Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats

Abstract

Vibrational echo experiments were performed on the IR active CO stretching modes (∼2000 cm−1) of rhodium dicarbonylacetylacetonate [Rh(CO)2acac] and tungsten hexacarbonyl [W(CO)6] in dibutylphthalate and a mutant of myoglobin-CO (H64V-CO) in glycerol–water using ps IR pulses from a free electron laser. The echo decays display pronounced beats and are nonexponential. The beats and nonexponential decays arise because the bandwidths of the laser pulses exceed the vibrational anharmonicities, leading to the excitation and dephasing of a multilevel coherence. From the beat frequencies, the anharmonicities are determined to be 14.7, 13.5, and 25.4 cm−1, for W(CO)6, Rh(CO)2acac, and H64V-CO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the v=0–1 and v=1–2 transitions are determined. At the lowest temperatures, T2≈2T1, so the v=2 lifetimes are obtained for the three molecules. These are found to be significantly shorter than the v=1 lifetimes. Although the v=1 lifetimes are similar for the three molecules, there is a wide variation in the v=2 lifetimes.