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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]

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]

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]

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 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]

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]

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 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]

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 adiabatic potential energy surfaces for the lowest five electronic states of 3A″ symmetry for the H++O2 collision system have been obtained at the multireference configuration interaction level of accuracy using Dunning’s correlation consistent polarized valence triple zeta basis set. The radial nonadiabatic coupling terms and the mixing angle between the lowest two electronic states (1 3A″ and 2 3A″), which adiabatically correlate in the asymptotic limit to H(2S)+O2+(X 2Πg) and H++O2(X 3Σg-), respectively, have been computed using ab initio procedures at the same level of accuracy to yield the corresponding quasidiabatic potential energy matrix. The computed strengths of the vibrational coupling matrix elements reflect the trend observed for inelastic vibrational excitations of O2 in the experiments at collision energy of 9.5 eV. The quantum dynamics has been preformed on the newly obtained coupled quasidiabatic potential energy surfaces under the vibrational close-coupling rotational infinite-order sudden framework at the experimental collision energy of 9.5 eV. The present theoretical results for vibrational elastic/inelastic excitations of O2 are in overall good agreement with the available experimental data obtained from the proton energy-loss spectra in molecular beam experiments [F. A. Gianturco et al., J. Phys. B 14, 667 (1981)]. The results for the complementary charge transfer processes are also presented at this collision energy. [ABSTRACT FROM AUTHOR]

Valence excitation spectra for the linear isomers of He–, Ne–, and Ar–Br2 are reported and compared to a two-dimensional simulation using the currently available potential energy surfaces. Excitation spectra from the ground electronic state to the region of the inner turning point of the Rg–Br2 (B,ν′) stretching coordinate are recorded while probing the asymptotic Br2 (B,ν′) state. Each spectrum is a broad continuum extending over hundreds of wavenumbers, becoming broader and more blueshifted as the rare gas atom is changed from He to Ne to Ar. In the case of Ne–Br2, the threshold for producing the asymptotic product state reveals the X-state linear isomer bond energy to be 71±3 cm-1. The qualitative agreement between experiment and theory shows that the spectra can be correctly regarded as revealing the one-atom solvent shifts and also provides new insight into the one-atom cage effect on the halogen vibrational relaxation. The measured spectra provide data to test future ab initio potential energy surfaces in the interaction of rare gas atoms with the halogen valence excited state. [ABSTRACT FROM AUTHOR]

A new potential energy surface involving the antisymmetric Q3 normal mode of CO2 for the He–CO2 van der Waals complex is constructed at the coupled-cluster singles and doubles with noniterative inclusion of connected triple [CCSD(T)] level with augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set plus bond functions. Two vibrationally adiabatic potentials with CO2 at both the ground and the first excited vibrational states are generated from the integration of the three-dimensional potential over the Q3 coordinate. The potential has a T-shaped global minimum and two equivalent linear local minima. The bound rovibrational energy levels are obtained using the radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm. The observed band origin shift of the complex (0.0946 cm-1) is successfully reproduced by our calculation (0.1034 cm-1). The infrared spectra of the complex are also predicted. The fundamental band is in excellent agreement with the experiment. Most of the transitions corresponding to the observed hot band [M. J. Weida et al., J. Chem. Phys. 101, 8351 (1994)] are assigned reasonably. [ABSTRACT FROM AUTHOR]

An accurate and efficient method for automated molecular global potential energy surface (PES) construction and fitting is demonstrated. An interpolating moving least-squares (IMLS) method is developed with the flexibility to fit various ab initio data: (1) energies, (2) energies and gradients, or (3) energies, gradients, and Hessian data. The method is automated and flexible so that a PES can be optimally generated for trajectories, spectroscopy, or other applications. High efficiency is achieved by employing local IMLS in which fitting coefficients are stored at a limited number of expansion points, thus eliminating the need to perform weighted least-squares fits each time the potential is evaluated. An automatic point selection scheme based on the difference in two successive orders of IMLS fits is used to determine where new ab initio data need to be calculated for the most efficient fitting of the PES. A simple scan of the coordinate is shown to work well to identify these maxima in one dimension, but this search strategy scales poorly with dimension. We demonstrate the efficacy of using conjugate gradient minimizations on the difference surface to locate optimal data point placement in high dimensions. Results that are indicative of the accuracy, efficiency, and scalability are presented for a one-dimensional model potential (Morse) as well as for three-dimensional (HCN), six-dimensional (HOOH), and nine-dimensional (CH4) molecular PESs. [ABSTRACT FROM AUTHOR]

We calculated the dynamics of nuclear wave packets in coupled electron-vibration systems and their nonlinear optical responses. We found that the quantized nature of the vibrational modes is observed in pump-probe spectra particularly in weakly interacting electron-vibration systems such as cyanine dye molecules. Calculated results based on a harmonic potential model and molecular orbital calculations are compared with experimental results, and we also found that the material parameters regarding the geometrical structure of potential energy surfaces are directly determined by accurate measurement of time-resolved spectra. [ABSTRACT FROM AUTHOR]

The Li+–(H2)n n=1–3 complexes are investigated through infrared spectra recorded in the H–H stretch region (3980–4120 cm-1) and through ab initio calculations at the MP2/aug-cc-pVQZ level. The rotationally resolved H–H stretch band of Li+–H2 is centered at 4053.4 cm-1 [a -108 cm-1 shift from the Q1(0) transition of H2]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li+ ion attached to a slightly perturbed H2 molecule. Around 100 rovibrational transitions belonging to parallel Ka=0-0, 1-1, 2-2, and 3-3 subbands are observed. The Ka=0-0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 Å increasing by 0.004 Å when the H2 subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li+–H2 potential energy surfaces [Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Sˇpirko, Chem. Phys. 330, 190 (2006)]. The H–H stretch band of Li+–(H2)2, which is centered at 4055.5 cm-1 also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H2–Li+–H2 geometry, in which the two H2 molecules are disposed on opposite sides of the central Li+ ion. The two equivalent Li+...H2 bonds have approximately the same length as the intermolecular bond in Li+–H2. The Li+–(H2)3 cluster is predicted to possess a trigonal structure in which a central Li+ ion is surrounded by three equivalent H2 molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm-1. [ABSTRACT FROM AUTHOR]

A highly accurate and efficient method for molecular global potential energy surface (PES) construction and fitting is demonstrated. An interpolating-moving-least-squares (IMLS)-based method is developed using low-density ab initio Hessian values to compute high-density PES parameters suitable for accurate and efficient PES representation. The method is automated and flexible so that a PES can be optimally generated for classical trajectories, spectroscopy, or other applications. Two important bottlenecks for fitting PESs are addressed. First, high accuracy is obtained using a minimal density of ab initio points, thus overcoming the bottleneck of ab initio point generation faced in applications of modified-Shepard-based methods. Second, high efficiency is also possible (suitable when a huge number of potential energy and gradient evaluations are required during a trajectory calculation). This overcomes the bottleneck in high-order IMLS-based methods, i.e., the high cost/accuracy ratio for potential energy evaluations. The result is a set of hybrid IMLS methods in which high-order IMLS is used with low-density ab initio Hessian data to compute a dense grid of points at which the energy, Hessian, or even high-order IMLS fitting parameters are stored. A series of hybrid methods is then possible as these data can be used for neural network fitting, modified-Shepard interpolation, or approximate IMLS. Results that are indicative of the accuracy, efficiency, and scalability are presented for one-dimensional model potentials as well as for three-dimensional (HCN) and six-dimensional (HOOH) molecular PESs. [ABSTRACT FROM AUTHOR]

We report here the first observation of the D 1Δg state of the C3 radical, which provides the first comprehensively analyzed example of the dynamic Renner-Teller splitting in Δ symmetry. Two color double resonance spectroscopy via the à 1Πu state was employed to experimentally probe an extensive range of vibronic levels in this D 1Δg state, covering all three modes of vibration of C3. The analysis was supported by ab initio potential energy surface calculations on the C3 radical to outline the lowest eight singlet electronic states. Two methods were used to analyze the Renner-Teller effect. The first method is an empirical Hamiltonian based on normal modes, using harmonic oscillator functions as a basis, with Renner-Teller and other terms added as required, which allows conventional vibrational parameters to be determined. The second is a much larger program that uses the exact kinetic energy operator for a triatomic molecule to calculate vibronic energy levels directly from the Renner-Teller pair of potential energy surfaces. Both methods give a good fit to the experimental results, with only a small adjustment to the ab initio surfaces required for the latter. One of the overall conclusions is that the Renner-Teller effect is rather smaller in the D 1Δg state than in the à 1Πu state. [ABSTRACT FROM AUTHOR]

The B 1A1 electronic state of silylene (SiH2) is the second excited singlet state of the molecule and, like the analogous c state of methylene (CH2), it is quasilinear with symmetry 1Σg+ at linearity. This state dissociates to Si(1D)+H2(1Σg+). At equilibrium, the B state of SiH2 has an energy that we calculate to be 0.71 eV above that of the dissociation products. However, there is a barrier to dissociation that allows quasibound rovibrational levels to occur, and some have been observed recently [Y. Muramoto et al., J. Chem. Phys. 122, 154302 (2005)]. Starting with our analytical ab initio potential-energy surface, we adjusted it in a fitting to the experimental term values in order to determine the optimum potential-energy function in the bound region. This potential has a C2v equilibrium structure with a SiH bond length of 1.459 Å and a bond angle of 165.4°; the barrier to linearity is only 129 cm-1. Using the optimized potential-energy surface we calculate B-state term values, and using our calculated y and z dipole moment surfaces, we simulate the rotation-vibration spectrum of the state in order to assist in the detection of the matrix isolation spectrum. [ABSTRACT FROM AUTHOR]

A new four-dimensional intermolecular potential-energy surface for the H2–CO complex is presented. The ab initio points have been computed on a five-dimensional grid including the dependence on the H–H separation (the C–O separation was fixed). The surface has then been obtained by averaging over the intramolecular vibration of H2. The coupled-cluster supermolecular method with single, double, and noniterative triple excitations has been used to calculate the interaction energy. The correlation part of the interaction energy has been obtained from extrapolations based on calculations in a series of basis sets. An analytical fit of the ab initio potential-energy surface has the global minimum of -93.049 cm-1 at the intermolecular separation of 7.92 bohr for the linear geometry with the C atom pointing toward the H2 molecule. For the other linear geometry, with the O atom pointing toward H2, the local minimum of -72.741 cm-1 has been found for the intermolecular separation of 7.17 bohr. The potential has been used to calculate the rovibrational energy levels of the para-H2–CO complex. The results agree very well with those observed by McKellar [A. R. W. McKellar J. Chem. Phys. 108, 1811 (1998)]: the discrepancies are smaller than 0.1 cm-1. The calculated dissociation energy is equal to 19.527 cm-1 and significantly smaller than the value of 22 cm-1 estimated from the experiment. Predictions of rovibrational energy levels for ortho-H2–CO have also been done and can serve as a guidance to assign recorded experimental spectra. The interaction second virial coefficient has been calculated and compared with the experimental data. [ABSTRACT FROM AUTHOR]

A heterodyned fifth-order two-dimensional (2D) IR spectrum of a model coupled oscillator system, Ir(CO)2(C5H7O2), is reported. The spectrum is generated by a pulse sequence that probes the eigenstate energies up to the second overtone and combination bands, providing a more rigorous potential-energy surface of the coupled carbonyl local modes than can be obtained with third-order spectroscopy. Furthermore, the pulse sequence is designed to generate and then rephase a two-quantum coherence so that the spectrum is line narrowed and the resolution improved for inhomogeneously broadened systems. Features arising from coherence transfer processes are identified, which are more pronounced than in third-order 2D IR spectroscopy because the transition dipoles of the second overtone and combination states are not rigorously orthogonal, relaxing the polarization constraints on the signal intensity for these features. The spectrum provides a stringent test of cascading signals caused by third-order emitted fields and no cascading is observed. In the Appendix, formulas for calculating the signal intensities for resonant fifth-order spectroscopies with arbitrarily polarized pulses and transition dipoles are reported. These relationships are useful for interpreting and designing polarization conditions to enhance specific spectral features. [ABSTRACT FROM AUTHOR]

Rotational spectra of the weakly bound He–HCCCN and He–DCCCN van der Waals complexes were observed using a pulsed-nozzle Fourier-transform microwave spectrometer in the 7–26-GHz frequency region. Nuclear quadrupole hyperfine structures due to the 14N and D nuclei (both with nuclear-spin quantum number I=1) were resolved and assigned. Both strong a and weaker b-type transitions were observed and the assigned transitions were used to fit the parameters of a distortable asymmetric rotor model. The dimers are floppy, near T-shaped complexes. Three intermolecular potential-energy surfaces were calculated using the coupled-cluster method with single and double excitations and noniterative inclusion of triple excitations. Bound-state rotational energy levels supported by these surfaces were determined. The quality of the potential-energy surfaces was assessed by comparing the experimental and calculated transition frequencies and also the corresponding spectroscopic parameters. Simple scaling of the surfaces improved both the transition frequencies and spectroscopic constants. Five other recently reported surfaces [O. Akin-Ojo, R. Bukowski, and K. Szalewicz, J. Chem. Phys. 119, 8379 (2003)], calculated using a variety of methods, and their agreement with spectroscopic properties of He–HCCCN are discussed. [ABSTRACT FROM AUTHOR]

An improved three-dimensional potential energy surface for the H2–Kr system is determined from a direct fit of new infrared spectroscopic data for H2–Kr and D2–Kr to a potential energy function form based on the exchange-Coulomb model for the intermolecular interaction energy. These fits require repetitive, highly accurate simulations of the observed spectra, and both the strength of the potential energy anisotropy and the accuracy of the new data make the “secular equation perturbation theory” method used in previous analyses of H2–(rare gas) spectra inadequate for the present work. To address this problem, an extended version of the “iterative secular equation” method was developed which implements direct Hellmann–Feynman theorem calculation of the partial derivatives of eigenvalues with respect to parameters of the Hamiltonian which are required for the fits. [ABSTRACT FROM AUTHOR]

Parity resolved state-to-state cross sections for inelastic scattering of OH (X 2Π) by HCl were measured in a crossed molecular beam experiment at the collision energy of 920 cm-1. The OH (X 2Π) radicals were prepared in a single quantum state, Ω=3/2, J=3/2, MJ=3/2, f, by means of electrostatic state selection in a hexapole field. The rotational distribution of the scattered OH radicals by HCl was probed by saturated LIF spectroscopy of the 0-0 band of the A 2Σ+–X 2Π transition. Relative state-to-state cross sections were measured for rotational excitations up to J=9/2 within the Ω=3/2 spin–orbit manifold and up to J=7/2 within the Ω=1/2 spin–orbit manifold. A propensity for spin–orbit conserving transitions was found, but no propensity for excitation into a particular Λ-doublet component of the same rotational state was evident. The data are presented and discussed in comparison with results previously obtained for collisions of OH with CO (Ecoll=450 cm-1) and N2 (Ecoll=410 cm-1) and with new data we have measured for the OH+CO system at a comparable collision energy (Ecoll=985 cm-1). This comparison suggests that the potential energy surface (PES) governing the interaction between OH and HCl is more anisotropic than the PES’s governing the intermolecular interaction of OH with CO and N2. [ABSTRACT FROM AUTHOR]

The infrared (IR) spectrum and vibrational predissociation of the proton-bound H2O+–Ar ionic complex are investigated within an ab initio and quantum dynamical study. For this purpose, a two-dimensional potential energy surface (2D PES) is determined as a function of the HO–H and OH–Ar coordinates. This PES is then employed in a wave-packet calculation to determine spectral properties of the system and to calculate the IR absorption spectrum. The vibrational energy levels and relative IR intensities agree well with the experimental spectrum reported earlier. On the other hand, the predissociation lifetimes in the nanosecond regime derived from the 2D PES are in disagreement with the experimental observations, indicating the importance of the neglected degrees of freedom for a correct description of the dynamics of the complex. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

New low-lying isomeric structures of ArnHF clusters are reported for n=6–13. They were determined using simulated annealing and evolutionary programing, for pairwise additive intermolecular potential energy surfaces. New global minima were found for the clusters with n=7, 10, 11. The new lowest-energy structure of Ar7HF and several new local minima for n=6, 7 clusters have the HF bound on a threefold surface site, consistent with the recent spectroscopic data for ArnHF clusters in helium nanodroplets. A new type of low-energy local minima were determined for n=9–13 clusters. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio multireference configuration interaction potential energy surfaces are computed for the eight lowest singlet surfaces of C3. These reveal several important features, including several conical intersections in linear, nonlinear, and equilateral triangle geometries. These intersections are important because, particularly for the excited à 1Πu state, reasonable ab initio results could only be obtained by including nearby, near degenerate, 1Σu- and 1Δu states that cross the à 1Πu state around 4500 cm-1 above the equilibrium geometry, and a 1Πg state whose potential in turn crosses the other states about 2000 cm-1 further up. These states are probably responsible for the complexity of the shorter wavelength UV absorption spectrum of C3. The computed potential energy surface for the ground, X 1Σg+, state and for the lowest two excited singlet surfaces (which both correlate with the à 1Πu state in a collinear geometry) are fitted to analytic functional forms. Vibrational energy levels are calculated for both states, taking account of the Renner-Teller coupling in the excited à 1Πu state. The potential parameters for both states are then least-squares fitted to experimental data. The ground-state fit covers a range of ∼8500 cm-1 above the lowest level, and reproduces 100 observed vibrational levels with an average error of 2.8 cm-1. The à 1Πu state surfaces cover a range of 3250 cm-1 above the zero-point level, and reproduce the 44 observed levels in this range with an average error of 2.8 cm-1. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The methyl rotational tunneling spectrum of p-xylene confined in nanoporous zeolite crystals has been measured by inelastic neutron scattering (INS) and proton nuclear magnetic resonance (NMR), and analyzed to extract the rotational potential energy surfaces characteristic of the methyl groups in the host-guest complex. The number and relative intensities of the tunneling peaks observed by INS indicate the presence of methyl-methyl coupling interactions in addition to the methyl-zeolite interactions. The INS tunneling spectra from the crystals (space group P212121 with four crystallographically inequivalent methyl rotors) are quantitatively interpreted as a combination of transitions involving two coupled methyl rotors as well as a transition involving single-particle tunneling of a third inequivalent rotor, in a manner consistent with the observed tunneling energies and relative intensities. Together, the crystal structure and the absence of additional peaks in the INS spectra suggest that the tunneling of the fourth inequivalent rotor is strongly hindered and inaccessible to INS measurements. This is verified by proton NMR measurements of the spin-lattice relaxation time which reveal the tunneling characteristics of the fourth inequivalent rotor. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report an ab initio intermolecular potential energy surface of the Ar-HCCCN complex using a supermolecular method. The calculations were performed using the fourth-order Møller–Plesset theory with the full counterpoise correction for the basis set superposition error and a large basis set including bond functions. The complex was found to have a planar T-shaped structure minimum and a linear minimum with the Ar atom facing the H atom. The T-shaped minimum is the global minimum with the well depth of 236.81 cm-1. A potential barrier separating the two minima is located at R=5.57 Å and θ=20.39° with the height of 151.59 cm-1. The two-dimensional discrete variable representation was employed to calculate the rovibrational energy levels for Ar-HCCCN. The rovibrational spectra including intensities for the ground state and the first excited intermolecular vibrational state are also presented. The results show that the spectra are mostly b-type (ΔKa=±1) transitions with weak a-type (ΔKa=0) transitions in structure, which are in good agreement with the recent experimental results [A. Huckauf, W. Jager, P. Botschwina, and R. Oswald, J. Chem. Phys. 119, 7749 (2003)].© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A set of analytical potential energy surfaces (PESs) for six singlet excited states of NOCl are constructed based on multireference configuration interaction calculations. The total absorption cross section at the energy range of 2–7 eV is calculated by quantum dynamics calculations with the present PESs and transition dipole moments. The calculated absorption spectrum agrees well with the experiment. It is also found that the A band with the absorption maximum at 6.3 eV is attributed to the transition to the 4 1A′ state, though the excitations to the 3 1A′ and 3 1A″ states contribute to the spectrum at the energy range between 4 and 5 eV. The spin-forbidden transitions are concluded to be negligibly weak. The mechanism of photodissociation reaction at the energy region corresponding to the A band is examined. The nonadiabatic transition rates from the 4 1A′ state to lower singlet and triplet states are estimated by Fermi’s golden rule, and the transitions to the 1 1A′ and 3 1A′ states induced by vibronic coupling are found to be the predominant dissociation pathways. The experimentally observed energy dependence of the recoil anisotropy of the fragments is discussed based on the calculated nonadiabatic transition rates. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurement of room temperature absorption by HCl—Ar mixtures in the 1–0 and 2–0 bands have been made for pressures between 10 and 50 atm. Fits of these spectra are made for the determination of the width, spectral shift, asymmetry, and intensity of individual lines. The broadening and shifting parameters are in satisfactory agreement with previous determinations but provide the first complete and self-consistent sets covering P(15)–R(14) and P(7)–R(8) in the 1–0 and 2–0 bands, respectively. The asymmetries of the profiles, which have been studied for the first time, are smaller than typically 10-3 atm -1 and cannot be determined experimentally. On the other hand, the intensities of the low j lines show a significant linear decrease with increasing Ar pressure. Calculations of all measured quantities are made with a classical path approach and an accurate vibrational-dependent HCl—Ar potential energy surface (PES). Comparisons with experimental values show that widths and shifts are well predicted, confirming the quality of the PES and of the theoretical model, and the calculations confirm that asymmetries are small. The damping factors of the intensities are analyzed by considering three contributions: The first is due to the formation of van der Waals complexes, the second results from the finite duration of collisions, and the last comes from initial correlations. Calculations indicate that the last process has negligible consequences but that the first two processes lead to effects of the same order and explain most of the observed decrease of the intensities, even if some discrepancies persist for the |m|=1 rotational components. [ABSTRACT FROM AUTHOR]

Supersonic jet investigations of the (HBr)2 dimer have been carried out using a tunable diode laser spectrometer to provide accurate data for comparison with results from a four-dimensional (4-D) ab initio potential energy surface (PES). The near-infrared v1 ±, v2 ±, and ( v1 + v4)- bands of (H 79Br)2 , (H 79Br—H 81Br), and (H 81Br)2 isotopomers have been recorded in the range 2500–2600 cm-1 using a CW slit jet expansion with an upgraded near-infrared diode laser spectrometer. The 4-D PES has been calculated for (HBr)2 using second-order Møller—Plesset perturbation theory with an augmented and polarized 6-311G basis set. The potential is characterized by a global minimum occurring at the H bond structure with the distance between the center of masses (CM) of the monomer being RCM = 4.10 Å with angles θA = 10°, θB = 100° and a well depth of 692.2 cm-1, θA is the angle the HBr bond of monomer A makes with the vector from the CM of A to the CM of B, and θB is the corresponding angle monomer B makes with the same CM—CM vector. The barrier for the H interchange occurs at the closed C2h structure for which RCM = 4.07 Å, θA = 45°, θB = 135°, and the barrier height is 73.9 cm-1. The PES was fitted using a linear-least squares method and the rovibrational energy levels of the complex were calculated by a split pseudospectral method. The spectroscopic data provide accurate molecular parameters for the dimer that are then compared with the results predicted on the basis of the 4-D ab initio PES. [ABSTRACT FROM AUTHOR]

The double photoionization spectrum of SO[sub 2] has been measured using the TOF–PEPECO technique and contains one resolved band. Detailed electronic structure calculations and experimental comparisons allow the resolved band to be identified as the A [sup 1]A[sub 2] state of the SO[sub 2][sup 2+] dication, with its adiabatic ionization energy at 35.284±0.02 eV. According to the most accurate calculations, the ground state level of SO[sub 2][sup 2+] must be located near 33.48 eV, well below the range accessed by vertical transitions from neutral SO[sub 2]. Transient SO[sub 2][sup 2+] molecules detected by mass spectrometry may be identified either as the sharp levels of the A [sup 1]A[sub 2] state or as ground state levels populated by nonvertical ionization pathways. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using a potential energy curve (based primarily on the RKR potential of Amiot and Vergès [J. Chem. Phys. 112, 7068 (2000)]) and a dipole moment function (based primarily on ab initio calculations of Park et al. [Chem. Phys. 257, 135 (2000)]), we have calculated radiative transition probabilities (Einstein A coefficients), radiative lifetimes, and dipole moment expectation values involving all vibrational levels (for several rotational quantum numbers) of the X [sup 1]Σ[sup +] ground state of [sup 39]K[sup 85]Rb. We observe that the radiative lifetimes of vibrationally excited levels, in particular, are ∼10[sup 3]–10[sup 6] seconds, far too long to be significant in most ultracold experiments involving [sup 39]K[sup 85]Rb or its isotopomers. Comparison with other molecules (LiH and HF) suggests that simple scaling (A∼μ[sup 2]ν[sup 3]∼τ[sup -1]) will predict similarly long lifetimes for many other heteronuclear molecules, e.g., RbCs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Fourier transform emission spectra of D[sub 2]O vapor were recorded at a temperature of 1500 °C in the wavenumber range 380–1880 cm[sup -1]. 15 346 lines were measured, of which the majority were identified as belonging to D[sub 2]O. The spectrum was analyzed using variational nuclear motion calculations based on spectroscopically determined potential-energy surfaces. Initial assignments were made using a potential surface obtained by fitting a high accuracy ab initio potential. The new assignments were used to refine the potential surface, resulting in additional assignments. A total of 6400 D[sub 2]O transitions were assigned and 2144 new D[sub 2]O energy levels were obtained. Transitions involving the 4ν[sub 2] and 5ν[sub 2] bending states, with band origins of 4589.30 (±0.02) and 5679.6 (±0.1) cm[sup -1], respectively, were assigned for the first time.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodetachment spectroscopy of the ClO[sub 2][sup -] ion is studied theoretically with the aid of a time-dependent wave packet method. The theoretical findings in conjunction with the experimental observations unambiguously established the existence of a conical intersection in the excited [sup 2]B[sub 2]–[sup 2]A[sub 1] electronic manifold of ClO[sub 2]. The highly diffused vibrational structure of this electronic manifold observed in the experimental recording is attributed to the nonadiabatic effects associated with this conical intersection. In our study we employed the near-equilibrium potential energy surfaces reported by Peterson and Werner [J. Chem. Phys. 96, 8948 (1992)] along the symmetric stretching and bending vibrations of ClO[sub 2]. The potential energy surfaces along the asymmetric stretching vibration are assumed to be harmonic. The dynamics of the system is studied within a linear vibronic coupling scheme, and the strength of the coupling is explicitly determined by ab initio methods. The effect of the next higher [sup 2]A[sub 2] electronic state of ClO[sub 2] on the above photoelectron band is also discussed. The nonradiative decay dynamics of the system mediated by the vibronic coupling is examined, and the findings are correlated with the femtosecond time-resolved experiment. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]