15min:

THOMAS A. BLAKE, Pacific Northwest National Laboratory, P. O. Box 999, Mail Stop K8-88, 3020 Q Avenue, Richland, WA 99352 (PNNL is operated for the US Department of Energy by the Battelle Memorial Institute under contract DE-AC06-76RLO 1830).

Using our pulsed slit-jet, infrared diode laser spectrometer, we have observed a rotationally-resolved parallel band centered at 741 cm^-1 in a molecular beam formed by expanding a mixture of 4% HF in helium through a 200 µm x 12 cm slit with a backing pressure of ca. 1000 Torr. A set of excited and ground state spectroscopic constants were fit simultaneously to a data set of 75 transitions using a symmetric top, rigid rotor Hamiltonian. The fit constants are ₀ = 740.9696(7) cm^-1, B' = 0.07507(5) cm^-1, (C' - B') - (C'' - B'') = 0.0014(1) cm^-1, and B'' = 0.07570(5) cm^-1. Recently, we have reported the observation of two librational bands of HF tetramer in the 710 to 775 cm^-1 region: one band is a perpendicular band centered at 752 cm^-1 and the other band is a parallel band centered at 714 cm^-1. The ground state B value of the tetramer was found to be 0.132081(1) cm^-1. For hydrogen fluoride tetramer and pentamer, theory predicts (Maerker, et al.) planar, oblate symmetric top structures at their global minima with B_e values of 0.136 and 0.0767 cm^-1 for (HF)₄ (C_4h symmetry) and for (HF)₅ (C_5h symmetry), respectively. We have assigned the 741 cm^-1 band as the (A'') HF out-of-plane librational fundamental of the pentamer, the 714 cm^-1 band as the (A_u) HF out-of-plane librational fundamental of the tetramer, and the 752 cm^-1 band as the (E_u) HF in-plane librational fundamental of the tetramer.