Spectroscopy of pendular molecules in strong parallel electric and magnetic fields

The authors report high-resoln. laser-induced fluorescence spectra of the A3P1, n' - 19 <- X1S0+, n = 0 system of ICI subjected to strong parallel elec. and magnetic fields, scanned independently up to 50 k V cm-1 and 0.7 T. Either field induces coherent superposition or hybridization of the rotational states within the A3P1 electronic state, which possesses both an elec. and a magnetic dipole moment along the internuclear axis. Only the elec. field induces such hybridization within the ground state X1S0+, since it is polar but diamagnetic. Comparisons with computed spectra illustrate several salient features. The hybridization creates directional pendular states in which the mol. 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 elec. and magnetic fields conjoin in producing hybridization for half of the ensemble but they counteract for the other half. For the latter component, the elec. and magnetic effects can be tuned to balance out, yet the concomitant field-induced hyperfine splitting remains, despite the resulting free rotation of the mol. The dependence of the transition intensities on the combined fields provides an unequivocal means to det. the relative sign of the elec. dipole moments in the upper and lower electronic states. The sign is the same for the X and A states (Cl more neg. than I)