Franck-Condon factors based on anharmonic vibrational wave functions of polyatomic molecules

Rodriguez-Garcia Valerie

Yagi Kiyoshi

Hirao Kimihiko

Iwata Suehiro

Hirata So

Journal of Chemical Physics

Franck-Condon (FC) integrals of polyatomic molecules are computed on the basis of vibrational self-consistent-field (VSCF) or configuration-interaction (VCI) calculations capable of including vibrational anharmonicity to any desired extent (within certain molecular size limits). The anharmonic vibrational wave functions of the initial and final states are expanded unambiguously by harmonic oscillator basis functions of normal coordinates of the respective electronic states. The anharmonic FC integrals are then obtained as linear combinations of harmonic counterparts, which can, in turn, be evaluated by established techniques taking account of the Duschinsky rotations, geometry displacements, and frequency changes. Alternatively, anharmonic wave functions of both states are expanded by basis functions of just one electronic state, permitting the FC integral to be evaluated directly by the Gauss-Hermite quadrature used in the VSCF and VCI steps [Bowman et al., Mol. Phys. 104, 33 (2006)]. These methods in conjunction with the VCI and coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] method have predicted the peak positions and intensities of the vibrational manifold in the X̃ 2B1 photoelectron band of H2O with quantitative accuracy. It has revealed that two weakly visible peaks are the result of intensity borrowing from nearby states through anharmonic couplings, an effect explained qualitatively by VSCF and quantitatively by VCI, but not by the harmonic approximation. The X̃ 2B2 photoelectron band of H2CO is less accurately reproduced by this method, likely because of the inability of CCSD(T)/ cc-pVTZ to describe the potential energy surface of open-shell H2CO+ with the same high accuracy as in H2O+.

Franck-Condon factors

vibrational states

wave functions

SCF calculations

configuration interactions

harmonic oscillators

molecular electronic states

coupled cluster calculations

perturbation theory

photoelectron spectra

water

positive ions

organic compounds

spectral line intensity

potential energy surfaces

eng

American Institute of Physics

Copyright (c) 2006 American Institute of Physics.

[ISSN] 0021-9606

[DOI] 10.1063/1.2209676

[NCID] AA00694991

[DOI] http://dx.doi.org/10.1063/1.2209676