Orienting molecules via an ir and uv pulse pair: Implications for coherent Raman spectroscopy

Spatial orientation of molecules is a pervasive issue in chemical physics and, by breaking inversion symmetry, has major consequences in nonlinear optics. In this paper, we propose and analyze an approach to molecular orientation. This extracts from an ensemble of aligned diatomic molecules (equally AB and BA, relative to the E vector) a subensemble that is oriented (mostly AB or BA). Subjecting an aligned molecule to a tailored infrared (ir) laser pulse creates a pair of coherent wave packets that correlate vibrational phase with the AB or BA orientation. Subsequent, suitably phased ultraviolet (uv) or visible pulses dissociate one of these vibrational wave packets, thereby weeding out either AB or BA but leaving intact the other orientation. Molecular orientation has significant implications for coherent Raman spectroscopy. In the absence of orientation, coherence between vibrational levels is generated by a pair of laser pulses off which a probe pulse is scattered to produce a signal. Orientation allows direct one-photon ir excitation to achieve (in principle) maximal Raman coherence.