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The near-equilibrium potential energy surface (PES) of the ã 3 B 1 state of SO2 is developed from explicitly correlated spin-unrestricted coupled cluster calculations with single, double, and perturbative triple excitations with an augmented triple-zeta correlation-consistent basis set. The lowest-lying ro-vibrational energy levels of several sulfur isotopologues have been determined using this PES. It is shown that the new ab initio PES provides a much better description of the low-lying vibrational states than a previous PES determined at the multi-reference configuration interaction level. In particular, the theory-experiment agreement for the three lowest-lying vibrational transitions is within 1–3 cm−1. [ABSTRACT FROM AUTHOR]

The low-energy wing of the C¹B² ←X¹A1 absorption spectra for SO2 in the ultraviolet region is computed for the 32S,33S,34S and 36S isotopes, using the recently developed ab initio potential energy surfaces (PESs) of the two electronic states and the corresponding transition dipole surface. The stateresolved absorption spectra from various ro-vibrational states of SO2(X ¹A1) are computed. When contributions of these excited ro-vibrational states are included, the thermally averaged spectra are broadened but maintain their key characters. Excellent agreement with experimental absorption spectra is found, validating the accuracy of the PESs. The isotope shifts of the absorption peaks are found to increase linearly with energy, in good agreement with experiment. [ABSTRACT FROM AUTHOR]

We determine from first principles two sets of four-dimensional diabatic potential energy surfaces (PES's) for the interaction of NO(X2Π) with H2, under the assumption of fixed NO and H2 bond distances. The first set of PES'swas computed with the explicitly correlated multi-reference configuration interaction method [MRCISD-F12 + Q(Davidson)], and the second set with an explicitly correlated, coupled-cluster method [RCCSD(T)-F12a] with the geometry scan limited to geometries possessing a plane of symmetry. The calculated PES's are then fit to an analytical form suitable for bound state and scattering calculations. The RCCSD(T)-F12a dissociation energies (D0) of the NO-para-H2(ortho-D2) and the NO-ortho-H2(para-D2) complexes are computed to be 22.7 (31.7) and 23.9 (29.2) cm-1, respectively. The values calculated with the MRCISD-F12 + Q PES's are 21.6 (31.1) and 23.3 (28.4) cm-1, respectively. [ABSTRACT FROM AUTHOR]

Transport properties for collisions of oxygen atoms with hydrogen atoms and hydrogen molecules have been computed by means of time-independent quantum scattering calculations. For the O(3P)-H(2S) interaction, potential energy curves for the four OH electronic states emanating from this asymptote were computed by the internally-contracted multi-reference configuration interaction method, and the R-dependent spin-orbit matrix elements were taken from Parlant and Yarkony [J. Chem. Phys. 110, 363 (1999)]. For the O(3P)-H2 interaction, diabatic potential energy surfaces were derived from internally contracted multi-reference configuration interaction calculations. Transport properties were computed for these two collision pairs and compared with those obtained with the conventional approach that employs isotropic Lennard-Jones (12-6) potentials. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

We report new and more accurate adiabatic potential energy surfaces (PESs) for the ground X¹A¹ and electronically excited C¹B2(2¹A') states of the SO2 molecule. Ab initio points are calculated using the explicitly correlated internally contracted multi-reference configuration interaction (icMRCI-F12) method. A second less accurate PES for the ground X state is also calculated using an explicitly correlated single-reference coupled-cluster method with single, double, and non-iterative triple excitations [CCSD(T)-F12]. With these new three-dimensional PESs, we determine energies of the vibrational bound states and compare these values to existing literature data and experiment. [ABSTRACT FROM AUTHOR]

We report new and more accurate adiabatic potential energy surfaces (PESs) for the ground X¹A¹ and electronically excited C¹B2(2¹A') states of the SO2 molecule. Ab initio points are calculated using the explicitly correlated internally contracted multi-reference configuration interaction (icMRCI-F12) method. A second less accurate PES for the ground X state is also calculated using an explicitly correlated single-reference coupled-cluster method with single, double, and non-iterative triple excitations [CCSD(T)-F12]. With these new three-dimensional PESs, we determine energies of the vibrational bound states and compare these values to existing literature data and experiment. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical investigation of rotationally inelastic transitions of OH, prepared in the X²π, v = 0, j = 3/2 F1f level, in collisions with molecular hydrogen (H2 and D2). In a crossed beam experiment, the OH radicals were state selected and velocity tuned over the collision energy range 15-155 cm-1 using a Stark decelerator. Relative parity-resolved state-to-state integral cross sections were determined for collisions with normal and para converted H2. These cross sections, as well as previous OH-H2 measurements at 595 cm-1 collision energy by Schreel and ter Meulen [J. Chem. Phys. 105, 4522 (1996)], and OH-D2 measurements for collision energies 100-500 cm-1 by Kirste et al. [Phys. Rev. A 82, 042111 (2010)], were compared with the results of quantum scattering calculations using recently determined ab initio potential energy surfaces [Ma et al., J. Chem. Phys. 141,114309 (2014)]. Good agreement between the experimental and computed relative cross sections was found, although some structure seen in the OH(j = 3/2 F1f → j = 5/2 F1e) + H2(j = 0) cross section is not understood. [ABSTRACT FROM AUTHOR]

This is the second in a series of papers detailing a MATLAB based implementation of the finite element method applied to collinear triatomic reactions. Here, we extend our previous work to reactions on coupled potential energy surfaces. The divergence of the probability current density field associated with the two electronically adiabatic states allows us to visualize in a novel way where and how nonadiabaticity occurs. A two-dimensional investigation gives additional insight into nonadiabaticity beyond standard one-dimensional models. We study the F(²P) + HCl and F(²P) + H2 reactions as model applications. Our publicly available code (http://www2.chem.umd.edu/groups/alexander/FEM) is general and easy to use. [ABSTRACT FROM AUTHOR]

For the interaction of OH(X²Π) with H2, under the assumption of fixed OH and H2 bond distances, we have determined two new sets of four-dimensional ab initio potential energy surfaces (PES's). The first set of PES's was computed with the multi-reference configuration interaction method [MRCISD+Q(Davidson)], and the second set with an explicitly correlated coupled cluster method [RCCSD(T)-F12a] sampling the subset of geometries possessing a plane of symmetry. Both sets of PES's are fit to an analytical form suitable for bound state and scattering calculations. The CCSD(T) dissociation energies (D0) of the OH-para-H2 and the OH-ortho-H2 complexes are computed to be 36.1 and 53.7 cm-1. The latter value is in excellent agreement with the experimental value of 54 cm-1. [ABSTRACT FROM AUTHOR]

We present the experimental and simulated (2+1) REMPI spectrum of the C 2Π state of the NO-Ar complex, in the vicinity of the 3p Rydberg state of NO. Two Rydberg states of NO are expected in this energy region: the C2Π (3pπ) and D2Σ+ (3pσ) states, and we concentrate on the former here. When the C2Π (3pπ) state interacts with Ar at nonlinear orientations, the symmetry is lowered to Cs, splitting the degeneracy of the 2Π state to yield C(2A″) and C(2A′) states. For these two states of NO-Ar, we calculate potential energy surfaces using second order Mo\ller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction to describe the excited states, the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectrum, which is in excellent agreement with experiment, providing assignments for the observed spectral lines from the calculated rovibrational wavefunctions. [ABSTRACT FROM AUTHOR]

We present detailed calculations on resonances in rotationally and spin-orbit inelastic scattering of OH (X 2Π, j = 3/2, F1, f) radicals with He and Ne atoms. We calculate new ab initio potential energy surfaces for OH-He, and the cross sections derived from these surfaces compare well with the recent crossed beam scattering experiment of Kirste et al. [Phys. Rev. A 82, 042717 (2010)]. We identify both shape and Feshbach resonances in the integral and differential state-to-state scattering cross sections, and we discuss the prospects for experimentally observing scattering resonances using Stark decelerated beams of OH radicals. [ABSTRACT FROM AUTHOR]

We present converged exact quantum wave packet calculations of reaction probabilities, integral cross sections, and thermal rate coefficients for the title reaction. Calculations have been carried out on the ground 12A′ global adiabatic potential energy surface of Deskevich et al. [J. Chem. Phys. 124, 224303 (2006)]. Converged wave packet reaction probabilities at selected values of the total angular momentum up to a partial wave of J = 140 with the HCl reagent initially selected in the v = 0, j = 0-16 rovibrational states have been obtained for the collision energy range from threshold up to 0.8 eV. The present calculations confirm an important enhancement of reactivity with rotational excitation of the HCl molecule. First, accurate integral cross sections and rate constants have been calculated and compared with the available experimental data. [ABSTRACT FROM AUTHOR]

Rotationally inelastic collisions of the CH3 molecule in its ground X2A2′′ electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH3, frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RCCSD(T)]. The anisotropy of the PES is dominated by repulsion of the helium by the hydrogen atoms. The dissociation energy De was computed to equal 27.0 cm-1. At the global minimum, the helium atom lies in the CH3 plane between two C-H bonds at an atom-molecule separation R = 6.52 bohr. Cross sections for collision-induced rotational transitions have been determined through quantum scattering calculations for both nuclear spin modifications. Rotationally inelastic collisions can cause a change in the rotational angular momentum n and its body-frame projection k. Because of the anisotropy of the PES due to the hydrogen atoms, there is a strong propensity for Δk = ±3 transitions. Thermal rate constants for state-specific total collisional removal have also been determined. [ABSTRACT FROM AUTHOR]

Quantum reactive scattering calculations on accurate potential energy surfaces predict that at energies below ∼5 meV, the reaction of F atoms with H2 is dominated by the Born-Oppenheimer (BO) forbidden reaction of the spin-orbit excited F(2P1/2) atom. This non-BO dominance is amplified by low-energy resonances corresponding to quasi-bound states of the HF(v = 3, j = 3) + H product channel. Neglect of non-adiabatic coupling between the electronic states of the F atom leads to a qualitatively incorrect picture of the reaction dynamics at low energy. [ABSTRACT FROM AUTHOR]

Rotationally inelastic collisions of the CH2 molecule in its ã1A1 electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH2(ã), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RSSCD(T)]. The PES is quite anisotropic, due to lack of electron density in the unoccupied CH2 non-bonding orbital perpendicular to the molecular plane. Quantum scattering calculations have been carried out to compute state-to-state rotational energy transfer and elastic depolarization cross sections at collision energies up to 2400 cm-1. These cross sections were thermally averaged to derive room-temperature rate constants. The total removal and elastic depolarization rate constants for the ortho ka = 1 levels agree well with recent experimental measurements by Hall, Sears, and their co-workers. We observe a strong even-odd alternation in the magnitude of the total rate constants which we attribute to the asymmetry splitting of the ka = 1 levels. [ABSTRACT FROM AUTHOR]

Rotational excitation of the CN(X 2Σ+) molecule with He is investigated. We present a new two-dimensional potential energy surface (PES) for the He–CN system, calculated at an internuclear CN distance frozen at its experimental equilibrium distance. This PES was obtained using an open-shell, coupled-cluster method including all single and double excitations, as well as the perturbative contributions of connected triple excitations [RCCSD(T)]. Bond functions were placed at mid-distance between the center of mass of the CN molecule and He atom for a better description of the van der Waals interaction. State-to-state collisional excitation cross sections of the fine-structure levels of CN by He are calculated for energies up to 2500 cm-1, which yield after thermal averaging, rate coefficients up to 350 K. The exact spin splitting of the energy levels is taken into account. The propensity rules between fine-structure levels are studied, and it is shown that the rate constants for Δj=ΔN transitions are much larger than those for Δj≠ΔN transitions, as expected from theoretical considerations. Our calculated rate coefficients are compared to experimental results at 295 K of Fei et al. [J. Chem. Phys. 100, 1190 (1994)]. The excellent agreement confirms the accuracy of the PESs and of the scattering calculations. [ABSTRACT FROM AUTHOR]

Elastic tensor and depolarization cross sections are computed for the collision of two exemplary diatomic molecules with 2Π electronic ground states—OH and NO—with argon. The interaction of a diatomic molecule in a Π state with a spherical collision partner must be described by two potential energy surfaces (PESs), corresponding to the two asymptotically degenerate electronic states, of A′ and A″ symmetry. Quantum scattering calculations are most naturally based on the average (Vsum) and half-difference (Vdif) of these two PESs. When Vdif is neglected, the OH(X 2Π)-Ar depolarization cross sections are found to be significantly reduced in magnitude, while the NO(X 2Π)-Ar cross sections are relatively unaffected. In addition, treating the molecules as closed-shell 1Σ+ species with a corresponding rotational level structure and using (Vsum) to model the PES, we predict depolarization cross sections which differ significantly from those based on full inclusion of the electronic degeneracy and fine structure of these 2Π molecules. This indicates that any single-PES-based simulation of the collisional depolarization of these two molecules would be subject to significant error. [ABSTRACT FROM AUTHOR]

Tensor cross sections for collisions of OH(X 2Π) with helium are computed using accurate ab initio potential energy surfaces computed by Lee et al. We use these cross sections to predict rate constants for collisional depolarization of specific rotational fine-structure levels of OH(X 2Π) in collisions with helium. Good agreement is found with the results of the two-color polarization spectroscopy experiments of Paterson et al. [J. Chem. Phys. 129, 074304 (2008)]. By examination of partial cross sections, the range of impact parameters at which collisional depolarization occurs is shown to be similar to that for rotationally inelastic collisions. [ABSTRACT FROM AUTHOR]

We present the results of an ab initio study of the interaction of electronically excited NO(A 2Σ+) with rare gas (Rg) atoms. The bound states of each NO(A)–Rg species are determined from potential energy surfaces calculated at the RCCSD(T) level of theory. Making use of the NO(X 2Π)–Rg vibrational wavefunctions, we then simulate electronic spectra. For NO–Kr and NO–Xe we obtain good qualitative agreement with the previously published experimental spectra. For NO–Ar, the shallowness of the surface gives rise to agreement that is less satisfactory, but a global scaling provides better qualitative agreement. The assignment of the spectra is far from straightforward and is only possible with guidance from the calculated energies and wavefunctions of the energy levels of the complex. Previous assignments are discussed in the light of this conclusion. [ABSTRACT FROM AUTHOR]

Collisions of the vibrationally excited OH(v=1) molecule with atomic oxygen are investigated theoretically using a coupled-states, statistical capture (CS-ST) model. Vibrational relaxation can occur by inelastic scattering, and the vibrationally excited molecule can also be removed by reaction to form O2 in both the ground (X 3Σg-) and first excited (a 1Δg) state. In the former case, reaction occurs on the lowest potential energy surface of 2A″ symmetry, and, in the latter case, by reaction on the lowest potential energy surface of 2A′ symmetry. We report new ab initio potential energy surfaces for both these states in the product and reactant regions necessary for application of the coupled-states, statistical method. Comparison with exact, reactive scattering calculations within the J-shifting approximation indicate that the CS-ST rate constants for removal of OH(v=1) can be expected to be reasonably accurate. Our calculated rate constants at 300 K agree well with the experimental results of Khachatrian and Dagdigian [Chem. Phys. Lett. 415, 1 (2005)]. Reaction to yield O2 (X 3Σg-) is the dominant removal pathway. At subthermal temperatures, the rate constants for the various vibrational quenching processes all increase down to T≅60 K and then decrease at lower temperature. [ABSTRACT FROM AUTHOR]

The CN–Ar van der Waals complex has been observed using the B 2Σ+-X 2Σ+ and A 2Π-X 2Σ+ electronic transitions. The spectra yield a dissociation energy of D0″=102±2 cm-1 and a zero-point rotational constant of B0″=0.067±0.005 cm-1 for CN(X)–Ar. The dissociation energy for CN(A)–Ar was found to be D0′=125±2 cm-1. Transitions to vibrationally excited levels of CN(B)–Ar dominated the B-X spectrum, indicative of substantial differences in the intermolecular potential energy surfaces (PESs) for the X and B states. Ab initio PESs were calculated for the X and B states. These were used to predict rovibrational energy levels and van der Waals bond energies (D0″=115 and D0′=183 cm-1). The results for the X state were in reasonably good agreement with the experimental data. Spectral simulations based on the ab initio potentials yielded qualitative insights concerning the B-X spectrum, but the level of agreement was not sufficient to permit vibronic assignment. Electronic predissociation was observed for both CN(A)–Ar and CN(B)–Ar. The process leading to the production of CN(A,ν=8,9) fragments from the predissociation of CN(B,ν=0)–Ar was characterized using time-resolved fluorescence and optical-optical double resonance measurements. [ABSTRACT FROM AUTHOR]

The photoelectron spectra of the ClH2- and ClD2- anions have been simulated using a Franck–Condon model involving vertical excitation to the four coupled quasidiabatic potential energy surfaces that correlate with Cl(2P)+H2(1Σg+). A careful analysis of the excitation process is presented. All electrostatic, spin-orbit, and Coriolis couplings in the photodetached ClH2 (ClD2) neutral are included. At a resolution of 1 meV, the resulting spectra are dominated by the bound and resonant states of the Cl...H2 and Cl...D2 van der Waals complexes, along with contributions from the associated continua. Only small differences occur between these spectra and those simulated under the assumption that each of the three electronically adiabatic Cl(2P)+H2 states can be treated separately. In particular, photodetachment to form the Cl*(2P1/2)H2 complex leads to very low intensity of spectral features associated with Cl(2P3/2)H2. This clearly implies that, while nonadiabatic effects do have some influence on the bound and resonance state energies of both complexes as shown recently by Garand et al. [Science 319, 72 (2008)], nonadiabatic transitions between the two complexes are extremely rare. [ABSTRACT FROM AUTHOR]

We describe fully quantum, time-independent scattering calculations of the F+H2→HF+H reaction, concentrating on the HF product rotational distributions in v′=3. The calculations involved two new sets of ab initio potential energy surfaces, based on large basis set, multireference configuration-interaction calculations, which are further scaled to reproduce the experimental exoergicity of the reaction. In addition, the spin-orbit, Coriolis, and electrostatic couplings between the three quasidiabatic F+H2 electronic states are included. The calculated integral cross sections are compared with the results of molecular beam experiments. At low collision energies, a significant fraction of the reaction is due to Born–Oppenheimer forbidden, but energetically allowed reaction of F in its excited (2P1/2) spin-orbit state. As the collision energy increases, the Born–Oppenheimer allowed reaction of F in its ground (2P3/2) spin-orbit state rapidly dominates. Overall, the calculations agree reasonably well with the experiment, although there remains some disagreement with respect to the degree of rotational excitation of the HF(v′=3) products as well as with the energy dependence of the reactive cross sections at the lowest collision energies. [ABSTRACT FROM AUTHOR]

The photoelectron spectra of the ClH2- and ClD2- anions have been simulated using a Franck–Condon model involving vertical excitation to the four coupled quasidiabatic potential energy surfaces that correlate with Cl(2P)+H2(1Σg+). A careful analysis of the excitation process is presented. All electrostatic, spin-orbit, and Coriolis couplings in the photodetached ClH2 (ClD2) neutral are included. At a resolution of 1 meV, the resulting spectra are dominated by the bound and resonant states of the Cl...H2 and Cl...D2 van der Waals complexes, along with contributions from the associated continua. Only small differences occur between these spectra and those simulated under the assumption that each of the three electronically adiabatic Cl(2P)+H2 states can be treated separately. In particular, photodetachment to form the Cl*(2P1/2)H2 complex leads to very low intensity of spectral features associated with Cl(2P3/2)H2. This clearly implies that, while nonadiabatic effects do have some influence on the bound and resonance state energies of both complexes as shown recently by Garand et al. [Science 319, 72 (2008)], nonadiabatic transitions between the two complexes are extremely rare. [ABSTRACT FROM AUTHOR]

We present the results of new ab initio calculations of the three FH2 potential energy surfaces (PESs) which are necessary for a complete description of the F+H2 reaction. These are the result of high-precision multireference configuration-interaction calculations, with an additional scaling of the external correlation energy. The results of these calculations have been fitted to a multiparameter form for use in subsequent quantum scattering calculations. With a scaling factor of s=1.078, the calculated exoergicity agrees nearly exactly with experimentally determined values. With a slightly reduced scaling factor of s=1.05, the fitted PESs give excellent agreement with the position and modulation depth of the F+HD→FH+D transition state resonance observed by Skodje et al. [J. Chem. Phys. 112, 4536 (2000)]. This suggests that these new PESs can be used with confidence in the simulation of the reactivity of the ground and excited spin-orbit states of the F atom in reactions with H2. [ABSTRACT FROM AUTHOR]

The statistical model of atom-diatom insertion reactions is combined with coupled-states capture theory to calculate integral cross sections for formation of specific rotational/fine-structure states of the SD product of the title reaction. The four electronic potential energy surfaces that correlate with the products (1,3A′ and 1,3A″) and an accurate description of the electronic and spin-orbit couplings between them have been determined from ab initio calculations. The dependence of the cross sections upon the product rotational quantum number shows a statistical behavior similar to that computed with the simple prior statistical model. We predict a significant preference for formation of the lower (F1) versus the upper (F2) spin-orbit manifold but essentially equal A′ and A″ Λ-doublet populations. The computed SD v=0 rotational/fine-structure state distribution is in good agreement with the distribution measured experimentally for this reaction by Khachatrian and Dagdigian [J. Chem. Phys. 122, 024303 (2005)]. The calculations predict the F1: F2 spin-orbit population ratio to be slightly larger than experimentally observed. [ABSTRACT FROM AUTHOR]

Vibrational relaxation of HF(v) in collisions with H atoms can occur by three pathways: inelastic scattering with and without H atom exchange, and, for v>=3, the HF+H→F+H2 reaction. Fully quantum, reactive scattering calculations on the Stark-Werner FH2 potential energy surface reveal narrow peaks in the energy dependence of the integral cross sections for each of these processes. By means of an adiabatic-bender analysis, we show that each of these peaks corresponds to the position of quasibound HF-H vibrational states trapped in the weak van der Waals well. The width of these resonances indicates that the lifetime of the quasibound states is up to 30 periods of the HF-H van der Waals vibration. [ABSTRACT FROM AUTHOR]

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A 2Σ+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X 2Π population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A 2Σ+ (ν′=0, N′=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH X 2Π (ν″, N″) population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N″) of the quenched OH X 2Π products observed in ν″=1 and 2 as well as a pronounced propensity for quenching into the Π(A′) Λ-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A 2Σ+ by H2 proceeds through conical intersections between two potentials of A′ reflection symmetry (in planar geometry) that correlate with the electronically excited A 2Σ+ and ground X 2Π states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H2. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000)], these calculations reveal a steep gradient away from the OH–H2 conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X 2Π products. [ABSTRACT FROM AUTHOR]

We report state-to-state and overall thermal rate constants for the isotope exchange reaction D(2S)+OH(2Π)→OD(2Π)+H(2S) for 0 K≤T≤500 K. The reaction cross sections were determined by a statistical capture model which includes the electronic degrees of freedom of the OH(OD) radical. Both the D+OH and OD+H channels are described by four ab initio potential-energy surfaces (1,3A′ and 1,3A″). All electronic and spin-orbit couplings are included. We predict a reaction rate constant of 14.22×10-11 cm3 molecule-1 s-1 at T=100 K and 10.78×10-11 cm3 molecule-1 s-1 at T=300 K. At lower temperatures, around 50 K, the value rises to k(T)=15×10-11 cm3 molecule-1 s-1. The predicted rate constants confirm the earlier theoretical and experimental results. A negative temperature dependence in the rate constant is observed. The state-resolved cross sections and rate constants predict a significant propensity toward the formation of the OD (A′) Λ-doublet level and the ground spin-orbit manifold F1 [ABSTRACT FROM AUTHOR]

We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B 2Σ-, ν=0, N→N′) with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)]. Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and ΔN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments. [ABSTRACT FROM AUTHOR]

We report quantum mechanical calculations of excitation functions (relative reaction cross sections) for the F+HD reaction. We include three potential energy surfaces and an accurate treatment of all couplings (non-adiabatic, spin-orbit, and Coriolis). Comparison with experimental results [Dong, Lee, and Liu, J. Chem. Phys., 113, 3633 (2000)] show excellent agreement for the DF product channel and an improved but not perfect agreement for the HF product channel. In the former case, when weighted by the (16%) fractional population of the spin-orbit excited state (F*) in the beam, the overall reactivity of the F* is small (∼5%). For the HF product channel and with the same (16%) fractional weight, F* reactivity makes a contribution of ∼12% in the high-energy tail of the resonance peak. As a result, averaging over the population of F spin-orbit states in the beam changes the shape of the resonance. The greater the fraction of F* in the beam, the less pronounced will be the resonance modulation of the reaction excitation function. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The statistical model of atom-diatom insertion reactions is combined with coupled-states capture theory and used to calculate product multiplet-resolved integral cross sections for the title reaction. This involves an ab initio determination of the four electronic potential energy surfaces that correlate with the products (1,3A′ and 1,3A″), and an accurate description of the electronic and spin-orbit couplings between them. The dependence of the resulting cross sections on the final-state rotational quantum number shows a statistical behavior similar to that observed in earlier studies of the reaction in which only the lowest (1A′) potential was retained. In addition, however, the present calculations provide information on the branching between the OH(2Π) multiplet levels. Although the two spin-orbit manifolds are predicted to be equally populated, we find a strong propensity for the formation of the Π(A′) Λ-doublet states. These two predictions confirm the experimental results of Butler, Wiesenfeld, Gericke, Brouard, and their co-workers. The nonstatistical population of the OH Λ-doublet levels is a consequence of the bond breaking in the intermediate H2O complex and is preserved through the multiple curve crossings as the products separate. This exit-channel coupling is correctly described by the present theory. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Coupled-cluster [CCSD(T)] calculations of the potential energy surface of the Cl[SUP-]H[SUB2] complex were carried out, subsequently averaged over the vibrational motion of the H[SUB2] moiety and then fitted. The lower bend-stretch levels of the anion complex were then determined, for the H[SUB2] molecule in its ground and first excited vibrational levels. The resulting rotational constants agree with earlier experimental values, almost to within the experimental precision. The predicted dissociation energies are 5-8arger than the experimentally derived values. The bend-stretch wave function of the lowest state of the Cl[SUP-]oD[SUB2] complex encompasses a considerably larger range of bending motion than in the complexes with oH[SUB2] and pD[SUB2] . [ABSTRACT FROM AUTHOR]

We present here a novel approach to the determination of the interaction between two atoms, each in a ²P electronic state, embedded in a cluster of spherical atoms. The model requires accurate ab initio potential energy curves for the M[sub 2] system, for all the 36 electronic states which correlate with dissociation into ground-state M(²p) atoms. Consequently, making use of a valence-bond-like model, we transform these 36 molecular orbital states into a set of 36 Cartesian (q[sub a]q[sub b]) states which correspond to assigning the two p electrons to Cartesian orbitals centered on either atom. It is then easy to use the earlier Balling and Wright model [L. C. Balling and J. J. Wright, J. Chem. Phys. 79, 2941 (1983)] to determine, in this 36 state basis, the matrix elements corresponding to the interaction of each ²P atom with any number of surrounding spherical ligands. The lowest eigenvalue of the resulting 36×36 matrix defines, in an adiabatic approximation, the potential governing the motion of the atoms. We apply this approach to the determination of the interaction of two Al atoms embedded in solid pH[sub 2], site-substituted in the center of two adjacent hexagons. We find the interaction between the two Al atoms to be significantly modified by the presence of the intervening pH[sub 2] molecules. [ABSTRACT FROM AUTHOR]

Presents coupled-cluster ab initio calculations of the potential energy surfaces of the Ar-NO complex. Scattering of NO by Ar; Results of scattering calculations; Similarity between the cross sections based on the CEPA and CCSD(T); Spectroscopic investigation of the bound states of the potential energy surfaces for the ArNO system.

New coupled-cluster ab initio potential energy surfaces (PES’s) were determined for the interaction of Ar with a rigid acetylene molecule. These PES’s were in addition modified by scaling the correlation energy. Based on both the original and scaled PES’s, close-coupled calculations of the total differential scattering cross section were carried out. Rovibrational energy levels of the Ar–C2H2 complex were computed variationally. In addition, we simulated the ir spectra corresponding to excitation of the upper diad of the ν3/ν2+ν4+ν5 excited molecular vibrational states. The comparison of all these quantities with experiment shows generally good agreement for the several scaled PES’s. In addition, the sensitivity of the PES to the experimental data are investigated by varying the scaling factor. The original and scaled PES’s are also compared with several phenomenological PES’s and a previously published ab initio PES [F.-M. Tao, S. Drucker, and W. Klemperer, J. Chem. Phys. 102, 7289 (1995)]. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Continuing an earlier study [M. H. Alexander, J. Chem. Phys. 99, 6014 (1993)] of the B(2P)...H2 complex, we report here a further investigation into the adiabatic→diabatic transformation for this system, characterized by three potential energy surfaces (PES’s) which become asymptotically degenerate. The diabatic PES’s are used to determine the energies of the lowest bend–stretch levels of complexes of B(2P) with either o- or pH2. The predicted dissociation energies (D0) are 27.9 and 38.6 cm-1 for the complexes with pH2 and oH2, respectively, and 37.3 and 48.5 cm-1 for the complexes with oD2 and pD2, respectively. The motion of the oH2 moiety within the cluster cannot be described accurately using a single electronic potential energy surface. In addition, new ab initio calculations are reported for the complex of H2 with B in its first electronic excited state (...2s23s). The PES for this B(2S)...H2 complex is repulsive and nonreactive, at least in the region sampled by vertical electronic excitation of the B(2P)...H2 complex. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

New ab initio potential energy surfaces (PES’s) are presented for the interaction of He with the NO radical in its ground (X 2Π) electronic state, determined within the coupled electron pair approximation (CEPA) with a large atomic orbital basis set. The dynamics of the collisions of NO with He are then investigated, in particular the coupling between nuclear motion (rotation and translation) and the internal electronic motion of the open-shell partner. State-to-state integral and differential cross sections are calculated using full close coupling and coupled states methods. These cross sections are compared with the results of the two separate measurements at different initial collision energies, 508 and 1186 cm-1 (63 and 147 meV). Excellent agreement is obtained in both cases. Also comparisons with previous calculations, based on an earlier local density potential energy surface, are made at 508 and 2420 cm-1 (63 and 300 meV). © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

New ab initio potential energy surfaces (PESs) are reported for the interaction of Ne(1S0) with the OH radical in its ground (X 2Π) electronic state. These are then used in the variational calculation of the bound vibrational states of the NeOH(X) complex. The calculated dissociation energy (D0) is 26.2 cm-1, which lies within the experimental estimate (23–30 cm-1). The ab initio PESs are also used to determine the positions and widths of the metastable levels of the complex which correlate with the first excited rotational state (j=5/2, ω=3/2) and the first excited spin–orbit state (j=1/2, ω=1/2) of OH(X 2Π). The predissociation rates are strongly dependent on the bending motion, the intermolecular stretching vibrational quantum number and the parity. The predissociation lifetimes are in good agreement with estimates from stimulated emission pumping experiments of [Chuang, Andrews, and Lester, J. Chem. Phys. 103, 3418 (1995)]. A quantum flux method is used to study the redistribution of the predissociation flux as a function of the fragment separation. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We use slightly modified versions of earlier ab initio potential energy surfaces for Ar+NH(a 1Δ) and Ar+NH(c 1Π) to investigate the bend–stretch levels of ArNH complexes, both within an adiabatic bender model and in full variational calculations. The positions of these levels are used to reinterpret the fluorescence excitation spectra reported previously [R. W. Randall, C.-C. Chuang, and M. I. Lester, Chem. Phys. Lett. 200, 113 (1992)]. The first 15 observed bands can be unambiguously assigned, with near-quantitative accuracy. The weak anisotropy in the lower electronic state [ArNH(a 1Δ)] complicates the rotational structure, since each band involves overlapped transitions out of several, nearly degenerate bend–stretch levels of the complex. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Multireference, configuration interaction potential energy surfaces (PES’s) are reported for the interaction of Ar with the AlH radical in its ground (X 1Σ+) and first singlet excited (A 1Π) electronic states. To assist in the assignment of the van der Waals bend–stretch vibrational quantum numbers in the accompanying experimental spectroscopic study of the ArAlH complex [E. Hwang and P. J. Dagdigian, J. Chem. Phys. 102, 2426 (1995)], slightly modified versions of these PES’s have been employed to calculate the vibrational energy levels of the ArAlH(X,A) van der Waals complex, both within the adiabatic bender model and in full variational calculations. Additionally, the band strengths for electronic transitions out of the ground and first vibrationally excited level of ArAlH(X) have been calculated for comparison with the relative band intensities measured by Hwang and Dagdigian. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By means of full quantum close-coupling and coupled states calculations based on an ab initio potential energy surface for the Ar–CH system, we confirm a propensity seen experimentally by Hancock, Stuhl, and their co-workers. During the rotational relaxation of high rotational levels of the CH(X 2Π) radical, produced by photolysis of a suitable precursor, there appears a clear population imbalance in favor of the Λ-doublet levels of Π(A‘) symmetry. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of both the temporal evolution and the pressure dependence of this Λ-doublet asymmetry. This asymmetry is a consequence of both an enhanced depletion of high N Π(A’) levels and the enhanced formation of Π(A‘) levels in the next lower (N-1) manifolds. The physical origin of this propensity involves a crossing between two adiabatic bender potentials which follow, respectively, the A’ and A‘ potential energy surface (PES). This crossing occurs only for the ‘‘helicopter-like’’ approach of the CH molecule, in which its rotational angular momentum is aligned along the initial relative velocity vector. Thus, a strong v, N correlation in the reactant channel results in a strong Λ, N correlation in the product channel. [ABSTRACT FROM AUTHOR]

New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the CH radical in its ground (X 2Π) and second excited (B 2Σ-) electronic states. These potential energy surfaces are then used in an adiabatic analysis of the rovibronic levels of the ArCH(X) and ArCH(B) van der Waals complexes. A qualitative discussion of the expected features in the B←X electronic spectrum of ArCH is presented, and these are compared with the experimental spectrum reported earlier by Lemire et al. [J. Chem. Phys. 99, 91 (1993)]. [ABSTRACT FROM AUTHOR]

New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the BH radical in its ground (X 1Σ+) and first excited (A 1Π) electronic states. These potential energy surfaces are then used with an adiabatic bender model for the calculation of the vibrational energy levels of the ArBH van der Waals complex in its ground and first excited singlet electronic states. Comparison of vibrational energies calculated using this adiabatic bender model with computed exact vibrational energies indicates that the former provides a very useful description of the bound levels of the ArBH complex. A qualitative discussion of the expected features in the A 1Π-X 1Σ+ electronic spectrum of ArBH is also presented, to facilitate comparison with the experimental ArBH spectrum reported in the following paper [E. Hwang and P. J. Dagdigian, following paper, J. Chem. Phys. 101, 2903 (1994)]. The most strongly bound ArBH(A) levels, with Ar–BH separations less than in the ground state ArBH(X) complex, correspond to motion described primarily by the more attractive VA‘ potential energy surface and to a helicopterlike internal motion of the BH moiety. For the more weakly bound states supported by higher bender curves, the vibrational motion cannot be described as occurring on either the VA‘ or VA’ potential energy surfaces separately. [ABSTRACT FROM AUTHOR]

We report a series of full close-coupling calculations of integral cross sections for fine structure resolved, rotationally inelastic transitions of CH induced by collisions with He. These calculations use the necessary two CH(X 2Π)+He potential energy surfaces as determined by a variety of ab initio techniques described in the preceeding paper. The calculated N=1→N=2-7 cross sections confirm a previous prediction of preferential population of final state levels in which the electronic wave function of the CH molecule is antisymmetric with respect to reflection in the plane of rotation of the molecule. A generally good overall agreement is found between the energy-dependent cross sections determined in earlier experiments of Macdonald and Liu and appropriate averages of the calculated cross sections. However, small, systematic, qualitative discrepancies persist between theory and experiment. Diagnostic calculations were carried out to identify the cause of these discrepancies. These calculations explored the influence of restrictions in basis set, configuration interaction, and functional forms used in fitting the potential energy surfaces. They also explored the consequences of the dynamical approximations of CH as a rigid rotor and the independence of the CH spin–orbit constant on the approach of the He partner. All these diagnostic calculations generally confirm the anticipated marginal influence of these approximations. [ABSTRACT FROM AUTHOR]

We report the results of fully quantum close-coupled studies of vibrational excitation in collisions of NO(X 2Π) with a Ag(111) surface. The interaction potentials used were an extension, to include the dependence on the NO bond distance, of the recent corrected effective medium potential energy surfaces (PES) of Depristo and Alexander. [J. Chem. Phys. 94, 8454 (1991)]. The final state rotational distributions show evidence of at least four rotational rainbows, corresponding to scattering on (and interference between) the two PES which arise when the degeneracy of the NO molecule is lifted upon approach to the surface. A strong tendency is seen to populate the lower spin–orbit manifold at low to moderate final J, which disappears as J rises beyond 30.5 and the final states are better described in Hund’s case (b). Simultaneously, there exists a propensity to populate those Λ-doublet levels in which the electronic-rotational wave function is symmetric (ΠA’) with respect to reflection of the electronic coordinates in the plane of rotation of the scattered NO molecule. This feature is similar to what has been seen experimentally. [ABSTRACT FROM AUTHOR]

New ab initio potential energy surfaces for the Ar–NO (X 2Π) system are reported based on correlated electron pair approximation (CEPA) calculations. The fitted, rigid-rotor surface was then used in full close-coupling calculations of differential and integral cross sections for excitation of NO at a center-of-mass energy of 442 cm-1 (0.0548 eV), as well as differential cross sections at lower energies of 119 and 149 cm-1 (0.0145 and 0.0185 eV). The calculated cross sections are compared with those determined using earlier electron-gas potential energy surfaces and with the results of available experimental measurements. In general, the new CEPA potential energy surfaces yield very good agreement with available experimental integral and differential cross sections. Both theory and experiment reveal a significant tendency for population of final rotational states of Π(A‘) reflection symmetry. [ABSTRACT FROM AUTHOR]

We report multireference configuration-interaction calculations for the lowest potential energy surfaces of the B(2s22p 2P)...H2 van der Waals molecule. The degeneracy of the 2p orbital implies that there exist three adiabatic potential energy surfaces (two of A’ symmetry and one of A‘ symmetry in Cs geometry) which become degenerate at large B–H2 separation. By assuming that the two adiabatic states of A’ symmetry correspond primarily to an orthogonal transformation of the in-plane B 2p orbitals, one can use calculated matrix elements of the electronic orbital angular momentum to transform to an approximate diabatic representation, which involves four potential energy functions. The proper angular expansion of these functions in terms of reduced rotation matrix elements is discussed and an analytic representation of the calculated points is obtained. The minimum energy of the B(2s22p 2P)...H2 van der Waals molecule is predicted to occur in C2v geometry with an electronic symmetry of 2B2, at a B–H2 distance of 3.11 Å, and a dissociation energy De of 121 cm-1. For the interaction of B(2P) with p-H2, assumed spherical in j=0, the zero-point corrected dissociation energy is D0=25 cm-1. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for the interaction of Li2(A 1Σ+u) with Ne has been computed using highly correlated multiconfiguration–reference configuration expansions (MRCI) and a large basis set. From the calculated points an analytical fit of the potential was obtained. Particular care was used to ensure a smooth fit to the angular dependence of this highly anisotropic potential. This PES has been used in exact close-coupling (CC) quantum scattering calculations of cross sections for rotationally inelastic collisions. The dependence of the calculated cross sections on velocity, as well as on the initial and final states, is found to be in excellent agreement with the measurements of Smith, Scott, and Pritchard [J. Chem. Phys. 80, 4841 (1984);81, 1229 (1984)]. For comparison, cross sections were also computed within the coupled-states (CS) approximation. At low collision energies the CS results deviate significantly from both the exact CC results and the experimental data. [ABSTRACT FROM AUTHOR]

We present a new potential energy surface for the interaction of the NO molecule with a close-packed Ag(111) surface obtained using the recently developed corrected effective medium (CEM) method. The dual NO–Ag(111) interaction potentials, which reflect the electronic orbital degeneracy of the NO molecule in its ground(X 2Π) electronic state, were determined at a large variety of distances and orientations of the NO molecule above the surface, and for approach of the molecule over three different sites on the surface unit cell, described by a cluster of greater than 10 Ag atoms. The more than 600 calculated interaction energies were then fit to a compact analytic form, which allows the determination of the interaction potential at any position above the surface. The strongest binding corresponds to a nearly flat orientation of the NO molecule, with approach of the N end slightly preferred, and with the single antibonding π orbital of NO pointing toward the surface. In this geometry, the binding energy above, respectively, the atop, two-fold bridge, and three-fold center sites site is 2200, 1773, and 1574 cm-1. By contrast when the antibonding π electron is oriented in a plane parallel to the surface, the binding energy diminishes dramatically, and the dependence of the interaction energy on the orientation of the NO molecule is also diminished. Here, the three-fold center site is preferred, but with a well depth of only 885 cm-1 (0.110 eV). [ABSTRACT FROM AUTHOR]