The role of dielectric friction in solvation dynamics and rotational motion is examined. Using the connection between solvatiOn dynamics and time dependent dielectric friction developed by van der Zwan and Hynes, it is demonstrated that solvation times determined from time dependent fluorescence studies can be used to accurately gauge dielectric friction contributions to the rotational diffusion of an oxazine dye in alcohol solutions. The generality of these conclusions requires that additional probe molecules, spanning a large range of permanent dipole moments, be studied. A series of amino substituted fluorenones are shown to be an excellent choice of molecules for such studies.

This paper describes new ultrafast fluorescence measurements on the excited state dynamics of polar molecules in polar solvents. Two processes are studied, excited state solvation dynamics and excited state intramolecular electron transfer.

Picosecond photon echo (PE) experiments are used to examine the optical dephasing and intermolecular interactions of organic chromophores in a variety of glassy organic and polymer hosts. The inherent disorder in these systems leads to dynamics which span times from picoseconds to hours and longer. The temperature dependent optical dephasing as measured by the PE follows a power law characteristic of tunnelling twolevel systems (TLS) at low temperatures (1 °K to 4 °K) and is exponentially activated at higher temperatures due to dephasing by additional modes. The nature of these modes will be addressed. These data and those from other line narrowing experiments such as hole burning allow the study both of fast dephasing processes and spectral diffusion due to slow fluctuations in the host. The observed lineshapes and holewidths require dipolar coupling between the chromophore and TLS and offer information on the distribution of relaxation rates. Furthermore, techniques sensitive to different timescales are used to examine host dynamics by removing the guest-host coupling from consideration. The data are discussed in conjunction with recently developed theoretical treatments of dephasing in disordered systems.

This paper pertains to rotational coherence spectroscopy, a time-domain method of high resolution rotational spectroscopy based on the quantum beat phenomenon. We present a brief discussion of the theoretical basis of the method and a consideration of its advantages over spectral domain methods. We also present experimental results on two species - 1,2-diphenyl acetylene and the tryptamine-(methanol) hydrogen-bonded complex - as further examples of the utility of the method in studies of large species.

Rates of chemical reactions in molecular clusters were measured by picosecond spectroscopy in a molecular beam mass spectrometer. We report on two classes of reactions. First, we consider acid-base proton transfer from an aromatic species to a basic solvent cluster . The chemistry involving the aromatic acids aniline cation and S phenol depended strongly on the solvent type and solvent cluster size. For example phenol*(NH3)n exhibited proton transfer for n 5. The rate constant for dissociation was ka (60 10 ps)' for n = 5 - 7. A geminate recombination rate of k_a (350 lOOps)1 occurs for n = 5. No proton transfer occurred in (CH3OH) solvent cluster up to n = 11. In another class of reactions, free radical chemistry in clusters was initiated by A state excitation in (CH3I) clusters. Extensive demethylation and formation of 12 and I2(CH3I)_2 fragments occured on the order of 10 ps. A mechanism is proposed for a chain of reactions driven in part by the energy of intracluster recombination of two CH3 radicals and the elimination of ethane.

Time-resolved techniques are applied to issues of vibrational energy transfer at surfaces. Primary attention is given to the relaxation of vibrationally excited diatomic adsorbates on metals. The sensitivity of the vibrational decay rate to the number of metal atoms in the solid is demonstrated. Preliminary results for the transient response of CO on Pt(111) are also reported. This latter measurement demonstrates the recently developed capability of monitoring adsorbate energy transfer processes at well defined surface sites. The observations appear to be consistent with relaxation through the excitation of electron/hole pairs in the substrate.

Time-resolved techniques are applied to issues of vibrational energy transfer at surfaces. Primary attention is given to the relaxation of vibrationally excited diatomic adsorbates on metals. The sensitivity of the vibrational decay rate to the number of metal atoms in the solid is demonstrated. Preliminary results for the transient response of CO on Pt(111) are also reported. This latter measurement demonstrates the recently developed capability of monitoring adsorbate energy transfer processes at well defined surface sites. The observations appear to be consistent with relaxation through the excitation of electron/hole pairs in the substrate.

The surface restricted transient grating technique has been found to be sensitive to the Fermi levelpinning surface states at the atomic interface of the native oxide layer of (100) GaAs. The sensitivity to these states is better than io of a monolayer. The high sensitivity arises from a surface enhancement effect that is attributedto the delocalized two dimensional character of the electronic state at the surface. The surface enhancement is eliminatedby photoinduced removal of the oxide layer and hole transfer to Se2 ions adsorbed to the surface. These resultssupport the assignment of the signal to electronic factors associated with surface state species. The coulombic bindingenergy of the minority hole carrier, into a 2-d hydrogenic state centered around these negatively charged surfacestates, is .16 eV. This coulombic trapping must be the first step in the surface state trappingprocess and rationalizes the picosecond surface state trapping dynamics observed at GaAs surfaces. In addition, the in-situ studies of hole transfer to Se2 at liquid junctions found the hole transfer time to be less than 30 psec. Relative to the thermalization time scale of space charge accelerated hole carriers, this result demonstrates that hot hole transfer contributes at least a fewpercent. to this surface reaction mechanism.

Fluorescence decay profiles are used to monitor carrier dynamics at the interface between single crystalline CdSe electrodes and 0.5M KOH solution. The fluorescence decay profiles are found to be independent of temperature over the range from 277K to 337K. It is also shown that a diffusion model of the carrier dynamics with a single rate constant for surface recombination does not adequately describe the observed dynamics.

A picosecond transient absorption spectroscopy technique has been employed to probe the charge transfer processes in Ti02 semiconductor colloids. The trapping of electrons at the TiO surface (Ti4+ sitesY was characterized from the appearance of a broad absorption in the region of 550-750 nm following the 355-nm laser pulse excitation of Ti02 colloids. The lifetime of these trapped charge carriers increased upon incorporation of a hole scavenger in the colloidal semiconductor system. The mechanistic and kinetic details of the charge injection from excited CdS into a large bandgap semiconductor such as AgI and Ti02 have also been inves-' t i ga ted.

valuable information on the interaction of an intense pulse with an absorbing medium can be gathered from a measurement of the reflected amplitude and phase as a function of time (in fs). Correlation techniques and algorithms have been developed to provide a complete (amplitude and phase) display of a fs signal. A sensitive method to study index nonlinearities is to use the large change in reflection occurring near critical angle. Picosecond switching and energy limiting has been demonstrated at the interface between glass and dye solution.

A simple model for dispersive optical systems is given. This method uses 4 by 4 matrices to model the system and is analogous to the ubiquitous ABCD matrices used for monochromatic optical systems. This model is exact up to quadratic phases and gives a simple prescription for the propagation of generalized gaussian beams.

Dynamic absorption spectroscopy takes advantage of the spectral width of ultrafast femtosecond pulses to measure complete time-resolved absorption spectra with a single pulse. To quantitatively analyze these experiments, we have developed a time-dependent theory for dynamic absorption spectroscopy which generalizes the usual wavepacket picture of optical absorption to the unique case of nonstationary initial states. In our treatment, the spectrum of an initial nonstationary state is seen to arise from the time-dependent overlap of wavepackets propagating on both the final and initial potential surfaces of the transition. Calculations modelling the 6 fs experiments on Nile Blue and bacteriorhodopsin are presented.

The optical dynamics of J-aggregates of 1,1'-diethyl-2,2'-cyanine in solution and adsorbed on 4-nm-diameter colloidal silica particles has been studied by using picosecond polarized pump-probe spectroscopy. The absorption anisotropy and the excitation-intensity dependence of the ground-state recovery kinetics are compared in these two systems. The results are discussed in terms of the dynamics of delocalized and localized excitons.

A new picosecond time-resolved fluorescence microscope (PFM) has been developed for measuring and mapping the lifetime of laser excited fluorescence from a sample placed under a micro— scope. A compact laser diode is used to generate excitation pulses. The combination of a spectrograph and a streak camera performs time resolved fluorescence spectroscopy with a resolution of 40 ps ( 5 ps with deconvolusion). The lifetime and the intensity of laser excited fluorescence are analyzed b a computer and are displayed on a monitor as either a lifetime or intensity map.

Using amplified 100 fs, 616 nm pulses, we employed pump-probe spectroscopy to investigate strong field interactions with gases and metal surfaces. The ionization dynamics of a gas with a pump pulse foeused to 1016 W/cm2 were deduced from spectral shifts in a probe pulse. In addition to above threshold ionization, impact ionization and a nonlinear dependence of the collision frequency on pulse intensity are proposed as mechanisms for the spectral shifts. Also, the cooling time of a high-temperature solid density plasma was measured specifically, the nonequiibrium electron energy distribution excited by a resonantly absorbed pulse focused to 5 x 1012 W/cm2 on a metal surface was measured with a photoelectron time-offlight detector.

Ultrafast processes which are spatially inhomogeneous are best studied by optical techniques which provide simultaneous temporal and spatial resolution. The ultrafast microscope developed in our lab provides picosecond time resolved images with micrometer scale spatial resolution. The principles of ultrafast microscopy, the apparatus itself, and the applications of this apparatus to several important processes, including polymer ablation and dynamic laser beam distortion, are described herein.

Timed sequences of femtosecond pulses produced by pulse-shaping techniques have been used to achieve improved optical control over molecular motion in crystalline solids. Selected lattice vibrational modes in an organic molecular crystal have been driven repetitively by appropriately timed pulse sequences in a manner analogous to that in which a child on a swing is pushed repetitively with timed mechanical forces. Repetitive driving with a pulse sequence results in larger lattice vibrational amplitudes and improved modeselectivity compared to driving with a single pulse. Numerous applications of pulseshaping techniques in femtosecond spectroscopy are anticipated.

Subpicosecond time-resolved transient grating and transient absorption experiments were performed in mercuric iodide, a wide bandgap semiconductor. Sub-bandgap excitation in the Urbach tail induces an index grating which forms in about 2 ps and which exhibits biexponential decay with time constants 60 ps and 1 .5ns. We ascribe this change in the index of refraction to the formation of defect states. In addition, coherent Raman excitation of the 1 14 cm1 Aig lattice mode was observed.

We report transient infrared spectra for the addition of perfluoroheptane to Cr(C0)5 which is created by the ultraviolet photodissociation of Cr(C0)6 in liquid perfluoroheptane. We compare both 266 nm and 297 nm excitation wavelengths and demonstrate that two product vibration bands at 1971 and 1938 cm1 are forming with a nominal 60 p5 rate constant, while fast rising and slowly decaying (80-220 ps) transients have absorption over the 1920-1983 cm1 range. These transient absorption features are assigned to Cr(C0)5 with vibrational quanta of 0,1,2 which are broadened from geometric distortions. These features decay from reaction, vibrational relaxation of GO, and vibrational relaxation of low frequency vibrational modes.