Spectroscopy of pendular molecules in strong parallel electric and magnetic fields

We report high-resolution laser-induced fluorescence spectra of the A3Π1, [Formula: see text] ν″ = 0 system of ICl subjected to strong parallel electric and magnetic fields, scanned independently up to 50 kV cm−1 and 0.7 T. Either field induces coherent superposition or hybridization of the rotational states within the A3Π1 electronic state, which possesses both an electric and a magnetic dipole moment along the internuclear axis. Only the electric field induces such hybridization within the ground state [Formula: see text], since it is polar but diamagnetic. Comparisons with computed spectra illustrate several salient features. The hybridization creates directional pendular states in which the molecular axis is confined to librate over a limited angular range about the field direction. For a polar paramagnetic electronic state, such as the A state, parallel electric and magnetic fields conjoin in producing hybridization for half of the ensemble but they counteract for the other half. For the latter component, the electric and magnetic effects can be tuned to balance out, yet the concomitant field-induced hyperfine splitting remains, despite the resulting free rotation of the molecule. The dependence of the transition intensities on the combined fields provides an unequivocal means to determine the relative sign of the electric dipole moments in the upper and lower electronic states. We find that the sign is the same for the X and A states (chlorine more negative than iodine).