ULTRAFAST INFRARED STUDY OF THE ULTRAVIOLET PHOTODISSOCIATION OF MN-2(CO)(10)

The 310 nm photodissociation of dimanganese decarbonyl, Mn-2(CO)(10), in cyclohexane has been studied using ultrafast infrared spectroscopy. Subpicosecond IR detection near 5 mu m, in the region of carbonyl str etch vibrations, is carried out using a frequency-resolved broad (> 10 0 cm(-1)) IR probe pulse. The evolution of infrared spectra provide in formation on Mn-2(CO)(10) and its photoproducts. In the terminal carbo nyl region (1970-2050 cm(-1)), bleach signals are observed due to loss of the parent compounds. A broad, featureless absorption appears prom ptly 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 Mn-2(CO)(9). The bleach signals p artially recover, indicating ground state recovery of vibrationally ex cited Mn-2(CO)(10), which is consistent with previous reports of gemin ate recombination. Mn-2(CO)(9) signal appearance times are 54(4) ps fo r 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 establish es 31 ps as an upper limit for the bridge bond formation. The time dep endence of the signal is probably due primarily to vibrational cooling of low frequency modes. The times are somewhat longer than the shorte r of two times measured in the visible region and attributed to low fr equency mode thermalization by Zhang and Harris [J. Chem. Phys. 95, 40 24 (1991)]. Vibrational coherence effects were observed at early times (close to the vibrational dephasing time).