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The 310 nm photodissociation of dimanganese decarbonyl, Mn2(CO)10, in cyclohexane has been studied using ultrafast infrared spectroscopy. Subpicosecond IR detection near 5 μm, in the region of carbonyl stretch vibrations, is carried out using a frequency-resolved broad (>100 cm−1) IR probe pulse. The evolution of infrared spectra provide information on Mn2(CO)10 and its photoproducts. In the terminal carbonyl region (1970–2050 cm−1), bleach signals are observed due to loss of the parent compound. A broad, featureless absorption appears promptly and narrows upon vibrational cooling (on the 50 ps time scale) to reveal IR spectral features consistent with those previously assigned to two photoproducts, Mn(CO)5 and Mn2(CO)9. The bleach signals partially recover, indicating ground state recovery of vibrationally excited Mn2(CO)10, which is consistent with previous reports of geminate recombination. Mn2(CO)9 signal appearance times are 54(4) ps for the terminal CO band at 2052 cm−1 and 31(5) ps for the bridge band at 1760 cm−1. The direct observation of the bridging band establishes 31 ps as an upper limit for the bridge bond formation. The time dependence of the signal is probably due primarily to vibrational cooling of low frequency modes. The times are somewhat longer than the shorter of two times measured in the visible region and attributed to low frequency mode thermalization by Zhang and Harris [J. Chem. Phys. 95, 4024 (1991)]. Vibrational coherence effects were observed at early times (close to the vibrational dephasing time).

Absolute infrared vibrational band intensities of molecular ions determined by direct laser absorption spectroscopy in fast ion beams J. Chem. Phys. 93, 3111 (1990); 10.1063/1.458845The structure of the nitrous oxide dimer from subDoppler resolution infrared spectroscopy J. Chem. Phys. 89, 5408 (1988); 10.1063/1.455592 SubDoppler laser spectroscopy of SO2 in a supersonic beamWe report the development of a new general technique for measuring vibration-rotation spectra of molecular ions with sub-Doppler resolution and with accurate determination of the mass and number density of the carriers of all spectral features. With this method, called direct laser absorption spectroscopy in fast ion beams (DLASFIB), we have carried out the first observation of direct absorption of photons by ions in a fast ion beam. Hyperfine-resolved vibration-rotation transitions ofHF+ have been measured, and along with optical combination differences and laser magnetic resonance data, have been analyzed to yield the fluorine hyperfine parameters a, b, c and d for both v = 0 and v = 1 in the X 2IT state.Comparisons with many-body perturbation theory results are presented.

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