OPTICAL-SPECTRA OF SPATIALLY ORIENTED MOLECULES - ICI IN A STRONG ELECTRIC-FIELD

Strong electric fields can hybridize rotational states of polar molecu les and thus create pendular states in which the molecules are confine d to librate over a limited angular range about the field direction. I n this way, substantial spatial orientation can be attained for the lo west rotational states of many linear, symmetric and asymmetric top mo lecules. A large fraction of a molecular ensemble can often be condens ed into these low rotational states by cooling in a supersonic expansi on. 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 ICl A 3P I1 <-- X1SIGMA0 measured as a function of the electric field strength up to 36 kV cm-1. These spectra are compared with calculated transitio n 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 fl uctuate as the angular lobes of the two pendular wavefunctions involve d come in and out of phase when the field strength is varied. The obse rved fluctuations are particularly pronounced in the present case beca use the dipole moment changes sign between the two electronic states.