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

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 electronicall y 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 th e ground (X) over tilde (2)A' state to the linear (2)A " component of the ( A) over tilde (2)Pi state. The bands are attributed to two overlapping vibr ational progressions: one progression involves excitation of the F-C-O bend ing mode (upsilon (3)'), the other consists of a combination of upsilon (3) ' and one quantum of the C-F stretch (upsilon (')(2)). Sharp rotational str ucture is only observed for sub-bands with K'=0; bands with K'>0 are diffus e, indicating rapid, rotation induced predissociation. Band origins, rotati onal constants for the excited state, and spectral linewidths have been der ived from the K'=0-K " =1 sub-bands. All rotational lines are somewhat broa dened and there is evidence of linewidths that increase with N', and hence an additional rotation-induced predissociation mechanism. Vibrational frequ encies and rotational constants are in excellent agreement with the predict ions of ab initio calculations by Krossner , J. Chem. Phys. 101, 3973 (1994 ); 101, 3981 (1994). The (A) over tilde (2)Pi>(*) over bar * (A('))-(X) ove r tilde (2)A(') absorption shows characteristics of a transition between tw o Renner-Teller components and this interpretation is confirmed by careful examination of the electronic structure of the FCO ground state. Implicatio ns for assignments of absorption features at higher energy than the spectra l region of the current study are discussed, and comparisons are drawn with the much studied electronic spectroscopy of both the HCO radical and the i soelectronic NO2. (C) 2000 American Institute of Physics. [S0021-9606(00)01 541-5].