Spectroscopy detecting interstellar molecules

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. 

CO has been detected in the interstellar absorption spectrum of f Ophiuchi and has thus become the second interstellar molecule to be detected by rocket ultraviolet spectroscopy. Smith and Stecher (1971) have detected eight transitions in the fourth positive system of 12C160 and four of 13C160, which yielded a 12C/13C ratio of 105. It seems likely that interstellar molecular detections in the vacuum ultraviolet will follow, especially of polyatomic molecules like H20. 

A Laboratory in Applying Molecular Spectroscopy for the Detection of Interstellar Molecules... 

Table 2.3 List of detected interstellar and circumstellar molecules, radicals and ions, grouped by the number of atoms (N) they contain. Species detected with UV, visible or infrared spectroscopy are indicated in italics...

The detection and identification of molecules in interstellar space is possible by millimeter wave spectroscopy. The independent synthesis and detection of such reactive species, e.g. by flash vaccuum thermolysis and mm wave spectroscopy, provides proof for their cosmochemical existence. The detection of the J 6 —> 7 rotational transition in the decomposition products of t-Bu2HSi—NH(CH2— C=CH) indicated the formation of HNSi (Table 20)333. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

Gravitational instability can occur in a cloud characterised by high density and low temperature (Jean s criterion). More precisely-, the mass of the cloud must be of the order of 20 M0 (where M0 is the solar mass) if the density (expressed as the number of dihydrogen molecules per cubic centimeter) is around 103 and the temperature around 10 K. Such a density is precisely what is observed in so-called dense clouds. These dense clouds are also regions of space where a lot of complex molecules are detected by spectroscopy, and where dust particles are observed. Readers interested in interstellar chemistry will find an excellent review of the subject in the recent book by Duley and Williams 13). 

Silicon dicarbide has been identified by Thaddeus et al. (1984) as a circumstellar molecule on the basis of 9 hitherto unassigned millimeter wave lines observed in the late type star IRC + 10216. The molecule is the first ring molecule detected in space, and its rotational spectrum is that of a near prolate asymmetric top with C2v symmetry. The molecule had been detected in the laboratory prior to the interstellar detection by optical laser spectroscopy (Michalopoulous et al. 1984). 

Measurement of DR branching ratios is perhaps the most problematic and contentious topic in experimentally-based interstellar chemistry. As the chief means of positive ion neutralization, DR is crucial in determining the eventual outcome of most, if not all, sequences of synthetic ion/molecule steps. Two fundamentally different techniques have been used for DR product analysis. The FALP technique of Smith and Adams, used with considerable success in the study of ion/electron recombination kinetics [171,176,177], has been adapted to permit subsequent neutral product detection by LIF (laser-induced fluorescence) or VUV (vacuum ultraviolet) spectroscopy, as shown in Fig. 13. Such studies, first... 

Rotational transitions correspond to photon wavelengths in the microwave region. The radiation sources in microwave spectrometers are klystron tubes, which were originally developed for radar apparatuses in World War II. Hollow metal wave guides carry the radiation to the sample cell, which is a hollow metal cavity, and the resonant radiation in the cavity is sampled to detect absorption. Microwave spectroscopy has played an important role in identifying molecules in interstellar space, but it is not a common tool in many chemical laboratories. 

Typical abundances of PAHs are of the order of 10 relative to hydrogen, which makes these species the most abundant polyatomic molecules present in space. Vibrational spectroscopy is eminently suited for detecting the presence of classes of molecules, but it is much more difficult to identify specific molecules within a collection of species. However, while the gross characteristics of these stellar and interstellar spectra are very similar, when examined in detail, they vary from source to source. Analysis of these variations may well provide us with a tool to identify specific molecules within the circumstellar and interstellar PAH family and such efforts, supported by extensive laboratory studies, are now underway. 

It is almost a certainty that you are familiar with microwave radiation—have you ever cooked or heated food in a microwave oven Microwave radiation plays a prominent role in a branch of science called rotational (or microwave) spectroscopy. For a discussion of rotational spectroscopy and how it is used not only to make precise measurements of bond lengths but also to detect molecules in interstellar space, go to the Focus On feature for Chapter 10, Molecules in Space Measuring Bond Lengths, on the MasteringChemistry site. 

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