Laser induced fluorescence (LIF) and multiphoton ionization (MPI) spectroscopic tech-niques have been used to investigate the HCO radical produced in the UV photolysis of formaldehyde and acetaldehyde. The LIF study has yielded vibrational and rotational information on the ground and first excited electronic states. The MPI experiment has enabled the rotational energy distribution of HCO to be examined under collision-free conditions.

Studies of energy transfer in the OH molecule are of both fundamental and practical importance. A recent series of measur9ments in this laboratory, on collisional processes involving the electronically excited A²Σ⁺ state of OH, has generated several interesting and surprising results. We find the total removal (electronic quenching) cross section of the v' = 0 level to decrease with increasing initially excited rotational level N' and temperature.

The combination of high resolution (<10^-3 cm^-1) cw tunable difference frequency generation (2.2-4.2 μm) with high sensitivity (10^-6/√Hz) long path length absorption methods in pulsed slit supersonic jets has permitted spectroscopic investigation of many weakly bound molecular complexes. Discussion will focus on three complementary areas of experimentation. 1) Cluster formation in the molecular beam is probed via sub-Doppler, velocity resolved absorption profiles of monomer species. Spatially dependent beam clustering is strongly manifested through loss of monomer absorption intensity at line center. 2) IR spectra of simple van der Waals molecules such as ArHF are obtained in the υ₁ HF stretching region. Information on all modes in the complex is extracted. 3) IR spectra of hydrogen bonded complexes such as HFCO₂ are observed which exhibit large changes in average molecular geometry as a function of vibrational state. Surprisingly low intermolecular bending frequencies are evidenced in the spectra via hot bands, and provide dynamical information on coupled vibrational-rotational motion in floppy molecular systems.

Highly time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to obtain nascent rotational, vibrational, and electronic state distributions of photofragments. These distributions are used to elucidate the state-to-state dynamics of the molecular photodissociation process. Selected results for the UV and visible photodissociation of ozone are presented and discussed as illustrative examples of this approach.

The variation of the collisionless absorption spectral shapes for vibrationally excited SF₆ and C₂F₅C1 as a function of the picosecond probe pulse duration was determined using a two-IR frequency pump-probe scheme. The experiments revealed a spectral narrowing with decreasing pulse duration, similar to a recent observation in C₃F₇I with a single laser pulse. The observed experimental results are shown to be consistent with a dynamic multi-tier classification of energy levels in the QC of polyatomic molecules. Specifically, the time constants for such a coupling are evaluated to be >20 ps for all the molecules studied.

Recent advances in laser and optical detector technology have made it possible to perform time resolved Raman scattering experiments on opaque semiconductors with sub-picosecond temporal resolution. This has allowed us to directly study the initial stages in the relaxation of energetic carriers in 111-V semiconductors such as GaAs and Gal_1-xAl_xAs. The generation of a non-equilibrium population of longitudinal optic (LO) phonons by a well characterized population of energetic carriers has been temporally resolved in GaAs. Changes in the screening of the LO phonons due to the creation of a high density of hot carriers and their cooling have also been observed. These studies have been extended to the alloy system Gal_1-xAl_xAs to characterize the changes introduced into the hot carrier relaxation processes by alloy disorder effects. The modification of the carrier relaxation processes due to the presence of strong carrier-carrier scattering was observed using the hot luminescence which accompanied the Raman scattering at high excitation levels.

Multi-color CARS approaches, which permit the simultaneous measurement of temperature and two or more species, are reviewed. Four different techniques will be discussed and compared. These involve multiple pump or Stokes lasers and synergistic combinations thereof. A new method to generate pure rotational CARS from pump and broadband Stokes lasers having arbitrary spectral separation will be described. Combined vibrational/pure rotational CARS approaches will also be discussed.

We have shown theoretically and experimentally that specially shaped laser pulses can give enhanced excitation selectivity, compensate for experimental complications such as inhomogeneities and pulse amplitude jitter, and cleanly pump forbidden transitions. A new approach to generating picosecond pulses, which does not require modelocking, permits software controlled, arbitrarily shaped (phase and amplitude modulated) pulses with roughly 1 ps resolution.

The optical Kerr response of liquid CS₂, nitrobenzene and chlorobenzene is probed with 65 femtosecond optical pulses centered at 633 nm. We observe a clearly separable instan-taneous signal contribution in each system that is attributed to the incoherent (dephased) electronic part of the third order nonlinear susceptibility χ⁽³⁾. In addition to this instantaneous component, each system exhibits a multicomponent decay characterized by two components in CS₂ and three in the substituted benzenes. Finally, we note that the prevailing theory of the optical Kerr effect fails to describe the short time behavior of the measured response in each liquid, apparently requiring the inclusion of an inertial term in the equation of motion for the induced birefringence.

A stabilized laser emits a light beam in a coherent state: a light field with properties very similar to a classical field, but with quantum zero-point fluctuations (vacuum noise) of the field superposed. The nonlinear refractive index of optical fibers can be used to alter the quantum statistics of the light to produce a beam with non-classical properties: a squeezed state. Upon detection, squeezed states exhibit a noise level which is phase-dependent and which can be well below the coherent state level ("shot-noise limit"). The nonlinear refractive index also provides a means for experimental demonstration of the related concept of "quantum non-demolition detection": The measurement of a quantum observable without adding uncertainty to that variable.

Picosecond transient grating experiments in sodium and iodine vapors, involving the 3S → 3P and X → B transitions respectively, are discussed. Population gratings in sodium demonstrate that the technique can be used to measure velocity distributions in the gas phase. It is shown that the time dependent transient grating signal is related to the Fourier transform of the velocity distribution. Similar experiments on iodine illustrate the effect of state changing collisions on the grating signal. In addition, the sodium experiments are used to illustrate a new type of time domain high resolution spectroscopy. When the grating excitation pulses have perpendicular polarizations, a polarization grating, rather than the usual population grating, is formed. Diffraction from the sodium polarization grating shows large time dependent oscillations in the diffraction efficiency. These oscillations yield the ground state and excited state hyperfine frequencies (1.77 GHz and 189 MHz, respectively). The results suggest that polarization grating spectroscopy can have applications in other areas, such as molecular rotational dynamics.

In the mid-sixties, when the laser was a new exotic devise it was believed to be oy many no more than a powerful light bulb, many of the most spectacular scientific experiments were performed. From optics to non-linear phenomena, quantum electronics to molecular spectroscopy saw an incredible growth. Phenomena which were only postulated theoretically such as two photon processes and optical harmonic generation became common experimental events in laser laboratories. Lasers made stimulated light processes rather easy to demonstrate and provide a means for their in depth studies. A new lasing medium. was discovered practically every day, and new effects" were exposed in each physics journal issue.

Experimental evidence pertaining to the structure of the hydrated electron is reviewed. In agreement with recent picosecond optoelectronic data, it is concluded that at low or moderate temperatures the hydrated electron is not an electron at all! Rather, it is very likely a hydrated semi-ionic pair (OH^...H₃0)(aq), having the chemical properties of either OH^-(aq) or H(aq). However, under certain conditions, where the hydrogen-bond structure of the solvent is weak, the hydrated electron may delocalize somewhat into the surrounding water medium.to become "its old self", behaving more like an electron in a cavity. This fragmented personality of one of chemistry's most celebrated fundamental particles is further substantiated by ab initio quantum mechanical calculations.

The high sensitivity of intracavity laser spectroscopy (ILS) for direct, quantitative detection of species at low concentrations and in special environments has been well established. In this paper, we demonstrate the use of ILS to obtain rotationally-resolved absorption spectra of (i) reactive radical species in situ during chemical vapor deposition (CVD) processes used to produce silicon films and (ii) ultra-cold gas phase molecules and van der Waals (vdW) complexes produced in supersonic jet expansions. Information on the chemistry of CVD processes for a number of silicon source materials and the formation efficiencies, stability and excited-state dissociation dynamics for a series of I₂-noble gas vdW complexes is presented.

Picosecond time-resolved fluorescence spectroscopy is used to probe the formation of the twisted intramolecular charge transfer (TICT) state of 4,4'-diaminophenyl sulphone in alcohol solvents. Emission kinetics were measured in 10 nm intervals from 400 nm to 560 nm. Dynamics were measured both as a function of alkyl chain length (methanol to hexanol) and temperature in ethanol solution (25°C - 60° C). Examination of the Stoke's shifted TICT emission revealed wavelength dependent rise and decay kinetics.

We have measured one color multiphoton dissociation/ionization (MPD/MPI) spectra for a series of arene molybdenum tricarbonyls. These experiments measure the relative timescales of multiphoton excitation, intramolecular energy redistribution (IER) and unimolecular dissociation under collision free conditions. The results indicate that IER is accelerated by the presence of low frequency vibrations and by molecular structures and bonding which enable a molecule to remain intact longer during the course of multiphoton excitation. The results for the organomolybdenum molecules are consistent with our earlier results obtained in a similar study of organochromium molecules.

Hydrogen-bonded solute-(solvent)_n, n = 0, 1, 2 complexes have been produced and spectro-scopically characterized in cryogenic rare gas matrices. The solutes studied can undergo excited state proton transfer and include 2-hydroxy-4,5-benzotropone, 3-hydroxyflavone and 3-hydroxychromone. The solvents include water and alcohols. The fluorescence excitation and dispersed emission spectra of each complex stoichiometry have been determined by dilution and matrix annealing studies. Excited state proton transfer in each type of complex has been studied by picosecond emission spectroscopy. The proton transfer dynamics were found to vary dramatically with the degree of solvation.

Recent development of the laser vaporization technique combined with mass-selective detection has made possible new studies of the fundamental chemical and physical properties of unsupported transition metal clusters as a function of the number of constituent atoms. A variety of experimental techniques have been developed in our laboratory to measure ionization threshold energies, magnetic moments, and gas phase reactivity of clusters. However, studies have so far been unable to determine the cluster structure or the chemical state of chemisorbed species on gas phase clusters. The application of infrared multiple photon dissociation IRMPD to obtain the IR absorption properties of metal cluster-adsorbate species in a molecular beam is described here. Specifically using a high power, pulsed CO₂ laser as the infrared source, the IRMPD spectrum for methanol chemisorbed on small iron clusters is measured as a function of the number of both iron atoms and methanols in the complex for different methanol isotopes. Both the feasibility and potential utility of IRMPD for characterizing metal cluster-adsorbate interactions are demonstrated. The method is generally applicable to any cluster or cluster-adsorbate system dependent only upon the availability of appropriate high power infrared sources.

Using a laser vaporization flow reactor apparatus, small niobium clusters (Nbx, x = 1-15) are reacted with benzene and detected with an excimer laser (ArF) photoionization time-of-flight mass spectrometer. At low ionizing laser intensities, the probability of dehydrogenation of Nb_xC₆H₆ clusters to Nb_xC₆ is found to be a maximum for x = 5, 6 and 11 and a minimum for x < 4 as well as for clusters with 8 and 10 niobium atoms. As the ionizing laser intensity increases, it is possible to photochemically drive the dehydrogenation reaction of the x = 7, 9, 12, 13 clusters with one extra photon above ionization while the x = 8 and 10 clusters are found to require two photons above ionization. These results are discussed in terms of the stabilities of these clusters relative to their photochemical ionic products.

The new supersonic beam techniques using laser vaporization have now permitted detailed study of clusters of refractory elements. These are molecular species which offer qualitatively new challenges to theories of molecular dynamics. Aside from their size and complexity, a key new feature of these "hard" clusters is that electronic degrees of freedom are often as active in the molecular dynamics as vibrations. Three examples are given of these new dynamical problems: (1) the competition between photodissociation and photodetachment in mass-selected negative metal cluster ions, (2) the rate of desorption of small molecules chemisorbed on the surface of small metal clusters, and (3) photofragmentation pathways and rates of large positive carbon cluster ions.

The combined application of supersonic jet methods and laser spectroscopic techniques has created an outpouring of experimental studies of the internal dynamics of atomic clusters. This article reviews highlights of this work for the special case of metal atomic clusters. Examples include the dynamical Jahn-Teller effect in X₃ systems (X = Na, Cu), and analogous effects (shape isomerization) in larger Group I clusters. Certain "magic" metal clusters with closed electron shells are of special interest, because semiclassical descriptions of their interatomic motions may be valid. Finally, we comment on the metal evaporative cooling processes, using analogies to ongoing molecular cluster work.

Surface-enhanced Raman spectroscopy has been used to study the adsorption behavior of imidazole and imidazolium ions on Cu and Ag electrode surfaces under different applied potentials, pH's and concentrations of the solutions. At neutral or basic pH's the unprotonated imidazole is the predominant species adsorbed on the electrode surfaces. On the other hand, the surface adsorbates are predominantly protonated ones (imidazolium ions) in acidic solutions. In slightly acidic solutions, both the protonated and unprotonated forms are observed on the electrode surfaces and they are interconvertible, depending on the applied electrode potentials. At neutral or basic pH's, two different molecular orientations are identified on two different metal surfaces in dilute solutions of imidazole. The imidazole molecule is bonded through its electron pair on the N atom to the Cu electrode surface whereas it is bonded through the π-system to the Ag electrode surface. The adsorbed imidazolium ions are bonded through the π-system of the imidazole ring to both Cu and Ag electrode surfaces.