No abstract

The ultraviolet photoelectron spectrum of F 2 O was recorded with a higher resolution than previously published. New vibrational structure was observed in the second and third bands. Near state-of-the-art molecular orbital calculations were performed on the

CASSCF and QCISD calculations were carried out on the X̃A1 state of Cl2O and the X̃B1, Ã2B2, B̃2A1, and C̃2A2 states of Cl2O+ in order to obtain their minimum-energy geometries and harmonic vibrational frequencies. Cl2O+ ← Cl2O X̃1A1 vertical and adiabatic ionization energies were also computed at the RCCSD(T)//QCISD level. Spectral simulations based on the ab initio results were performed for the first four bands of the He I photoelectron spectra of Cl2O. Iterative Franck−Condon analyses were also carried out. Comparison between the simulated and the observed spectra gave, for the first time, experimentally derived geometries, for the X̃2B1, Ã2B2, and C̃2A2 cationic states. The adiabatic ionization energy position and the vibrational assignments of the Cl2O+ Ã2B2 ← Cl2O X̃1A1 band were revised, based on spectral simulations, which included “hot” bands.

A variety of correlated molecular orbital methods and basis sets have been employed to obtain the minimumenergy geometries, harmonic vibrational frequencies, and relative energies of the X ˜1A 1 , a ˜3B 1 , and A ˜1B 1 states of SiCl 2 . The ab initio results obtained have been compared with experimental values, where available. It was found that ab initio methods which are based on unrestricted-spin (UHF) wave functions employing spin-unprojected energies, including the QCISD(T) and CCSD(T) methods and the composite methods of G1 and G2, failed to give a reliable A ˜1B 1 -X ˜1A ˜1 separation, whereas methods using spin-projected energies or the restricted multireference method MR-CISD/6-311G (2df) gave reliable A ˜-X ˜and a ˜-X ˜separations. The A ˜1B 1 -X ˜1A ˜1 and a ˜3B 1 -X ˜1A 1 emission spectra of SiCl 2 were simulated, employing MP2/6-311G (2df) force constants and compared with available experimental spectra. The geometry of the X ˜state was held fixed at the geometry determined by microwave spectroscopy, and the geometries of the a ˜3B 1 and A ˜1B 1 states were adjusted via an iterative Franck-Condon analysis (IFCA) procedure until the simulated spectra matched best with the observed spectra. The IFCA derived geometry for the A ˜1B 1 state is r(SiCl) ) 2.055 ( 0.008 Å and θ(ClSiCl) ) 119.4°( 0.4°. For the a ˜3B 1 state, r(SiCl) ) 2.041 ( 0.005 Å, while the (ClSiCl) angle can have a value of either 115.4°or 114.5°, depending on the vibrational assignments of the experimental spectra.