High-Resolution Spectroscopy of Small Transient Species Containing Metals and Phosphorus: Applications to Fundamental Chemical Physics and Astrochemistry

Abstract

Pure rotational spectroscopy is one of the highest resolution techniques available for studying gas-phase molecules and allows for understanding very weak, fundamental interactions within molecules. For radical species, the coupling of angular momenta is readily observed in their microwave spectra, which can be used to infer about the electronic state manifold, describe the intramolecular bonding, as well as guide theoretical models. Pure rotational spectroscopy is also the best technique for determining the structures and geometries of unsolvated molecules. Because of its sensitivity to isotopic differences, measurement in the millimeter-wave also provides rest frequencies for radio astronomical searches of astrophysically relevant molecules. Both direct absorption spectroscopy and Fourier transform microwave/millimeter-wave spectroscopy have been used to study the spectra of several transient molecules, including ZnBr, KO, CrP, SiP, ScC2, ClZnCH3, LiNH2, NaNH2, and CoS as well as several isotopic variations. The difficulty in measuring pure rotational spectra often stems from the synthesis of the molecule, for which three techniques were employed. These methods involve the laser ablation of metal rods, melting of pure metal in Broida-type ovens, and vaporization of organometallic precursors or phosphorus. The measured data were analyzed with effective Hamiltonians consisting of rotational, fine structure, hyperfine structure, and occasionally deperturbation terms. For some species, such as ZnBr, KO, CrP, and SiP, these results provide the first-ever high-resolution molecular parameters; for others, including LiNH2, NaNH2, and CoS, low-frequency measurements were made to establish additional hyperfine constants. Also, the structures of ScC2 and ClZnCH3 have been established through the non-trivial measurements of isotopically substituted versions.