Absorption spectrum of intense chirped pulse by molecules in solution and the time evolution of vibrationally non-equilibrium populations

We have calculated the absorption spectrum of an intense chirped pulse exciting a solute molecule in a solvent. In general it depends on both the real and imaginary part of the susceptibility (a phase-dependent absorption in the non-stationary media). We have shown that the absorption spectrum directly reflects the time evolution of a vibrationally non-equilibrium population difference in the ground and excited electronic states at the configuration coordinate corresponding to instantaneous Franck–Condon transition, when measured using high-power and strongly chirped pulses. A method has been proposed for extracting this time evolution from the measured absorption spectrum.