THE CHEMISTRY OF INTERSTELLAR SPACE

A view of the interstellar medium in the region of the constellation Chamaeleon (NASA/ Photo Researchers, Inc.) 

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Herbst E (1995) Chemistry in the interstellar medium. Annu Rev Phys Chem 46 27-53 Herbst E (2001) The chemistry of interstellar space. Chem Soc Rev 30 168 Howard JB, Das Chowdhury K, Vander Sande JB (1994) Nature 370 603 Iglesias-Groth S (2003) Physisorption and photoabsorption of fullerenes. Implications physics and astrophysics, Ph.D. thesis. Universidad de La Laguna, Spain Iglesias-Groth S (2004) Astrophys J 608 L37 Jeolaika L, Sidis V (1999) Chem Phys Lett 300 157... 

Astronomers have now determined that much of the chemistry of interstellar space occurs via ion-molecule reactions. Cosmic rays (fast-moving protons and electrons pervading all of interstellar space) ionize molecular hydrogen (H2) and the resulting ions (H2 ) react quickly with more H2 to... 

In this chapter, we describe the current status of theoretical kinetics for chemical reactions at low temperature, i.e., from 1 to 200K. The desire to understand the chemistry of interstellar space and of low temperature planetary atmospheres provides the general motivation for studying chemical kinetics at such temperatures. For example, the chemistry of Titan s atmosphere is currently a topic of considerable interest. This motivation led to the development of novel experimental techniques, such as the CRESU (cinetique de reaction en ecoulement supersonique uniforme) method, which allows for the measurement of rate coefficients at temperatures as low as lOK (see Chapter 2 by Canosa et ai). Such measurements provide important tests for theory and have sparked a renewed interest in theoretical analyses for this temperature range. ... 

Herbst E (1990) The chemistry of interstellar space. Ange-wandte Chemie, International Edition (English) 29 595-608. 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

The interstellar medium is thus a chemically diverse medium fed nearly all of the chemical elements by supernova explosions. Conditions in the interstellar medium produce a cocktail of molecules that ultimately find themselves back on the surface of planets during the formation of the new star and solar system. Does the interstellar medium seed life with molecules from space The nature of interstellar medium chemistry might then add credibility to the formation of life in many places within the Universe and act as a panspermia model for the origins of life. 

The discovery of C6o came about from an attempt by Kroto et al. to understand the adsorption spectra of interstellar space. Although they failed in this attempt, the discovery of C6o won them the Nobel Prize for chemistry in 1996. 

Naphthalene and anthracene radical cations are the two simplest members in the family of the PAH radical cations. Investigation of the photophysics and photochemistry of the latter are of major concern in contemporary chemical dynamics. The radical cations of PAHs are of fundamental importance in the chemistry of the interstellar space, environmental, biological processes and combustion [103-106]. Radical cations of PAHs are most abundant in the interstellar and extragalactic environments [41]. They absorb strong UV radiation emitted by the young stars and get electronically excited. Examination of the fate of electronically excited PAH radical cations invited critical measurements of their optical spectroscopy in the laboratory in recent years [42 4]. Attempt is made to understand the important issues like, (1) photostability and lack of fluorescence emission and (2) the origin of the enigmatic... 

The present work is only the very first step towards a comprehensive and systematic understanding of the fundamental elementary processes involved in the chemistry of hydrocarbon-rich planetary atmospheres and interstellar medium. Our experiments explicitly identified synthetic routes to nitriles — the alleged precursor molecules to amino acids. The experimental data can be employed to set up a systematic database of reaction products and can predict the formation of hitherto unobserved gas phase molecules. The applications of the crossed beam method to astrochemical problems have just begun. Many interesting problems remain to be studied. In the coming century, laboratory experiments of the kind we have presented here combined with observations and planetary-space missions will undoubtedly unravel the complex chemical processes which extend from atoms and simple molecules to large molecules and aggregates. 

The chemistry of dense, dark molecular clouds prior to planet formation is the topic of this paper. Dr. Ziurys has discussed the inventory and measurement of gas phase interstellar molecules associated with dense molecular clouds in the chapter, "Identifying Molecules in Space" 8). Here, the focus is on the chemistry in and on the ices and the interaction of these ices with species in the gas. Since these ices represent the largest repository of interstellar molecules in dense clouds, they tie up a large fraction of the chemical inventory in molecular... 

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