Ab initio calculations and franck-condon simulation of the absorption spectra of GeCl₂ including anharmonicity

Geometrical parameters, vibrational frequencies and relative electronic energies of the X̃1A1, ã3B 1 and Ã1B1 states of GeCl2 have been calculated at the CCSD(T) and/or CASSCF/MRCI level with basis sets of up to aug-cc-pV5Z quality. Core electron correlation and relativistic contributions were also investigated. RCCSD(T)/aug-cc-pVQZ potential energy functions (PEFs) of the X̃1A1 and ã3B1 states, and reported. Anharmonic vibrational wavefunctions of these electronic states of GeCl2, obtained variationally using the computed PEFs, are employed to calculate the Franck-Condon factors (FCFs) of the ã-X̃ and Ã-X̃ transitions of GeCl2. Simulated absorption spectra of these transitions based of the computed FCFs are compared with the corresponding experimental laser-induced fluorescence (LIF) spectra of Karolczak et al. [J. Chem. Phys. 1993, 98, 60-70]. Excellent agreement is obtained between the simulated obsorption spectrum and observed LIF spectrum of the ã-X̃ transition of GeCl2, which confirms the molecular carrier, the electronic states involved and the vibrational assignments of the LIF spectrum. However, comparison between the simulated absorption spectrum and experimental LIF spectrum of the Ã-X̃ transition of GeCl2 leads to a revision of vibrational assignments of the LIF spectrum and suggests that the X̃1A1 state of GeCl2 was prepared in the experimental work, with a non-Boltzmann vibrational population distribution. The X̃(0,0,1) level is populated over 4000 times more than expected from a Boltzmann distribution at 60 K, which is appropriate for the relative population of the other low-lying vibrational levels, such as the X̃(0,1,0) levels. KGaA, Weinheim.