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Fourier transform microwave spectroscopy of lanthanum monohalides Rubinoff, Daryl Simon

Abstract

The pure rotational spectra of the X¹Σ⁺ ground states of the lanthanum monohalides, ¹³⁹LaF, ¹³⁹LaCl, ¹³⁹LaBr and ¹³⁹Lal, have been measured using a pulsed-jet cavity Fourier transform microwave spectrometer in the range of 5-24 GHz. The molecules were prepared by ablating solid La with the second harmonic of a pulsed Nd:YAG laser and allowing the vapor to react with SF₆ , CI₂, Br₂ or CH₃I precursor present as < 0.01% in an Ar carrier gas. Only the J=1-0 rotational transition of LaF was measurable within the frequency range of the spectrometer, while multiple J transitions have been recorded for both La³⁵Cl and La³⁷Cl. The ground and first excited vibrational levels (v=0 and u=l) of LaF, La³⁵Cl and La³⁷Cl were recorded with an additional vibrational level, v=2, measured for LaF. Equilibrium geometries of LaF and LaCl have been evaluated and indicate a significant mass dependency indicating Born-Oppenheimer breakdown. The vibrational frequency, ωe, vibrational anharmonicity constant, ωeXe, and the dissociation energies have been estimated for both molecules and agree very well with the literature values. Several rotational transitions have been measured and are reported for La⁷⁹Br and Lal in both the ground and first vibrational levels with only v=0 data collected for La⁸¹Br. For LaBr, this is the first reported observed spectrum and analysis thereof. Equilibrium geometries have been evaluated along with estimates of the vibrational frequency, ωe, vibrational anharmonicity constant, ωeXe, and the bond dissociation energy, De, for La⁷⁹Br and Lal. Hyperfine structure due to lanthanum and halogen nuclei has been observed and used to determine nuclear quadrupole coupling constants and spin-rotation coupling constants for all nuclei with exception of fluorine which has no quadrupole moment. Using nuclear quadrupole coupling data, LaCl, LaBr and Lal were all found to be highly ionic and generally follow trends predicted based on electronegativity differences of related species. Magnetic shielding parameters have been calculated from nuclear spin-rotation coupling constants and are reported herein.

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