Rotational and vibrational structure in the 288Â nm band system of the FeCl2 radical

The laser excitation spectrum of the 288 nm band system of FeCl2, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. Vibronic transitions are observed from the ground state to two close-lying excited electronic states that differ in inversion (g, u) parity. Two extensive progressions in the symmetric stretching vibration have been identified, referred to as Progressions A and B. The main features of Progression A, which is based on the ν00 band, are allowed transitions to the excited electronic state of ungerade symmetry. Progression B is built on the 301 band and consists of vibronically induced transitions to the gerade excited state. A substantial decrease in the symmetric stretching vibrational wavenumber is observed on excitation (ν1′=200.43(18),ν1″=352.39(13)cm-1). Local perturbations are found to cause relative shifts between the different isotopomers. Several vibronic bands have been recorded and analysed at rotational resolution for the three isotopomers Fe35Cl2, Fe35Cl37Cl, and Fe37Cl2 in natural abundance. All bands show perpendicular rotational structure of a linear molecule, and have been unambiguously assigned to a Ω = 5-4 transition, consistent with the inverted 5Δg ground state predicted by ab initio and DFT calculations. The zero-point averaged FeCl bond length is determined to be r0′=2.24506(46)andr0″=2.13198(38)Å in the upper and lower electronic states. The results show that the molecule is linear in both states.