Generation of Highly Vibrationally Excited CO in Sequential Photodissociation of Iron Carbonyl Complexes

Ultraviolet photochemistry of iron pentacarbonyl, Fe(CO)(5), was investigated with resonantly enhanced multiphoton ionization (REMPI) spectroscopy and ion imaging. The REMPI spectrum of CO photofragments, generated by ultraviolet irradiation of Fe(CO)s, showed the generation in the highly vibrationally excited states with v = 11-15. Analysis of the band intensities observed in the 213-235 nm region indicated that the high-v CO generation was maximized at around 220 nm. Generation yields of the coordinatively unsaturated intermediates, Fe(CO)(n=1-4), were measured as a function of the photolysis wavelength using a nonresonant detection scheme. The yield spectrum of FeCO was correlated with that of the high-v CO fragments, suggesting high-v CO generation in the photo- dissociation of FeCO. The density functional theory calculations of the excited states of FeCO showed an intense photoabsorption to the metal-centered state near 220 nm. The theoretical results were consistent with the interpretation of FeCO + by -> Fe + high-v CO, which was experimentally indicated. The momentum distribution obtained from the velocity distributions of Fe, Fe(CO)(4), and CO fragments further supported that Fe is the counter-product of the high-v CO fragment. The present results provided selective observation of the photochemistry of the unsaturated iron carbonyl complexes, which has not been well elucidated in laser-based experiments because of the uncontrollable sequential photodissociation producing mixed Fe(CO)(n) intermediates.

Automated summary

The ultraviolet photochemistry of iron pentacarbonyl (Fe(CO)(5)) was investigated using REMPI spectroscopy and ion imaging. The study revealed that the photofragments of CO generated by ultraviolet irradiation of Fe(CO)(5) were in highly vibrationally excited states with v = 11-15, with maximum generation occurring at around 220 nm. The yield spectrum of coordinatively unsaturated intermediates (Fe(CO)(n=1-4)) was found to correlate with that of the high-v CO fragments, suggesting high-v CO generation in the photo-dissociation of Fe(CO). The theoretical calculations of the excited states of FeCO supported the experimental indication of FeCO + converting to Fe + high-v CO.