Femtosecond dynamics and vibrational coherence in gas-phase ultraviolet photodecomposition of Cr(CO)6

The initial dynamics of single-photon UV decomposition of Cr(CO)(6) to Cr(CO)(5) and CO in the gas phase was investigated by using femtosecond weak pump pulses at 267 nm and delayed, intense ionizing probe pulses at 800 nm with time-of-flight detection of ions. The parent and all the fragment ions show different temporal behavior at delay times of <200 fs, indicating that the molecules already go through several different configurations during this short time interval. With longer delays the heavy ions disappear, but light fragmention signals from Cr(CO)(n)(+), n = 3-0, exhibit coherent oscillations with a wavenumber of 96 cm(-1), which can be assigned to the C-ax-Cr-C-eq bending vibration of the neutral precursor Cr(CO)(5). The coherent oscillations are damped with a time constant of 815 fs. Our observations support a photochemical mechanism of chromium hexacarbonyl photodecomposition, proposed by Burdett et al. (Inorg. Chem. 1978, 17, 147): M(CO)(6) ((1)A(1g))--> M(CO)(6) (T-1(1u)) --> M(CO)(6) (a(1)T(2u)> -M(CO)(5)(C-4 upsilon, 1(1)A(1))-->M(CO)(5) (D-3h, 1(1)E')-->M(CO)(5) (C4(upsilon), 1(1)A(1)). The first step is induced by the pump pulse, and the next steps we have found to take 23, 10, 20, and 40 fs, respectively. A ground-state product M(CO)(5) (C-4 upsilon, 1(1)A(1)) is produced within 100 fs and exhibits a coherent C-ax-Cr-C-eq bending oscillation. Ionization of vibrationally hot Cr(CO)(5) followed by thermal dissociation and photodissociation of the Cr(CO)(5)(+) ion transfers these oscillations to Cr(CO)(n)(+), n = 3-0. We point out that a conical intersection of the Jahn-Teller kind between the 1(1)E' and 1(1)A(1) states of chromium pentacarbonyl in a trigonal bipyramid geometry provides a path for ultrafast internal conversion from an electronically excited state to the ground state and also for the excitation of coherent oscillations at the C-ax-Cr-C-eq bending coordinate, corresponding to Berry pseudorotation.