We discuss recent progress in the attempt to gain complete control over the internal state population of atoms and molecules for collision dynamic studies and spectroscopy. We describe a method which relies on stimulated Raman scattering with pulses delayed in a counter-intuitive way. This method allows highly efficient transfer of population between atomic or molecular levels. It allows, in particular, selective population of highly excited vibrational levels.

The orientation dependence in the spin changing collisions C₂H₂O₂(S₁)+AryieldsC₂H₂O₂(T₁)+Ar and C₂H₂(S₁)+AryieldsC₂H₂(T)+Ar has been examined by time-resolved laser induced fluorescence studies of the intersystem crossing rates in the C₂H₂O₂-Ar and C₂H₂-Ar complexes with different isomeric structures. It was found that when Ar interacts primarily with the n(O) orbital the S₁yieldsT₁ transition rate is about two orders of magnitude faster than that induced by Ar interacting primarily with the (pi) ^*(CO) orbital. On the other hand, the studies in acetylene show that the Ar induced intersystem crossing rate is nearly identical for both the (pi) and (pi) ^* orbitals.

The evolution of lasers from their early conception to a general laboratory tool has been earmarked by periodic quantum jumps in the technology. Such advances include tunability, high peak power, and ultra-short temporal pulses. In this paper we describe an ultra-short laser system incorporating all of the above qualities while achieving kilohertz repetition rates. This system uses an approach based on a unique class of solid state multi-pass optical amplifiers known as regenerative amplifiers. We specifically present details of a 250 fsec tunable gigawatt laser system capable of operating at 3 kHz repetition rate. This system is based on cw-pumping technology and Nd:YLF pump lasers. A general review of other high repetition rate systems is also discussed and contrasted. Preliminary results are presented on a titanium sapphire regenerative amplifier.

The application of ultraviolet resonance Raman spectroscopy to the electronic excitations of the simple (pi) -electron systems butadiene, cyclopentadiene, benzene, acetylene, dimethylacetylene (2-butyne) and diacetylene (1,3-butadiyne) is described. Raman scattering resonant with electronic excitations of these species provides new information that permits a check on the interpretation of the corresponding absorption transitions. The determination of the depolarization ratio for Raman scattering of totally symmetric modes and its variation with excitation wavelength is shown to be a useful way to demonstrate that an electronic band consists of two or more transitions with orthogonal polarization components. Raman spectra of acetylene, dimethyl-acetylene, and benzene show evidence of strong vibronic coupling. A quantitative analysis has been developed for the benzene case where pseudo-Jahn-Teller distortion is observed. The general utility of resonance Raman spectroscopy using ultraviolet radiation as a tool in molecular spectroscopy is illustrated with these studies. Highly excited vibrational levels not seen by other methods are often observed with high intensity, overlapping electronic transitions can be detected, and the nature of Franck-Condon displacements and vibronic coupling mechanisms can be determined.

Fluorescence excitation and emission spectra of the S₁ and S₂ states of model trienes and tetraenes are measured in free jet expansions. The barriers to cis-trans isomerization in the S₁ state are < 200 cm^-1 for trienes and approximately 2000 cm^-1 for tetraenes. < 250 fs nonradiative decay of the S₂ state of tetraenes is deduced from the observed Lorentzian linewidths.

Photoelectron spectroscopy of isomeric C₃H₂ and C₃H₃ molecules, produced by flash pyrolysis in a supersonic jet nozzle, is used to test a valence-bond picture for radical bond strengths and carbene singlet-triplet gaps. The good agreement suggests that ionization potential measurements can be used to determine these hard-to-obtain numbers.

The dependence of vibrational energy transfer on vibrational excitation has been studied using the stimulated emission pumping technique to efficiently prepare a large range of specific vibrational states of the nitric oxide molecule in its ground electronic state. Laser-induced fluorescence was used to detect collisionally relaxed NO. The self-relaxation rate constants of NO(v >> 1) were up to two hundred times larger than that of NO(v equals 1). Multiquantum relaxation was found to be important at high energy and was quantified at 3.8 eV. Theoretical explanations of our experimental results were attempted and it is shown that at vibrational energy up to approximately 3 eV the qualitative trends observed in these experiments such as: the mass effect and the linear dependence of the relaxation constant on v can be explained by Schwartz-Slawsky-Herzfeld theory. A simple explanation of the anomalously high NO self-relaxation rate is given. The large acceleration of the vibrational relaxation rate above 3.0 eV is coincident with the energetic onset of high energy (NO)₂ isomer-complexes. More theoretical and experimental work is needed to explain the quantitative aspects of these observations.

Time-resolved infrared diode laser spectroscopy has been used to state selectively monitor CO produced in the `hot' atom reaction of H with CO₂ and OCS. For center of mass (CM) kinetic energies of 2.4 and 1.4 eV, respectively, CO internal excitations are substantially colder than predicted by statistical theory, with energy preferentially channeled into CM translation. In the collision energy regime of these experiments, approximately 10,000 cm^-1 above the barrier, statistical rate theory is not applicable even with an intermediate.

Crossed-beam chemical reactions of excimer laser-generated metal atoms (Al, Ba, La, Ce, Gd, and Dy) with oxidants are studied under the condition of high collisional energy. Electronically excited metal monoxides are positively identified by their chemiluminescent spectra. From time- and spectrally-resolved emission studies, the dominant role of high-speed, ground state metal atoms in these chemiluminescent reactions is revealed. State-resolved excitation functions at high collisional energy are obtained for Ba + O₂ reaction. To the exclusion of Al + O₂ reaction, all the systems display anisotropic angular distributions of chemiluminescence. A Newton diagram analysis of the observed chemiluminescent spatial patterns infers direct reactions with backward scatterings for these metal atom + O₂ reactions at high collisional energy.

State-to-state collision dynamics of molecular radicals were investigated by the laser-induced fluorescence technique in a pulsed, crossed-beam apparatus. Dramatically different product state distributions were observed for two prototypical radicals, NCO(X²$PI) and CH(X²$PI). Based on a quantum scattering formalism and general considerations of the potential energy surfaces these observations were interpreted as generic features for the inelastic scattering of ²$PI radicals. The differences observed for NCO and CH are the results of well-known Hund's coupling classification of linear molecules.

Molecular beam reactive scattering experiments are performed with H2+ prepared in selected vibrational state distributions via resonantly enhanced multiphoton ionization. The technique allows a systematic exploration of the effects of reactant energy on product angular and energy distributions. Cross sections for the exoergic reaction H2+ + H2 yields H3+ + H, differential in scattering angle and recoil energy, are measured at c.m. collision energies of 1 - 5 eV. The endoergic reaction He + H2+ yields HeH+ + H is investigated at c.m. collision energies of 0.3 - 1.9 eV.

Infrared-optical double resonance incorporating vibrational overtone excitation of a light atom stretch mode prepares reactant molecules in single quasibound rovibrational states at energies above their decomposition threshold on the ground potential surface. When combined with laser induced fluorescence detection of the dissociation fragments, this approach permits fully quantum state resolved studies of unimolecular dissociation reactions. This report describes the application of this technique to study the unimolecular dissociation of hydrogen peroxide from a variety of vibrational and rotational states. We emphasize here the spectroscopy associated with the reactant excitation process and demonstrate how highly resolved excitation spectra provide detailed information about the dynamics of the ensuing dissociation. We then describe a new spectroscopic technique for detecting vibrational overtone excitation in low pressure environments based upon selective CO₂ laser infrared multiphoton dissociation of highly vibrationally excited molecules. We show preliminary results applying this approach to CH₃OH.

Detection of nominal `zero kinetic energy electrons' in resonant two-photon ionization with (omega) ₁ fixed and (omega) ₂ scanned permits photoelectron spectroscopy with 1 - 5 cm^-1 resolution. We report vibrational bands from 0 - 650 cm^-1 for benzyl⁺-h₇, benzyl⁺-(alpha) d₂, and benzyl⁺-d₇. The exocyclic C-C bond of benzyl cation has substantial double bond character. Band assignments from ab initio frequencies illuminate the mechanism of vibronic coupling in the 1²A₂-2²B₂ system of neutral benzyl. In toluene⁺, we observe internal rotor frequencies that fix the V₆ barrier height of the CH₃ rotation at 20 +/- 5 cm^-1, which is remarkably low. In VO yields VO⁺ + e^-, we measure state-to-state relative photoionization cross sections from the intermediate spin-rotational state N' J' to different cation spin-rotation states N⁺J⁺. Remarkably strong lines corresponding to large changes in angular momentum ((Delta) N equals -6 to +7 and (Delta) J equals -6.5 to + 6.5) are observed. Accurate values of the adiabatic ionization potentials of each molecule are reported.

We have made ultrafast time resolved pump probe measurements on the intramolecular electron transfer (ET) of the betaines, specifically betaine3O and penta-t-butyl betaine. The data have been analyzed to determine the ET rate in a range of solvent environments, at various temperatures and at a variety of pump and probe wavelengths. In all cases, the observed ET rate is fast, often faster than predicted by common ET theories. As a result, we have extended some common theories to successfully predict the measured ET rates. In addition to the ET dynamics, the data also display evidence for local heat deposition in the immediate vicinity of the betaine molecule which our extended model qualitatively predicts.

The title reaction was studied by probing the CO(v,J) product state distributions. Oxygen atoms were formed by 355 nm photolysis of NO₂. Photolysis produces approximately equal populations of NO(v equals 0) and NO(v equals 1). The collision energy of oxygen atoms corresponding to NO(v equals 0) is 1570 cm^-1. This is above the O + OCS activation barrier of 1540 cm^-1. Oxygen atoms corresponding to NO(v equals 1) do not have sufficient energy to proceed over the activation barrier, thus insuring monoenergetic collisions. CO product was probed using an IR tunable diode laser. Nascent CO distributions were extracted from the transient absorption signals using an initial slope approximation. A vibrational branching ratio of [v equals 1]/[v equals 0] = 2) was not detected. The CO(v equals 0) rotational Boltzmann plot was bimodal. The distribution for 0 = 15, the plot had a temperature of 4400 +/- 390 K. The low J population is the result of rotational relaxation of the nascent CO distribution. The high J signals are direct measure of the nascent CO population. Surprisal analysis resulted in a parameter of (Theta) _R equals 3.7 +/- 0.5. Hence, the CO(v equals 0) distribution is colder than a `prior' statistical model.

We report the structural characterization of the gas phase adducts HCN and CH₃CN with BF₃. Both have symmetric top structures with the nitrogen end of the R-CN toward the boron, reminiscent of the well known dative bond chemistry of BF₃ with nitrogen donors. The B-N bond lengths and N-B-F angles, however, are intermediate between those expected for van der Waals or covalent interactions. Moreover, in CH₃CN-BF₃, where comparison with x-ray crystallographic studies is possible, the gas phase adduct shows a markedly longer bond length and smaller N-B-F angle. We show that in a series of related BF₃ and BH₃ adducts, the bond length and bond angle can, in fact, be tuned almost continuously between the covalent and van der Waals limits. By analogy with classic crystallographic work by Buergi and Dunitz and coworkers, we discuss how members of such a series can be interpreted as snapshots along a generalized reaction path for the formation of the dative bond. Finally, in the context of such a path, we examine the evolution of other (non-structural) properties of the BF₃ adducts as the donor-acceptor bond formation proceeds.

The photoelectron spectrum of Cr₂^- shows vibrational levels in the ¹(Sigma) _g⁺ ground state of the neutral molecule up to 7300 cm^-1 above its zero point level. These data, obtained at an instrumental resolution of 5 meV (40 cm^-1), reveal a panoramic view of the controversial ground state potential curve of Cr₂. Low-lying vibrational levels are found to fit a Morse potential with (omega) _e equals 479 +/- 2 cm^-1 and (omega) _e(chi) _e equals 13.5 +/- 1.0 cm^-1. This unusually large anharmonicity extrapolates to a dissociation asymptote of only 0.5 eV, considerably lower than the true 1.44 eV value. Between 4875 and 7320 cm^-1 above the zero point level, we observe twenty peaks at 130 +/- 15 cm^-1 intervals, which we assign as transitions from the ground electronic and vibrational state of the anion to high vibrational levels of the Cr₂ ground state. Using an RKR inversion procedure, we have obtained a potential curve that fits all of the observed vibrational levels to within our experimental uncertainty. This potential curve is compared with the predictions of Goodgame and Goddard's modified GVB calculation. Transitions to highly excited vibrational levels of the Cr₂ ground state are far more intense than would be expected for a direct photodetachment process, and are also strongly wavelength dependent. These non-Franck-Condon intensities are attributed to a resonance of the laser with one or more metastable states of the negative ion far above the electron detachment threshold. The electron affinity of Cr₂ is measured to be 0.505 +/- 0.005 eV. An excited electronic state of Cr₂ with a vibrational frequency of 580 +/- 20 cm^-1 is observed 14,240 +/- 30 cm^-1 above the ground state. For Cr₂^-, we obtain (omega) _e equals 470 +/- 25 cm^-1, (omega) _e(chi) _e equals 20 +/- 10 cm^-1, and r_e equals 1.71 +/- 0.01 angstrom. Tentative state assignments of ¹(Sigma) _u⁺ or ³(Sigma) _u⁺ for the excited Cr₂ state, and ²(Sigma) _u⁺ for the anion, are discussed. Preliminary results for Cr₂H^- and Cr₂D^- are also presented. The photoelectron spectra of these anions reveal the Cr-Cr and Cr-H stretching frequencies in the neutral molecules, and exhibit partially resolved rotational structure.

The influence of vibrational excitation and collision energy on the reaction NH₃⁺((nu) ₂) + ND₃ has been investigated using a quadrupole-octopole- quadrupole mass spectrometer. The NH₃⁺ reagent ions are prepared state- selectively with 0 - 7 quanta in the (nu) ₂ umbrella bending mode by (2 + 1) resonance enhanced multiphoton ionization. The mass-filtered reagent ion beam interacts with a thermal distribution of neutral ND₃ molecules at controlled center-of-mass collision energies (0.5 - 10.0 eV) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH₃⁺ with ND₃ has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. The product branching ratios and relative cross sections for each of these channels exhibit strong dependences on ion vibrational excitation and collision energy. Briefly, both deuterium abstraction and charge transfer are enhanced by vibrational excitation, whereas proton transfer is suppressed. As the collision energy is increased, the branching fraction for charge transfer increases sharply while proton transfer decreases. The branching ratio for deuterium abstraction does not exhibit a significant dependence on collision energy. The influence of ion vibrational excitation is discussed in terms of its relationship to the reaction coordinates for the three product channels. The behavior of this reaction points to a short-lived collision complex in which vibration and translation play inequivalent roles.

While laser ablation is often used to create clusters, laser desorption at lower fluences can be used to probe clusters already present on a surface or van der Waals clusters formed between desorbed species and gas phase atoms. As an example of the former we discuss a study of para amino benzoic acid dimers. The dimers are formed by laser desorbing the monomer into a supersonic expansion. With the technique of laser desorption jet cooling it is also possible to form van der Waals clusters and species in the drive gas. As an example we discuss the spectroscopy of triphenylamine/argon clusters. Finally, the same technique allows the study of species on the surface, rather than clusters formed by the volatilization itself. A dramatic example is the formation of carbon clusters by laser ablation in a separate step and their subsequent study by laser desorption.

We describe the current status of coherent radiative control, a quantum-interference based approach to controlling molecular processes by the use of coherent radiation. In addition to providing an overview of proposed laboratory scenarios, ongoing experimental studies and recent theoretical developments, we call attention to recent theoretical results on symmetry breaking in achiral systems.

Photoelectron spectra are reported for Cr(CO)₃^-, Mo(CO)₃^- and W(CO)₃^- anions prepared from the corresponding metal hexacarbonyls in a flowing afterglow ion source. The 488.0 nm spectra were obtained at an electron kinetic energy resolution of 5 meV using a newly constructed apparatus. The spectra exhibit transitions between the ground electronic states of the anions and the neutral molecules, and they show weak activity in the symmetric CO stretching, MC stretching, MCO bending and CMC bending vibrational modes. The observed vibrational structure indicates that the anions, like the neutral molecules, have C_3v equilibrium geometries. Force constants estimated for the neutral M(CO)₃ molecules from the fundamental vibrational frequencies measured here are consistent with stronger metal-ligand bonding in the coordinatively unsaturated complexes than in the corresponding hexacarbonyls. Franck-Condon analyses of the spectra indicate only small differences between the equilibrium bond lengths and bond angles of the anions and the corresponding neutral molecules. The electron affinity pattern observed among the three group VI metal tricarbonyls is compared with characteristic trends within triads of transition metal atoms, and within the coinage metal dimer series. This comparison, combined with the results of previously reported theoretical calculations, suggests that the extra electron in the M(CO)₃^- anions occupies an sp hybrid orbital. Electron affinities of 1.349 eV, 1.337 eV, and 1.859 eV (all +/- 0.006 eV) are obtained for Cr(CO)₃, Mo(CO)₃, and W(CO)₃ respectively.

A mass-selected, partially rotationally resolved, resonant multiphoton ionization spectrum of the allyl radical, C₃H₅, is reported. Photoelectron spectroscopy, isotopic labelling, and rotational analysis establish that the band system corresponds to the 2²B₁ $IMP 1²A₂ transition, with an origin band at 248.15 nm. Spectral simulation indicates that the equilibrium CCC bond angle of the radical decreases from 124.6 degree(s) in the ground state to 117.5 degree(s) in the excited state.

High resolution HeI and HeII excited inner valence photoelectron spectra of the oxygen molecule have been recorded between 20 and 26 eV. In this range, three photoelectron bands are clearly seen; they are associated with the B ²(Sigma) _g^-,3 ²$PI_u and c ⁴(Sigma) _u^- states of O₂⁺. The state of ²$PI_u symmetry observed around 24 eV shows a long vibrational progression, contrary to earlier work, with spacings that decrease successively toward higher electron binding energies. The assignment is confirmed by ab initio calculations. These calculations show that if the potential curve is followed along the electron configuration rather than the adiabatic curve, the vibrational structure can be accounted for.

The Raman echo experiment, which can distinguish homogeneous and inhomogeneous broadening of vibrational spectra, is reported for the first time in liquids. The symmetric methyl stretch of acetonitrile, which has been extensively studied with contradictory results, is shown to be homogeneously broadened. The vibration interacts only with rapid collisional motions of the solvent.

An unusual slow rise of fluorescence intensity from Coumarins 102, 314, and 334, observed under conditions of high intensity laser excitation prompted further investigation of their photophysics by photoquenching, time-resolved transient absorption spectroscopy and time- resolved amplified stimulated emission. The slow rise was found to be the consequence of multi-photon excitation. Principal fate of coumarin molecules so excited was shown to be intersystem crossing to the nonemissive triplet manifold. Rate of intersystem crossing was estimated as ca. 10¹⁰ s^-1; energy matching between a higher excited singlet state, S_n, and a higher triplet, T_n, is implicated.