Ultrafast Rotational Anisotropy Measurements: Strong-Field Nonlinear Saturation Effects

Abstract

Nonlinear saturation effects in ultrafast time-resolved rotational anisotropy measurements are explored as a function of increased pump laser intensity. Femtosecond pump−probe experimental data were obtained over 3 orders of magnitude in pump laser intensity ranging from no saturation to highly nonlinear saturation. Data were obtained for molecular iodine vapor at room temperature following resonant excitation of the B←X transition. For low pump laser intensities, the data are shown to fit the conventional anisotropy formalism for unidirectional detection well. For higher pump laser intensities, the overall anisotropy diminishes because of saturation and the conventional fits diverge from the experimental data. In this regime, a saturation parameter can be introduced in the formulation to improve the fit between the model and the experimental data, thereby improving the accuracy of the resulting rotational temperatures. At the highest laser intensities, an additional photochemical pathway arising from the...