Our procedure for employing analytical gradients of ab initio potential energy hypersurfaces in the description of centrifugally distorted molecules is applied for the first time to an asymmetric top, namely ozone. Both single determinantal (HF/6-31G*) and analytically fitted multiconfigurational self-consistent field surfaces were utilized. The focus of the HF/6-31G* study is upon the centrifugal distortion pathway. Quartic centrifugal spectroscopic coefficients are obtained in both cases and are in reasonable agreement with experiment.

Theory of anharmonically modified Coriolis coupling in the S 1 state of benzene and relation to experiment J. Chem. Phys. 95, 872 (1991); 10.1063/1.461095Erratum: Vibronic spectra of the benzene crystal at 4.2 K using twophoton fluorescence excitation [J.Photochemistry of fluoro(trifluoromethyl)benzenes. II. Fluorescence spectra and quenching of singlet state emission in 1fluoro2, 1fluoro3, and 1fluoro4(trifluoromethyl)benzenes in the gas phase

A treatment of one-or two-photon fluorescence excitation spectra is described using the vibration-rotation coupling of zeroth order states in the excited electronic state and nonadiabatic coupling to the ground state. Using perturbation theory, experimental harmonic frequencies, an anharmonic force field, and various theoretical Coriolis coupling constants, a quasistationary molecular eigenstate in an excited electronic state S, is first calculated. The S, eigenstate is then coupled via the nonadiabatic nuclear kinetic energy operator (internal conversion) to rovibronic states in the ground state manifold, the latter states approximated in a simple manner. A search algorithm is used to select the S, dark states and the Sc states. Both the perturbation theory coefficient and the Franck-Condon factors are employed in the evaluation function used in the search. The results are applied in part II to the channel three problem in benzene.

A b i n i t i o SCF and CI calculations of the dipole moment function of ozonePotential energy surfaces for the ground and excited electronic states responsible for the Hartley continuum of ozone are used to obtain quadratic, cubic, and quartic force constants. Vibrational dependence of rotational constants to sixth order is calculated by perturbation theory. The spectroscopic constants enable computation of rovibronic energy levels. Overlap of ground state and excited state perturbed vibrational wave functions yield Franck-Condon factors. Electric dipole allowed rovibronic transitions are generated under the I r representation. The entire set of results generate the ultraviolet absorption spectrum. It is shown that inclusion of anharmonic terms in the vibrational Hamiltonian has a small effect upon the final spectrum, whereas rotational broadening plays a greater role in achieving agreement with experiment. a Frequencies are the harmonic frequencies; the fundamentals VI' V2' and V3 are given in parentheses. bReference 5. C Imaginary frequency.

A voided crossings between quasidegenerate rovibrational states in the Doppler-free twophoton excitation of the 141 mode in the SI excited state of benzene are treated theoretically. Two sets of avoided crossings in plots of spectral line frequency vs J at a given K and !::J( have been reported experimentally between an initially prepared "light" state (14 1 in zeroth order) and dark states, namely, one which in zeroth orderis a 5 1 10 1 16 1 state, the other being in zeroth order a 6 2 111 and/or possibly a 3 1 16 1 state, implicated earlier by Neusser et al. The identification of these states makes the phenomenon an excellent candidate for treatment of the avoided crossing via a Van Vleck transformation, no other basis set states being needed for the diagonalization in order to extract the important features. Two successive transformations are used for handling direct coupling and coupling via virtual states. The dominant calculated contribution to the coupling is, jointly, Coriolis plus cubic--cubic anharmonic interactions between vibrational modes. Playing less of a role are Coriolis terms in which the inverse moment of inertia tensor is expanded up to quadratic terms in the coordinates. There results a 5 X 5 (for coupling to 5 1 10 1 16 1 ) and a 3 X 3 (for coupling to 6 2 111 or 3 1 16 1 ) matrix of the transformed Hamiltonian, each of which can also be described, if desired, to a very good approximation by a 2 X 2 matrix. The coupling element Vo and the difference of the rotational constants for the light and dark states (AB) are obtained from the plots of line position vs J(J + 1) obtained. For the 141 to 5 1 10 1 16 1 and for the 141 to 6 2 111 couplings the theoretical results are in reasonable agreement with the experimental results, no adjustable parameters being employed. For a coupling of 141 to 3 1 16 1 the calculated Vo would be much too high compared with experiment (a factor of 10), the coupling involving the exchange of only three instead off our vibrational quanta. A situation in which the 141 state is coupled to the 6 2 111 state to yield an avoided crossing and off-resonantly coupled to the 3 1 16 1 state would be consistent with some experimental results and not affect the reasonable agreement of the slope difference and splitting for the avoided crossing plots. 872 J. Chem. Phys. 95 (2), 15