The pure rotational spectra of the lanthanum monohalides, LaF, LaCl, LaBr, LaI

Pure rotational spectra have been measured for all the major isotopomers of the lanthanum monohalides, LaF, LaCl, LaBr, and LaI, in their ground and (except for (LaBr)-Br-81) excited vibrational states. The spectra were observed with a cavity pulsed jet Fourier transform microwave spectrometer in the frequency range 5-24 GHz. The molecules were prepared by laser ablation of La metal and allowing the resulting plasma to react with SF6, Cl-2, Br-2, or CH3I precursor in an Ar carrier gas of the pulsed jet. For LaBr this is the first reported spectrum of any kind. Rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants, and nuclear spin-rotation constants have been determined for all the molecules. Accurate equilibrium (r(e)) internuclear distances have given an indication of where the Born-Oppenheimer approximation is beginning to fail. From the centrifugal distortion constants and vibration-rotation (alpha(e)) constants good estimates of the harmonic vibration frequencies and bond dissociation energies have been obtained. The halogen nuclear quadrupole coupling constants indicate the molecules to be highly ionic. (C) 2003 Elsevier Science (USA). All rights reserved.