Quantitative results on photodissociation of CH2(X̃ 3B1) through the coupled 2 A″ and 3 A″ states are presented. A three-dimensional, hybrid time-dependent quantum dynamical method was used, employing diabatic potential energy surfaces taken from ab initio calculations. In agreement with experiment and recent approximate dynamical calculations, a small product fraction (2.7%) was computed for the CH(A2Δ) + H channel. The dissociation proceeds mostly on an A2-like diabatic surface, into CH (a4Σ−) + H (93.3%) and C(3P) + H2 (4.0%). Model calculations show that the initial motion on the ‘bright’ B1-like surface biases the outcome of the dissociation in favour of CH + H.

We present quantitative results on photodissociation of CH2(X̃ 3B1) through the coupled 2A′′ and 3A′′ states. A three-dimensional, hybrid quantum dynamical method was used, employing hyperspherical coordinates. The diabatic potential energy surfaces (PES’s) used in the dynamics were derived from ab initio calculations. A small product fraction (2.7%) was computed for the CH(A 2Δ)+H channel, in agreement with experiment and approximate dynamical calculations. The dissociation proceeds mostly on a A2-like diabatic surface, into CH(a 4Σ−)+H(93.3%) and C(3P)+H2(4.0%). Resonances of widths in the range 0.1–10 meV affect the photodissociation. Pre-exciting a vibrational mode of CH2(X̃ 3B1) prior to photodissociation does not alter the picture, except if the antisymmetric stretch mode is excited: In this case the product fractions for the C(3P)+H2 and CH(A 2Δ)+H channels collapse to values of 1% or lower, and the resonances disappear. Model calculations show that the large product fraction found for CH(a 4Σ−)+H is due to the initial motion on the “bright”B1-like surface, which biases the outcome of the dissociation in favor of CH(a 4Σ−)+H.

Ab initio MRD-CI calculations are presented for the photodissociation of the HCO + ion, which plays an important role in the chemistry in interstellar space and in combustion flames. One-dimensional potential energy curves along the C-H bond length coordinate are presented for the lowest eight electronic singlet states. The calculations show that the 1 1 Π state is the lowest excited state through which dissociation is possible and which can be reached by dipole-allowed transitions. The photodissociation dynamics on the 1 1 Π potential and through the higher-lying 2 and 3 1 Σ + states are studied in the one-dimensional approximation, using a time-independent quantum mechanical treatment. The geometry dependence of the electric dipole transition moment is taken into account

Cross sections and rate constants are presented for the rotational excitation of OH in collision with ortho and para-H2, using a new ab initio interaction potential [Offer and Van Hemert, J. Chem. Phys. 99, 3836 (1993)]. The cross sections are given at a number of energies and are compared with those calculated using an earlier potential energy surface, and with the available experimental results. A strong oscillatory behavior is found in the cross sections for collisions with ground state para-H2 which was not apparent in earlier calculations. The oscillatory behavior is very much reduced in collisions with ortho-H2. Rate constants obtained by averaging the cross sections over a Maxwell–Boltzmann velocity distribution are given at a temperature of 300 K. Expressions for calculating the hyperfine resolved cross sections by transforming the S-matrices are discussed for the case where H2 is no longer constrained to its rotational ground state, and cross sections for transitions between the hyperfine resolved levels are given for collisions with both para and ortho-H2.

We present quantitative results on photodissociation of CH2 (($) over tilde X B-3(1)) and its isotopomers CHD and CD2 through the first excited triplet state (1 (3)A(1)). A two-dimensional wave packet method employing the light-heavy-light approximation was used to perform the dynamics; The potential energy surfaces and the transition dipole moment function used were all taken from ab initio; calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks,in experimental CH2 and CD2 3 + 1 resonance enhanced multi-photon ionization spectral provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape Of the total absorption spectra of vibrationless and vibrationally excited CH2, trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD.

A new two-configuration self-consistent field (SCF) plus dispersion potential for the interaction of OH with H2 is presented. The ground state of OH is a 2Π state leading to a two component potential which is diagonal in the adiabatic electronic basis in which the quantum chemical calculations are done. The transformation between the adiabatic basis and the electronic basis used in collision calculations is described. The transformed potential matrix is Hermitian in this electronic basis when OH–H2 configurations with no plane of symmetry are considered. The orientation dependence of the elements of the potential matrix is given in the form of a spherical expansion and the results are compared with previous potential calculations. The comparison is discussed in terms of the dependence on the orientation of the H2 molecule.

The photodissociation processes of CH2 into CH and H have been studied using ab initio multireference configuration-interaction methods. Two-dimensional potential energy-surfaces of the ten lowest triplet states correlating with the seven lowest states of CH have been calculated as functions of bond angle and one C-H bond distance, keeping the other C-H distance fixed at the equilibrium CH2 value. Transition dipole moments connecting the excited states with the ground state have been obtained as well. It is shown that efficient photodissociation of CH2 into CH (X 2PI)+H can occur by absorption from the ground X 3B1 (1 3A") state into the 1 3A1 (1 3A') state at about 6.3 eV. Photodissociation into excited CH (a 4SIGMA-) +H can take place through the 1 3A2 (2 3A") and 2 3B1 (3 3A") states, although in a more complex manner since several avoided crossings occur along the reaction path. The 1 3A2 state is a so-called low-angle state, which has an equilibrium bond angle of less than 60-degrees and correlates directly with C(3P)+H-2. At 180-degrees, when the molecule has D(infinity h) or C(infinity v) symmetry, interesting crossings between the ground and low-lying surfaces are found. Altogether, these crossings and correlations are predicted to lead to complicated dissociation dynamics for most of the states. The higher-lying states of CH2 can photodissociate either directly into excited states of CH, or they can be predissociated by the repulsive 1 5A2 (1 5A") state, which correlates with CH (a 4SIGMA-)+H.

Rovibrational levels of HeH+ have been calculated by numerical integration of the Schrödinger equation with a potential taken from the literature. After corrections were applied to the potential by using experimental data of the v = 1 ← 0 and v = 2 ← 1 bands of HeH+, rovibrational levels with 0 ≤ J ≤ 15 and 0 ≤ v ≤ 4 for HeH+ and HeD+ were predicted. The v = 3 ← 2 band of HeH+ and the v = 2 ← 1, v = 3 ← 2, and v = 4 ← 3 bands of HeD+ have been observed with a tunable diode laser system. The mean difference between experimental measurements and theoretical predictions is less than 0.1 cm−1. Molecular constants for both isotopic species were determined by a fit to all available data.

Calculations of the rotational excitation of H3O+ in collisions with H2 take the same form as calculations for NH3H2 collisions. However, calculations of the rotational excitation of NH3 in collisions with H2 usually neglect the tunnelling motion of the N nucleus. For the H3O+ molecule the inversion frequency is much higher and the question arises whether the inversion motion has a significant effect on the collision cross sections for H3O+H2 collisions. The effect of the tunnelling motion has been investigated using a model calculation for para-H3O+-para-H2 collisions. It is found that the effect of the tunnelling motion on the rotational cross sections is small.

Potential energy curves are calculated for the ten lowest states in HeH which correlate with the hydrogen asymptote in the n=1, 2, 3 occupation; these are X, A, C, D, 5 2Σ+, 6 2Σ+, and B, E, 3 2Π as well as the 1 2Δ states. Multireference configuration interaction calculations are employed thereby in an atomic orbital (AO) basis of contracted Gaussians. Extensive calculations of the ∂/∂R, ∂2/∂R2, Lx, and L2 matrix elements are carried out to account explicitly for the effects beyond the Born–Oppenheimer approximation. The positions of rovibrational levels are thereby determined in pairwise close-coupling calculations for the X/A and C/D states of 2Σ+ symmetry for the four isotopomers 4HeH, 3HeH, 4HeD, and 3HeD. Radial, angular, and mass polarization corrections affect the A and C states differently, so that the A–C energy gap increases by 39 cm−1 in 3HeD and by 53 cm−1 in HeH upon introduction of these terms, e.g., whereby the contribution of the mass polarization is by far the smallest. By employing a two-parameter correction function to the calculated electronic potential energy and making use of the calculated non-Born–Oppenheimer terms, a large number of levels for the A, C, and D states as a function of (v,J) quantum numbers are computed which agree with those, which are experimentally available for the C–A and D–A transitions within wave number accuracy.

Potential energy curves for states of HBr2+ which correlate with the first six dissociation limits are calculated employing multi-reference configuration-interaction (MRD-CI) treatments in a Gaussian AO basis. Tunneling rates for the X 3Σ−, a 1Δ and b 1Σ+ vibrational levels in HBr2+ and DBr2+ are calculated as well as predissociation rates for vibrational levels of the a 1Δ and b 1Σ+ states resulting from spin-orbit interaction with the repulsive 3Π state. The spin-orbit coupling elements are evaluated explicitly by employing the Breit-Pauli operator and the MRD-CI wavefunctions.

Potential energy curves for all HCl2+ states which correlate with the first five dissociation limits are calculated employing multi-reference configuration-interaction (MRD CI) treatments in a Gaussian AO basis containing up to four d and one f function for chlorine and two p functions for hydrogen. Tunneling rates for the X 3Σ1, a1Δ and b 1Σ+ vibrational levels in HCl2+ and D are calculated as well as predissociation rates for vibrational levels of the a 1Δ and b 1Σ+ states due to spin-orbit interaction with the 3Π state. The spin-orbit coupling elements are evaluated explicitly by employing the Breit-Pauli operator and the MRD CI wavefunctions. Comparison with previous works is given.

The photoisomerization of cyclohexadiene to cis-hexatriene has been investigated using the multi-reference configuration interaction (MRD CI) method involving all active valence electrons. It has been determined that the initial absorption process in cyclohexadiene is nonvertical, and that the excitation energy and oscillator strength obtained are in good agreement with gas phase absorption measurements. The excited state which is initially populated in the absorption process can undergo rapid nonradiative relaxation to a lower-lying excited state with a barrierless potential leading to formation of the ground state hexatriene photoproduct. The large absorption linewidth which is experimentally observed is correlated with rapid nonradiative relaxation in the excited state which is both photophysical and photochemical in nature. The effect of orbital symmetry upon the reaction, as well as the dipole moments of the excited states are also discussed.

The most recent ArH potential energy curves and transition dipole moments are employed to study the emission spectrum of ArH and ArD. The transition probabilities for radiation from the υ' = 0, 1 and 2 levels of the bound A2Σ+, 32Σ+, 12Π and 22Π states to the X2Σ+ ground state are calculated and the radiative lifetimes of the upper states determined. It is found that the A2Σ+ and 32Σ+ states interact strongly with the ground state via radial coupling. The predissociation (non-radiative) lifetimes of the A2Σ+ and 32Σ+ states are calculated in the Fermi-Wentzel golden rule expression and are found to be orders of magnitude smaller than the corresponding radiative lifetimes so that no observable emission spectra for the A2Σ+ or 32Σ+ is to be expected. The observed continous emission in the 2000–4000 Å region is assigned as resulting from a 12Π(υ′ = 0)-X2Σ+ transition with a radiative lifetime for the upper state of 21 ns; the calculated intensity as a function of wavelength matches very well the measured spectrum. A close coupling treatment for the 22Π-(A2Σ+, X) transition is also carried out and emission line profiles are calculated; the line widths are in good agreement with those derived from the golden rule predissociation rates and suggest that the latter perturbative approach is applicable in the present system. The observed discrete emission spectrum with line broadening in the 7600–7800 Å region must be assigned to the 22Π-A2Σ+ transition. The calculated rotational coupling matrix elements between 22Π and the X2Σ+ state yield linewidth in the order of 10-7 cm-1 and suggest no predissociation of the 2Π states. Hence emission 12Π-A2Σ+ around 4600–4900 cm-1, 22 Π-12Π and 22Π-32Σ+ in the 8000 cm−1 range should be observable.

Charge-transfer (sublevel) cross sections for ${\mathrm{He}}^{2+}$+H collisions have been calculated in the 20-eV--to--10-keV center-of-mass energy region. Both a time-independent quantum-mechanical close-coupling method (20--500 eV) and a semiclassical impact-parameter method (0.1--10 keV) were used. The close-coupling formalism is developed in terms of a molecular-state description of the ${\mathrm{HeH}}^{2+}$ system and is extended to include both radial and rotational couplings. The potentials and radial and rotational couplings for the 2s\ensuremath{\sigma}, 2p\ensuremath{\sigma}, 3d\ensuremath{\sigma}, and 2p\ensuremath{\pi} molecular states were computed by expressing the molecular orbitals as variationally determined linear combinations of Slater-type orbitals centered at the atoms. Common molecular-electron translation factors were introduced to ensure proper asymptotic behavior of the wave functions and were found to be crucial for agreement between calculated and experimental cross sections. The usual practice of compensating for the neglect of these factors by using couplings evaluated at an atomic origin rather than at the center of mass of the nuclei is analyzed. Quantum-mechanical and semiclassical results are compatible only if Coulomb trajectories instead of straight-line trajectories are used. In particular the sublevel cross section for ${\mathrm{He}}^{+}$(2${\mathrm{p}}_{\ifmmode\pm\else\textpm\fi{}1}$) production is found to be extremely sensitive to the trajectory. At all energies above 50 eV radial and rotational couplings are about equally important. Below 100 eV cross sections fall off rapidly, and radiative charge transfer becomes the dominant process below 25 eV.

A systematic investigation of the excited states that may participate in the photodissociation of the OH molecule has been carried out by ab initio calculations which use two different self-consistent field with configuration-interaction methods. Potential energy curves for states of 2Σ+, 2Σ−, 2Π, and 2Δ symmetries have been computed and the transition dipole moments connecting the states have been obtained. Photodissociation cross sections for absorption from the v″=0 vibrational level of the ground X 2Π state into the repulsive 1 2Δ, B 2Σ+, and 2 2Π states are reported. It is shown that the 1 2Δ and B 2Σ+ states, together with the 1 2Σ− state considered in an earlier study, are important channels for photodissociation. Photodissociation may take place also following absorption into bound electronic states and absorption oscillator strengths for these transitions have been determined. The mechanisms by which the bound states can be dissociated are discussed and estimates of the dissociation efficiencies of the mechanisms are made. Absorption into the bound 3 2Π state may be an important dissociation channel. Calculations of the interstellar photodissociation rate have assumed that photodissociation proceeds by absorption into the C 2Σ+ state for which a large oscillator strength had been adopted. We find that the C 2Σ+ channel is not significant in photodissociation and we present new estimates of the interstellar photodissociation rates. Our calculations demonstrate that OH is dissociated by the absorption of Lyman alpha radiation, a channel of particular significance in shocked interstellar gas and in cometary atmospheres.

The authors compare the emission spectral of a Na+Hg high-pressure discharge with spectral calculations using NaHg potentials recently reported by Huwel et al. (1981-2). The spectral calculations are based on, respectively, classical, semiclassical and quantum mechanical theory. They focus on the red part of the emission spectrum (630-1000 nm) and identify a satellite near 671 nm associated with the NaHg B 2 Sigma to X2 Sigma transition; the satellite is accompanied by an undulation structure which is accounted for by the quantum calculation. Based on the comparison between measured and calculated spectra the authors suggest some corrections to the NaHg A and B potentials of Huwel et al.