Coherent Excitation of Polyatomic Molecules by Ultrashort Laser Pulses — a Model Calculation

Abstract

In a recent series of experiments,^1–4 we have found that the interaction of polyatomic molecules excited into the quasicontinuum (QC) with ultrashort CO₂ laser pulses can be characterized by the following observations: (1) the spectral lineshape is well-defined for short pulses, but becomes broader and more diffused as the pulse duration increases, and (2) the line center absorption cross section decreases with the interaction time. A qualitative explanation for these observations in terms of the multi-tier energy level structure⁵ and the development of the homogeneous ensemble of energy levels in the QC⁶ has been given and was found to be quite satisfactory. In the present article, we wish to present a model calculation, based on the coherent Bloch equation⁷, on the change in small signal absorption cross section as a function of the laser pulse duration.