Sub-picosecond 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<). At t_D>), 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 rapid v-v coupling.