Volume 111, Issue 1, 01 July 1999
 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Ultraviolet photodissociation of furan probed by tunable synchrotron radiation
View Description Hide DescriptionThe photodissociationdynamics of furan at 193 nm have been studied using photofragment translational spectroscopy with tunable vacuum ultraviolet (VUV) probe provided by synchrotron radiation on the Chemical Dynamics Beamline at the Advanced Light Source. Three primary channels are observed: and The evidence suggests that the two closedshell channels occur on the groundstatepotential energy surface (PES) following internal conversion, while the radical channel likely takes place on an excited PES. All channels exhibit a barrier for dissociation with the channel having the largest value at about 25 kcal/mol. Angular distribution measurements show anisotropy only for the radical channel. These findings are consistent with a rapid excited state dissociation for the radical channel and slow dissociation for the other two pathways. The two groundstatedissociation channels— and acetylene+ketene—should be important in the thermal decomposition of furan as was found in pyrolytic studies [A. Lifshitz, M. Bidani, and S. Bidani, J. Phys. Chem. 90, 5373 (1986)] and theoretical investigations [R. Liu, X. Zhou, and L. Zhai, J. Comput. Chem. 19, 240 (1998)].

Positron and positronium chemistry by quantum Monte Carlo. IV. Can this method accurately compute observables beyond energy?
View Description Hide DescriptionMany different properties of the positron containing systems PsH, LiPs, and were computed using both variational Monte Carlo and fixed nodediffusionMonte Carlo methods, and explicitly correlated trial wave functions. Our results show that these techniques can accurately compute not only energy values, but also other observables. Our values for PsH, and LiPs are in good agreement with the most recent state of the art correlated calculations, while for our calculations are the first to give reliable results.

Energy partitioning in two kinds of NO molecules generated from the reaction of with Vibrational state distributions of “new” and “old” NO’s
View Description Hide DescriptionThe reaction of with produces two kinds of NO molecules, the “old” one which originally exists in and the “new” one which includes the attacking O atom. Using the isotopically labeled reagent, we determined the vibrational state distributions of these NO’s ( ) separately. To obtain the distributions, two types of experiments were performed with the laserinduced fluorescence(LIF) technique via the NO and transitions. First, the relative populations of NO molecules (the sum of the two kinds of NO’s) in levels were measured with unlabeled reagents. Then, isotopically labeled reaction, was utilized to determine the relative ratio between the two kinds of NO’s in the vibrational levels of and 12–15. Combining the above results with previously determined vibrational state distribution of NO in high vibrational levels [J. Chem. Soc., Faraday Trans. 94, 1575 (1998)], we were able to obtain a complete set of vibrational state distributions. It was found that the old NO dominantly populated in and 1 whereas the new NO extended its population toward higher vibrational levels However, in high vibrational levels, the old NO still have a considerable population due to the rapid energy transfer to the old NO. The observed efficient energy transfer to the old NO is attributed to the absence of light atoms in the present reacting system. Compared with the system including hydrogen atoms, the state density and the momentum coupling among the vibrational modes are much larger and accelerate the energy redistribution in spite of the short lifetime.

The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCl in the region, probed by infraredvisible double resonance overtone excitation
View Description Hide DescriptionWe use infraredvisible double resonance overtone excitation to promote HOCl molecules to single, wellcharacterized rotational levels of high OH stretching states just above the dissociation threshold on the ground potential energy surface. Double resonance spectra are monitored by laser induced fluorescence detection of the OH dissociation products. We present here the results obtained in the region of where we have studied states with J ranging from 4 to 25, from 0 to 5 and energy up to 300 cm^{−1} above the dissociation threshold. In the spectra for states, the zerothorder level is split by mixing with a nearby dark state. Because the two states have very different Arotational constants, their separation increases with but the effects of the mixing remain observable in the spectrum up to Comparison with preliminary results from together with analysis of the rotational constants, allows us to identify the perturbing state as (4,4,2). The lack of further strong perturbations compared to the average density of states allows us to infer that most of the matrix elements for couplings between the (6,0,0) bright state and other dark states are less than ∼0.1 cm^{−1}. The average intramolecular vibrational energy redistribution (IVR) rate implied by these matrix elements is two orders of magnitude longer than the predictions of statistical rate theory, indicating that IVR is likely to be the rate limiting step in the unimolecular dissociation process from (6,0,0). The present work provides the spectroscopic foundation for direct timeresolved studies of the unimolecular dissociation dynamics presented in a forthcoming paper.

Resonant twophoton ionization spectra of van der Waals complexes p, m,
View Description Hide DescriptionWe have studied the resonant twophotonionization (R2PI) spectra of three van der Waals (vdW) complexes p, m, through the transition with mass selectivity. The stretching frequencies of the three vdW complexes were found to be quite close (86 cm^{−1}). From the photodissociation mechanism and the relationship between anharmonicity of the stretching vibration and the dissociation energy, we estimated the dissociation energies of all the three vdW complexes in the and states. A quantum ab initio calculation on at the level gave the following geometry: The N atom of is located on the symmetry axis (Zaxis) and 3.53 Å above the benzene ring; the axis of is at an angle of 52.5° with the Zaxis of with one of the hydrogen atoms pointing towards the benzene ring; the rotation of around the Zaxis is nearly free. The calculated bond dissociation energies and the expectation of internal rotation are consistent with our experimental results.

Symmetry and structure of rotating
View Description Hide DescriptionWe present a global study of how the relative equilibria of the ion change as the angular momentumJ increases. A relative equilibrium is a classical trajectory for which the molecule rotates about a stationary axis without changing its shape. The study confirms previous results which show that the geometry of the minimum energy relative equilibria changes from an equilateral triangle to a symmetric linear configuration at around The series of bifurcations and stability changes that accompany this transition is presented in detail. New results include the discovery that the rotating equilateral triangle remains linearly stable for a large range of angular momentum values beyond the point where it ceases to be a minimum of the total energy. A third type of relative equilibrium, a rotating isosceles triangle, is also found to be linearly stable in the approximate range Both the equilateral and isosceles triangle configurations lose stability via Hamiltonian–Hopf bifurcations. The frequencies and symmetry species of the normal modes of the stable relative equilibria are computed and harmonic quantization is used to predict how the symmetries of the lowest lying quantum states will change as J increases. Energy level clustering due to tunneling between symmetryequivalent relative equilibria is described.

Doppler line shapes in the photolysis of laser excited, aligned molecules: Application to the vibrationally mediated photodissociation of
View Description Hide DescriptionA general expression for the Doppler profile for fragments produced in the photodissociation of laser excited, aligned molecules, as in vibrationally mediated photodissociation (VMP), is presented. In contrast to onephoton dissociation, for which the Doppler profile depends only on the second moment of the fragment molecularframe angular distribution, the profile in the case of VMP is sensitive to several moments of the angular distribution, up to In addition, the profile for a nearprolate molecule depends on the angle between the electronic transition moment μ and the a inertial axis. This theory is applied to the analysis and interpretation of Doppler profiles in the laser fluorescence detection of fragments, of rotational angular momenta and 10, from the 532 nm VMP of excited to the second N–H stretch overtone level For both Λdoublets of these rotational levels, the second moment of the molecularframe angular distribution was found to be positive, in agreement with previous results for highJ fragments from onephoton photolysis of The profiles are consistent with a value of ∼0° for the angle These values for and are inconsistent with simple expectations based on planar, prompt dissociation upon excitation to the lowest singlet excited state and suggest the importance of nonplanar geometries in the dissociation dynamics.

Compton profile study of bonding in BeO
View Description Hide DescriptionThe isotropic Compton profile of BeO has been measured using a γray Compton spectrometer which employs a 5 Ci annular source. The measurement is compared with the theoretical HFLCAO Compton profile published by Lichanot et al. The measurement has also been compared with our calculations based on the RFA model employing different ionic wavefunctions. It is seen that the experimental Compton profile is in very good agreement with the HFLCAO calculations. To determine the nature of bonding, equalvalenceelectrondensity Compton profile of BeO has been compared with that of the isovalent compound MgO. It is observed that the bonding in BeO is partially covalent and less ionic than MgO in agreement with the conclusions of the HFLCAO calculation.

The and cluster ions: Another example of luminescent clusters?
View Description Hide DescriptionDirac molecular orbital calculations on the octahedral and cluster ions are reported. These cluster ions have singlet ground state. The estimated clusters electronegativity, χ, and their HOMOLUMO energy gap suggest that is softer and more reactive than The calculated relativistic molecular orbitals indicate that the manifold of closely spaced unoccupied energy levels is mainly localized on the octahedral rhenium core with some contributions from the ligands, while all the cluster highest occupied molecular orbitals are largely centered on the apical halide ligands. The electronic features of their ground and excited states are similar to the intensely luminescent hexanuclear tungsten halide cluster ions. Our calculations suggest that the and cluster ions should be diamagnetic and luminescent.

The Rydberg states of : Spectra, structures, and interactions
View Description Hide DescriptionThe results of a critical reexamination of previous experimental observations of the Rydberg states of are employed to optimize a coupledchannel Schrödingerequation (CSE) model describing rovibronic interactions among a number of Rydberg and valence states. The results of calculations using this CSE model are compared with the experimental energylevel and predissociationlinewidth data base. As a result of the optimization process, relevant potentialenergy curves and Rydberg–valence, Rydberg–Rydberg, and valence–valence interactions are characterized for states in the eV region above the ground state of The precision of these characterizations and the interpretation of the experimental observations are found to be limited by the quality of the existing data, which exhibit a number of inconsistencies that cannot be reconciled without new, wellcharacterized experimental studies. In spite of these difficulties, the present CSE model, as currently parametrized, represents a significant improvement over previous work, enabling identification of a number of previously unassigned spectral features associated with perturbation of the state by the valence state and an anomalous J sensitivity in predissociation branching ratios for the level. In addition, the electronic structure and predissociation of the states for are found to be strongly influenced by Rydberg–Rydberg interactions.

Perturbations in the Rydberg states of : Bound–bound interactions with the second and valence states
View Description Hide DescriptionExisting experimental REMPI spectra for transitions into rotationally resolved levels of the and Rydberg states of have been rotationally analyzed, resulting in the first characterization of rotational perturbations in and In addition, the results of this analysis have been interpreted with the aid of a coupledchannel Schrödingerequation (CSE) model of the interacting electronic states. The identification and characterization of perturbations in the d and C states have allowed the nature of the interactions between the Rydberg states and the valence state to be clarified and a realistic empirical potentialenergy curve for the state to be determined. While it is found that first and secondorder interactions with the valence state are responsible for the strongest perturbations observed in and additional weak perturbations found in are shown to result from a secondorder interaction with the valence state. These weak perturbations, including an extra level observed for appear to be the first experimental evidence for the state, long predicted theoretically. Finally, detailed comparisons between experimental spectra and (2,0) and (3,0) REMPI spectra calculated using the CSE model are presented which support and illustrate these conclusions.

Ground state benzene–argon intermolecular potential energy surface
View Description Hide DescriptionA highly accurate ab initiointermolecular potential energy surface for the benzene–argon van der Waals complex is evaluated using the coupled cluster singles and doubles model including connected triple excitations [CCSD(T)] model with an augmented correlation consistent polarized valence double zeta basis set extended with midbond functions. The vibrational energy levels obtained by full threedimensional dynamical calculations are in excellent agreement with the available experimental data.

Ab initio study of the electronic transition in acetone: Symmetryforbidden vibronic spectra
View Description Hide DescriptionAb initio calculations of geometry and vibrational frequencies of the first singlet excited state of acetone corresponding to the electronic transition have been carried out at the level. The major geometry changes in this state as compared to the ground state involve CO outofplane wagging, CO stretch and torsion of the methyl groups, and the molecular symmetry changes from to The most pronounced frequency changes in the state are the decrease of the CO stretch frequency by almost 500 cm^{−1} and the increase of the torsion frequency from 22 to 170 cm^{−1}. The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for calculations of Franck–Condon factors. Transition matrix elements over the oneelectron electric field operator at various atomic centers calculated at the stateaverage level are used to compute vibronic couplings between the ground and Rydberg and electronic states, and the Herzberg–Teller expansion of the electronic wave function is applied to derive the transition dipole moment for as a function of normal coordinates. The results show that the intensity for this transition is mostly borrowed from the allowed transition due to vibronic coupling between and through normal modes and and, to some extent, from the transition due to (CO inplane bend) which couples with The calculated total oscillator strength for the transition through the intensityborrowing mechanism, is in close agreement with the experimental value of Ninetyfour percent of the oscillator strength comes from the perpendicular component inducing modes) and 6% from the parallel component modes). Calculated spectral origin, 30 115 cm^{−1} at the level, underestimates the experimental value by ∼300 cm^{−1}. Calculated positions of the most intense peaks in the spectra also reasonably agree with the experimental band maximum. The presence of numerous weak vibronic peaks densely covering a broad energy range (∼12 000 cm^{−1}) explains the diffuse character of the experimental band. Most of the bands observed in fluorescence excitation spectra [Baba and Hanazaki, Chem. Phys. Lett. 103, 93 (1983); Baba, Hanazaki, and Nagashima, J. Chem. Phys. 82, 3938 (1985)] can be assigned based on the computed spectrum.

Classicalquantum correspondence in multiphoton dissociation of diatomic molecules by chirped laser pulses
View Description Hide DescriptionThe classical and quantum dynamics of diatomic molecules driven by chirped laser pulses are investigated, with particular attention given to the dependence of the classicalquantum correspondence on the microscopic parameters of the systems. For this purpose, several molecules with different effective Planck’s constants are employed and their respective results are compared. Based on the bucket dynamics which has been successfully applied to explain the dissociation mechanism, we propose a criterion that determines whether a particular molecule will show a good correspondence between classical and quantum calculations in a given parameter region. It is found that, when the size of the bucket is bigger than the effective Planck’s constant, the classical predictions of dissociation probabilities agree well with the quantum mechanical results.

The disilaketenyl radical (HSiSiO) in its ground and first excited electronic states
View Description Hide DescriptionThe disilaketenyl (HSiSiO) radical, an isovalent isomer of the ketenyl (HCCO) radical, has been investigated theoretically using ab initioelectronic structure theory. For the two lowestlying electronic states ( and ) of HSiSiO, total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were predicted at the selfconsistentfield (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with a wide range of basis sets. At the CISD optimized geometries coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] energies were also determined. The ground and first excited electronic states of HSiSiO were predicted to be transplanar bent structures, while the linear state was found to be a saddle point with two imaginary vibrational frequencies. The and states of HSiSiO are more distorted from linearity and more polar than the corresponding states of HCCO. In particular the HSiSiO ground state is predicted to have a peculiarly acute HSiSi bond angle of only 75°, almost suggesting an Si–Si bridging hydrogen. At the CCSD(T) level of theory with the largest basis set, Dunning’s ccpVQZ, the first excited state was predicted to lie 36.3 kcal/mol (1.57 eV, 12 700 cm^{−1}) classically above the ground state. With the same method the barriers to linearity were determined to be 45.2 kcal/mol (1.96 eV, 15 800 cm^{−1}) for the ground state and 8.9 kcal/mol (0.39 eV, 3100 cm^{−1}) for the first excited state, respectively. Due to their large dipole moments and relatively large vibrational infrared (IR) intensities, the two lowestlying electronic states of HSiSiO may be suitable for IR spectroscopic studies, and the ground state for microwave spectroscopic investigations.

Magic numbers in transition metal (Fe, Ti, Zr, Nb, and Ta) clusters observed by timeofflight mass spectrometry
View Description Hide DescriptionWe have measuredtimeofflight(TOF)mass spectra of transition metal free clusters, ( Ti, Zr, Nb, and Ta and n is the number of atoms per cluster), produced by a laser vaporization source. The size resolved TOF intensities at 13, 15 are much higher than those at the neighboring n values for all Such specific n values are assigned to the magic numbers of these transition metalclusters and can be related to pentagonal bipyramid, icosahedron, and bcc structure units. The other magic numbers are observed for larger and 23 for Fe, and 25 for Ti, being attributable to the polyicosahedron. The TOFspectra of Nb and Taclusters are similar to each other and display the common magic number of

Intramolecular vibrational redistribution and fragmentation dynamics of clusters
View Description Hide DescriptionIntramolecular vibrational energy redistribution and fragmentation dynamics in and clusters is studied by hybrid quantum/classical techniques and the results are compared with experiments. A vibrational version of the molecular dynamics with quantum transitions (MDQT) treatment is used in which the vibrational degree of freedom of is treated quantum mechanically while all the other degrees of freedom are treated classically. The potential energy surface is represented as a sum of pairwise interactions with parameters taken from the literature. The calculated product state distributions are in very good agreement with the experiments. Fragmentation lifetimes were also calculated and agree reasonably well with those measured in timedependent experiments. Fragmentation proceeds via sequential ejection of Ne monomers through three different mechanisms: (i) sequential intramolecular vibrational redistribution plus vibrational predissociation (in which the molecule loses more than one quantum of vibration); (ii) direct vibrational predissociation (in which the molecule loses only one quantum of vibration); (iii) evaporation (in which the molecule remains in the same vibrational state).

Total and elastic crosssections for electron and positron scattering from OCS molecule: A comparative study with
View Description Hide DescriptionWe have carried out a joint experimental and theoretical study on determination of total and elastic crosssections for electron and positron impact on OCS molecules. For total crosssection measurements, impact energies are from 0.7 to 600 eV for electron collisions and 0.8 to 600 eV for positron collisions. For elasticscattering, only electron impact has been studied from 1.5 to 100 eV. Our present measurements for total crosssections for electron impact are found to agree extremely well with the measurements of Dababneh et al. (1982) in the entire energy region studied. The results by Szmytkowski et al. (1994) are consistently larger by 20% above 2 eV, and, in particular, are larger by a factor of two in the 1.2 eV resonance region. The present theory for elasticscattering is in good agreement with the present measurement, and has been employed for understanding the dynamics. The total crosssection recommended here is probably the best in accuracy as of today although further studies would be helpful.
