SUBPICOSECOND TIME-RESOLVED IR SPECTROSCOPY OF THE VIBRATIONAL DYNAMICS OF RH(CO)2(ACAC)

Sub-picosecond pulses are used to measure the transient IR spectra of vibrationally excited Rh(CO)2(acac) in dilute solutions of n-hexane and CCl3H. Ground-state bleach and excited-state absorption features are observed at t(D) > 0, while interference-like spectra characteristic of the perturbed free induction decay (FID) of the probe polarization are seen at t(D) < 0. At t(D) > 0, the bleach signal exhibits an initial decay (n-hexane, 3-6 ps; CCl3H, 2.2-4 ps) attributed to rapid v-v coupling between the symmetric and asymmetric stretch modes of the dicarbonyl, followed by much slower (n-hexane, 61 ps; CCl3H, 101 ps) population relaxation. Calculated transient spectra and bleach decay curves obtained from a 5-level density matrix model of the coupled CO oscillators account for the data, including the effects due to perturbed FID and the rapid v-v coupling.