions and nuclear motions on the dissociative potential surface are inferred from the energy, internal state and angular distributions of the photoproducts. Of particular interest is the use of state-resolved methods to ellucidate the photodesorption mechanism which can involve substrate and/or absorbate excitations followed by facile energy transfer between the electronic and nuclear degrees of freedom. The primary concern of this work is desorption induced by photon energies well below the work function ( 1 —2 eV) which is nominally assumed to occur via a thermally activated process. From a dynamical standpoint, laser-induced surface heating results from the rapid thermalization of initially photoexcited electron-hole pairs which relax through inelastic e — e scattering and energy transfer to lattice modes of the substrate. Desorption results from random surface atom displacements which deposit vibrational energy in the absorbate—metal bond in excess of the binding energy. The desorption rate is highest at the maximum surface temperature induced by the laser pulse which can be determined with reasonable accuracy from a classical heat-diffusion modeL35 As a result, "thermally" desorbed molecules are expected to have internal and translational energy distributions characteristic of Tmax. State-resolved measurements performed by Buntin, et al.6 for NO/Pt(111) at a number of photon energies between 0.65 eV and 3.49 eV have identified two desorption channels, with the "slow" velocity component exhibiting near Boltzmann rotational and translational distributions. Although the measured angular distribution, photon energy dependence and translational energies of the slow channel are consistent with thermally activated desorption, the translational energies did not show a dependence on laser fluence as expected from the classical heat-diffusion model, i.e. T 4. [n addition, the rotational temperature (s100 K) was found to be significantly smaller than the expected surface temperature rise = 227 K). In a related study, Prybyla, et al. observed a Boltzmann rotational state distribution for NO desorbed from Pd(111) at 2.33 eV (532 nm), however, the derived rotational temperature was approximately half that of the surface temperature.7 Such "rotational cooling" has been attributed to strong coupling between rotation and translation induced by the molecule-surface potential and its anisotropy with respect to molecular orientation.7'8 In this work, we present state-resolved measurements for IR (1.17 eV, 1064 nm) photodesorption of CO physisorbed on a Ag(111) surface. In contrast to NO, there has been very little experimental work on CO photodesorption, partly due to the difficulties associated with multi.. photon or VtJV probes required to obtain state-resolved dynamics. Recent state-resolved measurements for CO/Pt (111) , CO/NiO (111) 10 and 211 have focussed on the observation of non-Boltzmann final state distributions induced by 1JV excimer radiation (308 nm, 248 nm and 193 nm) at laser fluences too low to induce substantial thermal desorption. The low desorption temperatures for both the monolayer (48 K) and multilayer (36 K) phases of CO/Ag(111), however, permit the study of photo-induced "thermal" processes at modest laser power densities. Furthermore, the low surfa.ce temperature and weak interaction between the CO molecule and the Ag(111) surface favors equilibration of the rovibronic and translational energies with State-resolved detection of desorbed CO is performed via (1 + 1') resonant multiphoton ionization (REMPI) using coherent VTJV radiation and time-.offiight mass spec trometry. The observed rotational and translational state distributions are well described by Maxwell-Boltzmann distributions with characteristic temperatures which indicate near equili.. bration of the rotational and translational degrees of freedom. These results are consistent with a photo-induced "thermal" desorption mechanism and are compared with the predictions of the classical heatdiffusion model.

A detailed computer examination of the excited state dynamics of stilbene, both isolated in the gas phase and physisorbed on an inert surface is presented. It is discussed how the surface affects stilbene's dynamics in comparison with the isolated molecule case.

Transient species upon pulsed laser excitation of ultrathin polymer and Langmuir-Blodgett (LB) films was detected sensitively by the optical waveguide (OWG) method. Tetratolylporphyrin in poly(methylmethacrylate) films and amphiphilic tetraphenylporphyrin in mixed LB films with dioctadecyldimethylammonium were prepared on the surface of OWGs. These films were excited with the second harmonics of a ns Nd:YAG laser in controlled environment. The transient absorption of excited triplet porphyrins was detected sensitively by Ar⁺ laser, He-Ne laser, or diode lasers as a probe. Transient absorption in LB films with only two monolayers was observed by the present method. It was about hundred times increase of sensitivity as compared with a conventional normal incidence measurement. Different decay dynamics were observed for excited triplet state porphyrins in polymer films and in LB films, which was attributed to the different spatial distribution of chromophores.

Experimental results, related to a novel laser-assisted technique for deposition of III-V semiconductor thin-films, are presented. The method involves precursors in the form of I-III- V Zintl-phase materials. While such compounds exhibit a variety of useful properties, the presence of the group I element is a major concern--in terms of the desired final product it can be considered a major impurity that has to be removed. To address this situation, we employ a strategy, based on the difference in the ionization potentials for the constituent elements, that can be described in brief as (1) laser ablation of a I-III-V compound, (2) removal of the group I element by selective gas-phase laser ionization and extraction in an electric field, and (3) subsequent re-deposition of the III-V compound. For a particular I-III-V Zintl-phase compound, potassium indium antimonide, K₄In₄Sb₆, time-of-flight mass spectra clearly demonstrate high yield selective gas phase ionization and removal of potassium from the ablation plume. Deposition conditions have been found for scaling of the above removal process macroscopically to thin film growth. The deposits have been studied by Auger electron spectroscopy and the results confirm significant depletion of potassium from thin films deposited via the proposed ablation/ionization removal technique. An assessment is made as to the viability of the process and implications for possible applications of this research as related to film growth are discussed.

Sum-frequency spectroscopy (SFS) has been used to elucidate the structure of surfactant monolayers at the hydrophobic solid-water interface. The polar orientation and conformational order of a series of surfactant molecules has been determined from the vibrational spectra of their hydrocarbon chains in the C-H stretching region. The SF spectra of inorganic (thiocyanate and cyanide) and aromatic (tosylate, benzoate, and salicylate) counterions indicate that they bind to a monolayer of charged surfactant with a net polar orientation. The spectra of the aromatic anions bound to a surfactant monolayer at the air-water interface are also presented. The interactions between co-adsorbed surfactant and polymer has been examined-- the spectra of the surfactant indicate that an oppositely charged polymer causes a monolayer of the surfactant to adsorb at much lower bulk concentrations than in the absence of the polymer. In one case, the bound polymer has a SF-active resonance indicating that the polymer is also orientationally ordered at the solid-water interface.

The excimer laser induced CVD of boron nitride films is reported. Basic dependencies of deposition rate on pulsed laser fluence, vapor pressure and substrate temperature are considered. Under optimum conditions h-BN film deposition rate of the order of 1 A per pulse was realized. Structure and composition of BN-films were characterized by AES and Raman spectroscopy. A simple deposition model is proposed taking into account chemisorption and partial decomposition of borazine molecules on the substrate surface between laser pulses and final decomposition of the intermediate product during the laser pulse.

Electromagnetic (EM) radiation propagating inside a dielectric particle will develop standing waves if certain boundary conditions are satisfied. The boundary conditions depend on, among other things, the shape of the particle. The standing-wave solutions are widely referred to as morphology-dependent resonances (MDRs). If MDR conditions are satisfied then the intensity of the electric field can be calculated everywhere inside the particle. It has been found that for certain MDR modes in micron-sized spheres the EM field strength is localized close to the particle surface. Energy can be transferred into these MDRs through Gaussian laser beam scattering from the sphere, and the energy becomes trapped in a semi-bound state. The localization and increased lifetimes of MDRs allows them to be used in stimulated Raman scattering experiments. Calculations have shown that the maximum energy is transferred into the MDR when the laser is focused outside the sphere surface. This paper analyzes the internal and external radial wavefunctions involved in the scattering to determine the optimum position of the Gaussian beam.

Surface second harmonic generation is used to investigate the behavior of solute molecules at the liquid water/vapor interface. This surface specific technique allows the determination of the relative energetics between molecules adsorbed at the interface and solute molecules in the bulk, as well as the energy barrier between them. Measurements of the surface excess of dimethyl sulfoxide on aqueous solutions are reported. These data, in combination with the results of other techniques, facilitate development of models for the mechanism of the uptake of gases by aqueous solutions. The results are discussed in reference to current models of the uptake of gases by aqueous solutions and their relevance to heterogeneous atmospheric processes.

The adsorption of pentapeptides to lipid monolayers spread at the air-water interface is investigated by optical second harmonic generation (SHG) in the reflection geometry. The nonlinear optical response of the tryptophan side chain present in each of the synthetic peptide sequences chosen was insufficient to allow determination of its molecular orientation within the surface layer at the surface densities obtain by adsorption from the subphase. A difference in the character of peptide adsorption to lipid monolayers of various compositions was observed which depends on the nature of the monolayer interface accessible to the peptides. Results from crude and purified synthetic peptides are compared to point out the impact of potential hydrophobic impurities on the surface properties of the monolayer and on the measured second harmonic signal intensity. SHG studies of peptides and proteins adsorbed to membrane-mimetic lipid and phospholipid monolayers may suggest differences in the chemical nature of the monolayers which impact the ability of peptides and proteins to penetrate and interact with the membrane surface.

For in-situ investigations of catalytic reactions with second harmonic generation (SHG) it is necessary to distinguish the adsorbate induced signal due to the presence of different species. A further requirement is the quantitative determination of coverages of the reactants and products. We present in this work, how one can distinguish different adsorbates by the use of phase sensitive signal detection and how their coverages can be determined. It turns out that the relative sensitivity for the adsorbates can be varied by changing the azimuthal angle of the sample. Systematic SHG studies of the effect of coadsorbing oxygen and hydrogen show that nonlocal effects are influencing the SHG signal strongly. In contrast these nonlocal effects are negligible during experiments using only one adsorbate. The coadsorption studies are carried out under controlled UHV conditions at a temperature of T equals 100 K. At such low temperatures no reaction between the adsorbates occurs. The experiments are necessary to calibrate the SHG signal to varying coverages of the reactants coadsorbed for the later use during the in-situ study of the catalytical reaction H₂ PLU 1/2O₂ yields H₂O.

The 266 nm UV laser-induced desorption and photofragmentation of CCl₄, CF₃I and CH₃COCOCH₃ absorbed on a quartz surface at 130 K have been investigated by time-of-flight mass spectrometry using electron impact ionization. The cryogenic molecular films are continuously condensed on a transparent quartz substrate and its thickness is monitored by a quartz crystal microbalance. The mass and the translational energy distributions are measured as a function of laser fluence and surface coverage. For CCl₄, the adsorbate-mediated explosive desorption with no photodissociation is evident at high surface coverage and high laser fluence. For CF₃I, the photochemical desorption associated with photofragmentation are observed at monolayer coverage on a quartz surface. For CH₃COCOCH₃, the electronic excitation with photodissociation of the adsorbate leads to CH₃CO and CH₃ radicals formation. These results reveal the characteristic electronic, thermal and explosive desorption mechanisms depending on the optical, thermal and chemical properties of the adsorbate, the film thickness and the laser irradiation conditions.

We have measured the absorption spectrum of the laser dye IR125 (also known as indocyanine green) at the water/air interface using resonant enhanced surface second harmonic generation. The spectra of the dye molecules at the interface are extremely sensitive to the bulk concentration. All the surface spectra reflect aggregation of the dye at the interface, even at the smallest concentrations detectable, below 1 (mu) M. Resonant enhanced second harmonic generation appears to be a good technique for measuring spectra at liquid interfaces.

The photodissociation dynamics of a molecular ion (HCl⁺) adsorbed on a single crystal insulator surface (MgO(001)) is investigated theoretically in a 2D dimensional model. Both quantum and classical methods have been used to study the dynamics. The structureless absorption spectra of the adsorbate are found to shift relative to their gas phase counterparts, due to the (de)stabilization of the excited states. The angular distributions of the desorbed hydrogen fragments are highly structured, in sharp contrast to the smooth, gaussian-like distributions obtained for the corresponding gas phase dissociation. A classical deflection function is introduced to illustrate the influence of rainbow scattering on the final hydrogen angular distributions.

The second harmonic generation from pyridine multilayers adsorbed on Ag(111) has been studied for the elucidating the structure of the adsorbed layers. An irreversible phase transition in the pyridine overlayers was observed when the pyridine film/Ag crystal is annealed above approximately 145 K. The phase transition is speculated to be from the formation of ordered domains in the pyridine multilayers.

Laser induced formation of CO₂ and desorption of O₂ are initiated with femtosecond and picosecond laser excitation of a Pt(111) surface prepared with coadsorbed CO and O₂ at 90 K. The nonlinear fluence dependent reaction yields were measured for 267, 400, and 800 nm wavelengths, and for pulse durations from 80 fs to 3.6 ps. Two-pulse correlation experiments measuring total O₂ desorption yield versus time delay between 80 fs pulses show a 0.9 ps HWHM central peak and a slower 0.1 ns time-scale. At 267 nm the relative yields of O₂ and CO₂ are found to depend on fluence. Comparison of results at different wavelengths and pulsewidths shows that nonthermalized surface electrons play a role in the laser-induced surface chemistry.

Total internal reflection sum frequency generation has been used to obtain the first vibrational spectrum of a simple surfactant at an oil-water interface. The system examined is sodium dodecyl sulfate (SDS) at the D₂O-CCl₄ interface. The spectrum of the monolayer is obtained at a series of concentrations which extend from concentrations above the critical micelle concentration of SDS in the D₂O solution to interfacial concentrations as low as 0.03 monolayers. SDS shows the highest degree of order at monolayer concentrations with decreasing order displayed at lower interfacial concentrations. Significant vibrational contributions from methylene stretches of the alkyl backbone of the molecule at all concentrations suggests less order for this surfactant at the liquid-liquid interface than has been observed in previous liquid-air and solid-air studies.

We theoretically analyze infrared-visible sum-frequency generation (SFG) from gas molecules near a solid. The incident IR radiation is supposed to be in resonance with one of the normal intramolecular vibrations. We separate the evolution of the molecule's internal and external degrees of freedom using an adiabatic approximation. We solve the equation of translational motion containing the `integral of collisions' with phonons in the basis of ingoing and outgoing waves. The SFG spectrum is shown to be consisting of two lines, one at the sum frequency and a satellite arising from the desorbing molecules. The line shapes obtained are determined by the rate of `collisions with phonons' as well as molecule-surface distance dependence of the vibrational frequency and linewidth.

Density functional theory calculations are performed to determine the nature of vibrational modes associated with carbon monoxide chemisorbed to the copper (100) surface. The electronic states and charge density are determined using a plane-wave pseudopotential method within the local density approximation. The surface is modeled using a periodic slab geometry, and the force constant matrix is computed from first principles by displacing each atom in turn and determining the resulting forces on it and all other atoms. This matrix is then diagonalized to yield normal-mode polarization vectors and frequencies. The eigenvectors provide information about the strength of coupling between copper atom motions and CO vibrations. The computed vibrational spectrum is used to predict the transient response of the system to nonequilibrium heating, and the results are compared with recent pulse-laser experiments on this system.

Colloidal metal films prepared by the covalent attachment of small silver particles from a colloidal suspension to a glass substrate exhibit enhancement of both Raman scattering of adenine and fluorescence of fluorescein-labeled phospholipid adsorbed on their surface. The SERS signal from the films was stronger than that from a citrate-reduced silver colloid and, therefore, from a roughened silver electrode or an island film based upon previous results. The conclusion is made that the Raman enhancement originates mainly from molecules adsorbed on the aggregated particles, whereas the fluorescence enhancement arises from molecules in the space between the metal particles.

Proteins that regulate inorganic crystal formation in organisms are in some instances irreversibly bound to the mineral phase. They are clearly resolved by atomic force microscopy, easily withstanding the forces of the AFM probe in contact mode. This is demonstrated in AFM images of proteins attached to fragments of calcite oyster shell. The proteins may take specific positions on the crystal surface, in effect becoming part of the surficial lattice, stereochemically matching the functional groups of the protein with lattice positions of the constituent ions of the crystal. Requirements for adsorbate binding that is strong enough for AFM viewing in contact mode are discussed, including comparisons of ionically driven binding of oyster shell protein to calcite and hydrogen-bonded antifreeze proteins to ice.

This paper reviews our understanding on the dynamics of vibrational heating and bond- breaking within a universal electronic mechanism, i.e. the multiple scattering mechanism induced by hot-electrons generated by femtosecond laser pulses. A few theoretical approaches, based on the reduced density matrix and a resonance model of electron-molecule coupling, are briefly sketched and discussed, and qualitative features of vibrational heating and bond- breaking are emphasized. Two illustrative examples of vibrational heating of CO stretch on Cu and O₂ desorption from Pt(111) demonstrate that the dynamics of electronic and vibrational heating can be partly revealed by the yield and final state distribution analysis.