Astronomy Microwave spectroscopy

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). 

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 ... 

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. 

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... 

One of the most fruitful application of laboratory microwave spectroscopy over the last twenty years is the analysis of the molecular content of interstellar clouds. These clouds contain gas (99% in mass) which has been mostly studied by radioastronomy, and dust, whose content has been analysed mostly by IR astronomy. The clouds rich in molecular content are dense or dark clouds (they present a large visual extinction), with a gas density of 10 -10 molecules cm", and temperatures of T < 50K. At these low temperatures only the low-lying quantum states of molecules can be thermally (or collisionally) excited, i.e. rotational levels. Spontaneous emission from these excited states occurs at microwave wavelengths. In some warm regions of dense clouds (star formation cores) the absorption of IR radiation produces rotational emission in excited vibrational states. Other rich chemical sources are the molecular clouds surrounding evolved old stars, such as IRC-i-10216, and called circumstellar clouds. 

Rotational spectroscopy and microwave astronomy are the most accurate way to identify a molecule in space but there are two atmospheric windows for infrared astronomy in the region 1-5 im between the H2O and CO2 absorptions in the atmosphere and in the region 8-20 xrn. Identification of small molecules is possible by IR but this places some requirements on the resolution of the telescope and the spacing of rotational and vibrational levels within the molecule. The best IR telescopes, such as the UK Infrared Telescope on Mauna Kea in Hawaii (Figure 3.13), are dedicated to the 1-30 xm region of the spectrum and have a spatial resolution very close to the diffraction limit at these wavelengths. 

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. 

Infrared Astronomy Infrared TTchnology Macromolecules, Structure Microwave Molecular Spectroscopy Radiometry and Photometry Raman Spectroscopy... 

Analytical Chemistry Collision-Induced Spectroscopy Hydrogen Bond Infrared Spectroscopy Interstellar Matter Ion Kinetics and Energetics Microwave Communications Millimeter Astronomy Quantum Chemistry Time... 

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