Millimeter and Submillimeter Wave Spectroscopy and Astrophysical Applications

Abstract

Millimeter and submillimeter spectroscopy has played a crucial role in the discovery and study of interstellar molecules by radio astronomy, as well as in the characterization of the rotational energy levels for numerous chemical compounds. A broad range of spectrometer designs have been developed for this basic direct absorption technique. Certain systems employ “free space” cells and Gaussian beam optics, while others incorporate mirrors in multipass configurations. Fast scanning methods have additionally been implemented. Synthetic techniques are also important in the astrophysical application of millimeter/submillimeter spectroscopy, as many potential interstellar species are free radicals and molecular ions. Many instruments employ glow discharges, ovens, and other exotic gas‐phase methods for the creation of transient molecules. Radical spectra usually exhibit a range of fine structure and hyperfine interactions which can complicate their analysis. Another challenge for millimeter/submillimeter spectroscopy is the study of simple organic molecules with functional groups that undergo hindered rotation. Millimeter and submillimeter spectroscopic measurements continue to provide the necessary rotational “rest frequencies” for molecule identification in space. Recent work has led to the interstellar discoveries of long‐chain carbon anions C 4 H − and C 6 H − , more complex organic molecules including CH 3 CONH 2 , and unusual refractory species such as CCP and AlO.

Line shape of amplitude or frequency-modulated spectral profiles including resonator distortions

“…We may mention here the highly accurate determinations of the ionization and bond dissociation energies of molecular hydrogen with submegahertz accuracy [4], the search for a time dependence of the fundamental constants of physics [5], the search for parity violation in chiral molecules [6,7], or fundamental questions related to the spectroscopy of atomic hydrogen [8]. While the quest for high precision concerns all spectral ranges from radiofrequency [9] to microwave and submillimeter wave spectroscopy toward XUV spectroscopy [10][11][12][13][14], microwave and gigahertz spectroscopy in the millimeter and submillimeter range has been traditionally a field of very high precision. High-precision analyses of gigahertz spectra have been discussed repeatedly for some decades, proposing a variety of techniques often in conjunction with the prototype molecules carbon monoxide (CO), carbon oxide sulfide (OCS), and others [15][16][17][18][19][20].…”