Alignment enhancement of a symmetric top molecule by two short laser pulses

The alignment dynamics of a symmetric top molecule, tert-butyliodide, induced by two picosecond, nonresonant, linearly polarized pulses is studied experimentally and numerically using a combination of time-resolved photofragment imaging and nonperturbative solution of the time-dependent Schrodinger equation. It is shown that the strongest degree of one-dimensional (single axis) field-free alignment obtainable with a single pulse can be enhanced using the two-pulse sequence in a parallel polarization geometry. The conditions for alignment enhancement are: (1) The second pulse must be sent near the time where the degree of alignment, due to the first pulse, peaks. (2) The intensity of the first pulse must be smaller than the intensity of the second pulse, the optimal intensity ratio being determined by the duration of the pulses. Numerically, we also discuss the existence of an optimal pulse duration, the effect of focal averaging on the degree of alignment, and compare two-pulse alignment to adiabatic alignment.