Observation of revival structure in femtosecond-laser-induced alignment of N2 with high-order harmonic generation

We report a new sensitive method using high-order harmonic generation to observe revival structure in fs-laser induced alignment of a rotational wave packet of molecules. Pump and probe fs-laser pulses with a time delay were focused collinearly into a pulsed N-2 gas jet, so that the pump pulse induces alignment of a ground-state rotational wave packet, and the delayed probe pulse produces harmonic radiation from the aligning molecules. The harmonic signal observed as a function of time delay has clearly demonstrated a typical time-dependent revival structure in the field-free alignment of molecules.