THEORETICAL-STUDY OF THE VIBRATIONAL STRUCTURE OF THE HE-I PHOTOELECTRON-SPECTRUM OF H2SE

CCSD(T)/6-311G(2d,2p) geometry optimization and harmonic vibrational f requency computation were carried out on the X(1)A(1) state of H2Se an d the X(2)B(1), A(2)A(1), and (BB2)-B-2 states of H2Se+. The B3LYP/6-3 11G*, B3LYP/6-311G(3df,2p), CCSD(T)/LanL2DZS++(2d,2p), and CCSD(T)/La nL2DZ++(3df,2p) calculations were also performed on the ground states of the neutral and cation. Furthermore, vertical and adiabatic ionizat ion energies were computed at the CCSD(T)/6-311G(3df,2pd)//CCSD(T)/6-3 11G(2d,2p) level for the ionization processes to the three low-lying c ationic states. Franck-Condon analyses and spectral simulations were t hen performed for the first two He I photoelectron bands of H2Se emplo ying the ab initio data and FC procedures with the use of the harmonic oscillator approximation and the Duschinsky effect. Comparing the obs erved and simulated spectra as obtained at different cationic geometri es, those of the two lowest-lying cationic states of H2Se+ that give t he best spectral agreement are as follows: the X(2)B(1) state: r(e) = 1.471 +/- 0.001 Angstrom and theta(e) = 91.5 +/- 0.2 degrees; the A(2) A(1) state: r(e) = 1.475 +/- 0.004 Angstrom and theta(e) = 126.3 +/- 0 .5 degrees.