The photofragmentation of rovibrational energy-level selected and magnetic-state polarized (X¹A₁) CD₃I was performed at 266 nm. The relative NK) rotational energy-level populations and the angular momentum polarization of the vibrationless (X²A₂') CD₃ photofragment were measured by (2 + 1) resonance-enhanced multiphoton ionization. The magnitude and relative phase of the transition dipole matrix (or T-matrix) were determined by relating the initial system (CD₃I plus photon) alignment to the experimentally measured alignment for the CD₃ fragment. This is believed to be the first reported measurement of the T-matrix elements for a chemical reaction. This is significant since the transition dipole matrix is the most fundamental observable for a photochemical reaction and as such it is the definitive quantity with which to judge the viability of various theoretical models used to describe photochemical reactions.

The quantum state-counting phase space theory commonly used to describe 'barrierless' dissociation is recast in a helicity basis to calculate photofragment v(DOT)j correlations. Counting pairs of fragment states with specific angular momentum projection numbers on the relative velocity provides a simple connection between angular momentum conservation and the v(DOT)j correlation, which is not so evident in the conventional basis for phase space state counts. The upper bound on the orbital angular momentum, l, imposed by the centrifugal barrier cannot be included simply in the helicity basis, where l is not a good quantum number. Two approaches for a quantum calculation of the v(DOT)j correlation are described to address this point. An application to the photodissociation of NCCN is consistent with recent classical phase space calculations of Klippenstein and Cline. The observed vector correlation exceeds the phase space theory prediction. We take this as evidence of incomplete mixing of the K states of the linear parent molecule at the transition state, corresponding to an evolution of the body-fixed projection number K into the total helicity of the fragment pair state. The average over a thermal distribution of parent angular momentum in the special case of a linear molecule does not significantly reduce the v(DOT)j correlation below that computed for total J equals 0.

Ionization-detected absorption spectra of autoionizing Rydberg series converging to (100), (010), (02⁰0), and 02²0) vibrational states of NO₂⁺ have been recorded in three-color triple resonant excitation. Resonances associated with decay by relaxation in symmetric stretch are found to be much broader than those observed for the core excited in one quantum of the bending vibration, consistent with earlier findings on mode- specificity of vibrational autoionization in this system. Series excited in (02⁰0), on the other hand, are found to behave much more like (100) than (010), indicating that the bending specificity that characterizes the fundamental is lost in the overtone. This loss of mode specificity with increasing vibrational energy is ascribed to (100) - (02⁰0) Fermi resonance in the NO₂⁺ core, exemplifying how even small vibrational coupling can redirect mode-specific fragmentation pathways in polyatomic molecules.

Hexapole state-selection and orientation of parent molecules is combined with two-dimensional ion imaging of photofragments to study the direct photolysis of deuterated methyl iodide molecules (CD₃I). These two techniques allow us to create an essentially single quantum state-selected beam of oriented molecules, which are subsequently photodissociated, and to measure the final state-, velocity-, and angle-resolved recoil distribution of fragments. For the prompt dissociation of CD₃I at 266 nm the angular recoil distribution reflects predominantly the initial spatial orientation of the state-selected parent. The dependence of the orientation of the parent as a function of the orientation fieldstrength has been investigated and the three-dimensional recoil distributions of fragments are compared with theoretically calculated distributions.

The lab frame correlation between the recoil of electron and daughter ions from dissociative photoionization of individual molecules can be observed using imaging and coincidence techniques. Since, however, each may have some preferred angular distribution in the anisotropic molecular field (which will depend on the specific dynamics of the fragmentation processes), their recoil directions will be mutually correlated with the molecular coordinate frame, permitting the molecule frame photoelectron angular distribution to be inferred. This effectively allows free, oriented species to be studied in the gas phase. A feature of these experiments is that by establishing molecular orientation (and not just alignment) odd harmonics of the electron distribution are revealed. It is shown that a reversal of orientation in the angular distribution can provide a clear signature of the existence of a shape resonance without requiring observation to be made directly at the resonant energy.

Starting from different fine structure levels in the metastable c³II_u^- triplet state of molecular hydrogen, we excite ro-vibrational levels in the n equals 3 triplet gerade complex. The linear polarization of the laser results in an alignment of the molecular plane with the polarization vector. This is then reflected in an anisotropy of the photofragments. We show that the anisotropy parameters are influenced by the presence of the electron spin, which causes the fine structure splitting. We compare our results to theoretical expressions for the anisotropy parameter which incorporates the fine structure of the initial and final states.

The introduction of the pulsed field ionization zero kinetic energy photoelectron spectroscopy technique (referred to as PFI-ZEKE spectroscopy) has resulted in a revolution in photoelectron spectroscopy, because of the tremendous improvement in resolution. This method of threshold photoelectron spectroscopy is based on field ionization of metastable high principal quantum number Rydberg states using a pulsed electric field, delayed from the laser excitation. The detailed mechanism for stabilization of the high principal quantum number Rydberg states has been the subject of a great deal of recent discussion in the literature, and is still somewhat controversial. It is well known that Rydberg state lifetimes scale as n^-3, for fluorescence, autoionization, or predissociation, under ideal conditions. This means that for a Rydberg series that can decay by autoionization, if the lifetime of a 5p Rydberg state is 10^-12 s, the lifetime of a 150p state will be 10^-7 s, which is an order of magnitude shorter than typical delay times used in PFI-ZEKE. The 150p state will be field ionized by an electric field of 0.7 to 1.5 V/cm, which is typical of the pulsed fields used for Stark ionization. This question about Rydberg state lifetimes becomes quite important if one wishes to carry out PFI-ZEKE spectroscopy of ion states well above the lowest ionization threshold, as many decay channels will be available to the Rydberg states converging to the high energy states, resulting in shorter lifetimes for these high energy Rydberg states. Our work in this area has focused largely on PFI-ZEKE spectroscopy at excited state thresholds in molecular ions, where problems of autoionization will be most severe. To reach these high energy thresholds, we have usually used single photon excitation with coherent vacuum ultraviolet light. This excitation method has many advantages.

High resolution photoelectron spectroscopy is applied to the study of molecular clusters. The primary species studied are fluorene-Ar_n complexes. Spectroscopy of the neutral S₁ state has been performed on clusters as large as n equals 30. In order to study the photoelectron spectra of the clusters size selectively mass analyzed threshold ionization (MATI) is used which is a mass resolved version of the ZEKE technique. MATI spectroscopy has been applied to clusters up to n equals 5. The spectral shifts in the S₁ origin and ion threshold are used as a measure of the relative stability of the different clusters. Using previous experimental and theoretical work on related clusters the structures of the clusters are inferred from the observed spectral shifts. In some cases multiple conformations of a particular cluster size are identified.

Mass selected pulsed field threshold ionization of optically excited high Rydberg states is a new method for the production of state-selected molecular and cluster ions. Exclusive detection of state-selected threshold ions is achieved by the kinetic energy analysis of ions in a reflectron mass spectrometer. This leads to vibrationally resolved spectra of molecular ions, like benzene, xylene, carbazole, etc. and ionic complexes of these molecules with noble gas atoms. The detected threshold cluster ions are state-selected with different internal energies depending on the excited vibrational state. Above a certain vibrational excess energy their decay is observed by the disappearance of vibrational peaks in the threshold ion spectrum at the parent mass and the simultaneous appearance of threshold daughter ions. Upper and lower bounds for the dissociation energies of the neutral and ionic complexes are deduced and compared with recent theoretical results. The coherent interaction of two narrowband Fourier- transform limited nanosecond laser pulses with gas phase molecular systems leads to the new technique of coherent ion dip spectroscopy (CIS) for rotationally resolved spectroscopy of polyatomic molecules and clusters. It is based on coherent effects with a special time sequence of the two pulses and yields a population dynamic in a three level system which is different from that of incoherent excitation experiments: At resonance, for different time sequences of the two pulses no or a complete population transfer from an initial to a final state is achieved and nearly 100% deep ion dips are observed in spectroscopic investigations.

A crossed beam chemical reaction, Al(²P_{1/2, 3/2}) plus O₂(X ³(Sigma) _g^-) yields AlO(X ²(Sigma) ⁺) plus O(³P_j), has been investigated by fluorescence imaging techniques. It has been found experimentally that only the ground spin-orbit state, i.e., Al atoms in the ²P_1/2 level are reactive towards O₂ molecules. An electrostatic interaction model is examined to reveal the dynamics in the region of large separation between the two reactants. It is shown that Al atoms in the ²P_3/2 spin-orbit state experience repulsive potential energy surfaces and are non-reactive. In the entrance valley of the allowed channel, Al atoms in the ²P_1/2 spin-orbit state are governed by a barrierless, attractive potential energy surface. From the microscopic mechanism of the Al plus O₂ reaction, its activation energy has been calculated to be -0.19 kcal/mol, in accordance with experimental measurements.

A preliminary report is presented on experiments using fast ion-beam translational energy spectroscopy to study dissociative photodetachment and photodissociation dynamics in the small cluster ions O₄^- and O₂^-(DOT)(H₂O). Translational energy and angular distributions of coincident molecular fragments were recorded from the photodestruction of O₄^- and O₂^-(DOT)(H₂O) at 523, 349, and 262 nm. At each wavelength, the O₄^- results confirm the existence of at least two distinct channels: dissociative photodetachment (O₄^- + h(nu) yields O₂ + O₂ + e^-) and photodissociation (O₄^- + h(nu) yields O₂ + O₂^-). Observation of strongly anisotropic angular distributions shows that dissociation occurs on the time-scale of molecular rotation in both processes. The photodissociation of O₄^- at 523 nm gives a new value for the O₂-O₂^- bond energy, D_O equals 0.39 +/- 0.05 eV. In O₂^-(DOT)(H₂O), a single dissociative photodetachment channel (O₂^-(DOT)(H₂O) + h(nu) yields O₂ + H₂O + e^-) is observed at all wavelengths. Angular distributions from this process are slightly anisotropic and exhibit a small wavelength dependence.

Vibronic state specific radiative lifetimes and rate constants for electronic quenching are reported for four different vibrational modes of CH₂CHO(B²A'). Vinoxy radicals are produced in the ground state by 193 nm excimer laser photolysis of methyl vinyl ether, and irradiated between 332 and 348 nm to generate the second excited state. Bimolecular quenching rate constants are determined in the presence of twelve collision partners: He, Ar, N₂, O₂, CO, H₂, HCl, N₂O, CO₂, C₂H₄, CH₃OCH equals CH₂ and SF₆. Radiative lifetimes are found to vary from 98 plus or minus 10 to 154 plus or minus 18 ns, depending on the vibronic level. Some possible mechanisms for lifetime shortening and collisional quenching are discussed.

The reaction of chlorine atoms with selectively-deuterated butanes to form HCl and/or DCl under single-collision conditions has been studied. Photolysis of Cl₂ with 351 or 308 nm light produces Cl atoms that subsequently abstract an H or D atom to form the corresponding hydrogen chloride product. HCl and DCl are detected via 2 plus 1 multiphoton ionization. For both n-butane and isobutane when viewed on a per hydrogen atom basis, the 'middle' site is preferred. When viewed on a per carbon site basis, this propensity is still observed in n- butane, but isobutane shows no dominant site specificity. Mechanistically, the data can be interpreted qualitatively in several ways, but currently the most convincing argument suggests that a direct, impulsive encounter is involved.

The gas phase photodissociation of the cyclic sulfoxides, (CH₂)_nSO (n equals 2, 3, 4), has been studied following 193 and 248 nm photolysis. The nascent vibrational state distribution of the SO(X³(Sigma) ^-) photofragment following photolysis of each cyclic sulfoxide contains detailed information about the photochemical mechanisms. The SO vibrational state distributions have been measured by laser induced fluorescence spectroscopy on the (B³(Sigma) ^- -- X³(Sigma) ^-) transition, after 193 and 248 nm photodissociation of each cyclic sulfoxide. The observed vibrational state distributions are compared with physical models to help reveal the fragmentation mechanisms.

Resonant ion-dip infrared spectroscopy (RIDIRS) is used to record near-infrared spectra of several size-selected (benzene)_m(solvent)_n molecular clusters. In benzene-H₂O and benzene-HOD, large-amplitude motions of the water molecule on the benzene ring produce unusual OH stretch infrared absorptions. In the former complex, seven resolved transitions appear in the OH stretch region, rather than the two expected of a rigid complex. In C₆H₆-HOD, several combination bands involving internal rotation and in-plane torsion of the HOD have intensities greater than the OH stretch fundamental. Similar spectra of the (C₆H₆)₂-H₂O complex show that such large-amplitude motions are effectively quenched by the presence of the second benzene molecule. RIDIR spectra of C₆H₆-(CH₃OH)_n with n equals 1 - 5 are also presented. In C₆H₆-(CH₃OH)₃, the OH stretch infrared spectrum provides clear evidence that the three methanol molecules attach to benzene as a hydrogen-bonded chain. This occurs despite the fact that the lowest energy structure for the free methanol trimer is a cyclic structure, indicating that the presence of benzene has caused a structural rearrangement of solvent molecules near it.

Clusters composed of solvated ions provide a valuable prototype system for studying molecular aspects of solvation. Vibrational spectroscopy can provide insight into how the solvent structure around an ion evolves with cluster size. We describe studies which illustrate the effects of adding small numbers of solvent molecules to anions. Halide ions are among the most ubiquitous and fundamental anions in aqueous chemistry. Recent calculations and photoelectron spectroscopy experiments suggest that hydrated halide ion clusters X^-(H₂O)_n have structures in which the anion binds to the surface of water clusters, rather than being surrounded by solvent water molecules (so-called interior states). We have observed infrared spectra for a series of solvated chloride ions which can be assigned with the aid of ab initio calculations. We find that for the chloride ion the water molecules tend to associate, but that water-water hydrogen bonds are not necessarily formed at the smallest size. Our results suggest the importance of entropic factors in these floppy clusters. Evidence is also found for a possible transition to liquid-like structures at a critical cluster size.

The size-dependent reactivity of several transition-metal clusters: Co_n, Nb_n, Rh_n, and W_n with CO has been investigated in a cluster beam experiment. The reactions occur at single-collision-like conditions and the results are evaluated in terms of the reaction probability (S) in a collision. For all the four metals, clusters with more than 10 - 15 atoms show a high reaction probability, S >= 0.4, rather independent of size. For smaller Nb_n and W_n, the reaction probability is lower, and for Nb_n, large variations in the CO reactivity are observed in the n equals 8 - 13 range with a distinct minimum at Nb₁₀. Using an LCAO approach within the local spin density approximation (LSDA) the adsorption of molecular CO on Nb_n has also been investigated theoretically. The geometries of the bare clusters were optimized and two different sites for CO were investigated. The discussion is based on a detailed analysis of Nb₄. The calculations show that compact structures with high coordination numbers are the most stable ones for the bare Nb clusters and hollow sites, also maximizing the coordination, are preferred for CO adsorption. The calculations indicate that a high CO-Nb_n bond strength is obtained for clusters with a high density of states close to the Fermi level and for which the HOMO level has a symmetry that allows for an efficient back-donation of electrons to the 2(pi) ^*-orbital of CO. A particularly low chemisorption energy was calculated for the Nb₁₀ cluster.

This paper presents an overview of our recent work on the excited-state dynamics of the neutral clusters of naphthalene and related aromatic hydrocarbons, as probed by laser-induced fluorescence and mass-selected multiphoton ionization in a supersonic jet.

A broad picture is emerging of the role played by methyl internal rotation on intramolecular vibrational redistribution (IVR) among ring modes of the benzene ring and on IVR between the ring modes and the van der Waals modes of complexes. The studies are based on the benchmark system p-difluorobenzene (pDFB) for which IVR at 300 K from many excited electronic state initial levels has been characterized long ago by oxygen quenching (chemical timing) of fluorescence. Replacement of a fluorine with a methyl group to make p- fluorotoluene (pFT) provides the methyl internal rotation that has strong interaction with the ring. A view of the methyl rotor effect on vibrational predissociation (VP) is obtained by studies of van der Waals complexes between these aromatics and Ar. The VP characteristics are qualitatively changed when a methyl internal rotation is introduced onto the ring. VP from lower initial levels of the pFT complex loses the high vibrational channel selectivity that is the signature of all other aromatic complexes. Those changes are traced back to an IVR process involving both ring modes and van der Waals modes that precedes the dissociation.

We report the first application of a multiphoton ionization based, mass selective implementation of psec time resolved rotational coherence spectroscopy (RCS) to molecular clusters. This implementation of rotational coherence spectroscopy retains all of the advantages of fluorescence based implementations of RCS and also allows determination of the moments of inertia of molecular species which cannot be size selected via excitation energy or which do not fluoresce. The method is described and rotational coherence transients obtained are presented.

The photolysis of some hypohalous acids and related molecules has been investigated by observing the OH rotational distributions by laser induced fluorescence and Cl atoms by REMPI and time-of-flight techniques. The results are consistent with a rapid and direct dissociation process with most of the available energy going into relative translational motion of the products. Only a small degree of rotational and vibrational excitation of the OH fragment is observed.

OH photofragments generated from the 220 nm photolysis of HO₂ are interrogated using laser induced fluorescence. Internal energy distribution measurement combined with Doppler line shape analysis confirm that the excited electronic state of HO₂ accessed at 220 nm is of A' symmetry and that 85 plus or minus 10% of the oxygen atoms resulting from the photodissociation are formed in their ¹D excited electronic state. Very little of the 23,200 cm^-1 available energy (less than 2%) appears as internal excitation of the OH fragment.

Two-color resonant four-wave mixing (RFWM) shows great promise in a variety of double- resonance applications in molecular spectroscopy and chemical dynamics. One such application is stimulated emission pumping (SEP), which is a powerful method of characterizing ground-state potential energy surfaces in regions of chemical interest. We use time-independent, diagrammatic perturbation theory to identify the resonant terms in the third- order nonlinear susceptibility for each possible scheme by which two-color RFWM can be used for double-resonance spectroscopy. After a spherical tensor analysis we arrive at a signal expression for two-color RFWM that separates the molecular properties from purely laboratory-frame factors. In addition, the spectral response for tuning the DUMP laser in RFWM-SEP is found to be a simple Lorentzian in free-jet experiments. We demonstrate the utility of RFWM-SEP and test our theoretical predictions in experiments on jet-cooled transient molecules. In experiments on C₃ we compare the two possible RFWM-SEP processes and show that one is particularly well-suited to the common situation in which the PUMP transition is strong but the DUMP transitions are weak. We obtain RFWM-SEP spectra of the formyl radical, HCO, that probe quasibound vibrational resonances lying above the low threshold for dissociation to H+CO. Varying the polarization of the input beams or PUMP rotational branch produces dramatic effects in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well described by our theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation confirm the predicted lineshape and give widths that are in good agreement with those determined via unsaturated fluorescence depletion SEP.

The nonlinear optical technique of degenerate four-wave mixing (DFWM) is shown to be an effective tool for the interrogation of nascent product molecules generated during a photodissociation event. By combining an absorption-based, beam-like response with the sensitivity afforded by full resonant enhancement, this scheme provides an attractive alterative to the ubiquitous laser-induced fluorescence (LIF) methodology. DFWM spectroscopy has been used to probe the unrelaxed hydroxyl (OH) radicals formed upon 266 nm photolysis of hydrogen peroxide (HOOH), with a sub-Doppler experimental configuration enabling extraction of both scalar and vector properties. In particular, the rovibrational population distribution of the ground electronic state fragments, as well as the spatial alignment of their rotational angular momenta, have been measured. These results are compared with those obtained in previous LIF studies, thereby demonstrating the utility of four-wave mixing techniques for the quantum state-specific characterization of nascent reaction products.

The scattering of vibrationally and rotationally state-selected H₂ from Cu(110) and Cu(111) surfaces has been studied. Stimulated Raman scattering is used to pump a fraction of the incident flux in a supersonic beam of H₂ into the J equals 1 rotational state of the first excited vibrational state. Resonance-enhanced multiphoton ionization (REMPI) is used to detect incident and scattered H₂ in this state. Time-of-flight spectra are recorded as a function of the distance from the REMPI probe to the surface and scattering angle. These data are fit to a model whose parameters describe the velocity and angular distribution of the scattered molecules. The model permits a determination of flux from the density-sensitive measurements. Its use to derive a quantitative estimate of the absolute survival probability for the pumped state is discussed.

A recently developed implementation of photofragment spectroscopy, based on specific properties of infrared multiphoton dissociation, has been applied for overtone spectroscopy of the CH stretch vibration in CF₃H. The selectivity and sensitivity of this detection technique, called infrared laser assisted photofragment spectroscopy (IRLAPS), is high enough to combine it with both cooling in a supersonic expansion and infrared-near infrared double resonance excitation. This combination allows one to perform spectroscopy of high vibrational overtones of polyatomic molecules prepared in single J rotational states. The resulting simplification of the overtone spectra of the 4₂ vibrational level in CF₃H provides a more accurate estimation of a timescale of intramolecular vibrational energy redistribution from CH chromophore group to the rest of the molecule.

Fluorine substitution can have a major effect on the electronic spectra and sometimes on the ground electronic state conformations of organic compounds. In this work we investigate the effect of perfluorination on the resonance Raman spectra of the simple diene, hexafluorobutadiene where the preferred ground state geometry is believed to be skew s-cis. A detailed comparison of theory with experiment is possible and excellent agreement is observed. In an ongoing study, we investigate the effect of fluorination on the photodissociation dynamics of methyliodide by investigation of CF₃I. The activity of the CH₃ umbrella mode observed in the resonance Raman spectrum of CH₃I obtained in resonance with the dissociative A state is attributed to a change in the geometry from pyramidal to planar as the C-I bond elongates. This should not be observed in the case of CF₃I since CF₃ is pyramidal. The consequences of this fluorine effect on the photodissociation product as revealed in the resonance Raman spectrum are investigated.

The ability of iron ions to activate methane is studied in detail by using a guided ion beam tandem mass spectrometer. State-specific results for ground state Fe⁺(⁶D) and first excited state Fe⁺(⁴F) with methane are determined by varying the source conditions for Fe⁺. A more complete picture of the potential energy surface for this reaction is obtained by examining one of the reverse processes, FeCH₂⁺ plus H₂. The influence of ligands on the metal reactivity is also investigated for this reaction system. The reactions of methane with both FeCO⁺ and Fe(H₂O)⁺ are examined with the latter exhibiting much greater reactivity. This result is attributed to the more favorable thermodynamics associated with the electronic configuration on Fe⁺ as induced by the H₂O ligand.

Laser excitation is used to create small polyatomic ions with excitation in selected vibrational modes. These are formed into a beam and collided with a neutral target gas. Reactions are studied and product recoil energy distributions are measured as a function of collision energy and vibrational state. A wide variety of dynamical behavior is observed: strong mode-selective vibrational effects, severe steric bottlenecks, mode-dependent barrier crossing, complex lifetimes ranging from 10 microsecond(s) ec to 100 fsec, and impact parameter dependent mechanism switching.

A new method for studying state-selected bimolecular ion-molecule reactions is presented here, based on the technique of zero-kinetic-energy (ZEKE) photoelectron spectroscopy. State selection of the molecular ions is achieved by two-color laser excitation to high Rydberg states, followed by pulsed-field ionization of the Rydberg molecules. The ions are produced in unique vibration-rotation levels with 100% discrimination against other ions present. They are formed in a supersonic beam and accelerated so as to react with other neutral molecules present in the beam, with controlled collision energy variable in the range 10 meV to 1 eV. The product ions are detected using a quadrupole mass-filter, and the reaction probability is determined as a function of collision energy and of reactant ion quantum state by measuring the product ion/parent ion ratio. State selection of N₂⁺, NO⁺, CO⁺ and H₂⁺ in a range of rotational levels has been achieved, all in the vibrational ground state, and preliminary measurements of the reaction H₂⁺ plus H₂ yields H₃⁺ plus H have been made. The study of other reactions is currently in progress. Some new observations have also been made in the same apparatus concerning the decay dynamics of high Rydberg states of N₂ which are of relevance to the state-selection process. The dynamics are followed as a function of principal quantum number and the effects of a weak electric field in promoting stabilization are observed.

Significant technical advances have been made in the application of the triple-quadrupole double-octopole mass spectrometer for absolute total state-selected cross section measurements of ion-molecule reactions involving O⁺(⁴S degree(s), ²D degree(s), ²P degree(s)). By controlling the collision energies for dissociative charge transfer collisions of He⁺ (Ne⁺, Ar⁺) plus O₂ in a radio frequency (rf) octopole ion guide gas cell, and by applying appropriate effective ion trapping potentials to the rf octopole ion guide, we have demonstrated that state-selected O⁺(⁴S degree(s)), O⁺(²D degree(s)), and O⁺(²P degree(s)) reactant ion beams with high purities and usable intensities can be prepared for scattering experiments.

Acetaldehyde supersonically cooled was excited to the S₁ state with a UV laser in the wavelength range of 300 - 319 nm. Fragment HCO was detected by laser-induced fluorescence at the transition of B - X O₀⁰. The appearance rate of HCO was detected by probing the ^QR₀ (10) transition. The formation threshold of HCO is determined to be 317.5 nm from the onset of the measured appearance rate. No vibrational promoting mode was found to produce HCO. Emission decay curves of excited acetaldehyde were recorded in the same wavelength region. The time constant of the slow component of acetaldehyde emission starts to decrease at an excitation wavelength 317.5 nm near the formation threshold of HCO. The decay rate of this slow component is approximately equal to the appearance rate of HCO at the same wavelength. Strong interaction between the S₁ and T₁ states in acetaldehyde is implied by these experimental results.

The rotationally resolved spectrum of the HCCl A¹A' implied by X¹A' transition at near infrared (IR) wavelengths was obtained at Doppler- limited resolution using a combination of transient absorption and frequency modulation (FM) techniques. The analysis of the observed spectrum reveals perturbations arising from both the Renner-Teller effect and the singlet-triplet interaction between these states and the low-lying a³A' state as well as information concerning the geometry and the vibrational structure in the excited singlet state.

Resonance Raman spectroscopy under conditions of resonance with the B_1u state of benzene exhibits strong binary overtone activity due to the vibronic activity of the e_2g vibrations, especially (nu) _g. Resonance with the analogous L_a state of benzene derivatives exhibits activity in both the fundamental and two-quantum overtone of this mode in a fashion that reflects the relative amounts of vibronic and substituent induced allowed character of the electronic transition. The Raman spectra can be used to test simple perturbative ideas concerning inductive effects originally proposed by Petruska. In this work we describe recent theoretical and experimental studies of such effects. The theory includes the Jahn-Teller and pseudo-Jahn-Teller effects in a non-perturbative fashion. The experiments are aimed at determining what happens when the interaction of the substituent with the benzene (pi) -electron system is strong so that perturbative arguments are no longer expected to apply.

We present a theoretical-computational method for studying the excited structure and dynamics of conjugated (pi) -electron systems using spectra calculations, a normal mode analysis and a theoretical derivation of the correct Hamiltonian for constrained systems. We illustrate the method for the example of benzophenone. It is possible to reproduce very well the S₁ implied by S₀ absorption spectrum using a model with only four active modes, which we identify as (nu) ₆₄, (nu) ₆₂, (nu) ₂₇, and (nu) ₂₅. We present compelling evidence for the activity of a symmetric bending mode ((nu) ₆₂) with frequency of approximately 140 cm^-1 in the excited state. This result contrasts previous work based on models with activity in the asymmetric twist with frequency of approximately 70 or 100 cm^-1. We analyze the effects of deuteration and fluorine substituents, and show how the results could be used for an experimental test of the identity of the optically active modes.

The jet-cooled fluorescence excitation spectra and dispersed fluorescence spectra of trans- stilbene (t-S) and 4-methoxy-trans-stilbene (MS) have been recorded and analyzed. Preliminary results for 4,4'-dimethyl-trans-stilbene have also been obtained. The high- temperature vapor-phase and liquid phase Raman spectra of these molecules along with cis- stilbene have also been recorded. The low-frequency vibrational modes have been assigned, and the vibrational progressions associated with the two phenyl torsions and the ethylinic internal rotation, which primarily governs the photoisomerization process, have been analyzed. For t-S and MS the two-dimensional phenyl torsion potential energy surfaces and the one-dimensional carbon-carbon double bond torsional potential energy functions for both the S₀ and S₁((pi) , (pi) *) states were determined. The barriers to simultaneous phenyl internal rotation are 3100 cm^-1 for the ground state and 3000 cm^-1 for the excited state. The data for the C equals C torsion are consistent with a trans yields cis barrier of 48.3 kcal/mole for the S₀ state and a trans yields twist barrier of 2000 to 3500 cm^-1 for the S₁ state. For MS the barrier for simultaneous phenyl torsion for both the S₀ and S₁ states is 2860 cm^-1. The C equals C torsional barrier for MS in both the S₀ and S₁ states is approximately 10% higher than that of t-S.

The dynamics of NO₂ dissociation at 309.1 nm have been explored by examining the nascent distribution of NO rotational, vibrational, spin-orbit and lambda-doublet states. The NO fragment is produced with a monotonically decreasing vibrational distribution over the energetically accessible vibrational states ((upsilon) equals 0 minus 3), and non-statistical rotational distributions within each vibrational manifold. The distribution within (upsilon) equals 0 and 1 is strongly peaked near J equals 25.5 with a fairly narrow spread; the distribution within (upsilon) equals 2 is fairly flat, terminating at the limit of available energy; and the (upsilon) equals 3 distribution is oscillatory, also terminating at the limit of available energy. The ²(Pi) _1/2 spin-orbit state is more strongly populated than the ²(Pi) _3/2 state by a factor of 1.9 for every vibrational state. The differences in lambda-doublet populations are in general minor; each (Lambda) -state being roughly equally populated, although oscillations are again evident. It is found that the results are intermediate between the previous data at low excess energy and at high available energy; the distributions showing aspects of both regimes. From the data it is inferred that the dissociation dynamics of NO₂ vary continuously from a regime where phase space theory considerations with quantum overtones dominate the product state distributions to the regime where dynamics on the exit channel determine the distributions.

The photodissociation dynamics of jet-cooled formaldehyde, acetaldehyde, and propionaldehyde have been investigated at wavelengths of 308 nm or 309 nm by monitoring the resultant nascent HCO fragments by laser induced fluorescence excitation spectroscopy (LIF). For acetaldehyde and propionaldehyde, the distribution of energy in the HCO fragment was deduced directly from intensities in the rotationally resolved spectrum while the Doppler widths of selected rotational lines provided an estimate of the translational energy release. This has revealed that for both reactions, at these excitation wavelengths, most of the excess energy is deposited as translational energy in the two fragments (80 - 90%) and rotation of the HCO fragment (10 - 20%) with apparently minimal internal energy deposited in the alkyl fragment. The experimental observations indicate that the radical channel photodissociation mechanisms for acetaldehyde and propionaldehyde are similar. The reaction occurs following intersystem crossing onto the lowest lying triplet surface. On this surface there is a loose transition state above a relatively high, late barrier. The fixed energy of the exit channel appears to be converted almost solely into translation and HCO rotation. Very preliminary results are also presented for the 309 nm photolysis of formaldehyde. For this system, the HCO spectrum demonstrates clearly a dominance of K_a equals 1 states over K_a equals 0 states. This is consistent with ab initio calculations which place the departing hydrogen atom almost perpendicularly above the carbon atom at the triplet transition state. However the results are inconsistent with the singlet transition state geometry which is planar.

The photodissociation dynamics of CF₂Br₂, producing the CF₂ radical, have been studied at six photolysis wavelengths: 218, 223, 238, 246, 266, and 274 nm. For a photolysis wavelength of 246 nm, the average energy of each of the CF₂ degrees of freedom was measured as E_vib equals 0.4 kJ mol^-1, E_rot equals 2.4 kJ mol^-1, and E_trans equals 25 kJ mol^-1 for a total average CF₂ energy of 28 plus or minus 6 kJ mol^-1. The distribution of rotational states showed a preference for low K_a. Three possible pathways were considered for the formation of CF₂: (1) CF₂Br₂ plus h(nu) yields CF₂ plus Br₂; (2) CF₂Br₂ plus h(nu) yields CF₂ plus 2 Br; and (3) CF₂Br₂ plus h(nu) yields CF₂Br plus Br; CF₂Br plus h(nu) yields CF₂ plus Br. It was found that neither reaction (1) nor reaction (3) satisfied conservation of energy given the observed energy distribution in CF₂. Hence reaction (2) was concluded as responsible for the formation of the CF₂. Possible mechanisms for this reaction were considered, including stepwise and concerted production of the three fragments. However, neither mechanism was found to be in accord with the experimental observations. The concerted mechanism seemed to fail to account for the observed J/K correlation of the nascent CF₂ (favoring in-plane rotation). The stepwise reaction is not consistent with the non-statistical behavior of the nascent product. A concerted mechanism was re-evaluated based on low level ab initio quantum mechanical calculations. These calculations showed that the lowest energy transition for CF₂Br₂ reaction is planar. If the elimination of the two bromine atoms occurs in-plane, asymmetrically, then this geometry may be used to describe the experimental results.

Strongly aligned samples of C₂H₂ were prepared in several rotational states of the 2¹ level by stimulated Raman pumping. The effects of acetylene self-collisions on alignment have been studied in detail. Laser induced fluorescence was used to verify the initial degree of alignment and follow its decrease via collisions. Measurements of the initially excited alignment agree well with theoretical calculations of that achievable by stimulated Raman pumping and the decay can be modeled by a simple kinetic scheme based on the selection rule (Delta) M equals 1. As the rotational quantum number increases, there is decrease in the alignment decay rate, but a rapid increase in collisional transfer from a particular M_J level to its nearest neighbors. The physical basis for these trends is discussed. Transfer of alignment to other angular momentum levels by inelastic collisions has been observed when (Delta) J equals 2, but is absent when (Delta) J is greater than 2. These experiments can be analyzed to yield an average angular change during a collision where (Delta) J equals 2.

We have studied the dynamics of ultrafast internal conversion processes using femtosecond time-resolved photoionization and photoelectron spectroscopy. In hexatriene, following femtosecond pulse excitation at near 251 nm, we use time-delayed photoionization to observe the formation and decay of an intermediate species on the subpicosecond time scale. With time-resolved photoelectron spectroscopy, the rapid evolution of vibrational excitation in this intermediate is observed, as electronic energy is converted to vibrational energy in the molecule. The photodynamics of cis and trans isomers of hexatriene are compared and found to be surprisingly different on the 2 - 3 psec time scale. These results are important for understanding the fundamental photochemical processes in linear polyenes, which have served as models for the active chromophores of many biological photosystems.