Dynamic Orientation of OCS Induced by Intense THz Pulses*

We have used intense, picosecond THz pulses to dynamically orient OCS molecules. Rotationally cooled molecules in a pulsed super-sonic expansion enter a time of flight mass spectrometer where they are exposed to a 50 fs, 780 nm ``pre-alignment'' pulse. This pulse coherently broadens the rotational probability distribution within each molecule, creating a rotational wavepacket which undergoes initial alignment along the laser polarization axis. After one-quarter of the fundamental rotational period, approximately 20 ps after the pre-alignment pulse, the molecules have rotated out of alignment and are maximally misaligned. At this time they are exposed to an intense THz pulse generated by tilted-pulse-front optical rectification of a 100 fs 780 nm laser pulse in Li:NiO$_{\mathrm{3}}$. The THz field drives transitions between rotational states of opposite parity, creating a rotational wavepacket that exhibits periodic head vs. tail orientation along the THz polarization direction. The time-dependent orientation is probed by detecting the up vs. down asymmetry in the S$^{\mathrm{3+}}$ ion fragments released during the Coulomb explosion of the OCS molecules in an intense 40 fs, 780 nm pulse. Substantial orientation is observed at a delay of one-half the fundamental rotational period after the THz pulse.