Rotational Transitions Of H2 Research Articles

Rotational Quenching Rate Coefficients for H2in Collisions with H2from 2 to 10,000 K

Rate coefficients for rotational transitions in H2 induced by H2 impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H2)2 potential energy surface were performed. The potential energy surface used here has been demonstrated to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels of -->J ≤ 8 were computed for collision energies ranging between 10−4 and 2.5 eV, and the corresponding rate coefficients were calculated for the temperature range -->2 ≤ T≤ 10,000 K. In general, agreement with earlier calculations, which were limited to 100-6000 K, is good, although discrepancies are found at the lowest and highest temperatures. Low-density-limit cooling functions due to para- and ortho-H2 collisions are obtained from the collisional rate coefficients. Implications of the new results for nonthermal H2 rotational distributions in molecular regions are also investigated.

SpitzerIRS High‐Resolution Spectroscopy of the 12 μm Seyfert Galaxies. I. First Results

The first high-resolution Spitzer 1RS 9-37 μm spectra of 29 Seyfert galaxies of the 12 μm Active Galaxy Sample are presented. The spectroscopy was obtained with off-source observations, allowing excellent background subtraction and thus accurate measurements of continuum levels and strengths of weak emission lines. Several new combinations of emission-line ratios and line/continuum ratios were identified as effective diagnostics of the strength of the AGN component in these galaxies. The line ratios [Ne v]/[Ne II], [O IV]/[Ne II], already known, but also [Ne III]/[Ne II] and [Ne v]/[Si II] can be used to measure the dominance of the AGN. The equivalent width of the 11.25 μm PAH feature, like that of the 6.2 μm PAH feature, already studied, also anticorrelates with the dominance of the AGN. Our measurements indicate that the 11.25 μm PAH feature has a constant ratio with H2 S(1) irrespective of Seyfert type, approximately 10 to 1. The ratio of the 19 μm fluxes observed with the two spectrometer channels, with aperture areas differing by a factor of 4, measures the source extendedness, giving another estimate of the AGN dominance, which correlates with both the EWs of [Ne n] and PAH emission. Using the rotational transitions of H2, we estimated temperatures (200-300 K) and masses (1 -10 × 106 M⊙) for the warm molecular component. Finally, we used the ratios of the doublets of [Ne v] and of [S III] to estimate the gas electron density, which appears to be of the order of ne ∼ 103-104 cm-3 for the highly ionized component and a factor 10 lower for the intermediate- ionization gas. © 2008. The American Astronomical Society. All rights reserved.

State-to-state rotational transitions in H2+H2 collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the superthermal region. The two most recently published potential energy surfaces for the H(2)-H(2) complex, the so-called Diep-Johnson (DJ) [J. Chem. Phys. 112, 4465 (2000); 113, 3480 (2000)] and Boothroyd-Martin-Keogh-Peterson (BMKP) [J. Chem. Phys. 116, 666 (2002)] surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H(2)+H(2) collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H(2)-H(2) interaction. We found significant differences in rotational excitation/deexcitation cross sections computed on the two surfaces in collisions between two para-H(2) molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment B. Mate et al., [J. Chem. Phys. 122, 064313 (2005)] for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.

Transient CARS spectroscopy of rotational transitions in H2: the statistical dependences of the Doppler and collision dephasing

The S0(0) and S0(1) rotational transitions in the H2 molecules are studied by the method of transient CARS spectroscopy at temperatures 296 and 80 K in a broad range of gas densities — from the density at which Doppler dephasing occurs to the density corresponding to collision dephasing. The slowest decay of the responses of the scattered probe pulse at the molar gas density ∼0.2 amagat corresponds to the Dicke line narrowing. It is shown that the dephasing of the S0(1) transition in the natural H2 mixture leads to a substantial discrepancy between the experimental results and the model of statistically independent perturbations of the rotational and translational motion of molecules at densities within a few tenths of amagat. This discrepancy becomes quite significant at liquid nitrogen temperature. At the same time, the dephasing of the S0(0) transition better corresponds to this model, as well as the dephasing of the S0(1) transition in the buffer He gas. The deviation from the model of statistically independent perturbations is described by introducing the correlation parameter. It is assumed that the features of the dephasing of the S0(1) transition observed in the natural H2 mixture are mainly caused by resonance dephasing collisions.

Effects of expanding compact H II regions upon molecular clouds - Molecular dissociation waves, shock waves, and carbon ionization

view Abstract Citations (81) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effects of expanding compact H II regions upon molecular clouds: molecular dissociation waves, shock waves, and carbon ionization. Hill, J. K. ; Hollenbach, D. J. Abstract The effects of young expanding compact H II regions upon their molecular environments are studied, emphasizing the simultaneous evolution of the molecular hydrogen dissociation front and the shocked shell of gas surrounding the nebula. For H II regions powered by O5 stars embedded in molecular clouds of ambient density between 1000 and 10,000 per cu cm, the dissociation wave initially travels outward much more rapidly than the shock, but later decelerates and is swept up by the shock about 100,000 yr after the expansion begins. The 21-cm line of atomic hydrogen will be optically thick in both the preshock and postshock gas for most of this period. The most important coolant transitions are the forbidden O I line at 63 microns and, for times not less than about 100,000 yr, the rotational transitions of H2 and/or the rotational transitions of CO. The vibrational transitions of H2 are excited predominantly by ultraviolet pumping. The preshock and postshock carbon recombination-line emission measures are estimated. Publication: The Astrophysical Journal Pub Date: October 1978 DOI: 10.1086/156501 Bibcode: 1978ApJ...225..390H Keywords: Gas Dissociation; H Ii Regions; Interstellar Chemistry; Nebulae; Shock Wave Propagation; Stellar Evolution; Absorption Spectra; Carbon Monoxide; Emission Spectra; Gas Expansion; Hydrogen Ions; Infrared Spectra; Microwave Emission; Molecular Spectra; Stellar Models; Astrophysics; Expansion:H II Regions; H II Regions:Molecular Clouds; Molecular Clouds:Shock Waves full text sources ADS |

Rotational transitions in H2 by Li+ collisions at Ec.m. = 0.6 eV; Comparison with experiment

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