Potential energy functions of the X [sup 2]B[sub 1], Ã [sup 2]B[sub 2], B [sup 2]A[sub 1], and C [sup 2]A[sub 2] states of Cl[sub 2]O[sup +] and the X [sup 1]A[sub 1] state of Cl[sub 2]O: Franck–Condon simulations of photoelectron bands of Cl2O which include anharmonicity

Abstract

Restricted-spin coupled-cluster single and double plus perturbative triple excitations [RCCSD (T)] and/or complete-active-space self-consistent-field multireference internally-contracted configuration interaction (CASSCF/MRCI) potential energy functions of the X˜ 2B1 , A˜ 2B2 , B˜ 2A1 , and C ˜ 2A2 states of Cl2O1 and the X˜ 1A1 state of Cl2O, with basis sets of up to the augmented-correlation-consistent-polarized-valence-quadruple-zeta quality, have been reported. For each of these states, vibrational wave functions of the symmetric stretching and bending modes have been computed, employing the potential energy function obtained at the highest level of calculation, with Watson’s Hamiltonian and anharmonic vibrational wave functions expressed as linear combinations of harmonic basis functions. The helium I photoelectron spectrum of Cl2O has been simulated with Franck–Condon factors calculated using computed anharmonic vibrational wave functions and allowing for Duschinsky rotation. The adiabatic ionization energies ~AIEs! to the four lowest cationic states of Cl2O1 have been evaluated at the RCCSD~T! level with basis sets of up to polarized-valence-quintuple-zeta quality and by various extrapolation techniques to the basis set limit. Revised equilibrium geometrical parameters of the X˜ 2B1 and C ˜ 2A2 states of Cl2O1 were obtained from the iterative Franck–Condon analysis procedure, and revised AIEs for the A˜ 2B2 and B˜ 2A1 states of Cl2O1 were estimated based on comparison between the simulated and observed photoelectron spectra. It was found that inclusion of anharmonicity in the Franck–Condon factor calculations for each electronic state improves the quality of the simulated spectrum. The computed T1 diagnostics from the RCCSD calculations suggest that the B˜ 2A1 state of Cl2O1, with the ¯(9a1)1(3b2)2(10a1)0 electronic configuration, possesses multiconfigurational character in the region of r(ClO)51.87Å and u~ClOCl!5125°. CASSCF/MRCI/aug-cc-pVQZ~no g! calculations show an avoided crossing between the B˜ 2A1 state and (2)2A1 state @with the ¯(9a1)2(3b1)0(10a1)1 electronic configuration#, in the region of 1.96.r.1.80Å and 137.0.u.132.0°.