Studies using picosecond spectroscopy and jet-cooled samples have been performed on anthracene and t-stilbene in an effort to characterize intramolecular vibrational energy redistribution in these molecules. Information pertaining to the time-scale and extent of the dynamics as a function of vibrational energy in the molecules has been obtained.

The vibrational dephasing of the 656 cm-1 mode (v1, alg) of carbon disulfide and the 991 cm-1 mode (v2, alg) of benzene have been studied as a function of concentration in mixtures with a number of solvents using a picosecond time-resolved CARS technique. This technique employs two tunable synchronously-pumped mode-locked dye lasers in a stimulated Raman pump, coherent anti-Stokes Raman probe time-resolved experiment. Results are obtained for CS2 in carbon tetrachloride, benzene, nitrobenzene and ethanol, and for benzene v2 in CS2. The dephasing rates of CS2 vl increase on dilution with the polar solvents and decrease or remain constant on dilution with the nonpolar solvents. The CS2/benzene solutions show a contrasting behavior with the CS2 vl dephasing rate being nearly independent of concentration whereas the benzene v2 dephasing rate decreases on dilution. These results are compared to theoretical models for vibrational dephasing of polyatomic molecules in solution.

Recent advances in laser technology have made available to chemists reliable sources of tunable, narrow bandwidth radiation in the ultraviolet and vacuum ultraviolet regions. It has thus become possible to detect with unprecedented sensitivity the diatomic products of bimolecular chemical reactions with resolution of electronic, vibrational, rotational, and even fine structure states. In our laboratory, special attention has been given to studies of the reactivity of optically metastable, electronically excited carbon an0 oxygen atoms, C( D) and 0('D). Examined in detail has been the reaction of C('D) with H2 in which the population of the product CH(v"=O) A doublet states is found to Be inverted. The reaction was further shcown to proceed via a long-lived 'CH collision complex. Extensive studies of 0(±D) + H2 ? OH + H using hydrogen isotopes have demonstrated the importance of angular momentum constraints on the dynamics of HOH collision complex dissociation. Most recently, we have been successful in detecting the CN product of C('D) + NO ? CN + 0('D), evidence for excited oxygen formation having been inferred from the observed CN product energetics. Also to be discussed will be the detection of highly vibrationally excited OH radicals by off-diagonal LIF in the Av = -1i -2 progressions. These experiments provide the first direct evidence that 0('D) + H2 proceeds exclusively by insertion/dissociation with no significant contribution from direct hydrogen abstraction.

Optical pumping of cesium vapor in a heat pipe oven in which the energy of two near-infrared photons is close to the 6D states results in high conversion efficiency (up to 1.7%) of the near-infrared pump light to intense blue, forward-propagating radiation. The blue light corresponds to the 7P-6S transition. The dependence of conversion efficiency upon focusing conditions, cell length, and pump intensity is presented. Absorption and gain coefficients are measured in a second heat pipe. The results are discussed in terms of optically pumped stimulated emission (OPSE), stimulated electronic Raman scattering (SERS), and subsequent nonlinear four-wave mixing of two pump-laser photons and one SERS or OPSE infrared photon leading to parametric generation at w = 2WL -wiR.

The detection of gas phase atoms from a caesium source has been demonstrated by two photon resonant ionisation spectroscopy (2RPI). An excimer pumped dye laser system was used in conjunction with a proportional counter, operated at low pressure. Two photon ionisation spectra of caesium have been recorded in the 455-460nm region with the two prominent transitions at 455.5 and 459.3nm being well resolved. The dependence of the widths and relative intensities of the two transitions on incident laser fluence has been investigated.

The chemistry of the solid-liquid interface is of intense practical and theoretical interest, since a vast majority of heterogeneous reaction, adsorption, and epitaxy processes occur there. It is also among the most difficult of systems to study. One must have a probe which is extraordinarily sensitive, in order to observe the first few atomic layers, and very selective, in order to separate the effects of the surface associated species from those of the solvent or neat liquid in close proximity. To fulfill these requirements and to retain the advantages of optical spectroscopic techniques requires that the electric fields employed for excitation of interfacial molecules be spatially localized. When the solid component of the interface is one of the non-lossy metals, the above constraints can be satisfied by exciting a surface plasma resonance of a thin metallic film. Both theoretical and experimental work has shown that enhancement of Raman signals at Ag-liquid interfaces can be as high as 4 x 104. In addition the excitation of surface plasma resonances at metallic interfaces in the Kretschmann configuration has been shown to be extremely sensitive to adsorption of components from the ambient.4-7 In general the position of the plasmon resonance along the wavevector axis shifts and broadens upon adsorption or overcoating. Thus the exci-tation of surface plasma resonances at metal-liquid interfaces offers the possibility of simultaneously studying both adsorption-desorption kinetics, via observation of the reflectivity, and interfacial geometries, via enhanced signal vibrational spectroscopy of the adsorbates themselves.

The reactivity and thermochemistry of ionic metal atoms and clusters has been explored using guided ion beam mass spectrometry. With this apparatus, cross sections of ion-molecule reactions can be measured from thermal energies to hundreds of eV. Application to metal cluster chemistry demonstrates its utility for elucidating reaction dynamics and mechanisms and for determining cluster binding energies and ligand binding energies for both ionic and neutral species. Early results compare the largely unreactive atomic Mn+ with the dimer Mn2+.

Photoionization measurements on potassium clusters, KN, with 3 ≤N ≤101 atoms, show the influence of the clusters' electronic shell structure throughout the range of cluster sizes. Steps in the photoionization thresholds at the major shell closings are observed, and features in the photoionization spectra are correlated with the clusters' electronic levels. The trend of the thresholds toward the bulk work function is also observed.

A theoretical model based on the laser rate equations is used to describe injection locking in pulsed dye lasers. It is shown that, in general, the frequency of the injection locked pulse is not equal to the frequency of the cw light-an unexpected result since the two are related through stimulated emission. We discuss how to accommodate this feature in the design of a specialized delay line used to create optical pulse sequences with variable phase shifts between the pulses.

The formation of the C6H4Cl+ ion from 1,4-dichlorobenzene is studied by two-color picosecond laser mass spectrometry. The ion current of C6H4C1 is recorded as a function of delay time between 266 nm and 532 nm picosecond laser pulses. Also the 532 nm power dependence of this frpment is obtained. Complex delay time and intensity dependence is observed for the C6H4C1 fragment. The competition of fragmentation with ionization is discussed.

The study of atomic and molecular clusters constitutes a rapidly growing cross-disciplinary field of research, whose current advances depend critically on the utilization of new laser and molecular beam techniques. This article surveys current advances in the production, state preparation, and size-separation of clusters using pulsed-laser and molecular beam techniques. Laser photoexcitation experiments are used to probe the unique dynamics of clusters using both high resolution spectroscopy and photo-fragmentation measurements. Examples given are the discovery of the adiabatic pseudorotation in excited Na3, and the search for ion-pair formation in the excitation of microscopic liquid drops. A general spectroscopic approach to the low-energy elementary excitations of metal clusters is presented.

The nature of the electron donor/acceptor (EDA) excited state surface leading to the charge transfer (CT) complex is investigated by picosecond techniques. Irradiation into the CT band of the EDA complexes cyanoanthracene/tetracyanoethylene [CNA, TCNE] and indene/ tetracyanoethylene [IN, TCNE] produces the respective radical ion pairs within 25 psec, which then decay within 60 psec, with the [CNA, TCNE] couple returning directly to the ground state, while the [IN, TCNE] couple passes through a 500 psec lived intermediate before returning to the ground state. To obtain structural information about these complexes, a 5 kHz amplified cavity dumped sync pumped dye laser (-v5 psec) resonance Raman spectrometer using a flowing sample jet has been developed. Preliminary work on trans stilbene shows excellent signal to noise and an S1 decay of ~1000 psec, in agreement with literature values.

Bacteriorhodopsin (BR) has become recognized as an important system for modeling proton and ion transport mechanisms across protein membranes. These membrane processes are initiated optically by visible excitation of the retinal chromophore which undergoes a cyclic series of structural and conformational changes including isomerization and depro-tonation. Vibrational resonance Raman spectroscopy derived from spontaneous scattering has become a major experimental technique in measuring molecular structures and conformations, including those of retinal contained in BR. Laser techniques designed to record time-resolved resonance Raman (TR3) spectra have made it feasible to obtain the vibrational Raman spectra of transient forms of retinal generated during the photocycle of BR and to elucidate the dynamical mechanisms which underlie transport across the BR membrane. The laser and optical methods utilized in a picosecond TR3 spectrometer and its application to retinal intermediates formed on the picosecond time scale are described in this paper.

The four aromatic nitrogenous bases which are found in nucleic acids (adenine,guanine, cytosine, and uracil or thymine) are chromophoric residues which exhibit two absorption bands in the region 200-280nm. The ultraviolet resonance Raman (UVRR) spectra of several guanine or cytosine containing compounds and some isotopic derivatives have been taken with an incident ultraviolet laser beam at either 213nm, 218nm, or 266nm which is generated from the higher harmonics and stimulated Raman shifted frequencies of a YAG laser. Each of these wavelengths falls within either the first or second absorption bands of the chromophores. These spectra have been compared with the UVRR spectra which are calculated based on an earlier theory of one of the authors and various normal mode calculations which are found in the literature. Differences are observed and calculated in the intensities of the Raman bands in the spectra taken at different incident laser frequencies when they are resonant with different excited states of the chromophore. These different intensities must arise from different Franck-Condon overlap integrals between the molecular vibrations in the ground state and in each of the two excited states. An interesting observation is the existence of Raman bands which change in-tensity but do not change frequency upon isotopic substitution -- an effect ascribed to the change in Raman intensity with vibrational kinetic energy changes. It is suggested that the measurement and calculation of the intensity changes of the UVRR spectrum upon isotopic substitution should provide a method of improving the force fields of the purine and pyrimidine chromophores in nucleic acids as well as other aromatic ring compounds.

Optical phonon decay in hydrogen bonded molecular crystals is investigated by time resolved coherent Raman spectroscopy. The importance of low frequency motion in chemically important systems is examined. The low temperature phonon lifetimes, in general, were found to decrease monotonically with phonon frequency. This reflects the density of accepting phonon modes and the decay mechanism. Phonon relaxation in the complicated hydrogen bonded crystals of nucleic acid bases indicate that a more detailed theory is needed than that used to describe phonon decay in hydrogen bonded crystals of amino acids. The measurement of the lifetime and frequencies of the low energy modes can be important to an understanding of chemistry in condensed phases. Picosecond CARS has been used to establish the timescales involved for these motions and has enabled us to uncover the relative lifetimes of many optical phonons in the same crystal.

Using the planar optical waveguide geometry, a CARS spectrum of a 25 A film of TiO₂ on a Nb205 waveguide has been observed. Spectra of monolayers of phenol and pyridine adsorbed on ZnO surfaces also have been observed under high vacuum conditions. When most of the CARS contribution from a waveguide is eliminated by an interference condition, submonolayer quantities are detectable.

The high resolution of Raman Spectroscopy ( 1 cm-1) has led to the observation of rich Raman spectra of small CO molecules adsorbed on flat Ni(100) surface. The low polarizability of the adsorbed molecules and the unenhancing characteristics of the surface used lead to the conclusion that the observed spectra are unenhanced Raman spectra. In this work we report the results obtained when 13C0 molecules were used to independently confirm the previously observed Raman bands due to 12CO molecules adsorbed on Ni(100) surface.

Surfaced-enhanced Raman spectroscopy has been used to study rotational isomers of succinonitrile and N-methyl-thioacetamide on Cu and Ag surfaces. Both the gauche and trans conformers of succinonitrile are found to chemisorb on the metal surface. The doubly degenerate v(CEN) in the free molecules is removed when succinonitrile adsorbs on copper, which indicates that the two CEN groups are no longer chemically equivalent. Both conformers are found to coordinate to the copper surface through the n-system of one of the two CEN groups. In the case of N-methyl-thioacetamide, the population of the cis isomer is greatly increased on Cu and Ag surfaces. This is probably due to surface-induced cistrans isomerization in which the predominant trans isomer is converted to the cis isomer.

Raman scattering studies from single crystals of ammonium tetrabromozincate (ATBZ) are being reported for various polarization geometries for three distinct phases; phase I (450 K), phase III (300 K) and phase V (85 K). Temperature dependence of spectra under five different excitation wavelengths --514.5, 501.7, 496.5, 488.0 and 457.9 nm -- and under specific polarization geometries have revealed; (a) an hitherto unreported phase below 130 K and (b) a photo-selective phenomenon in the temperature range 440 to 325 K for the above five wavelengths. The paper describes, in addition to the vibrational analysis in three phases, the occurrence of a local irreversible change in the crystal at a temperature range which depends on the (intensity and) frequency of the laser radiation used for Raman studies. The structural information would be definitive, if combined with these observations; however the occurrence of this phenomenon, not associated with trivial heating effect, is interesting and needs explanation. The various possibilities to explain the abnormal frequency dependence of the observed changes have been discussed.