Measurement of laser-induced molecular alignment in cold, low density targets using femtosecond degenerate four wave mixing

A femtosecond degenerate four wave mixing (DFWM) technique for measuring the laser induced non-adiabatic alignment of asymmetric top molecules in a dilute gas jet is reported. The alignment is probed using three beams in a folded BOXCARS geometry. By generating the three beams using a physical mask in a single probe beam, automatic overlap in space and time is obtained. By scanning the pump-probe delay and measuring the background-free DFWM signal, the alignment of rotationally-cold and low-density molecular targets can be rapidly characterized. We demonstrate the technique by measuring the alignment low-density iodobenzene in a high-pressure helium jet. This scheme can also be used with arbitrarily-shaped aligning pulses and pulse sequences, and for a wide variety of molecules.