The near ultraviolet spectrum of the FCO radical: Re-assignment of transitions and predissociation of the electronically excited state

Abstract

Cavity ring-down spectra of the FCO radical, recorded over the wave number range 29 500–31 600 cm−1 reveal rotational structure of the electronically excited state for the first time. The spectra demonstrate the need for a complete re-assignment of the vibronic features: The rotationally resolved bands are successfully simulated as arising from c-type transitions from the ground X̃ 2A′ state to the linear A19″ component of the à 2Π state. The bands are attributed to two overlapping vibrational progressions: one progression involves excitation of the F–C–O bending mode (v3′), the other consists of a combination of v3′ and one quantum of the C–F stretch (v2′). Sharp rotational structure is only observed for sub-bands with K′=0; bands with K′>0 are diffuse, indicating rapid, rotation induced predissociation. Band origins, rotational constants for the excited state, and spectral linewidths have been derived from the K′=0–K″=1 sub-bands. All rotational lines are somewhat broadened and there is evidence of linewidths that increase with N′, and hence an additional rotation-induced predissociation mechanism. Vibrational frequencies and rotational constants are in excellent agreement with the predictions of ab initio calculations by Krossner et al., J. Chem. Phys. 101, 3973 (1994); 101, 3981 (1994). The à 2Π(A″)–X̃ 2A′ absorption shows characteristics of a transition between two Renner–Teller components and this interpretation is confirmed by careful examination of the electronic structure of the FCO ground state. Implications for assignments of absorption features at higher energy than the spectral region of the current study are discussed, and comparisons are drawn with the much studied electronic spectroscopy of both the HCO radical and the isoelectronic NO2.