Molecular clouds spectroscopy

For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

Throughout the book I have tried to constrain the wonders of imagination inspired by the subject by using simple calculations. Can all of the water on the Earth have been delivered by comets if so, how many comets How do I use molecular spectroscopy to work out what is happening in a giant molecular cloud Calculations form part of the big hard-sell for astrochemistry and they provide a powerful control against myth. I have aimed the book at second-year undergraduates who have had some exposure to quantum mechanics, kinetics, thermodynamics and mathematics but the book could easily be adapted as an introduction to all of these areas for a minor course in chemistry to stand alone. 

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

Diffuse clouds are tenuous concentrations of interstellar gas and dust that do not block entirely the light of stars which are located behind them. They can be studied by absorption spectroscopy and as seen from Table 2, they were already studied as early as 1940. Although diffuse clouds are chemically simpler than are dense molecular clouds the assessment of the formation and destruction mechanisms has its own difficulties associated with it. Just because of the lower density, photoionization and photodissocitiiion processes play a significant role in altering the otherwise simple chemistry of the diffuse clouds. The fommtion of HD may serve as a standard example. 

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. 

Infrared spectroscopy enables us to obtain information on the chemical composition and structure of icy grains in interstellar molecular clouds [3], Table 9.3 summarizes the abundance of molecules identified [4]. Among these species, the predominance of H2O ice is clear, its abundance being one order of magnitude greater than that of all odier molecules. The molecules CO and CO2 are those next most abundant, following H2O. Small amounts of reduced molecules, hydrocarbons and NH3 are also observed. 

Dense molecular clouds, after further contraction, are the places where stars are born. The observation of protostars, stars still embedded in their placental cloud, is a probe of the presence of ices in the clouds the almost black-body continuum emitted from the young object is absorbed by grains whose temperature changes as a function of the distance from the object. These observations, which are mainly obtained by IR spectroscopy, may reveal the evolution of ices due to thermal and/or energetic (e.g. interaction with UV photons and/or stellar particle winds and cosmic rays) processing (e.g. Cox and Kessler [6]). 

The observation of molecular hydrogen by means of its electronic transitions in a sense follows classical optical interstellar spectroscopy. It is, however, considerably more complex, requiring essentially controlled satellite observatories. Thus, it serves to determine molecular-hydrogen column densities in translucent clouds, but cannot provide images of the dense molecular clouds. For these, carbon monoxide is the generally accepted tool. The reported results are in terms of H2 column densities under the assumption that the H2 CO ratio is the accepted value of 10. CO is observable by means of its many isotopomers. This is extremely useful, as the common isotopomer is frequently optically opaque, making... 

A large tenuous cloud surrounds the object and is seen as a reflection nebula illuminated by the starlight that escapes above and below the ring of dust. The discovery of this object has provided dramatic evidence supporting earlier hypotheses that circum-stellar envelopes of infrared stars must be flattened. The large optical depth of the toroid produces a featureless, mid-infrared spectrum (Forrest eit 1976) but the chemical nature of the cloud has been deduced from optical spectroscopy of the reflection nebula (Crampton, Cowley and Humphreys, 1975) and by detection of a molecular cloud association with the source (Lo and Bechis, 1976 and Zuckermann ad 1976). These observations show that the 0.1 M cloud is carbon-rich, and, in fact has led to the suggestion that the source may be the progenitor of a planetary nebula. 

Optogalvanic spectroscopy is a suitable technique for studies of excitation and ionization processes in flames, gas discharges and plasmas [6.82]. Of particular interest is the investigation of radicals and unstable reaction products which are formed by electron-impact fragmentation in gas discharges. These species play an important role in the extremely rarefied plasma in molecular clouds in the interstellar medium. 

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. 

Gas and dust in molecular clouds are very cold and hence vibrational spectroscopy is limited to absorption spectroscopy, where a newly formed star inside the cloud or a chance superposition of the cloud against a background star provides the continuum against which the spectra can be measured. Such spectra show a variety of broad absorption features due to simple molecular species H2O, CH3OH, CO, CO2, OCS and CH4. Figure 4... 

Another type of vibrational spectroscopy, which can be used to study molecules without a changing dipole, is Raman spectroscopy which also involves molecular vibrations and, in this case, an interaction between the molecular polarizability (the ease with which the electron cloud around a molecule can be distorted) and th IR radiation. 

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... 

With reference to absorption spectroscopy, we deal here with photon absorption by electrons distributed within specific orbitals in a population of molecules. Upon absorption, one electron reaches an upper vacant orbital of higher energy. Thus, light absorption would induce the molecule excitation. Transition from ground to excited state is accompanied by a redistribution of an electronic cloud within the molecular orbitals. This condition is implicit for transitions to occur. According to the Franck-Condon principle, electronic transitions are so fast that they occur without any change in nuclei position, that is, nuclei have no time to move during electronic transition. For this reason, electronic transitions are always drawn as vertical lines. 

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