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  • 1
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    Collisional excitation of interstellar water (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Rates for rotational excitation of water molecules in collisions with He atoms have been obtained from a new, accurate theoretical interaction potential. Rates among the lowest 40 ortho levels are given for kinetic temperatures to 1400 K and among the lowest 29 para levels for kinetic temperatures to 800 K.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 68; 287-318
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  • 2
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    The rotational spectra of HCNH/+/ and COH/+/ from quantum mechanical calculations (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A description is provided of ab initio molecular orbital calculations designed to provide accurate predictions for the J = 1 to 0 rotational line of the candidate interstellar molecules HCNH(+) and COH(+). The former is believed to be important in the formation of both HCN and HNC in the interstellar medium. The latter, a metastable isomer of HCO(+), was first proposed as an interstellar molecule by Herbst et al. (1976). Attention is given to thermochemical arguments that this molecule can be formed in the same reactions which are proposed to form HCO(+), taking into account theoretical data which establish its stability to intramolecular rearrangement. Rotational constants are derived by applying an empirical correction to the ab initio rotational constants.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 257
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  • 3
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    Theoretical investigation of the interstellar CH3NC/CH3CN ratio (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Calculations have been performed to determine the abundance ratio of the metastable isomer CH3NC to the stable isomer CH3CN in dense interstellar clouds. According to gas phase, ion-molecule treatments, these molecules are both synthesized via protonated ion precursors. A calculation has been made of the ratio of the formation rates of the protonated precursor ions, CH3NCH(+) and CH3CNH(+), synthesized via the radiative association reaction between CH3(+) and HCN, which is thought to be the dominant formation process of the two isomeric ions. The calculations, which involve both ab initio quantum chemistry and equilibrium determinations, lead to a predicted CH3NCH(+)/CH3CNH(+) formation rate ratio between 0.1 and 0.4. If this ratio is maintained in the neutral species formed from the precursor ions, theory predicts a sizable abundance for methyl isocyanide (CH3NC) and lends credence to its tentative observation.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 293; 236-242
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  • 4
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    Calculations concerning the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Calculations have been performed to determine the rate coefficients of several reactions involved in both the formation and depletion of interstellar HCO(+) and HOC(+). The abundance of HOC(+) deduced from these calculations is consistent with the tentative identification of HOC(+) in Sgr B2 by Woods et al. (1983). The large HCO(+)/HOC(+) abundance ratio observed by Woods et al. is due at least in part to a more rapid formation rate for HCO(+) and probably due as well to a more rapid depletion rate for HOC(+).
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 279; 322-334
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  • 5
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    Calculations concerning the reaction C + H3(+) - CH(+) + H2 (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The ion-molecule reaction of C + H3(+) - CH(+) + H2 has not been studied in the laboratory but is thought to be important in the gas phase synthesis of organic molecules in dense interstellar clouds. This reaction was studied theoretically by performing quantum chemical ab initio calculations on the potential surface. It is found that there is no activation barrier to the reaction and that it proceeds smoothly to the first excited electronic state of CH(+). The rate coefficient as a function of temperature can then be estimated using the proper long-range potentials. The rate coefficient at 10 K is calculated to be 2.9 x 10 to the -9th cu cm/sec.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 374; 390-393
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  • 6
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    A reanalysis of the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: New theoretical and experimental results have prompted a reinvestigation of the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC(+) and H2, which was previously calculated by DeFrees et al. (1984) to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC(+) does indeed react efficiently with H2 in interstellar clouds, the calculated HCO(+)/HOC(+) abundance ratio rises to a substantially greater value under standard dense cloud conditions than is deduced via the tentative observation of HOC(+) in Sgr B2.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 303; 392-400
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  • 7
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    Can interstellar H2S be formed via gas-phase reactions? - Calculations concerning the rates of the ternary and radiative association reactions between HS(+) and H2 (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 237; 1057-106
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  • 8
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    The sensitivity of gas-phase models of dense interstellar clouds to changes in dissociative recombination branching ratios (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The approach of Bates to the determination of neutral product branching ratios in ion-electron dissociative recombination reactions has been utilized in conjunction with quantum chemical techniques to redetermine branching ratios for a wide variety of important reactions of this class in dense interstellar clouds. The branching ratios have then been used in a pseudo time-dependent model calculation of the gas phase chemistry of a dark cloud resembling TMC-1 and the results compared with an analogous model containing previously used branching ratios. In general, the changes in branching ratios lead to stronger effects on calculated molecular abundances at steady state than at earlier times and often lead to reductions in the calculated abundances of complex molecules. However, at the so-called 'early time' when complex molecule synthesis is most efficient, the abundances of complex molecules are hardly affected by the newly used branching ratios.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 194; 1-2
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  • 9
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    Ion-molecule calculation of the abundance ratio of CCD to CCH in dense interstellar clouds (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Laboratory measurements and calculations have been performed to determine the abundance ratio of the deuterated ethynyl radical (CCD) to the normal radical (CCH) which can be achieved in dense interstellar clouds via isotopic fractionation in the C2H2(+) (HD)=C2HD(+)(H2) system of reactions. According to this limited treatment, the CCD/CCH abundance ratio which can be attained is in the range 0.02-0.03 for the Orion molecular cloud and 0.0l-0.02 for TMC-1. These ranges of numbers are in reasonable agreement with the observed values in Orion and TMC-1. However, the analysis of the CCD/CCH abundance ratio is complicated via the presence of competing fractionation mechanisms, especially in the low-temperature source TMC-1.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 312; 351-357
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  • 10
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    A detailed investigation of proposed gas-phase syntheses of ammonia in dense interstellar clouds (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The initial reactions of the Herbst and Klemperer (1973) and the Dalgarno (1974) schemes (I and II, respectively) for the gas-phase synthesis of ammonia in dense interstellar clouds were investigated. The rate of the slightly endothermic reaction between N(+) and H2 to yield NH(+) and H (scheme I) under interstellar conditions was reinvestigated under thermal and nonthermal conditions based on laboratory data. It was found that the relative importance of this reaction in synthesizing ammonia is determined by how the laboratory data at low temperature are interpreted. On the other hand, the exothermic reaction between N and H3(+) to form NH2(+) + H (scheme II) was calculated to possess significant activation energy and, therefore, to have a negligible rate coefficient under interstellar conditions. Consequently, this reaction cannot take place appreciably in interstellar clouds.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 321; 898-906
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