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Several commonly used approximate methods for the calculation of vibration–rotation–tunneling spectra for (HCl)2 are described. These range from one-dimensional models to an exact coupled four-dimensional treatment of the intermolecular dynamics. Two different potential surfaces were employed—an ab initio and our ES1 experimental surface (determined by imbedding the four-dimensional calculation outlined here in a least-squares loop to fit the experimental data, which is described in the accompanying paper [J. Chem. Phys. 103, 933 (1995)]. The most important conclusion deduced from this work is that the validity of the various approximate models is extremely system specific. All of the approximate methods addressed in this paper were found to be sensitive to the approximate separability of the radial and angular degrees of freedom, wherein exists the primary difference between the two potentials. Of particular importance, the commonly used reversed adiabatic angular approximation was found to be very sensitive to the choice for fixed R; an improper choice would lead to results very much different from the fully coupled results and perhaps to false conclusions concerning the intermolecular potential energy surface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The jet-cooled high resolution spectrum of the vibrationless Ã←X transition of the deuterated species of the methyl peroxy radical has been recently published in this journal (S. Wu, P. Dupré, P. Rupper, and T. A. Miller, J. Chem. Phys. 127, 224305 (2007)). The spectrum was analyzed using a rigid-rotor model with quadratic spin-rotation coupling. The analysis was based on the fit of ∼350 partially resolved line positions and was quite satisfactory. However, the full simulation of the spectral intensity clearly identifies a lack of ability to reproduce relatively small line clumps ('extra' lines) located between the two main central Q branches. This is indicating of an incomplete initial analysis. In the present paper we reanalyze this electronic transition by considering a reference-frame axis-switching resulting from the nuclear rearrangement associated to the electronic transition (spectra obtained at two different temperatures are considered). The potential energy hypersurfaces of the two electronic states are sufficiently dissimilar to induce changes in the molecule geometry, particularly, the angle COO⁁, which induces a rotation (∼1.7°) of the principal axes of inertia located in the molecule symmetry plane. The present analysis is supported by a global fitting of the spectrum intensity and gives rise to a slightly different set of molecular constants. Attention is paid to the wavefunction symmetry assignment of a non-orthorhombic molecule. Couplings due to the torsion of the methyl group are discussed in the following paper (P. Dupre, J. Chem. Phys. 134, 244309 (2011)). [ABSTRACT FROM AUTHOR]

The bound rovibronic levels of the He–HF[sup +] complex were calculated for total angular momentum J=1/2, 3/2, 5/2, 7/2, and 9/2 with the use of ab initio diabatic intermolecular potentials presented in Paper I and the inclusion of spin–orbit coupling. The character of the rovibronic states was interpreted by a series of calculations with the intermolecular distance R fixed at values ranging from 1.5 to 8.5 Å and by analysis of the wave functions. In this analysis we used approximate angular momentum quantum numbers defined with respect to a dimer body-fixed (BF) frame with its z axis parallel to the intermolecular vector R and with respect to a molecule-fixed (MF) frame with its z axis parallel to the HF[sup +] bond. The linear equilibrium geometry makes the He–HF[sup +] complex a Renner–Teller system. We found both sets of quantum numbers, BF and MF, useful to understand the characteristics of the Renner–Teller effect in this system. In addition to the properties of a “normal” semirigid molecule Renner–Teller system it shows typical features caused by large-amplitude internal (bending) motion. We also present spectroscopic data: stretch and bend frequencies, spin–orbit splittings, parity splittings, and rotational constants. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report calculations of the rate of isomerization of 3-phospholene based on the theory of Gray and Rice as extended by Zhao and Rice. This system is of interest for two reasons. First, the potential energy surface for motion in the isomerization coordinates is well known because of the spectroscopic studies of Harthcock and Laane. Second, the potential energy surface generates a Hamiltonian which has important off-diagonal kinetic energy coupling terms, which provides a different test of the accuracy of the approximations used in the Gray–Zhao–Rice theory than posed by previous applications. We find that the predictions of the Gary–Zhao–Rice theory are in good agreement with the results of simulations of the rate of isomerization of 3-phospholene. We also briefly discuss the relationship between the theory presented in this paper and the Reactive Island theory of isomerization. [ABSTRACT FROM AUTHOR]

Chemical reactivity theory provides a basis for predicting the reactive proclivities of molecular or condensed systems. The frontier-orbital concepts of Fukui, as generalized by Parr and collaborators within the framework of density-functional theory, were developed further by us in a previous paper (I) [J. Chem. Phys. 101, 8988 (1994)]. Nevertheless, five aspects of the theory still require further development; the reactivities are defined as local responses to global stimuli instead of nonlocal responses to local stimuli; there are ambiguities associated with the existence of energy bands in condensed systems; the theory is static and does not properly incorporate the internal dynamics of the reacting systems; the theory focuses on responses without a corresponding definition of chemical stimuli; and no connection is made with the potential energy surface and the reaction pathway. In the present paper, we concentrate on gapless systems extended in at least one dimension, metals, semimetals, and insulators. We show that taking as measures of chemical reactivity the reactivity kernels of Nalewajski [J. Chem. Phys. 78, 6112 (1983); 81, 2088 (1984); 89, 2831 (1985)] and of Berkowitz and Parr [J. Chem. Phys. 88, 2554 (1988)] in the form introduced by the latter meets the first two requirements. We find an explicit expression for the softness kernel which is a natural generalization of that found for the local softness in I.The response functions entering it are calculable via the Kohn–Sham formalism. As recognized by Nalewajski and Koninski [Z. Naturforsch. 42a, 451 (1987)], there is an infinite set of modes of chemical reactivity with corresponding spectra of softness and hardness eigenvalues. The softness spectrum has an upper bound corresponding to the most reactive mode or set of degenerate modes [Z. Naturforsh. 42a, 451 (1987)] and a vanishing lower bound. The softness kernel itself can be expressed simply in terms of the static dielectric function.... [ABSTRACT FROM AUTHOR]

Four potential energy surfaces are of current interest for the Ne–CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler–London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the 20Ne–12C16O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne–CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne–CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne–CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne–CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne–CO interaction. [ABSTRACT FROM AUTHOR]

Direct current slice and crush velocity map imaging has been used to probe the photodissociation dynamics of nitrogen dioxide above the second dissociation limit. The paper is a companion to a previous publication [J. Chem. Phys. 128, 164318 (2008)] in which we reported results for the O(3PJ)+NO(2ΠΩ) adiabatic product channel. Here we examine the O(1D2)+NO(2ΠΩ) diabatic product channel at similar excitation energies. Using one- and two-color imaging experiments to observe the velocity distributions of state selected NO fragments and O atoms, respectively, we are able to build a detailed picture of the dissociation dynamics. We show that by combining the information obtained from velocity map imaging studies with mass-resolved resonantly enhanced multiphoton ionization spectroscopy it is possible to interpret and fully assign the NO images. By recording two-color images of the O(1D2) photofragments with different polarization combinations of the pump and probe laser fields we also measure the orbital angular momentum alignment in the atomic fragment. We find that the entire O(1D2) photofragment distribution is similarly aligned with most of the population in the MJ=±1 magnetic sublevels. The similarity of the fragment polarizations is interpreted as a signature of all of the O(1D2) atoms being formed via the same avoided crossing. At the photolysis energy of 5.479 52 eV we find that the NO fragments are preferentially formed in v=1 and that the vibrationally excited fragments exhibit a bimodal rotational distribution. This is in contrast to the unimodal rotational profile of the NO fragments in v=0. We discuss these observations in terms of the calculated topology of the adiabatic potential energy surfaces and attribute the vibrational inversion and rotational bimodality of the v=1 fragments to the symmetric stretch and bending motion generated on excitation to the (2) 2B2 state. [ABSTRACT FROM AUTHOR]

The fluoroborane (HBF) free radical has a large vibronic interaction which splits the orbitally degenerate 2Π state in the linear configuration into two separate electronic states, one strongly bent and one linear. The observed vibrational structure of the electronic transition between the Renner–Teller pair of states is very complex. As an aid to understanding the spectrum, the vibronic energy levels of the ground and first excited states have been calculated from high-level ab initio potential energy surfaces using a variational method. The vibrational frequencies and anharmonicities have been derived from these energy levels and the boron and hydrogen isotope shifts have been predicted. Although the ground state energy levels are for the most part well behaved, the excited state levels show substantial Renner–Teller mixing with nearby ground vibrational states. The calculations in the present work have been successfully used in the companion paper to make vibrational assignments of the laser-induced fluorescence spectra of HBF and DBF. [ABSTRACT FROM AUTHOR]

Subsequent to our spectroscopic detection of the HBX (X=F, Cl, Br) free radicals (S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)), the electronic spectrum of the à 2A″Π-X 2A′ system of the fluoroborane (HBF) radical in the 600–745 nm region has been studied in detail using the pulsed discharge jet technique. The band system involves a linear-bent transition between the two Renner–Teller components of what would be a 2Π state at linearity. Using the results of our theoretical study of the ground and excited state vibrational energy levels and 11B–10B isotope shifts (see the companion paper), the vibrational quantum numbers of the bands in the laser-induced fluorescence (LIF) spectra have been assigned. Rotational and vibrational analyses of the LIF and wavelength resolved emission spectra have been carried out, from which the linear excited state and the bent ground state equilibrium configurations have been confirmed. The ground state molecular geometry of HBF has been determined as r0(BH)=1.214(2) Å, r0(BF)=1.303 4(5) Å, and θ=120.7(1)°. Based on high-level ab initio calculations and symmetry considerations, predissociation of the excited state into H(2S)+BF(1Σ+) on the ground state potential energy surface is identified as the cause of the breaking off of fluorescence in the LIF spectra. [ABSTRACT FROM AUTHOR]

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X 1Σg+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the ν7(πu) anharmonic fundamental has been calculated to be at 102 cm-1, which validates the assignment to C5 of the pattern of bands centered at 102 cm-1 observed with the ISO telescope. [ABSTRACT FROM AUTHOR]

All the pure rotational transitions reported in the previous studies [J. Chem. Phys. 113, 10121 (2000); J. Mol. Spectrosc. 222, 22 (2003)] and newly observed rotation-vibration transitions, P=1/2←3/2, for Ar–SH and Ar–SD [J. Chem. Phys. (2005), the preceding paper] have been simultaneously analyzed to determine a new intermolecular potential-energy surface of Ar–SH in the ground state. A Schrödinger equation considering the three-dimensional freedom of motion for an atom-diatom complex in the Jacobi coordinate, R, θ, and r, was numerically solved to obtain energies of the rovibrational levels using the discrete variable representation method. A three-dimensional potential-energy surface is determined by a least-squares fitting with initial values of the parameters for the potential obtained by ab initio calculations at the RCCSD(T)/aug-cc-pVTZ level of theory. The potential well reproduces all the observed data in the microwave and millimeter wave regions with parity doublings and hyperfine splittings. Several low-lying rovibrational energies are calculated using the new potential-energy surface. The dependence of the interaction energy between Ar and SH(2Πi) on the bond length of the SH monomer is discussed. [ABSTRACT FROM AUTHOR]

Infrared spectra of weakly bound hydrogen–krypton complexes have been studied at high spectral resolution (0.04 cm-1) using a long-path (154 m) low temperature (100 K) absorption cell and a Fourier transform spectrometer. In addition to spectra from the regions of the H2 and D2 fundamental vibrational bands in the midinfrared, the results also include the region of the pure rotational S0(0) transition of H2 in the far infrared. A total of 219 measured line positions from these spectra have been fully assigned to specific quantum transitions and form the basis for determining a greatly improved semiempirical three-dimensional intermolecular potential energy surface for hydrogen–krypton in an accompanying paper. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical study of the size evolution of equilibrium structures and approximate HF vibrational red shifts for ArnHF van der Waals clusters, with n=1–14. Pairwise additive ArnHF intermolecular potential energy surfaces were constructed from spectroscopically accurate Ar–Ar and anisotropic Ar–HF potentials. The latter depend on vibrational excitation of the HF monomer. The global and energetically close-lying local minima of ArnHF, n=1–14, for HF v=0 and v=1, were determined using simulated annealing followed by a direct minimization scheme. For ArnHF clusters with n≤8, the lowest-energy structure always has HF bound to the surface of the Arn subunit. In contrast, for n≥9, the global minimum of ArnHF corresponds to HF inside a cage. Ar12HF has the minimum-energy configuration of an HF-centered icosahedron, which appears to be unusually stable. Size dependence of the HF vibrational red shift in ArnHF (n=1–14) clusters was investigated by means of a simple approximation, where the red shift was represented by the energy difference between the global minima of a cluster obtained for HF v=0 and v=1, respectively. The approximation reproduced rather accurately the experimentally determined variation of the ArnHF red shift with the number of Ar atoms, for n=1–4, although it overestimated their magnitude. For larger ArnHF clusters, 4

We report here a new determination of the H216O potential energy surface from experimental data. The calculations have been carried out by means of the very accurate and highly efficient method proposed and applied to H216O in a previous paper [Polyansky, Jensen, and Tennyson, J. Chem. Phys. 101, 7651 (1994)]. This previous work has been significantly improved by inclusion of additional terms in the analytical expression used to represent the potential energy surface. Previously, 1600 rotation-vibration term values for H216O were fitted with a standard deviation of 0.36 cm-1. With the extended model of the present work, this standard deviation could be improved to 0.25 cm-1. With the extended model and the new fitted potential function we have calculated a data set comprising 3200 term values, all of which can be compared with experimentally derived values. The standard deviation for this data set is 0.6 cm-1. The data set contains rotationally excited energy levels for all the 63 vibrational states which have been characterized by high resolution spectroscopy. The potential energy function obtained in the present work improves drastically the agreement with experiment for the highly excited local mode stretching states above 20 000 cm-1. For the vibrational band origins of these states, the highest of which is measured at 25 118 cm-1, our previous fitted potential produced discrepancies of more than 100 cm-1. These deviations are reduced to less than 1 cm-1 by the potential energy function of the present work. We show that no significant improvement of the fit can be obtained by extending the analytical expression for the potential energy by further high-order terms. An analysis of the residuals shows that at the level of accuracy achieved, the major contribution to the error originates in the neglect of nonadiabatic correction terms in the Born-Oppenheimer kinetic energy operator. We conclude that any further improvement of the potential energy surface... [ABSTRACT FROM AUTHOR]

The determination of potential energy surfaces (PES) from values calculated ab initio at a set of points or from spectral data (vibration–rotation energy level information and rotation constants) are important and often difficult problems. The former is a ‘‘potential interpolation’’ problem, the latter a ‘‘potential inversion’’ problem. These are indeterminate problems in which the known data is insufficient to yield a unique solution. We present here a new constrained variational approach to the direct solution of these problems. The constraints are the known exact values of the potential or the exact perturbation corrections desired. The variational functional is chosen to provide control of the magnitude and smoothness of the correction function or potential. The method is very simple, very fast computationally, and yields exact solutions to the perturbation or interpolation equations in a single application. The potential inversion is completed by iteration to converge the perturbation solutions for a given set of assigned levels, and then by repeating with additional levels assigned in sequence to the data set to yield a physically acceptable PES very quickly. This procedure yields a smooth PES from which the energy levels in the dataset are calculated exactly. Information on rotational constants may also be used. Both interpolation and inversion procedures are applied to the the two dimensional (R,θ) PES for ArOH(A 2Σ+). A combined application of these two procedures is also presented in the paper, where we first interpolate a PES from ab initio points and then correct the ab initio fitted surfaces using spectral data. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)2 has been determined by a direct nonlinear least-squares fit to 33 microwave, far-infrared and near-infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (1991)] and a rigorous four-dimensional dynamical method (described in the accompanying paper). The global minimum (De=-692 cm-1) is located near the hydrogen-bonded L-shaped geometry (R=3.746 Å, θ1=9°, θ2=89.8°, and [lowercase_phi_synonym]=180°). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor–acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm-1) process involves a contraction of 0.1 Å in the center of mass distance (R) at the C2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)]. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The (sub-)millimeter wave spectrum of the non-rigid CH2OH radical is investigated both experimentally and theoretically. Ab initio calculations are carried out to quantitatively characterize its potential energy surface as a function of the two large amplitude ∠H1COH and ∠H2COH dihedral angles. It is shown that the radical displays a large amplitude torsional-like motion of its CH2 group with respect to the OH group. The rotation–torsion levels computed with the help of a 4D Hamiltonian accounting for this torsional-like motion and for the overall rotation exhibit a tunneling splitting, in agreement with recent experimental investigations, and a strong rotational dependence of this tunneling splitting on the rotational quantum number Ka due to the rotation–torsion Coriolis coupling. Based on an internal axis method approach, a fitting Hamiltonian accounting for tunneling effects and for the fine and hyperfine structure is built and applied to the fitting of the new (sub)-millimeter wave transitions measured in this work along with previously available high-resolution data. 778 frequencies and wavenumbers are reproduced with a unitless standard deviation of 0.79 using 27 parameters. The N = 0 tunneling splitting, which could not be determined unambiguously in the previous high-resolution investigations, is determined based on its rotational dependence. [ABSTRACT FROM AUTHOR]

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X 1Σg+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the ν7(πu) anharmonic fundamental has been calculated to be at 102 cm-1, which validates the assignment to C5 of the pattern of bands centered at 102 cm-1 observed with the ISO telescope. [ABSTRACT FROM AUTHOR]

We present several numerical applications based upon the effective-mode formulation for the short-time dynamics through conical intersections in macrosystems, as detailed in the preceding paper and first proposed by Cederbaum et al. [Phys. Rev. Lett. 94, 113003 (2005)]. The macrosystem, containing a vast number of nuclear degrees of freedom (modes), is decomposed into a system part and an environment part. Only three effective environmental modes are needed—together with the system’s modes—to accurately calculate the low resolution spectra and the short-time dynamics of the entire macrosystem. For the systems discussed here, results are compared to those of a full quantum wave-packet propagation. Some rules are extracted to provide general tendencies; these rules allow one to understand and predict the dynamical properties in more general situations where the exact quantum dynamics of the macrosystem is out of reach. [ABSTRACT FROM AUTHOR]

A potential-energy force field for methanol is calculated at the coupled-cluster singles doubles and noniterative triples correction level using the correlation-consistent polarized valence triple zeta basis set. The force field describes the coupled molecular vibrations, including the cubic and quartic anharmonicites, as a function of the torsional coordinate. The resulting molecular eigenstates are calculated using a combination of perturbative and variation calculations following the approach of Castillo-Chará and Sibert [J. Chem. Phys. 119, 11671 (2003)]. The energies, including torsional tunneling splittings, are compared with available spectroscopic data for all the fundamentals. Excellent agreement is found. Varying the torsional mass, correlation plots are constructed in order to elucidate the complex role of torsion–vibration coupling. [ABSTRACT FROM AUTHOR]

Adiabatic potential energy surfaces for the six lowest singlet electronic states of N2O (X 1A′, 2 1A′, 3 1A′, 1 1A″, 2 1A″ and 3 1A″) have been computed using an ab initio multireference configuration interaction (MRCI) method and a large orbital basis set (aug-cc-pVQZ). The potential energy surfaces display several symmetry related and some nonsymmetry related conical intersections. Total photodissociation cross sections and product rotational state distributions have been calculated for the first ultraviolet absorption band of the system using the adiabatic ab initio potential energy and transition dipole moment surfaces corresponding to the lowest three excited electronic states. In the Franck–Condon region the potential energy curves corresponding to these three states lie very close in energy and they all contribute to the absorption cross section in the first ultraviolet band. The total angular momentum is treated correctly in both the initial and final states. The total photodissociation spectra and product rotational distributions are determined for N2O initially in its ground vibrational state (0,0,0) and in the vibrationally excited (0,1,0) (bending) state. The resulting total absorption spectra are in good quantitative agreement with the experimental results over the region of the first ultraviolet absorption band, from 150 to 220 nm. All of the lowest three electronically excited states [1Σ-(1 1A″), 1Δ(2 1A′), and 1Δ(2 1A″)] have zero transition dipole moments from the ground state [1Σ+(1 1A′)] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The 1Δ(2 1A′)←X 1Σ+(1A′) absorption dominates the absorption cross section with absorption to the other two electronic states contributing to the shape and diffuse structure of the band. It is suggested that absorption to the bound 1Δ(2 1A″) state makes an important contribution to the experimentally observed diffuse structure in the first ultraviolet absorption band. The predicted product rotational quantum state distribution at 203 nm agrees well with experimental observations.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A sophisticated adiabatic ground electronic state potential energy surface for a pure nitrogen ring (cyclic-N3) molecule is constructed based on extensive high-level ab initio calculations and accurate three-dimensional spline representation. Most of the important features of the potential energy surface are presented using various reduced dimensionality slices in internal hyperspherical coordinates as well as full dimensional isoenergy surfaces. Very significant geometric phase effects are predicted in the spectra of rotational-vibrational states of cyclic-N3. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present in this paper an application of the Lanczos algorithm to the fitting of a potential energy surface from the experimental spectrum. This method presents two definite improvements on the conventional approach: (i) the Lanczos algorithm allows to treat ultra-large basis sets (120 000 states in this calculation) and (ii) it is possible to tune selectively the calculation to specific components of the spectrum (e.g., a given combination band nνi+mνj) thus reducing considerably the complexity of the fit. This method has been applied to the CD3H molecule, considering all the vibrational degrees of freedom. Converged line positions have been obtained for high overtones of the C–H stretching mode (up to 16 000 cm-1). The accuracy of the fitted surface is demonstrated by direct comparison of experimental and calculated spectroscopical parameters. [ABSTRACT FROM AUTHOR]

Single configuration self-consistent field (SCF) calculations near the Hartree–Fock limit, and singles and doubles configuration-interaction (CI)(SDCI) calculations from this single SCF configuration have been performed for the ground and selected excited states of the molecules B2+, B2, C2+, C2, N2+, N2, O2+, O2, F2+, F2, CN, CN-, CP, CS, PN, SiC, SiC-, SiN, SiN-, SiO, and SiP. Potential energy curves, with Davidson corrections, have been obtained around equilibrium separations. Equilibrium bond lengths, vibrational energies, ionization potentials, and dissociation energies are reported. [ABSTRACT FROM AUTHOR]

We demonstrate in this Comment that the quantum mechanical absorption spectrum of the Hartley band of ozone, calculated using the diabatic B state potential energy surface constructed from the ab initio data of the title paper, is strongly structured. This result, which is in agreement with previous theoretical studies of the Hartley band, contradicts the findings of Baloïtcha and Balint-Kurti and questions the accuracy of their calculations. [ABSTRACT FROM AUTHOR]

We report on a new approach to ease the computational overhead of ab initio "on-the-fly" semiclassical dynamics simulations for vibrational spectroscopy. The well known bottleneck of such computations lies in the necessity to estimate the Hessian matrix for propagating the semiclassical pre-exponential factor at each step along the dynamics. The procedure proposed here is based on the creation of a dynamical database of Hessians and associated molecular geometries able to speed up calculations while preserving the accuracy of results at a satisfactory level. This new approach can be interfaced to both analytical potential energy surfaces and on-the-fly dynamics, allowing one to study even large systems previously not achievable. We present results obtained for semiclassical vibrational power spectra of methane, glycine, and N-acetyl-L-phenylalaninyl-L-methionine-amide, a molecule of biological interest made of 46 atoms. [ABSTRACT FROM AUTHOR]

Variationally computed infrared spectra in the range [0-5000] cm−1 are reported for the deuterated PH2D and PHD2 molecules from accurate potential energy and dipole moment surfaces initially derived for the major isotopologue PH3( C 3 v ). Energy level and line intensity calculations were performed by using a normal-mode model combined with isotopic and symmetry transformations for the H → D substitutions. Theoretical spectra were computed at 296 K up to Jmax = 30 and will be made available through the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru). For the very first time, ab initio intensity predictions of PH2D/PHD2 are in good qualitative agreement with the literature. This work will be useful for spectral intensity analysis for which accurate spectral intensity data are still missing. [ABSTRACT FROM AUTHOR]

Vibrational levels of the recently observed high-spin transition (1 5Πu - 1 5&3928;g) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear doubleresonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d 3Πg, 3 = 8 and 1 5Πg, 3 = 3 states results in accurate molecular constants for the 3 = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d 3Πg, 3 = 8 are improved. The first excited vibrational state of 1 5Πu is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d 3Πg, 3 = 6 ~ 1 5Πg, 3 = 0 system. The rotationally resolved spectra yield 11 transitions to 1 5Πu, 3 = 1 that include four spinsubstates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, 3 = 1 and 2 of the lower quintet state (1 5Πg) are measured by performing perturbation-assisted double-resonant excitation to the 1 5Πu, 3 = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings. [ABSTRACT FROM AUTHOR]

We present new potential energy surfaces for the interaction of NO(C ²π) with each of Ne, Kr, and Xe. The potential energy surfaces have been calculated using second order Møller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction for the excited states: the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectra and their appearance is in good agreement with available (2+1) REMPI spectra. We discuss the assignment and appearance of these spectra, comparing to that of NO-Ar. [ABSTRACT FROM AUTHOR]

A new full-dimensional global potential energy surface (PES) for the acetylene-vinylidene isomerization on the ground (So) electronic state has been constructed by fitting ~37000 high-level ab initio points using the permutation invariant polynomial-neural network method with a root mean square error of 9.54 cm-1. The geometries and harmonic vibrational frequencies of acetylene, vinylidene, and all other stationary points (two distinct transition states and one secondary minimum in between) have been determined on this PES. Furthermore, acetylene vibrational energy levels have been calculated using the Lanczos algorithm with an exact (J = 0) Hamiltonian. The vibrational energies up to 12 700 cm-1 above the zero-point energy are in excellent agreement with the experimentally derived effective Hamiltonians, suggesting that the PES is approaching spectroscopic accuracy. In addition, analyses of the wavefunctions confirm the experimentally observed emergence of the local bending and counter-rotational modes in the highly excited bending vibrational states. The reproduction of the experimentally derived effective Hamiltonians for highly excited bending states signals the coming of age for the ab initio based PES, which can now be trusted for studying the isomerization reaction. [ABSTRACT FROM AUTHOR]

The photochemical isomerization of the trans-trans-cis to the trans-trans-trans isomer of the merocyanine form of 6-nitro BIPS, which has been studied with femtosecond transient absorption spectroscopy [S. Ruetzel, M. Diekmann, P. Nuernberger, C. Walter, B. Engels, and T. Brixner, J. Chem. Phys. 140, 224310 (2014)], is investigated using time-dependent density functional theory in conjunction with polarizable continuum models. Benchmark calculations against SCS-ADC(2) evaluate the applicability of the CAM-B3LYP functional. Apart from a relaxed scan in the ground state with additional computation of the corresponding excitation energies, which produces the excited-state surface vertical to the ground-state isomerization coordinate, a relaxed scan in the S1 gives insight into the geometric changes orthogonal to the reaction coordinate and the fluorescence conditions. The shape of the potential energy surface (PES) along the reaction coordinate is found to be highly sensitive to solvation effects, with the method of solvation (linear response vs. state-specific) being critical. The shape of the PES as well as the computed harmonic frequencies in the S1 minima are in line with the experimental results and offer a straightforward interpretation. [ABSTRACT FROM AUTHOR]

The laser control of the radiationless decay between the B3u(nπ*) and B2u(ππ*) states of pyrazine using the dynamic Stark effect has been investigated. A vibronic coupling model Hamiltonian in diabatic representation, including potential energy, transition dipole, and static polarizability surfaces as a function of the four most important vibrational modes of the molecule has been parametrized using multi-reference electronic structure calculations. The interaction of the molecule with a strong non-resonant laser pulse has been analyzed in terms of dressed potential energy surfaces. Because of the large polarizability difference between the vibronically coupled B3u(nπ*) and B2u(ππ*) states, the Stark effect induced by the non-resonant laser pulse shifts the conical intersection away from the Franck-Condon region. We have shown, by solving the time-dependent Schrödinger equation for the molecule interacting with a relatively weak pump pulse driving the electronic excitation from the ground state to the B2u(ππ*) state, and a strong non-resonant control pulse, that this control mechanism can be used to trap the wavepacket on the B2u(ππ*) potential energy surface for a much longer time than the natural B2u(ππ*) lifetime. [ABSTRACT FROM AUTHOR]

Quantum beat spectroscopy has been used to measure rate coefficients at 300 K for collisional depolarization for NO(A 2Σ +) and OH(A 2Σ +) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr. While the experimental data for NO(A) + Kr are in excellent agreement with single surface quasiclassical trajectory (QCT) calculations carried out on the upper 2A' potential energy surface, the equivalent QCT and quantum mechanical calculations cannot account for the experimental results for OH(A) + Kr collisions, particularly at low N. This disagreement is due to the presence of competing electronic quenching at low N, which requires a multi-surface, non-adiabatic treatment. Somewhat improved agreement with experiment is obtained by means of trajectory surface hopping calculations that include non-adiabatic coupling between the ground 1A' and excited 2A' states of OH(X/A) + Kr, although the theoretical depolarization cross sections still significantly overestimate those obtained experimentally. [ABSTRACT FROM AUTHOR]

We report a new three-dimensional ab initio intermolecular potential energy surface for the Kr-H2O complex with the H2O monomer fixed at its experimental averaged structure. Using the supermolecular approach, the intermolecular potential energies were evaluated at the coupled-cluster singles and doubles level with noniterative inclusion of connected triples with the full counterpoise correction for the basis set superposition error and a large basis set including bond functions. The global minimum corresponding to a planar H-bond configuration was located at the intermolecular distance of 3.82 Å with a well depth of 169.98 cm-1. In addition, two first-order and one second-order saddle points were also identified. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to calculate the rovibrational energy levels for 16 isotopic species of the Kr-H2O complexes. The rotational transition frequencies, structure parameters, and nuclear quadrupole coupling constants were also determined for the ground and first intermolecular vibrational excited states and are all in good agreement with the available experimental values. [ABSTRACT FROM AUTHOR]

We report the experimental and theoretical study of the decomposition of gas phase pentaerythritol tetranitrate (PETN) [C(CH2ONO2)4] following electronic state excitation. PETN has received major attention as an insensitive, high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. The initial decomposition mechanism of PETN is explored with nanosecond energy resolved spectroscopy and quantum chemical theory employing the ONIOM algorithm at the complete active space self-consistent field (CASSCF) level. The nitric oxide (NO) molecule is observed as an initial decomposition product from PETN at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A 2Σ+ ← X 2Π electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for PETN, which generates the NO product with a rotationally cold (∼20 K) and a vibrationally hot (∼1300 K) distribution. Potential energy surface calculations at the ONIOM(CASSCF:UFF) level of theory illustrate that conical intersections play an important role in the decomposition mechanism. Electronically excited S1 PETN returns to the ground state through the (S1/S0)CI conical intersection, and undergoes a nitro-nitrite isomerization to generate the NO product. [ABSTRACT FROM AUTHOR]

The global potential energy surfaces for both the ground (X1A′) and excited (Ã1A′′) electronic states of the HNO molecule have been constructed by three-dimensional cubic spline interpolation of more than 17 000 ab initio points, which have been calculated at the internal contracted multi-reference configuration interaction level with the Davidson correction using an augmented correlation-consistent polarized valence quadruple zeta basis set. The low-lying vibrational energy levels for the two electronic states of HNO have also been calculated on our potential energy surfaces including the diagonal Renner-Teller terms. The calculated results have shown a good agreement with the experimental vibrational frequencies of HNO and its isotopomers. [ABSTRACT FROM AUTHOR]

In this work, we report on our full results of the dynamics of the quasi-linear, predissociated S2 state of the prototypical halocarbene, CHF, and its deuterated isotopomer CDF using optical-optical double resonance spectroscopy through the S1 state. Homogeneous linewidths were determined for a total of 51 S2 state vibrational levels with angular momenta in the range ℓ = 0 - 3 for CHF, and 76 levels for CDF. Progressions involving all three fundamental vibrations were observed. The linewidth data reveal pronounced mode specificity for both CHF and CDF, where pure bending states have the largest linewidths. For CDF, the linewidths are uniformly narrower. Calculated (CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVQZ) stationary points on the CHF potential energy surface show that two dissociative pathways are available at the energies accessed in this experiment: dissociation on the triplet surface, over a barrier, to form C(3P) + HF, and dissociation to ground state CF + H products. The former is excluded as a primary channel based on the small spin-orbit coupling in this system. A 27-state dynamically weighted full-valence complete active space self-consistent field calculation was performed with maximal weight focused on the S2 state, which was then used as a reference for Davidson-corrected multireference configuration interaction calculations MRCI(+Q) of the three lowest A′ and two lowest A″ states. These calculations reveal the presence of multiple conical intersections in the singlet manifold. Consistent with our experimental results, the most important of these involves the repulsive S3 state, which conically intersects with S2 at linearity. [ABSTRACT FROM AUTHOR]

In this work, we report on our full results of the spectroscopic analysis of the quasi-linear S2 state of the prototypical halocarbene, CHF, and its deuterated isotopomer CDF using optical-optical double resonance spectroscopy through the S1 state. A total of 51 S2 state vibrational levels with angular momenta in the range ℓ = 0-3 were observed for CHF, and 76 levels for CDF. Progressions involving all three fundamental vibrations were observed, and rotational constants were determined for each of these levels by measuring spectra through different intermediate J levels of the S1 state. Our experimental results are in excellent agreement with the predictions of vibrational calculations using the discrete variable representation method. The variational vibrational calculations were performed with an analytic potential energy surface fit to ab initio data by the method of interpolating moving least squares. The ab initio data are Davidson-corrected multi-reference configuration interaction calculations based on a state-averaged multiconfigurational self-consistent field reference incorporating a generalized dynamic weighting scheme. [ABSTRACT FROM AUTHOR]

The near-ultraviolet band system of the jet-cooled boron difluoride free radical has been studied by a combination of laser-induced fluorescence and single vibronic level wavelength resolved emission spectroscopies. The radical was produced in a supersonic discharge jet using a precursor mixture of 1%-3% of BF3 or 10BF3 in high pressure argon. A large number of bands were found in the 340-286 nm region and assigned as transitions from the X2A1 ground state to the lower Renner-Teller component of the Ã2Π excited state, based on our previous ab initio potential energy surface predictions, matching the emission spectra Franck-Condon profiles of 11BF2 and 10BF2, and comparison of observed and calculated boron isotope effects. Several bands have been rotationally analyzed providing ground state structural parameters of r0′′ (BF) = 1.3102(9) Å and θ0′′ (FBF) = 119.7(6)°. The ground state totally symmetric vibrational energy levels of both boron isotopologues have also been measured and assigned up to energies of more than 8000 cm-1. Although BF2 might be considered to be a 'simple' free radical, understanding the details of its electronic spectrum remains a major challenge for both theory and experiment. [ABSTRACT FROM AUTHOR]

Analysis of surface fluctuation spectra is performed for a large cluster of particles interacting via a sum of the short-range Lennard-Jones potential and long-range ±1/r potential, where the positive sign corresponds to the gravity, and negative corresponds to the electrostatic interaction. The spectral amplitudes of thermally driven capillary modes in a self-consistent field induced by cluster particles including the modes with no axial symmetry are derived in the approximation of small amplitudes. It is demonstrated that within used approximation, the surface tension is independent of the field strength. The low wave vector amplitudes are damped by attracting field that compresses the cluster and magnified by repulsing field leading to cluster fission. The fission threshold is found to be different from that found by Bohr and Wheeler and Frenkel due to the replacement of the ordinary surface tension by the bare one. Molecular dynamics study of a cluster with the long-range interaction in the vapor environment is performed using a novel integrator for a multiscale system. Simulation scheme implies rotation of the long-range components of forces acting on cluster particles thus vanishing an artificial torque. Simulation results justify theoretical conclusion of modes damping and independence of the surface tension of the field strength. Fission threshold evaluated from simulation data is in a good agreement with theory. [ABSTRACT FROM AUTHOR]

We report rigorous calculations of rovibrational energies and dipole transition intensities for hydrogen peroxide using a new version of MULTIMODE as applied to molecules with torsional (reaction path) motion. The key features which permit such calculations for moderately sized polyatomic molecules of this general type are briefly described. A previous, accurate potential energy surface and a new high-level ab initio dipole moment surface are employed in these calculations. Detailed comparisons are made with high-resolution experimental spectral intensities from the HITRAN database. [ABSTRACT FROM AUTHOR]

We report a new 'spectroscopic' potential energy surface (PES) of formaldehyde (H212C16O) in its ground electronic state, obtained by refining an ab initio PES in a least-squares fitting to the experimental spectroscopic data for formaldehyde currently available in the literature. The ab initio PES was computed using the CCSD(T)/aug-cc-pVQZ method at 30 840 geometries that cover the energy range up to 44 000 cm-1 above equilibrium. Ro-vibrational energies of formaldehyde were determined variationally for this ab initio PES by means of the program TROVE [Theoretical ROtation-Vibration Energies; S. N. Yurchenko, W. Thiel, and P. Jensen, J. Mol. Spectrosc. 245, 126 (2007)]. The parameter values in the analytical representation of the PES were optimized in fittings to 319 ro-vibrational energies with J = 0, 1, 2, and 5. The initial parameter values in the fittings were those of the ab initio PES, the ro-vibrational eigenfunctions obtained from this PES served as a basis set during the fitting process, and constraints were imposed to ensure that the refined PES does not deviate unphysically from the ab initio one in regions of configuration space not sampled by the experimental data. The resulting refined PES, referred to as H2CO-2011, reproduces the available experimental J <= 5 data with a root-mean-square error of 0.04 cm-1. [ABSTRACT FROM AUTHOR]

Through a variety of highly correlated methods combined with large basis sets we have studied the electronic structure of FeO, FeO+, and FeO-. In particular, we have constructed complete potential energy curves for 48, 24, and 4 states for the FeO, FeO+, and FeO- species, respectively, at the multireference level of theory. For all states examined we report energetics, common spectroscopic parameters, and dipole moments. Overall our results are in good agreement with experiment, but we have encountered as well interesting differences between experiment and theory deserving further investigation. [ABSTRACT FROM AUTHOR]

Five-dimensional ab initio potential energy surfaces (PESs) for CO2–H2 that explicitly incorporate dependence on the Q3 asymmetric-stretch normal-mode coordinate of the CO2 monomer and are parametrically dependent on its Q1 symmetric-stretch coordinate have been calculated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for v3(CO2)=0, and 1 to the Morse/long-range potential function form. These fits to 23 113 points have root-mean-square (rms) deviations of 0.143 and 0.136 cm-1, and require only 167 parameters. The resulting vibrationally averaged PESs provide good representations of the experimental infrared data: for infrared transitions of para- and ortho-H2–CO2, the rms discrepancies are only 0.004 and 0.005 cm-1, respectively. The calculated infrared band origin shifts associated with the ν3 fundamental of CO2 are -0.179 and -0.092 cm-1 for para-H2–CO2 and ortho-H2–CO2, in good agreement with the (extrapolated) experimental values of -0.198 and -0.096 cm-1. [ABSTRACT FROM AUTHOR]

For the ground states of the diatomic carbide FeC(X 3Δ) and its ions, FeC+(X 2Δ) and FeC-(X 2Δ), we report on accurate multireference variational ab initio results employing augmented correlation consistent basis sets of quintuple cardinality. The dissociation energies and bond lengths are found to be D00=87±1, 95.2, and 84±1 kcal/mol at re=1.581, 1.556, and 1.660 Å for FeC, FeC+, and FeC-, respectively. All our final numbers are in agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

We report the first measurement of the spectrum of the NO–N2 complex in the region of the first vibrational NO overtone transition. The origin band of the complex is blueshifted by 0.30 cm-1 from the corresponding NO monomer frequency. The observed spectrum consists of three bands assigned to the origin band, the excitation of one quantum of z-axis rotation and one associated hot band. The spacing of the bands and the rotational structure indicate a T-shaped vibrationally averaged structure with the NO molecule forming the top of the T. These findings are confirmed by high level ab initio calculations of the potential energy surfaces in planar symmetry. The deepest minimum is found for a T-shaped geometry on the A″-surface. As a result the sum potential also has the global minimum for this structure. The different potential surfaces show several additional local minima at slightly higher energies indicating that the complex most likely will perform large amplitude motion even in its ground vibrational state. Nevertheless, as suggested by the measured spectra, the complex must, on average, spend a substantial amount of time near the T-shaped configuration. [ABSTRACT FROM AUTHOR]

The photolysis products of dibromodifluoromethane (CF2Br2) were characterized by matrix isolation infrared and UV/Visible spectroscopy, supported by ab initio calculations. Photolysis at wavelengths of 240 and 266 nm of CF2Br2:Ar samples (∼1:5000) held at ∼5 K yielded iso-CF2Br2 (F2CBrBr), a weakly bound isomer of CF2Br2, which is characterized here for the first time. The observed infrared and UV/Visible absorptions of iso-CF2Br2 are in excellent agreement with computational predictions at the B3LYP/aug-cc-pVTZ level. Single point energy calculations at the CCSD(T)/aug-cc-pVDZ level on the B3LYP optimized geometries suggest that the isoform is a minimum on the CF2Br2 potential energy surface, lying some 55 kcal/mol above the CF2Br2 ground state. The energies of various stationary points on the CF2Br2 potential energy surface were characterized computationally; taken with our experimental results, these show that iso-CF2Br2 is an intermediate in the Br+CF2Br→CF2+Br2 reaction. The photochemistry of the isoform was also investigated; excitation into the intense 359 nm absorption band resulted in isomerization to CF2Br2. Our results are discussed in view of the rich literature on the gas-phase photochemistry of CF2Br2, particularly with respect to the existence of a roaming atom pathway leading to molecular products. [ABSTRACT FROM AUTHOR]

The H3+ potential energy surface is sampled at 5900 geometries with the emphasis on the nonequilibrium and asymptotic points. Apart from the Born–Oppenheimer energy converged to the accuracy better than 0.02 cm-1, the adiabatic and the leading relativistic corrections are computed at each geometry. To represent analytically the potential energy surface, the parameters of a power series are determined by fitting to the computed energy points. Possible choice of nuclear masses simulating the nonadiabatic effects in solving the nuclear Schrödinger equation is analyzed. A set of theoretically predicted rovibrational transitions is confronted with the experimental data in the 10 700–13 700 cm-1 window of the spectra. [ABSTRACT FROM AUTHOR]

The six-dimensional potential energy surface of the electronic ground state of trans-HNNH+ (X 2Ag) is mapped at the RCCSD(T)/aug-cc-pV5Z level of theory. This potential energy surface is incorporated later into perturbative and variational treatments to solve the nuclear motion and to derive a set of spectroscopic data for trans-HNNH+, trans-HNND+, and trans-DNND+. Our vibrational spectra are compared with those deduced from the earlier photoelectron spectra by Frost et al. [J. Chem. Phys. 64, 4719 (1976)], for which a good agreement between the theoretical and experimental results is found. Our calculations reveal the presence of strong anharmonic resonances between the vibrational levels of these cations even at low energies, thus complicating even more their assignment by vibrational quantum numbers. These resonances should participate in the transfer of intensities between the active modes during the direct photoionization of the neutral molecule and the combination modes and overtones of the inactive modes belonging to the totally symmetric irreducible representation. [ABSTRACT FROM AUTHOR]

Seven ground state potential energy surfaces for the hydroperoxyl radical are compared. The potentials were determined from either high-quality ab initio calculations, fits to spectroscopic data, or a combination of the two approaches. Vibration-rotation calculations are performed on each potential and the results compared with experiment. None of the available potentials is entirely satisfactory although the best spectroscopic results are obtained using the Morse oscillator rigid bender internal dynamics potential [Bunker et al., J. Mol. Spectrosc. 155, 44 (1992)]. We present modifications of the double many-body expansion IV potential of Pastrana et al. [J. Chem. Phys. 94, 8093 (1990)]. These new potentials reproduce the observed vibrational levels and observed vibrational levels and rotational constants, respectively, while preserving the good global properties of the original potential. [ABSTRACT FROM AUTHOR]