Molecular radio astronomy

During the past fifty years extensive effort in many laboratories has led to enhancements of the simple system, turning microwave spectroscopy into an extremely sensitive and versatile tool. We now review some of these developments. We shall also describe the essential features of a radio telescope because almost thirty diatomic molecular species, many of which would be transient species in the laboratory, have been detected in interstellar gas clouds. Molecular radio astronomy is closely linked with and complementary to laboratory microwave spectroscopy. Or, if you wish, you can reverse the emphasis of that last statement ... 

The existence of molecular species in interstellar space has been known for almost seventy years. The first observations involved the electronic spectra, seen in absorption in the near-ultraviolet, of the CN, CH [28] and CH+ [29] species. Radiofrequency lines due to hydrogen atoms in emission [30] and absorption [31], and from the recombination of H+ ions with electrons were also known. However, molecular radio astronomy started with the observation of the OH radical by Weinreb, Barrett, Meeks and Henry [32] in 1963 in due course, this was followed by the discovery of CO [33]. In the subsequent years over 110 molecules have been observed in a variety of astronomical sources, including some in galaxies other than our own. Nearly a third of these are diatomic molecules, with both closed and open shell electronic ground states, and some were observed by astronomers prior to being detected in the laboratory. 

Widely used treatises and monographs on the theory and the applications of MRR-spectroscopy are available [8-//], also comprehensive reviews on all aspects of obtaining molecular structure from MRR-spectra [6,12,13], including rovib interactions [14] and the reliability of the results [15-17], Molecular structural and other data obtained from MRR spectroscopy have been compiled over the past decades [18,19], The most recent compilation is MOGADOC (short for Molecular Gas Phase Documentation ), a computerized database and retrieval system that is updated periodically and today contains more than 20,000 references, which were critically selected and evaluated by means of keywords. Included is work done by MRR-spectroscopy, electron diffraction of gases, and molecular radio astronomy the documentation refers to more than 6000 compounds. As an additional feature, MOGADOC contains explicit numerical data on the structure of approximately 2000 compounds. A detailed description can be found in ref. [20],... 

The determination of Tex and its interpretation in terms of the physical conditions which exist in interstellar clouds is an intriguing task of molecular radio astronomy. Two limiting cases are readily considered i) if the rate of collisionally induced transitions C(,i is small compared to the radiative rate then Tex is determined by the 2.7 °K radiation field and ii) if the reverse is true, then Tex % 7k, the kinetic temperature of the gas... 

Barrett A H 1983 The beginnings of molecular radio astronomy Serendipitous Discoveries in Radio Astronomy ed K Kellerman and B Sheets (Green Bank, WV NRAO)... 

The longest wavelengths of the electromagnetic spectmm are sensitive probes of molecular rotation and hyperfine stmcture. An important appHcation is radio astronomy (23—26), which uses both radio and microwaves for chemical analysis on galactic and extragalactic scales. Herein the terrestrial uses of microwave spectroscopy are emphasized (27—29). 

We have already discussed the high-resolution spectroscopy of the OH radical at some length. It occupies a special place in the history of the subject, being the first short-lived free radical to be detected and studied in the laboratory by microwave spectroscopy. The details of the experiment by Dousmanis, Sanders and Townes [4] were described in section 10.1. It was also the first interstellar molecule to be detected by radio-astronomy. In chapter 8 we described the molecular beam electric resonance studies of yl-doubling transitions in the lowest rotational levels, and in chapter 9 we gave a comprehensive discussion of the microwave and far-infrared magnetic resonance spectra of OH. Our quantitative analysis of the magnetic resonance spectra made use of the results of pure field-free microwave studies of the rotational transitions, which we now describe. 

Swings and Rosenfeld, 1937 McKellar, 1940). The discovery in 1963 of the XI8 cm radio spectrum of OH by Weinreb et al. was the first identification of an interstellar molecule by radio astronomy. Thus, up until 1968, only four interstellar molecules were known to exist, leading to the generally accepted conclusion that simple free radicals were the main interstellar molecular constituents in a highly dilute gas (< 1 particle cm-3), subject to ionizing ultraviolet radiation. 

In radio astronomy multichannel or autocorrelation (Fourier) spectrometers are used which simultaneously cover the whole line profile. Consider a molecular cloud observed against a source of continuum radiation of a given brightness temperature. The continuum brightness temperature is the sum of the 2.7 °K isotropic background radiation Tbh of a continuum source (such as an HII region or a supernova remnant) which may be in the line of sight and located behind the molecular cloud. A specific molecular transition with optical... 

Dense molecular clouds, often also called dark clouds, block entirely the light of stars which lie behind them, and can therefore be studied observationally only by radio astronomy or infrared techniques. These clouds have a visual extinction in excess of A 10 which corresponds to a gas density of n lO cm" and a kinetic temperature usually well below T 100 K, typically between 10 and 25 K. Within the last ten years, the investigation of these dark molecular clouds has become almost entirely the domain of radio astronomy although now the first very promising results by infrared astronomy reveal the power of this new branch of spectroscopy. 

If a sufficient number of resonances with laser lines can be found, the rotational constant, the fine structure parameters, and the magnetic moments can be determined very accurately. The identification of the spectra and the assignment of the lines are often possible even if the molecular constants are not known beforehand [144]. Most radicals observed in interstellar space by radio astronomy [141a] have been found and measured in the laboratory with LMR spectroscopy [141b]. 

Radio- and microwaves also have several other fields of application in spectroscopy. Molecular rotational transitions correspond to this wavelength region. Radiometers can be used for passive remote sensing, of e.g. temperature and air humidity, and radar systems can be utilized for active measurements of e.g. oil slicks at sea. Finally, radio astronomy is a fascinating field, yielding information on the most remote parts of the universe. 

Meth. DR LA LM MB MW RA method of measurement applied to obtain the reported value double resonance experiments (microwave-optical double resonance MODR or radiofrequency-optical double resonance RFDR) Doppler free laser spectrosefipy laser magnetic resonance molecular beam electric resonance or molecular beam resonance with laser detection method microwave spectroscopy radio astronomy... 

HNC was first detected in June 1970 by L. E. Snyder and D. Buhl using the 36-foot radio telescope of the National Radio Astronomy Observatory (NRAO). The main molecular isotope, was observed via its J=1 0 transition at 88.6 GHz in six different... 

The following molecular parameters have been determined by fitting to laboratory and radio astronomical rotational frequencies [05Mar], The investigation has aimed at a better predictability of rest frequencies, and in turn at an improved determinability of the velocities of radio sources (RAS = Radio Astronomy) ... 

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