Radio astronomy interstellar molecules

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. 

In this section, we shall restrict ourselves to those aspects of radio astronomy which are relevant to the study of the rotational spectra of diatomic molecules. We will not deal with the study of continuum sources, with cosmology, or with the detailed structure, dynamics and chemistry of interstellar clouds. These are important parts of astrophysics, covered in many research articles, reviews and books [34, 35, 36]. We will describe the main features of the dishes which collect radiation (i.e. the telescope), the detectors and signal processing equipment, and the analysis of the spectra. Many of the microwave spectra of diatomic molecules are now used as important probes to... 

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. 

Within the last four years, nearly 30 different molecules have been identified in the interstellar gas clouds of our Galaxy. This advance has been made possible in part by improved techniques of radio astronomy, and has added a large variety of new interstellar molecules to the list of the three radicals CN, CH, and CH+, known from their ultraviolet spectra since before 1940 (Adams,... 

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. 

Over the past few decades, it has become increasingly obvious that interstellar space has a rich and varied chemistry. Quantum mechanics is the basis by which molecules are investigated in space. Rotational spectra obtained from accurate laboratory measurements allow for unambiguous identification of such molecules, combined with the remote-sensing capabilities of radio astronomy. This symbiotic relationship has led to a new chemical field Astrochemistry. 

Onsala in 1973. The water molecule H2O, which lias an angle of 105° between the H atoms, has a complex rotational spectrum. Litense maser action has been observed for the 22 GHz transition, wliich is also seen in atmospheric absorption (Fig. 7.24). High-frequency radio astronomy observations have to be performed on dr y days. A large number of other molecules, some of them quite complex, have been observed in space. The (very) remote sensing of the physical conditions in the interstellar clouds has been discussed in [7.93-7.95]. 

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