Ultrafast rotational anisotropy measurements: Strong-field nonlinear saturation effects

Nonlinear saturation effects in ultrafast time-resolved rotational anisotro py measurements are explored as a function of increased pump laser intensit y. Femtosecond pump-probe experimental data were obtained over 3 orders of magnitude in pump laser intensity ranging from no saturation to highly nonl inear saturation. Data were obtained for molecular iodine vapor at room tem perature following resonant excitation of the B-X transition. For low pump laser intensities, the data are shown to fit the conventional anisotropy fo rmalism for unidirectional detection well. For higher pump laser intensitie s, the overall anisotropy diminishes because of saturation and the conventi onal fits diverge from the experimental data. In this regime, a saturation parameter can be introduced in the formulation to improve the fit between t he model and the experimental data, thereby improving the accuracy of the r esulting rotational temperatures. At the highest laser intensities, an addi tional photochemical pathway arising from the A-X transition is observed. I ncorporation of this pathway into the nonlinear rotational anisotropy model yields accurate rotational populations even for the highest laser intensit ies. Applications of this nonlinear anisotropy model to reactive and nonrea ctive ultrafast studies in order to extract quantitative information are di scussed.