Mode specific internal and direct rotational predissociation in HeHF, HeDF, and HeHCl: van der Waals complexes in the weak binding limit

Abstract

The near-infrared vibration–rotation spectra of the weakly bound complexes HeHF, HeDF, and HeHCl are observed in a slit supersonic expansion. The spectra correspond to simultaneous excitation of vibration and internal rotation of the H(D)X subunit within the complex. The HeHF and HeDF P/R branch transitions show J-dependent excess linewidths, which are attributed to rapid predissociation of the excited states from intramolecular rotation–translation energy transfer. The corresponding P/R branch transitions in HeHCl are not observed despite good S/N on the Q branch, suggesting even more rapid predissociation for the upper state of this complex. The Q branch transitions for all three complexes abruptly terminate at low J, yielding lower limits to the number of bound rotational states and good estimates of the dissociation energies D0=7.1±0.1 cm−1 for HeHF and HeDF, and 10.1±1.2 cm−1 for HeHCl. In addition to isotropic intermolecular potentials, the HeHF/HeDF data yield considerable information on the potential anisotropy in the region sampled by the bound and quasibound states. The information so obtained is complementary to results from scattering studies and provides sensitive tests for refining trial potential energy surfaces.