Optical spectra of spatially oriented molecules: ICl in a strong electric field

Strong electric fields can hybridize rotational states of polar molecules and thus create pendular states in which the molecules are confined to librate over a limited angular range about the field direction. In this way, substantial spatial orientation can be attained for the lowest rotational states of many linear, symmetric and asymmetric top molecules. A large fraction of a molecular ensemble can often be condensed into these low rotational states by cooling in a supersonic expansion. Pendular eigenstates differ qualitatively from those of a rotor or an oscillator and can be observed spectroscopically; this provides a means to characterize the extent of the orientation achieved. Here we present high-resolution laser-induced fluorescence spectra of ICI A 3Π1â†� X 1Σ0 measured as a function of the electric field strength up to 36 kV cm–1. These spectra are compared with calculated transition frequencies and probabilities between pendulum/pinwheel states. The field-induced mixing of J states changes the transition probabilities markedly and enriches the spectra with many transitions that would be forbidden in the absence of the field. The transition probabilities fluctuate as the angular lobes of the two pendular wavefunctions involved come in and out of phase when the field strength is varied. The observed fluctuations are particularly pronounced in the present case because the dipole moment changes sign between the two electronic states.