We use the photofragment ion imaging technique to investigate the 266 nm photodissociation dynamics of hydrogen iodide. We show the quantitative features of ion imaging by comparing determinations of photofragment recoil velocity distributions, product branching ratios and the HI bond dissociation energy with previously published results. Excellent agreement with previous experimental and theoretical results is obtained. The H atoms produced from this process are then used as reactants in the reaction of H + HI and H + D₂. Imaging techniques are used to measure the velocity distributions of the products of these reactions.

We report two new optical detection methods for the CH₂ (X ³B₁) radical and one new detection method for the HCF (X ¹A') radical. These detection methods are based upon resonance enhanced multiphoton ionization (REMPI) spectroscopy and require only commonly available lasers. Single laser pulse sensitivity for CH₂ and HCF is better than 10⁸ radicals(DOT)cm^-3. REMPI spectra of CH₂ between 380 and 440 nm arose from three-photon resonances with the B ³A₂ (3d), C (3d), D (3d), and 4d ³A₂ Rydberg states between 78,950 and 68,200 cm^-1 above the ground state. A fourth laser photon ionized the radicals and the signal was carried by CH₂⁺ (m/z 14) ions, i.e., CH₂ signal arises through a 3 + 1 REMPI mechanism. We have also discovered two new states, H(3p) and I(4p), that enable extremely sensitive detection of CH₂ through a 2 + 1 REMPI scheme. In two photons, the H(3p) state produces a strong band at 311.80 nm (64,126 cm^-1). The much weaker I(4p) state appears at 269.27 nm (74,254 cm^-1). Between 305 and 325 nm HCF and DCF radicals produce 2 + 1 REMPI spectra by excitation to 3p Rydberg states. The band origins for HCF and DCF are at 321.6 nm (62,180 cm^-1) and 321.7 nm, respectively.

When energized sufficiently either vibrationally or electronically, a molecule, RH, can dissociate to form H atoms and radicals. By partial deuteration of the molecule, one obtains information on isotope- and site-specific reactions, if they occur. We have studied reactions of O(¹D) with hydrogen, alkanes and alkyl chlorides that are partially deuterated, in addition to vacuum ultraviolet (VUV) photodissociation of these molecules.

The B[1²A₁] and C[2²B₁] electronic states of the allyl radical have been examined by laser spectroscopy. The spectra are classified according to the theory of resonance secondary radiation as predominantly resonance fluorescence in the excitation wavelength range from 242 nm to 250 nm. The observed emission spectra are indicative of the vibronic nature of the absorption bands. The excited state vibrational frequencies for (nu) ₇ and (nu) ₁₂ of the C[2²B₁] state are found to be 375 cm^-1 and 509 cm^-1.

The CH3SO radical was formed from reaction of dimethyl disulfide (CH-3)SSCH3 with microwave-discharged O2) in a flow tube. The A²A' IMP X²A" fluorescence excitation spectrum was observed in the wavelength region 615 - 692 nm. The origin was located at 14,542 cm^-1; the vibrational frequency for the SO stretching of the A state was 859 cm^-1. Dispersed fluorescence was observed in the 680 - 850 nm region. The main progressions gave vibrational frequencies of 1066 cm^-1(nu)5", SO stretching), 704 cm^-1 ((nu) 7)", CS stretching), and 1464 cm^-1 ((nu) 4)", CH3-deformation). The results agree with ab initio calculations. Infrared absorption lines of the CH3)S radical were observed after UV photolysis of a CH3)SH/Ar (1/500) matrix at 12 K. The results yielded (nu) 1) (CH stretching) equals 2948.6, (nu) 2) (CH3)-umbrella) equals 1315.1, ((nu) 3) (CS stretching) equals 719.4, (nu) 4) approximately equals 2873, and 2(nu) 6) (CH3) rocking) equals 1154 cm^-1.

We report a study of the photodissociation of the nitromethyl radical utilizing a fast beam photofragment translational spectrometer. A fast radical beam is prepared via the synthesis, acceleration and subsequent photodetachment of mass-selected, internally cold nitromethyl anions. Following ultraviolet photodissociation, the recoiling photofragments are detected in coincidence by a microchannel plate detector with a time- and position-sensitive wedge-and- strip anode. The data reveal that the electronically excited nitromethyl radicals dissociate into two mass channels: (I) CH₂NO₂$DAG yields CH₂NO + O, and (II) CH₂NO₂$DAG yields CH₂O + NO. The branching ratio of the two channels is nearly constant at 1:1 over the excitation wavelength range of 240 - 270 nm. The kinetic energy release (KER) of channel (I) peaks at 5 - 8 kcal/mol, suggesting that the N-O bond rupture does not occur directly, as it would on a purely repulsive potential surface. The observed KER for channel (II) peaks at 52 kcal/mol, with an average of over 100 kcal/mol going into internal degrees of freedom in the fragments. This channel requires a significant rearrangement of the nuclei and has a large exit barrier. The photofragment angular distributions for both channels show little anisotropy. The absence of the channel corresponding to C-N bond rupture provides evidence that the accessed excited state(s) of nitromethyl have C-N double bond character. These observations provide qualitative information about the mechanisms for O atom loss and for rearrangement and subsequent NO elimination.

We report the observation of stimulated emission pumping spectra in the NCO radical formed in a supersonic free jet expansion by the reaction between photolytically generated CN radicals and O₂. The spectra give rotationally resolved information on high lying vibrational levels that are difficult or impossible to detect by conventional single photon spectroscopic techniques. These new data provide detailed insight into the Renner-Teller, spin-orbit and Fermi-resonance coupling in the molecule. They also provide a solid basis for future state- selected chemical and dynamical studies involving this important radical species.

High-resolution infrared spectroscopy of molecular ions and free radicals has witnessed a dramatic progress in the past ten years. Spectroscopic identification and characterization of transient species are of great importance for understanding elementary processes in discharge plasmas. Infrared spectroscopy of molecular ions is a relatively new field. Nevertheless, up to the present time, more than 50 species (including negative ions) have been identified and the spectroscopic constants have been determined. In this presentation, some of the more recent results are reviewed, and a new observation of the infrared spectra of D₃ and formation mechanism of triatomic hydrogen radicals in hydrogen discharge plasma are discussed.

This paper describes investigations of fundamental photophysical processes in the small inorganic ionic complexes N₂⁺-He and N₂O⁺-Ar. Studies were carried out by observing the wavelength dependent photofragment yield of mass selected ionic species in a guided ion beam apparatus. Spectra of the N₂⁺-He complex exhibit several bands in the near UV that correspond with those of the N₂⁺ chromophore. By measuring the relative intensities of the B $IMP X origin and 1₁¹ hot bands as a function of ion flight time from ion source to the laser, a vibrational predissociation lifetime of 220 +/- 30 microsecond(s) has been determined for the (v equals 1) state of the N₂⁺-He complex. For the N₂O⁺-Ar complex, vibrationally structured electronic bands were observed which arise from charge transfer type transitions. In the ground state of the complex the charge is localized principally on N₂O. Lying above this ground state by the difference in the N₂O and Ar ionization potentials are several excited states where the charge is localized on Ar. Photodissociation to produce both N₂O⁺ and Ar⁺ fragments occurs. While the former fragment probably arises via vibrational predissociation from vibrationally excited levels in the ground state, the appearance of Ar⁺ fragments is evidence for rapid vibrational predissociation on the excited state surface. Dissociation energies (D₀ values) of 690 cm^-1 for the X state of the N₂O⁺-Ar complex and 1340 cm^-1 for the lowest charge transfer state dissociation energy were inferred from the appearance thresholds for N₂O⁺ and Ar⁺ fragments. Various possible dissociation routes are discussed.

Laser spectroscopic investigations of positive and negative molecular ions are described. The experiments include studies of anions in an ion beam apparatus and of cations in a velocity- modulated discharge. In the coaxial beams apparatus, an infrared laser beam overlaps a 3 keV beam of molecular anions. The laser beam induces vibrational-rotational transitions in a negative ion. The vibrationally excited ion then autodetaches, producing a fast neutral. The neutral is then detected by collisional ejection of secondary electrons. This autodetachment technique has a background count rate near zero, and is, therefore, very sensitive. The transitions are sub-Doppler because of a velocity-bunching effect occurring in fast ion beams. Molecular anions whose vibrational-rotational spectra were measured include NH^- and HNO^-.

Optical absorption spectra for size-selected Hg;t" and Hg;t+ clusters show an abrupt transition to a collective, plasmon-like absorption as a function of increasing cluster size. The position of the one plasmon peak as well as the width of the plasmon peak is studied in detail. A temperature dependance is conjectured for the width. A strong influence of electronic correlations on the cluster size dependence of the oscillator strength is observed.

We continue to perform high resolution infrared spectroscopic studies on the vibration-rotation spectra of small polyatomic cations: H₃⁺, C_mH_n⁺, H_mO_n⁺, and N_mH_n⁺, (m,n >= 1). All these ions have been proposed as having fundamental roles in the chemistry of interstellar molecular gas clouds, the birthplace of stars. In particular, H₃⁺ plays the part of the universal protonator: H₃⁺ + X yields H₂ + HX⁺. The work horse of our high resolution molecular ion spectroscopy has been our difference-frequency laser spectrometer with its tunable infrared source. The molecular ions are made in a Pyrex multiple-inlet-multiple-outlet discharge cell using a gas mixture with a high fraction of He (approximately 95%) and various types of cooling (air, water, and liquid nitrogen). The ion signals are detected by velocity modulation of the plasma with unidirectional multipassing and noise subtraction of the probing infrared radiation. This spectrometer has been used for our studies of the vibration-rotation spectra of H₃⁺ fundamental, overtone and hot bands, and of CH₂⁺, CH₃⁺, C₂H₂⁺ and C₂H₃⁺ fundamental bands. Most recently, we have used a water cooled, Cu hollow cathode discharge to re-examine C₂H₃⁺.

The IR spectra of CH₅⁺(H₂) and CH₅⁺(CH₄)_n(n equals 1, 2, 3) were obtained using vibrational predissociation spectroscopy. From the spectra the information on the structure and the intramolecular dynamics of CH₅⁺ with H₂ and CH₄ was obtained and discussed.

A new folded variant of optical-optical double resonance has been developed for the detailed characterization of polyatomic species that contain chemically significant quantities of vibrational excitation. Based upon a novel implementation of phase-conjugate degenerate four- wave mixing (DFWM) spectroscopy, this technique provides a quantum state-specific probe of molecular topography and dynamics that offers substantial advantages over more conventional methods. Despite the third-order nonlinearity inherent to the DFWM process, the tremendous resonant enhancement that accompanies this Doppler-free interaction permits facile detection of double resonance signals even under the rarified conditions present in a molecular beam environment.

The spectroscopy and dynamics of highly excited vibrational levels of the a¹A₁ and b¹B₁ states of CH₂ were studied using time-resolved Fourier transform emission spectroscopy. The use of a Fourier transform spectrometer allows efficient acquisition of dispersed fluorescence spectra over several thousand cm^-1 range in the visible, with better than 1 cm^-1 resolution, from this short lived and low concentration species. Furthermore, the temporal evolution of the dispersed fluorescence spectra due to collisional relaxation can be monitored with 50 ns time-resolution. The results presented and discussed in this paper are: (1) the state-to-state rotational energy transfer and reactive cross- sections for b¹B₁ (0, 16^O, 0) CH₂; and (2) rotational analysis of several previously unobserved high vibrational levels of the CH₂ a¹A₁ state.

We review the use of coherent anti-Stokes Raman scattering (CARS) spectroscopy for rotationally and vibrationally resolved detection of photofragments and reaction products, and the complementary use of stimulated Raman excitation (SRE) for vibrational excitation of diatomic and polyatomic reactants. We illustrate the advantages and disadvantages of these techniques by consideration of a few selected recent experiments in our lab.

A new experimental technique, based on high resolution time of flight analysis of ions, has demonstrated that one photon ionization of dimers, trimers, and tetramers, is accompanied by dissociation of the cluster ion to form either clusters of lower order, new ion structures, or other products formed by intra-cluster ion molecule reactions. The method is based measuring the inevitable kinetic energy released in the dissociation process. By maintaining a low electric field across the ionization region, and optimizing the instrumental parameters, it is possible to measure kinetic energy releases as low as 2 meV. Examples of systems investigated include clusters of acetylene, ethylene, NO, CH₃OH, CH₃SH, CH₃Cl, and C₂H₅Cl. The results showed that even at the lowest photon energies used, ionization without dissociation consisted of at most 5% of the total signal. In some ions, such as C₂H₂ clusters, 100% of the one photon ionization process is via dissociation.

We have investigated autoionization, predissociation, and ion-pair formation from highly excited states of molecular hydrogen by using double-resonance excitation via the E, F ¹(Sigma) _g⁺, v equals 6 level. The energetic threshold for ion-pair formation occurs just below the H₂⁺ X ²(Sigma) _g⁺, v⁺ equals 9 ionization threshold. The spectrum in this region was studied by using conventional and constant-ionic-state photoelectron spectroscopy, by monitoring the H^- production, and by detecting dissociation products by ionization with a third laser. The decay dynamics in this region are extremely rich, because the excited levels may decay by rotational and vibrational autoionization, by predissociation to neutral H + H^* (n equals 2, 3, 4), by predissociation to the ion pair H⁺ + H^-, and by fluorescence. In addition, the dissociative potential curve of the 2p(sigma) _u3s(sigma) _g ¹(Sigma) _u⁺ doubly excited electronic state crosses the H₂⁺ X ²(Sigma) _g⁺ potential curve in the same energy region, and the electronic autoionization of this state is found to significantly influence these decay processes.

We examine the strong-field dissociation behavior of diatomic molecules under two distinctive physical scenarios. In the first scenario, we discuss the dissociation of the isolated hydrogen and deuterium molecular ions. The dynamics of above-threshold dissociation (ATD) are investigated over a wide range of green and infrared intensities and compared to a dressed- state model. The second situation arises when strong-field neutral dissociation is followed by ionization of the atomic fragments. The study results in a direct measure of the atomic fragment's ac-Stark shift by observing the intensity-dependent shifts in the electron or nuclear fragment kinetic energy.

Superexcited Rydberg states (n equals 7-45, l equals s, p, and f, v equals 0, 1) of NO have been state-selectively produced with a two-color double resonance excitation method, and fragment atoms produced by predissociation and NO⁺ ions generated by autoionization have been directly detected by a resonance enhanced multiphoton ionization technique. As a result, not only N(²D)-generating predissociation predicted by previous studies has been confirmed, but unexpected generation of N(⁴S) has also been observed. Competition between vibrational autoionization and predissociation shows strong dependence on the orbital angular momentum and principal quantum number. Striking rotational state dependence of the decay dynamics in the superexcited 7f state (v equals 1) has been found. Through detailed analysis, it has been shown that the decay dynamics of the 7f state (v equals 1) is governed by predissociation processes due to direct coupling with (Sigma) ⁺ valence states. Furthermore, studying the competition between rotational autoionization and predissociation, it has also been demonstrated that the decay dynamics of the rotational superexcited states are predominantly governed by predissociation, not by rotational autoionization.

Zero kinetic energy (ZEKE) photoelectron spectroscopy has been employed to study the spectroscopy and the geometry of the sodium cluster Na₃⁺ with nanosecond tunable dye lasers. The fully resolved cation vibrational spectra prove the equilateral (D_3h) geometry of Na₃⁺. Due to the high resolution of the ZEKE method we derived a much improved value for the ionization potential of Na₃ of 31,363 +/- 5 cm^-1, which is about 300 cm^-1 lower than previous values. The real- time dynamics of two photon ionization and fragmentation of Na₃ has been studied applying femtosecond pump-probe techniques in combination with ion- and ZEKE photoelectron spectroscopy. Three dimensional vibrational wave packet motions in the excited state B and in the ground state X as well as pseudorotational wave packet motion in the B-state have been observed. Time resolved fragmentation studies show ultrafast picosecond decays of most of the excited states of Na₃.

\We report results on pulsed field ionization of NO and O₂. The experiments were carried out using tunable coherent vacuum ultraviolet radiation for single photon excitation of the NO or O₂ molecules. In the case of NO, light between 15.66 eV and 15.90 eV was used to probe the v equals 0, 1, and 2 levels of the NO⁺(a³(Sigma) ⁺) state. The resulting PFI-ZEKE spectra were interpreted using a model developed to explain rotational line intensities in photoelectron spectra. The rotational line intensities were found to vary systematically with v⁺, in disagreement with the model, and the intensities in the v⁺ equals 1 band were strongly affected by the presence of a complex resonance at that energy. In addition, the relative intensities of the v⁺ equals 0, 1, and 2 bands were very different from the Franck-Condon factors, with the intensity of the v⁺ equals 1 band strongly enhanced by the complex resonance, and the v⁺ equals 2 band having a much smaller intensity than expected. In oxygen, we were able to record rotationally resolved PFI-ZEKE spectra of the v⁺ equals 6 to 21 vibrational levels of the O₂⁺(X²$PRD_g) electronic state. As these levels have zero Franck-Condon factors with O₂(X³(Sigma) _g⁺)(v equals 0), their observation shows the presence of a type of `resonant autoionization,' where neutral continuum states provide a coupling between Franck-Condon allowed Rydberg states and the Franck-Condon forbidden levels observed.

Time resolved zero electron kinetic energy (ZEKE) photoelectron spectroscopy is applied to the study of molecular dynamics on the picosecond time scale. The dynamics processes studied include intramolecular vibrational redistribution (p-difluorobenzene, acenaphthene) and vibrational predissociation of van der Waals complexes (aniline-CH₄). The technique offers excellent time resolution as well as sensitivity to the detailed nature of the vibronic state undergoing dynamics.

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Recent advances in experimental techniques now make it possible to readily achieve rotational resolution in molecular photoelectron spectra. Here we discuss results of our recent theoretical studies of rotationally resolved photoelectron spectra at near-threshold energies for the nonlinear molecules H₂O, H₂S, and CH₂O (formaldehyde). These studies serve to reveal the rich dynamics of molecular photoionization and, where possible, to provide a robust description of key spectral features of interest in related experimental studies.

We have recorded the photoelectron spectrum of ⁷Li₂ at 488 nm and 457.9 nm. Photodetachment transitions are observed between the X²(Sigma) _u⁺ state of ⁷Li₂ and the X¹(Sigma) _g⁺, a³(Sigma) _u⁺, and A¹(Sigma) _u⁺ states of ⁷Li₂. The electron affinity of ⁷Li₂ was determined to be 0.437 +/- 0.009 eV. Additional spectroscopic parameters for the X²(Sigma) _u⁺ of ⁷Li₂ determined in this work are: B_e equals 0.502 +/- 0.005 cm^-1 which leads to r_e equals 3.094 +/- 0.015 Angstrom, (omega) _e equals 232 +/- 35 cm^-1, and D₀ equals 0.865 +/- 0.022 eV.

The Cd(DOT)H₂ and Cd(DOT)D₂ van der Waals complexes have been synthesized by expanding cadmium vapor and He/H₂ mixtures into a supersonic free jet. The `half-collision' process, Cd(5s5p ¹P₁)(DOT)H₂ yields Cd(5s5p ³P_J) + H₂, was studied by fixing a probe dye laser pulse (delayed 10 ns) onto one of the Cd(5s6s ³S₁ $IMP 5s5p ³P_J) transitions while exciting the Cd(DOT)H₂(Cd(DOT)D₂) complex with a pump dye laser pulse tuned across frequencies near that of the free Cd(5s5p ¹P₁ $IMP 5s5s ¹S₀) atomic transition. When the probe laser was tuned to detect Cd(5s5p ³P₂), an action spectrum to the red of the atomic transition was obtained for Cd(DOT)H₂, consisting of a broad continuum superimposed upon which was an anharmonic series of vibrational transitions with discernible, blue-shaded rotational structure. A similar spectrum was recorded for Cd(DOT)D₂, except that only very broadened blue-shaded rotational structure was observed.

Measurements of the translational and internal state distributions of OH products produced in the biomolecular reaction ¹⁶O(¹D) + H₂¹⁸O yields ¹⁶OH + ¹⁸OH are reviewed. These detailed measurements reveal relationships between the states of geminate fragments produced in individual reaction events. Preliminary measurements on the same reaction initiated in O₃(DOT)H₂O clusters formed in free jet expansions are reported. The dynamics of the cluster reaction are dramatically different than those of the biomolecular reaction. The results indicate that the third body present in the cluster (O₂), carries a significant amount of the energy produced in the reaction.

Wave packet calculations modeling vibrational predissociation in X₂BC(v') van der Waals clusters are discussed. A model involving three active degrees of freedom is used. Cluster lifetimes and BC vibrational product distributions are obtained, and compared with available experimental results for He₂Cl₂, and Ne₂Cl₂. Some preliminary results for He₂I₂ and Ne₂I₂ are also discussed. Mechanistic issues, including the role of direct versus sequential mechanisms in leading to the production of 2X + BC are addressed, as well as the role of intramolecular vibrational relaxation (IVR). Higher dimension extensions of the model are suggested.

Time-dependent and time-independent calculations are presented for the vibrational predissociation of Ar...Cl₂ in the B excited electronic state: Ar...Cl₂(B,v') yields Ar + Cl₂(B,v < v', j). The potential energy surface used is a sum of pairwise Morse atom-atom interactions adjusted asymptotically to a C₆/R⁶ + C₈/R⁸ anisotropic van der Waals form. The results presented here correspond to excitation in the energy region of Ar...Cl₂(B,v' equals 10 and 11). In agreement with the experimental findings, the final rotational distribution of Cl₂ is found to be strongly dependent on the initial v' state being excited, as well as on the number of quanta lost in the vibrational predissociation process. The role of intramolecular vibrational redistribution (IVR) is examined. It is shown that the vibrational predissociation (VP) dynamics are dominated by the coupling of a zero-order `bright' state with a single `dark' state from the v' - 1 manifold of van der Waals vibrationally excited states which then decays to the continuum, and that the product state distribution is determined by the dissociation of the dark state. This is characteristic of the sparse limit for intramolecular vibrational redistribution. The corresponding time evolution is examined. Time-dependent calculations are performed for the Ar...Cl₂ system frozen at its equilibrium angular configuration. It is found that the dissociation probability as a function of time exhibits oscillations due to interferences between IVR and dissociation. This result is general and simple models based on two bound zero-order levels (the `bright' and `dark' ones of the IVR process) are applied to generate the three- dimensional time-evolution of the system. This allowed us to analyze the effect of the rotation of the complex. It is shown that the oscillatory behavior of dissociation as a function of time can still be present. Moreover, the lifetime for the vibrational predissociation process is found to be dependent on the value of (Omega) , the quantum number for the projection of the overall rotation angular momentum onto the intermolecular axis.

Electronic spectra and predissociation dynamics for the CN-Ne van der Waals complex are reported. From an analysis of the bands associated with the CN B-X transition, we conclude that the equilibrium geometry of the complex is `T'-shaped in the ground state, and linear in the excited state. Electronic and vibrational predissociation of CN(B)-Ne was found to be too slow to compete with radiative decay. Excitation spectra for the CN-Ne A-X transition were strongly influenced by predissociation processes. Two channels were characterized for CN(A²$PRD, v equals 3)-Ne. One was spin-orbit induced (CN(A²$PRD_1/2, v equals 3)-Ne yields CN(A²$PRD_3/2, v equals 3) + Ne), and the other mediated by interstate transfer (CN(A²$PRD_3/2, v equals 3)-Ne yields CN(X, ²(Sigma) ⁺, v equals 7) + Ne). The former was approximately 10⁴ times faster than the latter. Symmetry based propensities were evident in the rotational population distributions of the CN fragments.

Intermolecular vibrational levels which lie above the OH (X ²$PRD) + Ar dissociation limit have been characterized by stimulated emission spectroscopy. OH-Ar complexes prepared in these levels undergo rotational or spin-orbit predissociation. The energies and lifetimes of these metastable states have been computed using a variational method and a flux projection technique. Analogous predissociative levels beyond the OH (A ²(Sigma) ⁺) + Ar dissociation limit have been detected by laser-induced fluorescence and confirmed by hole-burning experiments. Intermolecular vibrational levels of NH-Ar have been probed in the vicinity of the NH c ¹$PRD - a ¹(Delta) transition, revealing information on the NH (c ¹$PRD, a ¹(Delta) ) + Ar interaction potentials.

An overview of our recent work on the theory and simulation of the vibrational predissociation dynamics of I₂(B,v)-Ar₁₃ clusters is given. The decay of the intact cluster induced by vibrational relaxation of the excited impurity exhibits unusual nonexponential behavior, characterized by an instantaneous rate that increases with time. A hybrid statistical model is described, which explicitly takes into account the slow relaxation of the impurity while assuming rapid redistribution of energy among the low frequency cluster modes. This model provides a good description of the statistical dissociation dynamics. A direct mechanism for rapid and selective ejection of rare gas atoms from the system is identified, and understood in terms of correlated binary collisions between molecule and `solvent' atoms facilitated by a nonlinear resonance phenomenon.

The Si(111)/H surface has provided a model system for the vibrational dynamics of phonon coupled surfaces. It has also been an ideal testing ground for the time-resolved surface nonlinear optical probe.

We explore the link between the surface sensitive spectroscopic method, second harmonic generation (SHG), and a popular linear optical method, circular dichroism spectroscopy (CD), in order to study chiral structures within monolayers. The experiment involves utilizing circularly polarized light as an excitation source in the usual reflection SHG geometry. We show that circular dichroic information is conveyed through the SHG process. SHG-CD was used to study the adsorption of R-2,2'-dihydroxy-1,1'-binaphthyl (R-BN) at the air/fused quartz and air/water interfaces. The SHG-CD spectra of R-BN show a strong preference for left-circularly polarized light over right-circularly polarized light for adsorption at both the air/glass and air/water interfaces. The magnitude of the preference is 10³ times larger than that observed in ordinary CD spectroscopy, and is attributed to the fact that the species are oriented at the interfaces.

Surface grating and reflective electro-optic sampling are used to characterize interfacial hole transfer dynamics at n-GaAs(100)/[Se^-2/Se_n^-1] aqueous interface. Complementary femtosecond optical Kerr studies of the response of water to field changes are presented as a probe of the intermolecular solvent modes coupled to the reaction coordinate.

In this paper, single photon ionization time-of-flight mass spectroscopy (SPI-TOFMS) is used to monitor chemical fluxes of In, Ga, and As_n, relevant in molecular beam epitaxy of GaAs. With single photon ionization at 118 nm (10.5 eV), the photon energy is large enough to ionize the species, but not sufficient to ionize and fragment. The lack of molecular dissociation of species such as As₂ and As₄ greatly simplifies the interpretation of mass spectra. SPI-TOFMS provides the ability to measure densities, and hence fluxes, of multiple chemical species above a substrate noninvasively and in real time during conventional molecular beam epitaxy. The relative ionization efficiencies of Ga and the As_n species at 118 nm are determined. Additionally, this laser probing technique is used to study the isothermal and temperature programmed desorption of arsenic from Si(100). The catalytic cracking of As₄ on Si is also examined and discussed. This technique promises to be a valuable in-situ optical diagnostic for III-V and II-VI molecular beam epitaxy.

Photochemistry of N₂O adsorbed on a Pt(111) surface at 193 nm has been studied by temperature-programmed desorption (TPD) and time-of-flight (TOF) distribution analysis. Upon the irradiation of excimer laser pulses at 193 nm, adsorbed N₂O molecules are either desorbed or dissociated to produce oxygen adatoms and nitrogen molecules in the gas phase. In addition, a small amount of adsorbed NO is found by TPD after the irradiation. The production yields of oxygen adatoms depend on the polarization and angle-of-incidence of laser light. This suggests that electron transfer to adsorbed N₂O initiated by the surface excitation is a predominant primary step for the production of oxygen adatoms. Both TOF distributions of N₂O and N₂ reveal nonthermal multiple velocity components. The mechanisms of the photochemical processes and the origins of the multiple velocity components are discussed.

The dynamics of the 308 nm photochemical reaction between CH₃Br and H coadsorbed on Pt(111) are discussed. Photoinduced dissociative electron attachment of adsorbed CH₃Br produced energetic methyl radicals which abstracted surface H to give desorbing methane photoproduct. Photoproduct angular and translational energy distributions were derived from time-of-flight spectra to a quadrupole mass spectrometer following pulsed excimer laser irradiation of the surface.

CERISES, an apparatus recently designed to study state selected ion-molecule reaction by threshold photoelectron photoion coincidence (TPEPICO) was used to measure absolute cross sections at energies ranging from 0 to 20 eV in the laboratory. We report the study of three charge transfer systems of atmospheric interest [N₂ + O₂]⁺, [N₂ + NO]⁺, and [O₂ + NO]⁺. The ions were prepared either in their ground state or in a known distribution of internal energy states.

Using a three-photon (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique, we have demonstrated that 193 nm photodissociation of KI should lead to the predominant channel of K(5²P_J) and I(5²P_3/2) states. The quantum yield of the ground state I(5²P_3/2) amounts to (97 +/- 3)%. In terms of a three-level kinetic model, we have furthermore determined the fine-structure branching ratio of the resultant nascent K 5²P_J doublets in the presence of Ar, He, H₂, CH₄, and CO₂ to be 0.800, 0.798, 0.791, 0.797, and 0.785, respectively, with +/- 1% accuracy. This model takes into account the rapid energy transfer between the 5²P_J doublets and the relevant collisional quenching, thereby leading to a more accurate value than the measurement of fluorescence intensity at low pressure. Since a variety of foreign gases that cause different energy transfers and quenching capabilities have been considered in the system, the resulting branching ratios derived at the zero-pressure are identical, thus confirming reliability of our kinetic model. The relevant fine-structure mixing rate coefficients and the collisional quenching rate coefficients are also evaluated. In addition, the kinetic model has been extended to the case of photodissociation at 248 nm; given the appropriate mixing rate coefficients reported elsewhere, the average branching ratio of K(4²P_3/2) photofragment are then determined to be 0.610 in the presence of Ar, H₂, and N₂. It is advantageous to find that the model can also be used to inspect the evaluation of fine- structure mixing rate caused by various foreign gases.

When the CS₂ vapor of 350 to 450 mTorr in a tube was irradiated by the output of a pulsed laser at wavelength 343.6 nm, and was optically excited to the R(J) equals 29, v equals 0, 10, 0, R³B₂ state, collective emissions were observed at six wavelengths. The emissions are terminated on the highly vibrational states of the ground electronic state, and exhibit a third power dependence. A rate equation including stimulated emission is proposed to explain this phenomenon.

Molecular photoionization and (photo-)dissociation dynamics both result, in general, in anisotropic fragment spatial distributions reflecting the non-spherical potentials experienced in these processes. In dissociative photoionization events, where two such nominally sequential fragmentations take place, the photoelectron and photofragment ion recoil directions can be mutually correlated. It is demonstrated how this correlation may be exploited to select both the state and effective orientation of an ensemble of symmetric top molecular ions. The feasibility of this orientational selection is explored both with model calculations of the photoelectron angular distributions resulting from ionization of partially oriented molecules and with experimental photoelectron-ion coincidence measurements.

Highly resolved JKM> state-selected beams of ND₃ were produced and the experimentally determined hexapole focusing curves are in good agreement with simulated curves calculated with the ND₃ temperature as the only adjustable parameter. ND₃ molecules in the 111> state were photodissociated at 193.3 nm and the angular and kinetic energy distributions of the resulting D-atoms were measured by velocity-aligned Doppler spectroscopy.

The influence of temporal pulse length on laser induced PVD of diamond-like carbon (DLC) as well as Si and Cu films is investigated, using 30 ns and 500 fs UV laser pulses of 248 nm. For the case of the DLC films the laser generated plasma is analyzed by time of flight (TOF) and also by optical emission spectroscopy. The TOF measurements of ablated carbon particles exhibit striking differences between nanosecond and femtosecond laser irradiation. In the case of 30 ns ablation, cluster formation is quite evident in the TOF mass spectra, which coincides with the observation of micron-sized particulates on the deposited DLC films. No evidence for cluster formation is found in the TOF spectra obtained from the femtosecond plasma. The corresponding optical emission spectra indicate a high contribution of C⁺, rather than C₂, and larger molecules in the nanosecond case. In addition, much higher kinetic particle energies (in the keV range) have been measured, which are known to be favorable with respect to DLC film formation. Consequently, high optical quality DLC films without particulates can be grown by femtosecond laser induced PVD. The differences of the two pulse durations are also discussed with respect to ablation characteristics, plume formation, and deposition rates.

A mixture of methane and chlorine molecules in a helium carrier is expanded into a vacuum chamber using a pulsed valve. Polarized laser photolysis of Cl₂ at 355 nm is used to produce chlorine atoms with a sharply peaked speed distribution and a known angular distribution. Methane molecules are excited in the v₃ mode by infrared absorption on the R(O) fundamental transition, which results in methane in the v₃ equals 1, J equals 1 state. Following a 100 ns time delay to allow for reaction, HCl (v' equals 1, J') product molecules are detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI). The resulting photoions are detected with both mass and velocity resolution using a linear time-of-flight mass spectrometer (TOF-MS). An analysis of the time dependence of the ion signal allows us to determine the differential scattering cross section for the specific rovibrational state ionized. The TOF data show a change in the product angular distribution with J', and thus demonstrate the importance of measuring doubly differential cross sections for elucidating the dynamics of this reaction system.

Vibrational overtone excitation of single rovibrational eigenstates followed by laser-induced fluorescence (LIF) detection of the collisionally populated quantum states in single collision conditions provides a method for directly measuring state-to-state rotational and vibrational energy transfer rates in highly vibrationally excited acetylene. There are several advantages in collecting the data in vibrational overtone excitation spectra with LIF detection (scanning excitation laser wavelength with probe laser wavelength fixed) rather than collecting LIF excitation spectra (scanning the probe laser wavelength with the excitation laser wavelength fixed) of the collision-induced transitions. We compare the spectra produced by these two methods and use the technique to acquire a spectrum of state-to-state vibrational energy transfer in single collision conditions as well.

Silanium ions are an important class of hypervalent molecules, and the determination of their structure will yield insights into the nature of nonclassical bonding and provide a contrast to the bonding in carbonium ions. We report the infrared spectrum of the mass-selected silicon hydride cluster ion (formula available in paper) detected by vibrational predissociation spectroscopy. Silanium ions were formed in a pulsed high pressure glow discharge and cooled by the subsequent supersonic expansion. Photodissociation spectra were obtained using a tandem time-of-flight mass spectrometer: (formula available in paper) ions were mass-selected and excited by a tunable infrared laser. The resulting photofragments were detected using a reflectron as a mass analyzer. We observed a vibrational band at 3865 cm^-1, which was the only one observed from 3500 cm^-1 to 4200 cm^-1. This result suggests that the molecule might form a symmetric complex with the structure H₂(DOT)SiH₃⁺(DOT)H₂, in contrast to the (formula available in paper) which has the structure CH₅⁺(DOT)H₂.