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Notes: The photodissociation of H2 S in the first absorption band is studied by time-dependent wave packets evolving in two electronic states; the lower state is dissociative and the upper one is bound. The adiabatic potential energy surfaces and transition dipole functions are constructed from ab initio calculations while the nonadiabatic coupling is adjusted. The diffuse structure superimposed on the broad absorption spectrum is due to symmetric stretch motion in the upper (bound) electronic state which is strongly quenched by nonadiabatic coupling. This is different from the photodissociation of water in the first band.

Notes: The photodissociation of NH2→NH(A 3Π)+H was investigated by photolyzing NH2 in a flow system with tunable synchrotron radiation from 200 to 105 nm and other vuv light sources. The NH photofragments were analyzed by their triplet emission at 336 nm. Additionally, ab initio configuration interaction calculations were performed for the electronic states of NH2 involved in the photodissociation process. Vertical excitation energies, bending potentials for the excited states, Franck–Condon factors, and transition moments were calculated in order to interpret the experimental observations. The following picture evolves for the dynamics of the NH2 photodissociation: At about 7.8 eV, NH2 is excited to the 2 2A1(A') state, which possesses the same bending angle as the X˜ 2B1 ground state. The upper state correlates with the fragments NH(A 3Π)+H. Since the bending angle is not changed, the NH(A) radicals are formed with little rotational excitation. However, the symmetric stretch becomes excited at the beginning of the dissociation leaving the NH(A) fragment with vibrational excitation. Because of symmetry conservation, the formation of the Π(A') component of NH(A) is preferred. In the region of ∼9 eV, transitions to the 1 2A2 and/or 3 2B1 states (both have A‘ symmetry in Cs) occur. The bending potentials of both states have minima for linear configurations. Therefore, the structure of the excitation spectrum is determined by a progression in the bending motion and a preferred population of high rotational NH(A) levels is observed. Vibrational excitation is small suggesting that the unbroken NH bond stays unchanged during the dissociation process. According to symmetry conservation, the Π(A‘) component of NH(A) is preferably formed.

Notes: In order to interpret the experimental results of the state resolved UV-laser-induced desorption of NO from NiO(100) (rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.), we have performed ab initio complete active space self-consistent field (CASSCF) and configuration interaction (CI) calculations for the interaction potential between NO and the NiO(100) surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO(100)–NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO8−5-cluster embedded in a Madelung potential has been used for representing the NiO(100) surface. The excited states which are important for the desorption process, are charge transfer states of the substrate–adsorbate system, in which one electron is transferred from the surface into the NO-2π-orbital. The potential curves of these excited charge transfer states show deep minima (4 eV–5 eV) at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO−-like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fs≤τ≤25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential. © 1996 American Institute of Physics.

Notes: Quantum chemical ab initio calculations at the complete active space SCF level and with inclusion of correlation effects have been performed for the potential energy surfaces of PH in its X 3Σ− ground state and its first excited triplet state, A 3Π, colliding with He atoms. The PH distance was fixed at its experimental value (of the A 3Π state), the PH–He distance and the HePH angle were varied. All three potential energy surfaces [1 3A′′ for PH(X)–He and 1 3A,2 3A′′ for the two components of PH(A)–He] are purely repulsive, except for very shallow van der Waals minima with well depths of about 15–40 cm−1. The interaction potentials decay approximately exponentially with increasing PH–He distance and show large angular anisotropies. Legendre expansions for the angular dependence of the potential surfaces converge slowly for V(1 3A′′) and the sum potential 1/2[V(2 3A′′)+V(1 3A)], but rapidly for the corresponding difference potential 1/2[V(2 3A′′)−V(1 3A)]. The present PH(A)–He potentials have been used in the companion paper by Neitsch et al. [J. Chem. Phys. 106, 7642 (1997)], for the calculation of thermal state-to-state rate constants for inelastic PH(A)–He collisions. © 1997 American Institute of Physics.

Notes: We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A˜ state and a calculated SCF dipole moment function for the X˜→A˜ transition. The dynamical calculations are performed within the infinite-order-sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of "vibrational'' structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A˜ state potential energy surface and details of the dissociation dynamics. Vibrational excitation of the OH and OD products is significantly wavelength dependent. The distribution of the three lowest vibrational states at 157 nm is in good accord with recent LIF measurements. Particular attention is paid to the sensitivity of the final results with respect to the coordinate dependence of the transition dipole function, the parent nuclear wave function and the excited state potential energy surface.