Time-resolved investigation of the ν1 ro-vibrational Raman band of H2CO with fs-CARS

The technique of femtosecond time‐resolved coherent anti‐Stokes scattering (fs‐CARS) is used to investigate the strongly perturbed ν1 ro‐vibrational Raman band of formaldehyde (H2CO). The time‐dependent signal is simulated using a ‘Watson‐’Hamiltonian in A‐type reduction and Raman theory for asymmetric rotors. The results are compared with the experimental data. The fs‐CARS method measures the evolution of the polarization in a molecular ensemble via superposition of many states and is sensitive to spectral irregularities or line shifts of the involved transitions. ‘Coriolis’ interactions play a major role in the analysis of the ν1 band of formaldehyde. We successfully simulate the fs‐CARS transient signal from the ν1 band of formaldehyde including a model for multiple ‘Coriolis’ interactions, without the necessity of describing the complete interaction between all the vibrational levels. ‘Coriolis’ coupling coefficients and energy shifts are derived from the experiment by a least‐square fit. The results are discussed and compared to literature values. Copyright © 2006 John Wiley & Sons, Ltd.