Measurement of laser-induced alignment of molecules by cross defocusing.

The field-free alignment of CO2 produced in response to the excitation of a molecule by a high-intensity femtosecond pump pulse is measured with a simple coronography-like technique. The technique is based on the defocusing of a time-delayed probe pulse produced by the spatial distribution of aligned molecules. In the intensity regime explored here, the technique is shown to give valuable information about dynamic alignment. With the help of simulations, the degree of alignment is extracted from the data.