Solid state femtosecond lasers enable powerful new nonlinear optical spectroscopic characterization techniques for technologically relevant Column IV and III-V semiconductor interfaces and growth surfaces.

Most knowledge on catalytic reaction mechanisms has up until now been gathered with UHV-based techniques. The extrapolation of such obtained reaction parameters to the pressure range of real application conditions is called the pressure gap. The nonlinear optical technique Second Harmonic Generation (SHG) allows studies covering the whole pressure range between UHV and application pressures. Of special importance for this purpose is the possibility to distinguish coverages of different adsorbates. In this work we show how the application of phase sensitive SHG can be used to determine coverages of different adsorbates in-situ allowing the determination of important reaction parameters. Our choice of reaction is the catalytic water formation on platinum which is already widely investigated in the literature.

We have developed a Sum-Frequency Generation (SFG) spectrometer allowing sensitive and highly resolved (1.8 cm^-1) vibrational spectroscopy in the mid-infrared (2 - 10 micrometers ) using a dedicated picosecond optical parametric oscillator. Combining SFG and infrared absorption-reflection spectroscopy, we pinpoint the occurrence of dynamical charge transfer at the C₆₀/Ag(111) interface. The induced strong infrared activation of the C₆₀ A_g(2) mode (approximately 1445 cm^-1) and its softening are quenched upon K-doping. This allows us to infer the coupling strength of the A_g(2) vibration to the t_1u orbital of C₆₀.

We demonstrate the se of an optical differential reflectivity technique to measure the rate of a surface reaction. The technique can be easily extended to a temporal resolution of nanoseconds and a spatial resolution of micrometers . The technique is used to study the reactions of acetylene on Cu(100). In the limit of low coverage acetylene undergoes two reactions: the first channel is desorption and the second channel is isomerization to vinylidene, which remains irreversibly bound to the surface. The rates of both the desorption and isomerization reactions have been measured. The desorption rates are in quantitative agreement with the predictions of transition state theory; the isomerization rates are in qualitative agreement.

A new experimental technique, correlated optical reactivity and scanning tunneling microscopy (CORSTM), is shown to be a uniquely powerful tool for the study of spatially localized reactivity of surfaces. In particular, CORSTM measurements directly correlate electromagnetic field enhancements that affect chemical dynamics and reactivity with surface topography on the length scale of a few nanometers. These measurements are based on the detection of surface plasmon polariton mediated multi-photon ionization from metal surfaces using ultrafast optical excitation and scanning probe microscopy photoelectron detection. The CORSTM approach is extended to a pump-probe scheme facilitating spatially localized measurement of hot electron dynamics. The experimental results provide direct confirmation of the optimal structural topographies for surface enhanced spectroscopy predicted by electromagnetic theories. CORSTM will provide a better understanding of phenomena that involve plasmons through the direct measurement of structure-function correlations.

The tunneling of electrons between the tip of a scanning tunneling microscopy (STM) and a sample is accompanied by the emission of photons. This luminescence phenomenon can be used to study local radiative processes at surfaces by combining the high spatial resolution of the STM and optical techniques. In this way, spatial maps of photon intensity modulations can be measured with lateral resolutions of less than 1 nm. Here we review the basic concepts of STM induced light emission and discuss recent results from adsorbed molecules.

In this work we study metal surface-induced changes of lifetime and transition frequency of alkali atoms and clusters, deposited onto nanoscaled insulator-metal systems. The systems are made of rough metallic surfaces (characterized by atomic force microscopy), onto which ultrathin organic films as spacer layers (characterized by LEED) are epitaxially grown. We observe an unusually small red shift of the transition frequency of Na atoms of a few hundred Megahertz, which is due to the interaction with the metal surface. This is explained by the nonlocal response of the surface, i.e., the excitation of multipole surface plasmons (MSPs) in the selvedge region of the metal surface, which is influenced by surface roughness. The MSPs should become observable also via linear optical methods such as attenuated total reflection spectroscopy. As a first step in this direction, we present linear extinction spectra of alkali cluster films that are grown on top of organic spectra layers of different length. Due to the interaction with the gold films a red shift of the dipole plasmon resonance is observed, which increases with decreasing chain length.

Surface second harmonic generation (SHG) allows detailed molecular measurements on a surface previously difficult to study: the ice/vacuum interface in equilibrium with its vapor. The heterogeneous reaction ClONO₂ + H₂O yields HOCl + HNO₃, a key reaction in ozone-depletion mechanisms in the Antarctic stratosphere, was followed on the basal ice surface maintained under stratospheric conditions (185 K, 10^-4 Torr water). SHG signals characteristic of the product HOCl allow us to determine reaction kinetics at various ClONO₂ exposures and temperatures and in the presence of co-adsorbates. We find that the reaction is autocatalytic in HOCl.

The 367 nm photochemistry of chlorine dioxide, OClO, in and on thin films of amorphous ice at T equals 100 K has been investigated. Reflection Absorption Infrared Spectroscopy was used to probe the photoproducts formed in and on amorphous ice films upon 367 nm irradiation. Under dilute conditions, 367 nm irradiation of OClO in amorphous ice results in the formation of the isomer, chlorine superoxide, ClOO. Under more concentrated conditions, in which OClO clusters are presented, 367 nm irradiation OClO results in an additional photoproduct chloryl chloride, Cl-ClO₂. Irradiation of OClO adsorbed on ice results in the formation of a single chlorine-containing photoproduct identified as Cl-ClO₂. Similarities in the photoreactivity of concentrated amorphous ice films containing OClO clusters and OClO adsorbed on ice suggest that OClO clusters may play a role in ice surface photochemistry.

A microstructure and properties of polymer coatings of the steel details, generated on surfaces were investi gated. The outcomes of comparative researches of properties of coatings obtained are adduced at thermal structuring and laser irradiation. In the report the found relations of properties of coatings to modes oflaser treatment are shown. Keywords : polymer coatings, microstructure, laser treatment, IR-spectrum investigation, theogravimetrical investigation method, tribotechnical properties.

The advent of femtosecond laser technique has stimulated investigations of electron relaxation processes in solids'8. At last investigators have obtained a unique tool for direct monitoring of ultrafast electron dynamics in real time scale. These investigations are of considerable importance both from scientific and practical points of view. Ultrafast electron processes are fundamental for the physics of surfaces. They determine a variety of surface processes such as adsorption, desorption, phase transitions, catalytic chemical reactions on a surface, etc., which have broad applications. The first studies of the electron relaxation in metals with the use of femtosecond laser pulses revealed that the electron distribution in metals is not equilibrium in the coarse of irradiation"2. It was shown that thermalization of the non-equilibrium electron gas is determined by the electron-electron relaxation and can be described by the Landau's Fermi-liquid theory. The latest detailed measurements ofthe energy dependence of the electron-electron relaxation times have supported this conclusion6'7. Some discrepancy between the experimental data and the Fermi-liquid model has been also reported for the cases when electrons from d-bands were involved in the process of relaxation4'8. It should be mentioned that thermalization of the electron gas takes place for the times comparable to the characteristic time of electron4attice energy transfer1 ; that is, the relaxation of the optically excited electron gas in a metal can not be treated as a two-step process9'1° where the electron-electron thermalization and electronlauice energy transfer are separated in time. One can expect that the electron distributions not thermal for any regime of optical excitation (to be correct, here the electron distribution averaged over the laser period is meant). So, for example, a study of the lattice temperature dependence of the electron-electron relaxation time in Au and Ag showed that the electron distribution is indeed nonthermal on the time scale of the electron-lattice energy transfer time . Inthis work we present a quantum mechanical kinetic theory for the electron gas in a metal excited by a short laser pulse. This work is ideologically based on our previous theory" ,where the electron distribution was determined from the solution of the Boltzmann equation with the additional phenomenologically introduced integral of electron-phonon collisions. Here we will consider both excitation due to laser-stimulated absorption of laser photons in electron-phonon collisionless and relaxation due to electron-electron and electron-phonon collisions not referring to any model assumptions. No other theory can now do that. The model calculations in' consider only the relaxation of the initially nonthermal distribution due to the electron-electron relaxation and do not explain how this initial distribution is created. The theory by Bejan and Raseev 'is based on the classical Boltzmann equation and phenomenologically accounts of only one-quantum absorption in a phenomenological way.

We present a theoretical and experimental study of thermosetting resins used in thermal stereolithography. In usual practice, stereolithography makes use of photosensitive resins where HeCd (0.352 micrometers ) laser ultraviolet laser initiates the curing process. In this work we study the process of local curing through the application of infrared radiation, which has proved to be useful in a new technique for the making of prototypes by means of selective heating with a CO₂ laser (10.6 micrometers ). The sample consists of a thermosetting resins (epoxy) with the curing agent (diethylene triamine) and a filler (silica). The ideal composition of the thermosetting resins has proved to be 10 parts epoxy, 1.4 part diethylene triamine (the curing agent) and 0.7 part silica powder. A physical theoretical model is applied for control of the parameters which influence the confinement of the curing in the irradiated bulk. A mathematical model was developed through the solution of the time dependent heat conduction equation in cylindrical co-ordinates, which enables the determination fo the behavior of curing in terms of irradiation conditions. An experimental analysis has determined the temperature range at which the curing process starts and the optimum silica concentration for efficient curing.

In this paper we discuss theoretical investigations of UV- laserinduced desorption of NO-molecules from Nickeloxide- (100)-surfaces. We focus on the interpretation of experimental results (velocity-distributions of the desorbing molecules, rotational and vibrational distributions) by performing for the first time high quality ab initio configuration interaction calculations for the construction of 2D potential energy surfaces of the ground and excited states involved in the desorption process. We were able to characterize these states as charge transfer states, where an electron is transferred from the surface into the NO-2(pi) -orbital. Potential energy surfaces for the intermediate NO^--like states have been constructed by varying the molecule-surface distance and the tilt angle of the molecule axis with respect to the surface normal. The characterization of the potential energy surfaces allows for a mechanistic insight into the driving forces of nuclear motion. 3D wave packet calculations on the ab initio potential surfaces have been performed to simulate the experimentally obtained state resolved velocity distributions of the desorbing NO-molecules. It has been possible to simulate experimental details like desorption cross sections, bimodal velocity distributions and the coupling of rotational and translational degrees of freedom on the basis of our first principles calculations.

One effective approach is to destroy industrial waste and pollution is the use of a semiconductor photocatalysis system. To date such, photocatalysis systems have employed conventional linear light sources. Initial results from a study of a photocatalysis system incorporating a tripled Nd:YAG laser are reported. The laser light not only played a roll as alight source for activating the photocatalyst (TiO₂), but also destroyed the organic species directly via a photochemical process. The reaction intermediates and changes in their concentrations are monitored using HPLC. The relationship between the power of laser and kinetics of photoreaction are discussed.

We have measured translational and rotational energy distributions of N₂ molecules following desorption from a Ag(111) surface by infrared (1064 nm) radiation. The observed desorption yields were large even at laser fluences far below that required for laser-induced thermal desorption. State-resolved laser techniques using coherent VUV radiation showed that the rotational and translational energy distributions of the desorbing N₂ molecules are not consistent with the predictions of the heat diffusion model governing laser-induced surface heating. These results suggest that physisorbed adsorbates can couple directly to the nascent-phonon distribution or the nascent electron-hole pairs in the photoexcited substrate without heating of the surface.

Femtosecond time-resolved two photon photoemission has been used to investigate the dynamics of photoexcited electrons at a polycrystalline Al surface. The measured relaxation time data are very different from the behavior predicted for a Fermi liquid. We observed a distinct increase in the decay rate of the excited states. The origin of this strong deviation from the theoretical prediction may be transport effects or band structure effects induced by the periodic crystal lattice.

The ultrafast dynamics of electrons in image-potential states on a Cu(100) surface is studied by means of femtosecond time-resolved two-photon photoemission (2PPE). By coherently exciting several eigenstates of the Rydberg series we observe periodic oscillations of the 2PPE signal as a function of the delay time between pump and probe pulses. These quantum beats allow us to determine the spacing of high-order states (quantum number n >= 4) that are difficult to resolve by conventional electron spectroscopy. The superposition of several states around n equals 7 creates an electron wave packet that describes the quasi-classical periodic motion of weakly bound electrons. Its distance from the surface is reflected in the strength of the photoemission signal. The electron is observed to move about 100 atomic distances away from the surface and oscillates back and forth with a period of 800 femtoseconds. The results demonstrate the power of coherent laser spectroscopy for surface studies.

Femtosecond time-resolved two-photon photoemission is used to study the influence of physisorbed xenon and oxygen adlayers on the lifetime of image potential states and interfacial quantum well states on Cu(111). Adsorption of 0 to 3 layers of Xe leads to a pronounced increase of the n equals 1 image state lifetime from 22 fs to 300 fs, respectively. However, for adsorbate heterostructures consisting of one monolayer (ML) O₂ on top of Xe spacer layers with variable thickness it is found that the lifetime of an oxygen induced quantum well state (0.35 eV below E_vac) decreases from 650 fs to 90 fs when the number of spacer layers is raised from 1 ML to 5 ML. The results can be semiquantitatively reproduced by a model calculation which accounts for the modified image potential due to the Xe and O₂ adlayers. The changes of the lifetimes are explained by the differences in the penetration of the excited state wave function into the Cu substrate.

Using density functional theory, we have performed structural relaxations of Rh(111) and Cu(100). To obtain accurate results, these calculations must be converged with respect to all computational approximations. In particular, it is vital to treat Brillouin zone integration with care, taking into account the effect of finite k-point sampling on surface and bulk structural properties. A new method is described and demonstrated for minimizing the error of finite k-point sampling in predicting surface relaxations accurately.

We shall report the UV-laser induced desorption of NO/Cr₂O₃(0001) and the coadsorbate system NO/K/Cr₂O₃(0001). Resonance enhanced multiphoton ionization was used for state selective detection of the desorbing NO after excitation with pulses of nanosecond duration and desorption laser energies between 3.5 eV and 6.4 eV. There are two adsorbate species of NO, a chemisorbed and a physisorbed species. We shall focus on data of the chemisorbed species. The main emphasis within this paper will be put on electron spin effects, particularly the preferential population of a fast translational desorption channel for the ²(Pi) _3/2 state observed specifically in connection with surface state induced processes at desorption energies of 5.0 eV. For those processes changes within the final state distributions of desorbing NO are fond when modifying the electronic surface structure via adsorption of small amounts of potassium.

NO molecules have been desorbed from an epitaxial NiO(100) film with ultrashort laser pulses (3.95 eV, 550 fs). All degrees of freedom of the desorbed molecules are populated non-thermal. Time-resolved two-pulse correlation measurements yield an increase of the desorption signal with a maximum at short delays between both pulses. Total vibrational energies up to 1600 cm^-1 are observed.

The photo-induced processes of methane adsorbed on Pt(111) and Pd(111) surfaces have been studied by post-irradiation temperature-programmed desorption and angle-resolved time- of-flight measurements. Methane adsorbs weakly on those metals. Although gaseous methane does not show any appreciable absorption cross sections of 6.4 eV, methane weakly adsorbed on those metals is photodissociated to produce methyl and hydrogen by the irradiation of 6.4-eV photons. The incident angle dependence of cross sections of the photochemistry obtained with linearly polarized light indicates that direct electronic excitation of methane adsorbate plays an important role in the photochemistry of methane. We interpreted that the photochemistry is induced via the electronic transition from the ground state localized at methane to the excited state of the methane- substrate atom complex where the first excited Rydberg-like state of methane significantly mixed with substrate empty states. Photofragments of methane, H and CH₃, further react with preadsorbed methyl and hydrogen species, respectively. In particular, methane is desorbed via associative recombination between a `hot' hydrogen and a methyl adsorbate. The average translational energy of the desorbed methane is 0.26 eV and 0.53 eV for Pd(111) and Pt(111), respectively. This difference can be explained by the difference in the surface electronic structure between Pd(111) and Pt(111).

The corrosive adsorption of Br atoms on Pt(111) was studied by variable temperature STM, TPD, and AES. Photoinduced dissociative electron attachment to adsorbed CH₃Br at 193 nm was used to dose hot Br anions onto the surface of 90 K. Br formed two ordered structures on Pt(111): (3 X 3) and ((root)3 X (root)3) R30 degree(s). STM images show that both structures coexist at intermediate coverages. Photofragmentation of CH₃Br produced monovacancies on the Pt(111) surface which were attributed to abstractive attack by the hot Br anions. The initial monovacancy etching efficiency of the adsorbing Br anions was high, roughly 33%.

Photon stimulated desorption of NO molecules at modified Pt(111) surfaces are investigated by reflection-absorption infrared spectroscopy (RAIRS) and resonance-enhanced multiphoton ionization (REMPI). The initial adsorption state of NO is probed by RAIRS and the final-state distribution is measured by REMPI. Coadsorption with N, O, and Ge atoms on Pt(111) appreciably influences the initial adsorption state of NO and the final-state distribution of photodesorbed NO. The rotational temperatures are substantially lowered by coadsorption with O and N atoms. Effects of initial geometric and electronic states on the desorption dynamics are discussed.

Contact imaging by energy transfer was the first application of the optical near-field for imaging beyond the diffraction limit. It is a method by which surface nanostructures can be copied onto a monomolecular layer of a dye. Near field microscopy using tapered metal coated fibers with an aperture at their tip as a submicron source of light can be used as a tool to write structures at a resolution of 80 nm. These near-field optical methods are well suited to create-- by local photochemical reactions--patterns of locally differing chemical composition and reactivity. Such structures serve as matrices for a site selective binding of colloidal particles. Unlike other methods, light microscopy and near-field microscopy have a sensitivity to detect photochemical processes at the single molecular level. Near- field microscopy is not limited to a resolution of 50 nm. As recently shown, the resolution can be extended to the 1 - 10 nm range using the tetrahedral tip as a probe. We expect, that a convergence of these recent developments should result in a very powerful near-field optical toolbox to read, write and copy information at the 10 nm scale.

Second harmonic generation at the surface of Na and K clusters was studied using femtosecond laser pulses. The observed size dependence of the second harmonic intensity can be explained by resonant enhancement of surface plasmon polariton excitation. At different cluster sizes, the second order interferometric autocorrelation of a 25 fs pulse was recorded using the clusters as nonlinear medium. There is no broadening of the cluster autocorrelation with respect to a non resonant reference autocorrelation within the experimental error of 1 fs. This experimental finding is independent of particle size, resonance of the fundamental (potassium) or the second harmonic (sodium) or surface modification by chemisorption of oxygen.

Photoinduced electron-hole excitation and relaxation in bulk, at interfaces, and at surfaces of solid state materials play a key role in a variety of physical and chemical phenomena that are important for surface photochemistry, and device physics. The possibility of controlling the charge carrier dynamics by the means of the optical phase may open up new possibilities in these fields. The control of electron distribution excited in Cu(111) through optical phase of the excitation light is demonstrated. Two-photon photoemission from the Cu(111) surface is excited by a pair of approximately 15 fs laser pulses with a mutual delay fixed to an accuracy of +/- 0.025 fs. A consequence of the interference between several coherent excitation pathways in the two-photon excitation process, the photoemission spectra do not only depend on the frequency, as in conventional spectroscopy, but also on the phase of the excitation light. Though coherent control is demonstrated for electrons at a metal surface, the excitation scheme is identical to optical Ramsey fringe experiments, and therefore, is a general phenomenon in multi-photon ionization in atomic, molecular, and condense phase environments.

We investigate via optical (viz. linear extinction and nonlinear second harmonic generation) and non-optical methods (viz. helium atom scattering) rough alkali cluster films, which are formed on dielectric substrates. The application of nonlinear optical methods to these systems allows us to obtain real-time dynamical information on the time-constants for laser-induced desorption and for the decay of the elementary optical surface plasmon excitation into single electron excitations and finally into lattice oscillations of the clusters and the substrate. The combination of linear and nonlinear optics also enables us to deduce structural information about the morphology of the cluster films, which--in the submonolayer regime--is complementary to information deduced from atom scattering data.

The dynamics of photofragments produced in ordered molecular layers adsorbed on surfaces are intimately connected to the electronic properties of the surface and the physical structure of the layer. These dynamics have been most often studied by examining the gaseous fragments expelled into the vacuum by using angularly-resolved time-of-flight mass spectrometry (TOF-MS), with or without internal (vibrational and rotational) state selectivity. The angular distribution of these fragments can be correlated with collisional encounters that occur in specific oriented and aligned geometries. Controlling the adlayer structure, and hence the photodynamics, is therefore a possible method for control of surface chemical reaction. Examples from recent studies of methyl halides adsorbed on LiF(001), NaCl(001), MgO(001) and TiO₂(110) will be presented to illustrate how angle- resolved TOF-MS data can be used to provide insight into both the structure and photodynamics of adsorbed molecules. In at least one of these cases, the unique collision dynamics of the adsorbed state produced reaction products that were not observed in gas phase molecule photochemistry.

Quantum dynamics studies of dissociative chemisorption of diatomic molecules on metal surfaces have revealed some interesting features that may have important potential applications. The probability for dissociative adsorption of a diatomic molecule on relatively smooth metal surfaces is highly dependent on rotational orientation of the diatomic molecule with the parallel (helicopter) mode being most favorable in overcoming the dissociation barrier. For dissociation of homonuclear diatoms under certain conditions, there is a selection rule which forbids the dissociation of the molecule at low collision energies. In the model simulation of dissociation probability with a time-dependent barrier, the dissociation probability is found to have a maximum as a function of the oscillating frequency of the barrier.

We describe the design and performance of an all solid state uv laser source that produces several hundred milliwatts at 266 nm and up to one hundred milliwatts at 213 nm. Examples of using this source to ablate borosilicate glass and to expose (beta) -chloroethyl silsequioxane on silicon are discussed.

Near-field scanning optical microscopy (NSOM) combines the frequency-specific detection associated with optical spectroscopy with the improved spatial resolution of a near- field probe. In NSOM, a tapered, metal-coated fiber optic with a sub-wavelength aperture at the end is used to illuminate a surface placed in the near-field of the aperture. This technique is capable of resolution down to 20 nm, and is particularly suited for imaging >20 nm to micrometers sized domains that do not possess significant topography and are not well imaged by other scanning probe techniques, such as atomic force microscopy. Biological and biomimetic membranes have been shown to posses domains that can be well characterized by NSOM. In this study, the lateral distribution and shape of these domains in lipid monolayers were characterized at particular surface pressures and in the presence of other compounds, such as cholesterol, using NSOM. The species to be detected was labeled with an appropriate fluorophore and fluorescence emission was measured as the NSOM tip optically excited a local volume at each point of a raster scan across the sample surface.

The conceptual starting point in surface dynamics is the potential energy surface, the most celebrated of which is the one proposed by Lennard-Jones 65 years ago. The Lennard- Jones potential considers two adsorption states as a molecule approaches the surface: a molecular precursor state and a chemisorption state. While this simple model has contributed much to the development of surface reaction dynamics, its one-dimensionality is of significant limitation. Molecule-surface interactions are inherently multidimensional. For example, molecular internal motions, substrate motions, and the relative orientations of a molecule to surface atoms are all important in determining the chemical consequence of a surface process. These issues are illustrated using recent examples involving polyatomic species. In the first example, electronic excitation/de-excitation of adsorbed NH₃ results in internal vibrational excitation, which can couple efficiently to the adsorbate-surface coordinate and results in desorption. In the second example, the thermal desorption of CH₃ from GaAs can lead to vibrational excitation in the product. The third example shows that a substantial amount of energy released by adsorbate-surface bond formation can be transferred to the intramolecular modes, leading to the ejection of energetic products.

IN this paper, the engraving process with Q-Switched Nd:YAG laser is investigated. High power density is the pre- requisition to vapor materials, and high repetition rate makes the engraving process highly efficient. An acousto- optic Q-Switch is applied in the cavity of CW 200 W Nd:YAG laser to achieve the high peak power density and the high pulse repetition rate. Different shape craters are formed in a patterned structure on the material surface when the laser beam irradiates on it by controlling power density, pulse repetition rate, pulse quantity and pulse interval. In addition, assisting oxygen gas is used for not only improving combustion to deepen the craters but also removing the plasma that generated on the top of craters. Off-focus length classified as negative and positive has a substantial effect on crater diameters. According to the message of rotating angle positions from material to be engraved and the information of graph pixels from computer, a special graph is imparted to the material by integrating the Q- Switched Nd:YAG laser with the computer graph manipulation and the numerically controlled worktable. The crater diameter depends on laser beam divergence and laser focal length. The crater diameter changes from 50 micrometers to 300 micrometers , and the maximum of crater depth reaches one millimeter.

Using mass spectrometry flight-time measurements, photodesorption of HFCO, H₂CO, CH₂CO and CH₃Cl from Ag(111) under pulsed nanosecond laser irradiation has been investigated in the experimental photon energy range of 1.17 eV <EQ hv <EQ 4.67 eV. All these molecules are physisorbed on Ag(111). No threshold behavior has been established within this energy range. The translational energy distribution of the desorbing molecules is characterized by a Maxwell-Boltzmann temperature in the range 110 - 150 K. The low translational temperatures and low photon energy thresholds, though in striking contrast to the high average translational temperatures and threshold behavior reported for photodesorption of chemisorbed molecules from metal surfaces, can be understood in terms of the prevailing electron attachment model. However, it requires that the substrate electrons attach to the molecules with positive electron affinities. In an alternative model, electron scattering excites the high frequency, v equals 1 molecular vibrational levels, possibly through dipole interactions. In this case, desorption results from vibrational predissociation in the adsorbate- surface bond.

Reflective transient grating experiments were conducted using two different experimental configurations to study carrier dynamics. Using an 800 nm pump and 400 nm probe, a signal attributed to bleaching was observed, and the carrier energy relaxation time was measured to be approximately 600 fs. Experiments were also conducted with a 400 nm pump and 800 nm pump. For this configuration, the observed TG signal decay was attributed to carrier diffusion and recombination.