Ab Initio Calculations and Franck-Condon Simulation of the Absorption Spectra of GeCl_2 Including Anharmonicity

摘要

Geometrical parameters,vibrational frequencies and relative electronic energies of the X~1A_1:a~3B_1 and A~1B,states of GeCl_2 have been calculated at the CCSD(T) and/or CASSCF/MRCl 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 X'A,and a~3B_1 states,and a CASSCF/MRCI/aug-cc-pVQZ PEF of the A~1B_1 state of GeCl_2 are reported.Anharmonic vibrational wavefunc-tions of these electronic states of GeCl_2,obtained variationally using the computed PEFs,are employed to calculate the Franck-Condon factors (FCFs) of the a-X and A-X transitions of GeCl_2.Simulated absorption spectra of these transitions based on the computed FCFs are compared with the corresponding experimental laser-induced fluorescence (LIF) spectra of Karolczak et al.[J.Chem.Phys.7993,98,60-70].Excellent agreement is obtained between the simulated absorption spectrum and observed LIF spectrum of the a-X transition of GeCl_2,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 A-X transition of GeCl_2 leads to a revision of vibrational assignments of the LIF spectrum and suggests that the X~1A_1 state of GeCl_2 was prepared in the experimental work,with a non-Boltzmann vibrational population distribution.The X(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 X(1,0,0) and X(0,10) levels.