Time-resolved infrared diode laser spectroscopy of the ν3 band of the jet-cooled Fe(CO)2 radical produced by ultraviolet photolysis of Fe(CO)5

The infrared spectrum of the iron dicarbonyl radical Fe(CO)2 produced in a supersonic jet expansion by the excimer laser photolysis of iron pentacarbonyl Fe(CO)5 was observed by time-resolved infrared diode laser spectroscopy. About 170 transitions, each split into one or two fine structure components, were assigned to the ν3 (CO antisymmetric stretch) band of Fe(CO)2. The assignment was greatly facilitated by spectral simplification caused by rotational as well as vibrational cooling in the supersonic jet. It was observed that lines are missing at alternate J quantum numbers in each spin component, which confirmed that Fe(CO)2 is a linear molecule with D∞h symmetry and that the electronic ground state is of 3Σg− symmetry. The rotational and centrifugal distortion constants in the ground state were determined to be B0=1414.675(46) MHz and D0=0.3077(74) kHz, respectively. The spin–spin interaction constants obtained, λ0=655.3(42) GHz, is comparable with that of the FeCO radical, λ0=684.470(51) GHz. The ν3 band origin was determined to be 1928.184335 (82) cm−1. The figures in parentheses are uncertainties (1σ) in units of the last digit.