De-Chao Wang

Franck–Condon analysis of photoelectron and electronic spectra of small molecules

Abstract Currently available methods for the simulation of Franck–Condon envelopes of electronic and photoelectron spectra of small to medium-sized molecules are reviewed and discussed. The advantages of combining ab initio molecular orbital calculations with Franck–Condon calculations to simulate vibrationally resolved electronic and photoelectron spectra are outlined and the ability of such calculations to assign the vibrational components observed in these spectra and to derive excited or ionic state geometries is emphasized. Simulation of the emission spectrum of CF 2 is taken as an example where a harmonic oscillator model has been used for each electronic state to derive vibrational eigenvalues and eigenvectors and hence calculate Franck–Condon factors. The possibility of improving this model by the use of anharmonic potentials is identified as a likely future development.

The X̃ 2B1, 2B2, 2A1, and 2A2 states of oxygen difluoride cation (F2O+): High-level ab initio calculations and simulation of the ultraviolet photoelectron spectrum of F2O

The ultraviolet photoelectron spectrum of F2O 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 X 1A1 state of F2O and the X 2B1, 2B2, 2A1, and 2A2 state of F2O+, and their potential energy functions were computed. Spectral simulations based on Franck–Condon factor calculations including the Duchinsky effect were carried out within the harmonic oscillator model and also with the inclusion of anharmonicity, in order to assist spectral assignment. Based on the computed ionization energies obtained with the coupled cluster and multireference configuration interaction methods with basis sets of up to quintuple zeta quality, the order of the low-lying cationic states of F2O+ has been firmly established. However, the detailed assignment of the overlapping second and third photoelectron bands was only achieved with the aid of spectral simulation. The it...

Simulation of photoelectron and electronic spectra of small molecules

Abstract In this contribution, a description is given of the application of high-level ab initio calculations and Franck–Condon analysis to study some photoelectron and electronic spectra, where there were uncertainties in their assignments. The spectra chosen are the He I photoelectron spectrum of BrO 2 and the single vibrational level (SVL) dispersed fluorescence spectra of AlCN and AlNC. With suitable ab initio calculations and subsequent spectral simulations, assignments of these spectra can now be made. In addition, the iterative Franck–Condon analysis (IFCA) procedure was also applied to some of these spectra to derive excited and ionic state geometries. In the investigation of the He I photoelectron spectrum observed when O( 3 P) reacted with Br 2 ( 1 Σ g + ), calculations were performed on the low-lying triplet states of both BrOBr and BrBrO and their low-lying cationic quartet states for the first time. It was found that there are a number of weakly bound triplet radical-radical states and quartet radical-cation states for both BrOBr and BrBrO.

HeI photoelectron spectra of PH2 and PF2: comparison between simulation and experiment

Molecular orbital calculations on PH2 and PF2 and some of their low-lying cationic states, followed by Franck-Condon calculations, have been performed with the objective of simulating HeI photoelectron band of these radicals. The molecular orbital calculations involved MP2 and CCSD(T) geometry optimization and frequency calculations, with basis sets of size up to 6-311G(3df,2p), and as well as G1/G2 calculations. Franck-Condon simulations of photoelectron bands were performed using force constants derived from the ab initio calculations. Based on comparison between simulated and observed spectra, the first adiabatic ionization energy of PH2 has been confirmed as (9.84 ± 0.01) eV and the lowest singlet-triplet separation in PH2+ (1A1 − 3B1 has been deduced as (0.78 ± 0.04) eV. Also, the first adiabatic ionization energy of PF2, corresponding to the ionization PF2+ 1A2 ← PF2 X2B1, has been established as (8.84 ± 0.01) eV. The vibrational structure observed in the first band of PF2 has been assigned to excitation of the symmetric stretching mode (v1) in PH2 in PF2+(X1A1) and the vibrational structure observed in the second band of PH2 has been assigned to excitation of the deformation mode (v2) in PH2+(a3B1).

THEORETICAL STUDY OF THE HE I PHOTOELECTRON SPECTRA OF HBS AND FBS

Abstract MP2 and CCSD(T) geometry optimization and harmonic vibrational frequency calculations were performed on the X1∑+ states of HBS and FBS and the X2∏ and A2∑+ states of their cations with basis sets as large as 6-311G(2df,2p). Franck-Condon analyses and spectral simulation were carried out on the first two He I photoelectron bands of these species. The theoretical spectra obtained by employing the ab initio geometries, vibrational frequencies and force constants agree well with the observed ones.