Two-Dimensional Time-Frequency Ultrafast Infrared Vibrational Echo Spectroscopy

2D spectrally resolved ultrafast $(<200\mathrm{fs})$ IR vibrational echo experiments were performed on $\mathrm{Rh}(\mathrm{CO}{)}_2\mathrm{acac}$ [(acetylacetonato)dicarbonylrhodium $\left(I\right)$]. The 2D spectra display features that reflect the 0-1 and 1-2 transitions and the combination band transition of the symmetric $\left(S\right)$ and antisymmetric $\left(A\right)$ CO stretching modes. Three oscillations in the data arise from the frequency difference between the $S$ and $A$ modes (quantum beats) and the $S$ and $A$ anharmonicities. A new explanation is given for these “anharmonic” oscillations. Calculations show that spectral resolution enables the 0-1 and 1-2 dephasing to be measured independently.