Picosecond CARS as a probe of ground electronic state intramolecular vibrational redistribution

We describe a representation of coherent anti-Stokes Raman spectroscopy (CARS) suitable to the description of time-dependent measurements. This representation includes the standard energy frame formulation as a special case when cw fields are involved. The traditional CARS cw field frequency matching condition ω0=ω1−ω2+ω3 must be generalized for non-cw fields; in that case it refers to the Fourier component at ω0 of the convolution of the radiation field with the wave packet recurrences. The influence of resonance, both in the ground and electronically excited states, on the decay of time delayed CARS signals is discussed. As expected, intramolecular vibrational redistribution on the ground state potential surface of a molecule causes the CARS signal to decay in time. Model calculations show that quantum beats in the CARS signal may be observed, reflecting either a small number of coupled states (strong and regular recurrences) or sequential coupling of states (weak and early recurrences).