1,721,145 research outputs found
H2 excitation in turbulent interstellar clouds
No full textWe discuss the observational differences between lines of sight that intercept a group of turbulent dissipative structures and lines of sight that cross less-active regions. Using time-dependent calculations we show that the energy level distribution of the hydrogen molecule evolves in time in response to the local thermal phase. We find that relatively simple models can explain the observed properties of molecular hydrogen in diffuse interstellar clouds in terms of time evolution induced by collisional excitation in a low-density, high-temperature gas
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Near Resonance Charge Exchange in Ion-Atom Collisions of Lithium Isotopes
No full textCollisions of ions and atoms of (6)Li and (7)Li are explored theoretically over a wide range of energy from 10(-14) to 1 eV. Accurate ab initio calculations are carried Out of the Born-Oppenheimer potentials and the nonadiabatic couplings that are responsible for the near resonance charge exchange. Scattering studies show that the calculated charge exchange cross section follows Wigner's law for inelastic processes for energies below 10(-10) eV and that the zero temperature rate constant for it is 2.1 x 10(-9) cm(3) s(-1). At collision energies much larger than the isotope shift of the ionization potentials of the atoms, we show that the near resonance charge exchange process is equivalent to the resonance charge exchange with cross sections having a logarithmic dependence on energy. A comparison with the Langevin model at intermediate energies is also presented
Rotational and vibrational excitation of CO molecules by collisions with He-4 atoms
No full textFull close-coupled calculations are carried out of the cross sections for energy transfer between rotational levels of carbon monoxide in collision with He-4 atoms with energies between 5 and 600 cm(-1). At low energies, the cross sections are dominated by contributions from shape resonances. The calculated cross sections are in satisfactory agreement with the experimental data measured at an energy of 570 cm(-1). Calculations using the infinite order sudden approximation are carried out of cross sections for energy transfer between vibrational levels of CO. Vibrational energy transfer is dominated by transitions in which the vibrational quantum number changes by unity
Chemical reactions in the limit of zero kinetic energy: virtual states and Ramsauer minima in F+H-2 -> HF+H
No full textThe behaviour of reactive scattering at ultracold temperatures is explored by calculating the real and imaginary parts of the scattering length for the reaction of F with a molecule composed of a pair of pseudo-hydrogen atoms of arbitrary mass. The origin of a low energy feature in the cross section for the reaction of F with H-2 and its absence for the reaction with D-2 is investigated. Close-coupling calculations of the scattering matrix show that the F-H-2 feature arises from the presence of a virtual state associated with the van der Waals well in the entrance channel and that the virtual state is responsible for the enhanced zero temperature rate coefficient of the F-H-2 reaction. For a mass of about 1.12 hydrogen masses the virtual state turns into a
zero energy resonance and the corresponding zero temperature rate coefficient is I X 10(-9) cm(3) s(-1) despite an energy barrier of 300 K. Evidence in sup
port of the virtual state is also provided by the detection of a deep Ramsauer-Townsend minimum in the elastic component of the total cross section for F-H
-2 which the present calculations predict to occur at low energies
Relaxation of energetic S(1D) atoms in Xe gas: comparison of ab initio calculations with experimental data
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Quantal calculations on the rovibrational excitation of H(_2) and HD induced by H
Full text linkCross sections have been computed for non-reactive rovibrational transitions of H(_2) and HD, induced by collisions with H atoms, using the H+H(_2) interaction potential calculated recently by Mielke et al. [39] and an earlier potential of Boothroyd et d. [38]. The calculations relate explicitly to non-reactive scattering, excluding the proton-exchange channels. Cross sections derived using a simple harmonic oscillator approximation to the vibrational motion are compared with results obtained using numerically 'exact' solutions of the rovibrational eigenvalue equation. The convergence of the cross sections with respect to the size of the rovibrational basis set is investigated. Convergence is found to be slow, owing to the strength of the collisional coupling between vibrational manifolds. The cross sections are used to derive the rate coefficients for the rovibrational excitation of H(_2) and HD by H. For vibrationally inelastic transitions, the new rate coefficients at T ~ 1000 K are larger than the results of earlier calculations in which the vibrational motion was treated approximately, by means of a simple harmonic oscillator model. As a result, much better agreement is obtained with the empirical estimates by Allers et al. [59] of the rate coefficients for vibrational relaxation of the levels (v,j) = (1, 3) and (2, 3) of H(_2) . However, the effects of the new data on the resists of illustrative astrophysical models are less pronounced than the changes to the rate coefficients for vibrationally inelastic transitions might suggest
F+D-2 reaction at ultracold temperatures
No full textWe present a quantum mechanical study of the reaction F+D-2 at ultracold temperatures based on the potential energy surface of Stark and Werner. Th
e reaction cross section at low energies is controlled by the tunneling through the activation barrier, a mechanism that is favored by the long duration of
the collision at those energies. Differences are found in the behavior of the reactive cross section compared to that for F+H-2, due to the changed mass a
nd zero point energy. A new feature is detected in the reaction probabilities and is attributed to a Feshbach resonance corresponding to a metastable state
in the exit channel
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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