Volume 111, Issue 6, 08 August 1999
 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Vibrational spectroscopy of methanol and acetonitrile clusters in cold helium droplets
View Description Hide DescriptionInfrared molecular beam depletion spectroscopy of small methanol and acetonitrile clusters embedded in large helium clusters has been studied in the spectral region of the CO stretch and the rock mode from 1023 to 1059 The results are compared with the experimental spectra of the corresponding free clusters generated in adiabatic expansions and calculations based on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the tetramer and pentamer, namely a monomer and dimer attached to a cyclic trimer. For acetonitrile clusters in helium, aside from the dimer, different structures are observed. The spectra from the trimer to the hexamer are dominated by structures which contain the antiparallel dimer as building block with symmetry for the tetramer. They do not correspond to the minimum configurations observed for the free clusters. The fragmentation of the two cluster groups in helium droplets by electron impact ionization is discussed.

The near infrared, visible, and near ultraviolet overtone spectrum of water
View Description Hide DescriptionNew long path length, high resolution, Fourier transformspectrometer measurements for water are presented. These spectra cover the near infrared, visible, and near ultraviolet regions and contain water transitions belonging to all polyads from 3 to 8 Transitions in the range 13 100–21 400 cm^{−1} are analyzed using line lists computed using variational firstprinciples calculations. 2286 new transitions are assigned to These result in the observation of transitions in 15 new overtone and combination bands of water. Energy levels for these and other newly observed levels are presented. It is suggested that local mode rather than normal mode vibrational assignments are more appropriate for the vibrational states of water in polyads 4 and above.

Quantum scattering study of electronic Coriolis and nonadiabatic coupling effects in
View Description Hide DescriptionIn this paper we develop an approximate quantum scattering method capable of determining cross sections for reactive collisions, with A being an open shell atom and BC being a closed shell diatomic molecule. This method is based on timeindependent coupled channel calculations, and absorbing potentials are used to describe reaction. The coupled channel expansion includes all electronic states of the atom that correlate to a selected atomic term, and a converged set of rotational states of the diatomic. Diatomic vibration is approximated as an adiabatic degree of freedom. The method is used to study the title reaction, including all five of the electronic surfaces that correlate to as well as terms in the Hamiltonian that couple these surfaces. These couplings include: electronic and rotational Coriolis coupling, and electrostaticnonadiabatic coupling.Coriolis coupling causes all five states to interact and is most important at long range, while electrostatic coupling produces strong interactions between the and states at short range (where these states have a conical intersection) and weak but nonnegligible interactions between these states at long range. The most important three of the five surfaces and or and and the electrostaticnonadiabatic coupling between them are taken from the recent ab initio calculations of Dobbyn and Knowles [A. J. Dobbyn and P. J. Knowles, Mol. Phys. 91, 1107 (1997); Faraday Discuss. 110, 247 (1998)], while the other surfaces or and are based on a diatomicsinmolecules potential. Our results for the fully coupled problem indicate that Coriolis coupling is significant between the electronic fine structure levels so that electronic alignment is not strongly preserved as the reactants approach. However, the fine structure averaged reaction probability is relatively insensitive to the electronic Coriolis mixing. Averaged reaction probabilities from a centrifugal decoupled calculation where both electronic and rotational Coriolis interactions are neglected are in good agreement (10% or better) with the results of the fully coupled calculations. We find that electrostaticnonadiabatic coupling between the lowest Σ and Π states is significant, even at energies below the Π barrier where only the longrange nonadiabatic coupling between these states is important. As a result, the low energy cross section summed over electronic states receives a contribution from the Π state. We find that the total cross section decreases with energy for energies below and increases slightly at higher energies, with the increase due to reaction over the Π barrier. We find that the Π barrier contribution to the cross section is about twice that obtained by treating the reaction adiabatically, with the difference due to nonadiabatic dynamics on the state.

The electronic structure of MoC and WC by anion photoelectron spectroscopy
View Description Hide DescriptionPhotoelectron spectra of and are reported at two detachment energies, 532 and 355 nm. The electron affinities of MoC and WC were measured to be 1.358 (0.010) and 1.022 (0.010) eV, respectively. Seven lowlying electronic states were observed for MoC within 2 eV above its ground state whereas six were observed for WC within 2.4 eV above its ground state. The bonding and spectroscopy of MoC and WC were found to be different and were attributed to their different atomic configurations due to the strong relativistic effects in W. The current study reports the first spectroscopic information on WC.

ArHF vibrational predissociation dynamics using the diatomicsinmolecule potential energy surface
View Description Hide DescriptionVibrational predissociationdynamics of ArHF and ArDF complexes is investigated theoretically for the first time owing to the use of threedimensional potential energy surfaces (PES’s) based on the diatomicsinmolecule approach [J. Chem. Phys. 104, 5510 (1996)]. The original PES is improved empirically to yield a reasonable description of the lowest vibrational energy levels of the ArHF complex at Predissociationdynamics is studied by means of line shape and diabatic Fermi Golden Rule methods. The latter is found to provide excellent results for the total decay widths but only a qualitative estimate for the product rotational distributions. It is shown that predissociationdynamics is governed by vibrational to rotational energy transfer. The decay proceeds almost entirely into the highest accessible rotational product channel. This propensity manifests itself in the decrease of the predissociation lifetime upon increasing vibrational excitation of the diatomic fragment when the highest rotational channel appears to be closed. Another source of state specificity in the vibrational predissociation is the anisotropy of the PES. Absolute calculated lifetime values are likely too small, but exhibit some qualitative trends observed experimentally.

The oneatom cage effect in Evidence that caging is inefficient for the Tshaped isomer
View Description Hide DescriptionCaged recombination is observed when the binary complex is excited to energies above the B state dissociation limit. The recombination mechanism has been the subject of several previous investigations. Two alternative models have been proposed; Nonadiabatic recombination in a Tshaped complex, or adiabatic recombination in linear In the present study, fluorescence depletion measurements were used to test these models. Fluorescence resulting from caged recombination could not be depleted using the wellknown resonances of Tshaped Instead, depletion via a diffuse band system previously assigned to the linear was observed. These results are consistent with the adiabatic recombination model. Geminate recombination of in a He supersonic expansion is also reported.

Spectroscopic characterization of the weakly bound state: Evidence for a substantial maximum in the potential curve at long range
View Description Hide DescriptionThe weakly bound state has been characterized by means of R2PI (Resonant TwoPhotonIonization)spectroscopy, using transitions from the metastable state prepared in a laservaporization/supersonic jet source. Because several of the vibrational levels are above the dissociation limit [to it is concluded that there must be a substantial maximum in the potential curve (>200 cm^{−1}) at large R (>4.0 Å). This is discussed, and shown to be consistent with our earlier ideas of “penetration” of outerlobes of electron density of metal atom excited states by RG (raregas) atoms. Perturbations observed, due to possible potential curve crossings with states of different electronic symmetry, are also discussed.

Differential cross sections for at 1.55 eV
View Description Hide DescriptionThe photoloc technique with core extraction of the nascent product laboratory speed distribution in a Wiley–McLaren timeofflight spectrometer has been used to measure differential cross sections for the reaction at collision energies ∼1.55 eV. We find that the peak of each angular distribution shifts from complete backward scattering toward side scattering as the rotational excitation of the product increases. We found the same trend in our previous study of at ∼1.70 eV. We conclude that the same type of correlation exists between impact parameter and rotational quantum number in both product vibrational manifolds. Further analysis of the HD differential cross section data reveals, however, a clear tendency of this vibrational manifold to scatter sideways at lower than HD Within the framework of a lineofcenters model with nearly elastic specular scattering, this result implies that smaller impact parameters lead to more vibrationally excited products.

Timedependent density functional calculations on the electronic absorption spectrum of free base porphin
View Description Hide DescriptionThree highlevel correlated ab initio studies have recently been performed on the electronic absorptionspectrum of free base porphin (FBP), but significant differences between the various assignments of the lowlying bands remain. In view of the importance of FBP as the basic building block of the porphyrins, further reliable results are evidently required and are provided here, using timedependent density functional theory (TDDFT). Our results strongly support the recent CASPT2 interpretation which is consistent with the traditional interpretation, stating that the intense B band (or Soret band) is due to the two closelying excitations and As in the CASPT2 paper, we attribute all lowlying bands to pairs of excitations. The interpretation of the combined B–N band system is discussed in some detail. The effects of basis set, geometry, and choice of exchangecorrelation potential are considered as well.

Reaction of stateselected ammonia ions with methane
View Description Hide DescriptionWe have investigated the reaction of ammonia ions with methane molecules over the collision energy range of 0.5–10.0 eV and for ammonia ion vibrational states ranging from Under these conditions, the two main product channels are and The cross section for formation of is enhanced with increasing internal energy at collision energies below 6.0 eV, and independent of internal energy at higher collision energies. The enhancement is greater for forwardscattered products indicating that ammoniaion vibrational energy enhances reactivity at large impact parameters. The mechanism for formation of involves collisioninduced dissociation of (or ) which leads to the formation of a shortlived complex, which then decays to products. This reaction is found not to be vibrationally mode selective, which is consistent with the hypothesis that mode selectivity in reactions of ammonia ions is driven by the Franck–Condon overlap whenever charge transfer is involved.

Observation of linear isomers of the ionized rare gas tetramers and
View Description Hide DescriptionThe ionized rare gas clusters and have been studied by photofragmentation. Centerofmass velocities of the neutral and ionized photofragments have been measured for photon energies between 1.6 and 4.4 eV. In almost all of the examined photon energy region, both clusters exhibit fragment velocity distributions which show that they consist of a linear ionized trimer with an additional atom loosely attached to its side. For the lowest photon energies, however, fragmentation patterns are observed which can only be explained by a linear tetramer structure. This demonstrates that stable linear isomers of the ionized rare gas tetramers exist.

Rotational statetostate rate constants and pressure broadening coefficients for collisions: Theory and experiment
View Description Hide DescriptionConverged closecoupling and coupledstates calculations were used to obtain statetostate rate constants and pressure broadening coefficients for the collisional rotational (de)excitation of by He. The ab initio potential used in these calculations was previously computed by symmetryadapted perturbation theory. The computed pressure broadening coefficients and total rate constants agree well with the available experimental data. In the experimental part of the paper stimulated Ramanpumping has been used to prepare acetylene in selected rotational states to 18 and to 19 of the CC stretching mode). The population decay in the prepared state and the transfer to other rotational states was monitored by laser induced fluorescence. The experimental data can be described by an infiniteordersudden power law (IOSP) or directly compared with the ab initio derived rate constants. The influence of multiple collisions possible at the relatively large pressuredelayproducts employed has been taken into account by simulating the rotational energy transfer with a master equation. Experimentally we obtain a total rate constant for depopulation of and IOSP fitting parameters of and γ=0.96 in very good agreement with the ab initio calculated values. There we obtain and IOSP fitting parameters of and γ=0.96. Also the experimental statetostate rotational energy transfer constants, which vary from 5.83 to 2.32 for endothermic collisions, agree very well with the ab initio data.

The infrared spectrum of the nitric oxide dimer cation: Problems for density functional theory and a muddled relationship to experiment
View Description Hide DescriptionAb initio and density functional theory(DFT) methods have been used to study the geometries, vibrational frequencies, and infrared intensities for the trans, cis, and gauchestructures of the cation. Five different functionals were employed for comparison. Doubleζ plus polarization (DZP) basis sets and tripleζ plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the transONNO cation is of symmetry. The prominent infrared absorption is predicted as ∼1900 cm^{−1} based upon the DFT methods. However, this DFT prediction is suspect since exhibits inverse symmetry breaking, dissociating to the physically absurd limit plus This inverse symmetry breaking phenomenon was discussed in an important 1997 paper by Bally and Sastry [J. Phys. Chem. A 101, 7923 (1997)]. Therefore, a higher theoretical level, Brueckner coupledcluster method was ultimately applied, and the harmonic vibrational frequency of this mode was predicted to be about 1550–1650 cm^{−1}. The important matrix isolation infrared experiments of Jacox et al. [J. Chem. Phys. 93, 7609 (1990)], Lugez et al. [J. Chem. Phys. 110, 10345 (1999)], Hacaloglu et al. [J. Phys. Chem. 94, 1759 (1990)], Andrews et al. [J. Phys. Chem. A 103, 4167 (1999)], and Strobel et al. [J. Phys. Chem. 99, 872 (1995)] are carefully considered.

Bonding in hypohalous acids HOX Cl, Br, and I) from the topological analysis of the electron localization function
View Description Hide DescriptionThe bonding in hypohalous acids has been investigated from the topological analysis of the electron localization function (ELF) at the Becke3LYP and Hartree–Fock levels. The interaction between halogen and oxygen atoms has been characterized by the presence of bonding, disynaptic attractors V(O,X) Cl, Br, and I with the mean electron population of 0.32, 0.61, 0.45, and respectively. In the case of HOBr, the possibility of a strong contribution of bromine core electrons to the valence shell has been observed. On the base of the bonding evolution theory (BET), the O–F bond has been recognized as a covalent, polarized one whereas, the bonding between O and Cl, Br, and I atoms is of the electron donor–acceptortype with halogen donating the electron density to valence shell of oxygen. The observed difference between and Br, and I) polarizations is reflected in topology of ELF maps with a large localization domain surrounding the V(F) and V(F,O) attractors in HOF and a common superbasin encompassing the V(O,H), V(O), and V(X,O) attractors in HOCl, HOBr, and HOI. The very large values of the relative quantum fluctuation (λ), above 0.8, found for V(O,X) suggest that the covalent electron density is almost entirely delocalized over other basins. The comparison of the mean electron population of the V(H,X) and V(H,O) basins computed for hydrohalic (HX) and hypohalous acids (HOX) has revealed that the population alters in line with values of therefore can be used as a approximate measure of acidic properties of molecules. The topological analysis of the ELF function supports the concept of probonded electronegativity and its usefulness as a tool for prediction of the nature of the oxygen–halogen linkage.

Autoionizing Rydberg states of NO in strong electric fields
View Description Hide DescriptionWe report on an investigation on autoionization of Rydberg electrons of the nitric oxide molecule in strong, static electric fields. The excitation was done via various rotational states of the intermediate state and with polarization parallel or perpendicular to the electric field. The splitting of the autoionizing Rydberg states into overlapping Stark manifolds is resolved for excitation above the saddlepoint created by the field. We observe that the competing decay between photoionization and predissociation can lead to an incorrect interpretation of threshold energy. The photoionization spectrum of NO Rydberg series attached to various rotational thresholds is very similar owing to weak rotational coupling. The experimental results are accurately simulated by quantum calculations based on multichannel quantum defect theory (MQDT). A more intuitive formulation of the average behavior of the photoionization cross section is developed that accounts for the suppression of ionization near the threshold due to competing predissociating channels.

A new sixdimensional analytical potential up to chemically significant energies for the electronic ground state of hydrogen peroxide
View Description Hide DescriptionWe report calculations of the electronic ground state potential energy surface (PES) of hydrogen peroxide covering, in an almost global fashion, all six internal degrees of freedom by two different ab initio techniques. Density functional theory (DFT) calculations using the Becke 3 parameter Lee–Yang–Parr (B3LYP) hybrid functional and multiconfigurational second order perturbation theory (CASPT2) calculations, both using large basis sets, are performed for a wide range of geometries (8145 DFT and 5310 CASPT2 singlepoint energies). We use a combined data set of mostly DFT with additional CASPT2 ab initio points and the complete CASPT2 surface to fit a total of four different 6D analytical representations. The resulting potentials contain 70–76 freely adjusted parameters and represent the ground state PES up to 40000 cm^{−1} above the equilibrium energy with a standard deviation of 100–107 cm^{−1} without any important artifacts. One of the model surfaces is further empirically refined to match the bond dissociation energy for The potentials are designed for energy regions accessible by vibrational fundamental and overtone spectroscopy including the dissociation channel into hydroxyl radicals. Characteristic properties of the model surfaces are investigated by means of stationary point analyses, torsional barrier heights, harmonic frequencies, lowdimensional cuts and minimum energy paths for dissociation. Overall good agreement with highlevel ab initio calculations, especially for the CASPT2 based potentials, is achieved. The drastic change in geometry at intermediate O–O distances, which reflects the transition from covalent to hydrogen bonding, is reproduced quantitatively. We calculate fully 6D anharmonic zero point energies and ground state torsional splittings with the diffusion quantum Monte Carlo method in perfect agreement, within statistical error bars, with experiment for the CASPT2 based potentials. Variational vibrational calculations in the adiabatic approximation yield energy levels and torsional splittings from the ground state up to predissociative states, satisfactorily reproducing the experimental transition wavenumbers.

Electronic nonadiabaticity in highly vibrationally excited Spinorbit coupling between and
View Description Hide DescriptionWe report full quantumstateresolved spectra of highly vibrationally excited In addition to providing high precision molecular constants for several new vibrational levels, we observe a local spectral perturbation of We present a deperturbation analysis of the observed spectra and assign the perturber to We predict a crossing between the and state at an internuclear separation somewhat further extended and higher in energy than the outer classical turning point of Using the appropriate vibrational overlap integral, we are able to determine the spin–orbit interaction between these two electronic states, which is in the vicinity of the crossing. These results suggest that the collision dynamics of highly vibrationally excited may involve excited potential surfaces. Furthermore, they imply that present theoretical approaches to the problem, which use a single potential surface, may not be adequate. Possible implications regarding nonequilibrium models of stratosphericozone formation and the dynamics of the reaction are discussed.

Photodissociation of HBr molecules and clusters: Anisotropy parameters, branching ratios, and kinetic energy distributions
View Description Hide DescriptionThe ultraviolet photolysis of HBr molecules and clusters with average size around is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a timeofflightmass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character of the transitions is observed leading to the spin–orbit state The dissociation channel associated with the excited state is populated by a parallel transition with a branching ratio of At the wavelength of 193 nm, about the same value of is found, but both channels show a mainly perpendicular character with for Br and for The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster.

Photodissociation of Ar–HCl: An energyresolved study of the dynamics of total fragmentation into
View Description Hide DescriptionUVphotolysis of Ar–HCl is simulated by means of an exact wave packet treatment in three dimensions. The focus of the work is on the mechanism of indirect dissociation of the hydrogen atom, which leads to total fragmentation of Ar–HCl into H, Ar, and Cl. The results predict for this photodissociation path a probability of about 13% of the photolysis process. The remaining probability would be associated with direct photodissociation of the H fragment. Kineticenergy distributions of the hydrogen fragments produced by indirect photodissociation are calculated for different excitation energies of Ar–HCl. The distributions reflect a pronounced structure of peaks associated with broad and overlapping resonances of the system. The resonance structure is present in the whole energy range covered by the absorptionspectrum. Hydrogen atoms initially populating the resonances can dissociate from the cluster extensively cooled down, after several collisions with Ar and Cl. A mechanism is suggested for the fragmentation process due to indirect photodissociation, which involves successive jumps of the hydrogen to lowerenergy resonances, induced by the collisions. A classical collisional model is proposed to rationalize qualitatively the fragmentation dynamics.

A quantum mechanical view of molecular alignment and cooling in seeded supersonic expansions
View Description Hide DescriptionExperimental investigations on the collisional alignment of the rotational angular momentum, occurring in supersonic seeded beams and in drift tubes, have recently documented a strong dependence of the observed effects on the final molecular velocity. The present investigation aims at elucidating the possible mechanisms at the molecular collision level. Quantum statetostate differential scattering cross sections, calculated for the prototype system for an interaction potential previously obtained in this laboratory, exhibit propensities relevant to reveal nature and selective role of the elastic and inelastic scattering events, participating in the overall mechanisms which lead to molecular alignment and cooling. The present analysis shows that the dynamics of such phenomena crucially depends on the initial and final rotational state, on the collision energy, on the involved orbitalangular momentum and therefore alternative routes are possible for molecular polarization and relaxation. These routes lead to scattering into specific angular cones and therefore observations from different experiments provide complementary pieces of information which, exploiting studies of various molecular systems under diverse experimental conditions, can be correlated in a single mosaic.
