Molecules in the Interstellar Medium

The spectral features of molecules are different from those of atoms as well as their transition mechanisms. For atoms the electrons jump between different discrete energy levels. The spectral features of molecules are generated by different energy levels or by rotation and vibration transitions. This will be explained in the Appendix. 

The first such molecule detected in the interstellar medium was the methylidyne radical (CH) in 1937. While Saha, 1937 [289] proposed the identification of some 

The hydroxyl radical OH was the first interstellar molecule detected at radio frequencies (Weinreb et ah, 1963 [356]). 

Interstellar molecules are formed by chemical reactions within interstellar or cir-cumstellar clouds of dust and gas. By interaction with a cosmic ray particle, an atom becomes ionized. Such a positively-charged molecule attracts a nearby reactant (electrostatic force). Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower. 

In interstellar clouds a huge amount of different molecules can be found including also complex organic molecules. 

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Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

Ehrenfreund, P. and Charnley, S.B. (2000). Organic molecules in the interstellar medium, comets and meteorites. Annu. Rev. Astron. Astrophy., 38, 427-483... 

Energy levels within an atom or molecule can be populated in several ways to produce more target species in the higher energy excited state than in the ground state. The population can occur by collisional processes such as between molecules in the interstellar medium and a balance can occur between the excitation process and a number of deactivation processes (Figure 3.17a). The population of level 2 can be subjected to ... 

Indeed, the actual carbon fraction in fullerenes depend of the proper mixture of these molecules in the interstellar medium. It is likely that the number density of fullerenes and buckyonions will decrease with increasing radius (R). A distribution of the type N(full) a R m has been frequently considered in the literature on interstellar grain populations (Mathis et al. 1977). A mixture of fullerenes and buckyonions following such size distribution may reproduce the observed UV bump. The best fits to the shape, peak energy and width of the bump are obtained for m values in the range 2.5 4.5 (Fig. 1.6b). 

We have summarized the evidence supporting the existence of fullerene-based molecules in the interstellar medium. 

Millar T (2004) Organic molecules in the interstellar medium. In Ehrenfreund P (ed) Astrobiology future perspectives. Kluwer, Dordrecht, pp 17-31 Ninomiya I, Naito T (1989) Photochemical synthesis. Academic, London, p 221 Nossal J, Saini RK, Alemany LB, Meier M, Billups WE (2001) Eur J Org Chem 2001 4167-4180... 

It should be noted that the rotational spectroscopy of CO confined to a single vibrational level, usually the ground v = 0 level, provides only a limited amount of information about molecular structure. In the field of vibration-rotation spectroscopy, however, CO has been studied extensively and particular attention paid to the variation of the rotational and centrifugal distortion constants with vibrational quantum number. Vibrational transitions involving v up to 37 have been studied with high accuracy [78, 79, 80], and the measurements extended to other isotopic species [81] to test the conventional isotopic relationships. CO is, however, an extremely important and widespread molecule in the interstellar medium. CO distribution maps are now commonplace and with the advent of far-inffared telescopes, it is also an important... 

PAHs are believed to be a major class of carbon-bearing molecules in the interstellar medium 1138], They are found in carbonaceous chondrites tliat have fallen to Earth (see section 4.2.1) and in interplanetary dust particles [28]. Shock and Schulte [139] suggested that amino acids could be syntliesized by aqueous alteration of precursor PAHs in carbonaceous chondrites. We directed attention to shock reaction of PAHs [135,140,141], and conducted shock reactions using benzene, tire simplest aromatic hydrocarbon, as a starting material to simulate possible reactions occurring in interstellar space. Furtliermore, we examined the mechanism of shock reaction on the basis of quantum chemistry and discussed the implication for cosmocheniistiy. 

Kaiser R.L Experimental investigation on the formation of carbon-bearing molecules in the interstellar medium via neutral-neutral reactions. Chem. Rev. 2002, 102, 1309-1358. 

Cheung, A. C., Rank, D. M., Townes, C. H. et al. (1968). Detection of NH-, molecules in the interstellar medium by their nicrowave emission. Physical Review Letters. 21, 1701. 

Organic-rich extraterrestrial samples such as meteorites, micrometeorites, interplanetary dust particles (IDPs) and samples returned by spacecraft provide a unique record of the chemical processes in the early solar system and in the interstellar medium. In particular, detailed structural and isotopic analyses of carbonaceous meteorites have revealed a rich organic inventory and provided evidence of the synthesis of complex organic molecules in the interstellar medium and on the asteroidal parent bodies of meteorites [1,2], The organic matter in carbonaceous meteorites is present at levels of up to 5% and can be divided into solvent-soluble (l%-25%) and insoluble (75%-99%) fractions, the former characterized by considerable structural, isomeric, and isotopic diversity [2,3], and the latter characterized by a high molecular weight and complex aromatic network [4]. 

Chemical models are important for the analysis of observations of molecules in the interstellar medium and to make predictions for molecules that have not or cannot be directly observed. Modem models are able to compute the evolution of the chemical composition of a mixture of gas and dust taking into account a large number of processes including bimolecular gas phase reactions, interactions with cosmic-ray particles and UV (and X-ray) photons, interactions with interstellar... 

A fundamental role played by molecules in the interstellar medium is as one of the cooling catalysts for the star formation process. Molecular clouds that exceed the maximum stable mass for a cloud with only thermal support, known as the Jeans mass (see Section Al), are predicted to gravitationally collapse. In order for this collapse to proceed, molecules and dust are required to radiate away energy released by the gravitational collapse. Many of the stars formed will eventually produce novae and supernovae, which further eiuich the interstellar medium with molecules that can be used as catalysts to future star formation events. 

Werner (JPL), Becklin (UCLA), and Hacking (JPL) have detected Fe HI emission in the galactic center with CSHELL. This detection constrains the excitation in the region (near IRS16), but this emission is spatially segregated and kinematically distinct from nearby HI Br 7 emission, another traco of ionized gas. Other investigators have searched for H2, CO, and other molecules in the interstellar medium. 

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