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This paper is devoted to chemistry in the gas phase dealing firstly with ion-molecule reactions at extremely low temperature. The experimental techniques that have been used in this field are shortly presented and the reactions that have been studied using the CRESU(S) method reviewed. In the second part, the most recent measurements concerning dissociative recombination are discussed, including studies of branching ratio and new determination of the rate coefficient for H+3 ions.
ISO, the Infrared Space Observatory is capable of measuring spectra across most of the mid-infrared, the region from 4000 to 500 cm−1 (2.5-20 μm). Of these 3500 cm−1, about 670 have been inaccessible to astronomers due to strong atmospheric absorptions and about 750 cm−1 require an airborne platform, making access limited. ISO will provide high quality spectra in the 19% of the celestial mid-infrared which has never been studied and dramatically increase the number of spectra in the 21% with limited access. Thus ISO has the potential to nearly double our knowledge of the mid-IR spectroscopic properties of the cosmos. This knowledge, in turn, will revolutionize our understanding of its chemical make-up because of the mid-infrared's powerful compositional diagnostic capabilities.
Experimental laboratory work on simulation of the electrostatic fragmentation was started with loosely bound Al2O3 particles of 1 to 10 micrometers size. These particles were suspended in an electrodynamic quadrupole inside a vacuum chamber and electrically charged by ion beams of energies up to 5 keV. The electrostatic fragmentation was observed and derived tensile strengths of the particles range from 103 to 105 Pa what is compatible with theoretical estimates. A dependence of the tensile strength on the size of particles has been found. This dependence can be well fitted by inverse square power law.
Libraries of reactions used in astrochemistry modeling have seen an explosive increase in size in recent years. Their quality control by manual effort is almost impossible. Expert systems with artificial intelligence are now needed to ensure the quality of large scale astrochemistry libraries.
Absorption spectra of some gaseous PAHs, either pure species or natural mixtures, have been obtained in the VUV-visible region and compared to the interstellar extinction curve.
The assumption that free PAHs are ubiquitous in the ISM cannot be rejected by incompatibility between the interstellar extinction curve and the absorption spectra of such molecules. PAHs absorb in the FUV rise and may give an important contribution to the bump at 2200 Å. We have derived that about 15% of the cosmic carbon is involved in these molecules.
Astronomical searches for H2CCC and H2CCCC, based on frequencies from our laboratory identifications, have resulted in detections toward TMC-1 and IRC+10216. These new interstellar species are possibly the first of a new family of highly polar carbon chains; they are only the second and third carbenes (carbon molecules with two nonbonded electrons) known in space.
Laboratory spectra through the mid-infrared have been used to calculate the optical constants (n and k) for a variety of pure and mixed molecular ices. The ices studied were H2O, CH3OH, CO2, OCS, CH4, CO2+CH4, CO2+OCS, CO+CH4, CO+OCS, O2+CH4, O2+OCS, N2+CH4, N2+OCS, H2O+CH4, H2O+OCS, and H2O+CH3OH+CO+NH3.
Warm-up of astrophysical ice analogues containing formaldehyde produced organic residues in large abundances. It is argued that formaldehyde reactions at very low temperatures could be an important source of interstellar and cometary organic molecules.
This review paper presents the results of state of the art Quantum Chemistry calculations in the field of Astrochemistry. It provides selected examples to illustrate the possible contribution of molecular orbital theories to solving a number of problems of astrophysical interest ranging from identification of new molecules to IR emission analysis and rate constants determinations.
Nucleation is a non-equilibrium process: the products of this process are seldom the most thermodynamically stable condensates but are instead those which form fastest. It should not be surprising that grains formed in a circumstellar outflow will undergo some degree of metamorphism if they are annealed or exposed to a chemically active reagent. As a consequence of this processing in the laboratory one observes a continuous increase in the strength of the silicate absorption band at 20 microns relative to the 10 micron feature. In Section 1 we show that this ratio can be used as an indicator of the relative age of silicate condensates. Metamorphism of refractory particles continues in the interstellar medium (ISM) where the driving forces are sputtering by cosmic ray particles, annealing by high energy photons and grain destruction in supernova generated shocks. Studies of the depletion of the elements from the gas phase of the ISM tell us that if grain destruction occurs with high efficiency, then there must be some mechanism by which grains can be formed in the ISM. Laboratory studies of such a process (Moore, Tanabe, and Nuth, Ap. J. (Lett) 373, L31-L34, 1990) have shown that the frequency of the -SiH stretch can be used as an indicator of the oxidation state of the silicon in such grains. Highly reduced grains exhibit an SiH absorption near 2100 cm−1 whereas highly oxidized silicates absorb near 2300 cm−1: this point is discussed in Section 2.
From a physical point of view we analyzed the possibility of observation of the Lα-line of the positronium (Ps). In a broad range of temperatures the processes of recombination to states of Ps with different nl, collisions of Ps with electrons and protons of medium are examined.
The molecule C60, Buckminsterfullerene, was discovered1-3 during laboratory experiments motivated by problems associated with processes involving carbon in stars and space4,5. Astronomical puzzles also lay behind the experiments which led to the molecule's extraction and structure confirmation6-8. Although the resulting breakthrough has opened up exciting new avenues of chemistry, physics and materials science here on earth9 the original astrophysical questions still remain and are even more tantalising now than they were before. Some of the puzzles are here re-addressed in the light of the new understanding which the fullerene discovery has brought. Indeed we shall look at the questions through magenta coloured spectacles and note that there are new and even more intriguing parallels between the behaviour of carbon on earth and space. The article contains a brief account of the processes responsible for the synthesis of carbon in stars and its dissemination throughout the Galaxy as this information is deemed necessary to gain an intrinsic understanding of the amazing role carbon plays in nature.
The advancement of photoionization and photodissociation rates relevant for interstellar diffuse and dense clouds, circumstellar medium, cometary coma and planetary atmosphere has been reviewed with a mention of uncertainties involved in these rates.
Our fundamental knowledge of interstellar grain composition has grown substantially during the past two decades thanks to significant advances in two areas: astronomical infrared spectroscopy and laboratory astrophysics. The opening of the mid-infrared, the spectral range from 4000-400 cm−1 (2.5-25 μm), to spectroscopic study has been critical to this progress because spectroscopy in this region reveals more about a material's molecular composition and structure than any other physical property.