Rate Coefficients For Transitions Research Articles

Collision strengths and excitation rate coefficients for transitions in CA SV

Collision strengths and excitation rate coefficients for transitions in CA SV

Collision strengths and excitation rate coefficients for transitions in Fe XXI

Using the Dirac-Fock R-matrix program, collision strengths have been computed for all transitions belonging to the 20 fine-structure levels among the 1s 1 2s 2 2p 2 , 1s 1 2s 2 2p 3 and 1s 1 2p 4 configurations of Fe XXI. All partial waves with J ≤ 29/2 of both even and odd parities have been included, and collision strengths are tabulated at energies above thresholds in the range below 300 Ry. Resonances are highlighted in the threshold energy region, and effective collision strengths have been obtained after integrating collision strengths over a Maxwellian distribution of electron velocities.

Rotational energy transfer in OH (A 2Σ+, v′=0): A method for the direct determination of state-to-state transfer coefficients

We have determined state-to-state rate coefficients for rotational and fine structure transitions of OH (A 2Σ+, v′=0) in thermal collisions with He and Ar at 300 K. The temporal evolution of single fluorescence lines within the A–X, 0–0 band of OH were measured, exciting either the F2(4) or F2(5) state by a nanosecond laser pulse. The OH radical was produced in a discharge flow cell, containing predominantly He or Ar, at various pressures between 1 and 6 mbar. The time resolution in the experimental setup was sufficient to evaluate the rotational energy transfer coefficients directly from the time dependence of two fluorescence lines. The observed average rate coefficients for collisions of OH (A, v′=0) with Ar are approximately 3 times larger than those with He. The two rare gases show different qualitative behavior. Whereas the almost isoenergetic transitions with ΔJ=1 and ΔN=0 are favored in collisions with Ar, those with ΔJ=ΔN=−2 are favored in collisions with He. In addition, a strong preference for transitions conserving the parity of the OH, a propensity rule, previously reported for rotational relaxation in the A state of OH, was found for collisions with He but not for collisions with Ar. Our experimental results for He and for Ar are in good agreement with recent quantum mechanical calculations of the energy transfer coefficients.

Investigation of effective ionization rate coefficients of Al I and Zr I in a plasma using laser produced atomic beams

Al I and Zr I atomic as well as Zr II ionic beams were produced by laser irradiation of Al and Zr targets. The beams were injected into a plasma, and effective ionization rate coefficients were derived by analyzing the beam densities both with and without the plasma present. Because of the large number of intermediate levels, the Zr I atoms are ionized much faster than the Al I atoms under the same plasma conditions, in spite of the fact that the ionization energy from the ground state for Zr I is higher than that for Al I. The results are compared with theoretical effective ionization rate coefficients assuming a simplified two-step ionization process. For Zr I, the measured effective ionization rate coefficients were found to be two to five times the sum of the excitation rate coefficients for electric dipole-allowed transitions and for Al I one to three times. These results may represent a limiting case, in which the effective ionization rate coefficient is nearly equal to the sum of all excitation rate coefficients including electric dipole-allowed as well as multipole and forbidden transitions.

Direct and resonance contributions to electron-impact excitation of n=2 to n=3 transitions in neonlike ions.

Rate coefficients for electron-impact excitation have been calculated for transitions from the ground state to n=3 excited states of six neonlike ions with atomic numbers Z=15, 17, 26, 34, 42, and 54. The direct-excitation rate coefficients were calculated using relativistic multiconfiguration target wave functions and a relativistic distorted-wave approximation. The contributions from autoionizing resonances were evaluated using a multiconfiguration Dirac-Fock model. These resonances enhance the rate coefficients for transitions to the 2${p}^{5}$3s states by as much as a factor of 3. The peak rate coefficient and the relative importance of resonance excitation diminish with increasing atomic number. However, we find that resonance enhancements are generally quite large for electron temperatures less than the temperature where peak excitation occurs. Our total rate coefficients for ${\mathrm{Se}}^{24+}$ are in good agreement with close-coupling results obtained with R-matrix calculations. The resonance contributions from the present work differ from the results by Smith et al. [Astrophys. J. 298, 898 (1985)] for the ${\mathrm{Fe}}^{16+}$ ion by as much as a factor of 3--5.

Rotationally inelastic collisions between H2 molecules in interstellar magnetohydrodynamical shocks

We present the results of quantum mechanical calculations of the rates of rotational transitions in ortho- and para-H2 induced in collisions with other H2 molecules. Transitions ΔJ = 2 between rotational levels J ⩽ 7 are considered. Some possible approximations in the collision calculation are discussed and found to give results of an accuracy which is acceptable for the intended astrophysical applications, to the theoretical study of interstellar MHD shocks. Comparison is made with earlier calculations of H2–H2 scattering by Monchick & Schaefer and of He–H2 scattering by Schaefer. The rate coefficients for transitions induced by ortho-H2 (J = 1) are found to be remarkably similar to the rate coefficients for the same transitions induced by He, particularly at the lower temperatures considered.

Electron Excitation Rate Coefficients for Transitions from the IS21S Ground State to the 1S2S1,3S and 1S2P1,3P0 Excited States of Helium

view Abstract Citations (19) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron Excitation Rate Coefficients for Transitions from the IS21S Ground State to the 1S2S1,3S and 1S2P1,3P0 Excited States of Helium Aggarwal, K. M. ; Kingston, A. E. ; McDowell, M. R. C. Abstract The available experimental and theoretical electron impact excitation cross section data for the transitions from the 1s2 1S ground state to the 1s2s 1,3S and 1s2p 1,3P0 excited states of helium are assessed. Based on this assessed data, excitation rate coefficients are calculated over a wide electron temperature range below 3.0×106K. A comparison with other published results suggests that the rates used should be lower by a factor of 2 or more. Publication: The Astrophysical Journal Pub Date: March 1984 DOI: 10.1086/161856 Bibcode: 1984ApJ...278..874A full text sources ADS |

Electron collisional rate coefficients for low-level transitions in hydrogen

There exists presently no general agreement concerning the collisional excitation rates of atomic hydrogen. The present investigation is concerned with the results reported by various investigators, taking into account the semiempirical rates of Johnson (1972), a study conducted by Aggarwal (1983), empirically based rates of Crandall et al. (1974), and the rates tabulated in Osterbrock (1974), based on the close-coupling calculations of Burke et al. (1967). It is found that there is a major conflict between the 'best' theoretical and Johnson values on the one hand, and the empirical data on the other. A study of the data shows that the Johnson value for the 1 to 2 rate agrees well with other theoretical calculations, and within 25 percent of empirical measurements, but that the Johnson 1 to 3 rate is 40-75 percent of the 'best' theoretical calculations.

Collisional rates for vibrational-rotational transitions in circumstellar SiO masers

Calculations for rotational population inversion in circumstellar SiO masers require collisional rates for vibrational-rotational transitions. This paper reports quantum mechanical, state-to-state collisional rate coefficients for pure rotational and vibrational-rotational transitions among the first three vibrational states of SiO. These are expressed in terms of simple two-parameter power-law fits accurate in the temperature range 1000-3000 K.

Collisional excitation of OH by H2: transitions within the ground-state Λ doublet

The authors present the results of calculations of cross sections and rate coefficients for transitions induced by collisions with H2 within the ground-state Lambda doublet of the OH radical. The calculations are relevant to the 18 cm interstellar OH maser emission. The structure observed in the cross section at low energies is attributed to shape resonances. It is found that the combined effects of the rise in the cross section towards threshold and the contribution of the resonances sustain large values of the rate coefficients at low temperature. The authors suggest that the mechanism of collisional inversion of the populations of the ground state Lambda doublet (by rotational excitation followed by radiative decay) will be effectively quenched by collisions within the doublet at low temperatures.

Rate coefficients for fine structure transitions in [Ne IV

Rate coefficients for fine structure transitions in [Ne IV

Properties of the Io plasma torus inferred from Voyager EUV data

A physical model for the Io plasma torus is constructed to explain the EUV radiative emission observed by the Voyager UV spectrometer. Electron impact excitation rate coefficients for electronic transitions of S III, S IV, O II and O III are calculated by the method of distorted waves (Davis, Kepple, and Blaha, 1976); these coefficients account for the asymmetric shape of the 686 A feature. It is concluded that the electron gas must have a distribution function with a non-Maxwellian tail. An approximate representation of the distribution function as two temperature components requires a cold component of 3.5-4 eV and density of 2000 per cu cm and a hot component of about 100 eV and density of 50-100 per cu cm to satisfy observational constraints.

Spectroscopy and atomic physics of highly ionized Cr, Fe, and Ni for tokamak plasmas

We consider the spectroscopy and atomic physics for some highly ionized Cr, Fe, and Ni ions produced in tokamak plasmas. Specifically, we consider both allowed and forbidden transitions between levels of the 2s22pk, 2s2pk+1, and 2pk+2 configurations. Forbidden and intersystem wavelengths for Cr and Ni ions are extrapolated and interpolated using the known wavelengths for Fe lines identified in solar-flare plasmas. We present tables of transition probabilities for the B I, C I, N I, O I, and F I isoelectronic sequences, and give collision strengths and transition probabilities for Cr, Fe, and Ni ions of the Be I sequence. Proton excitation rate coefficients for fine-structure transitions in the Be I through F I sequences are tabulated, as well as populations of all levels of the 2s22pk, 2s2pk+1, and 2pk+2 configurations for Cr, Fe, and Ni for the Be I sequences, and for Cr and Ni for the B I-F I sequences. We discuss similarities of tokamak and solar spectra, and show how the atomic data we have presented may be used to determine ion abundances and electron densities in low-density plasmas.

The diffusion of an atom on a solid surface

A microscopic model for the diffusional motion of an atom chemisorbed on a solid surface is presented. The atom located at a given site is described by a set of states which can be divided into two classes. One class is composed of states whose energy is located below the intersite barrier. A particle described by one of these states can go to a neighboring site only through tunneling. The second class of states have higher energy than the intersite barrier. They are delocalized and an atom occupying such a state can change its site with relative ease. The vibrational motion of the lattice can transfer energy to or take it from the particle and it can also change the intersite transfer probability. As a result the particle may undergo three types of processes: 1) change state but not site (vertical transitions); 2) change site and state (oblique transitions); or 3) change site but not energy (horizontal transitions). The theory derives a kinetic equation for the probability that at a given time the particle is located at a given site and has a specified energy. This is then used to compute the mean square displacement of the particle and its diffusion coefficient. All these quantitities are expresed in terms of rate coefficients for horizontal, oblique, and vertical transitions. Microscopic expressions are provided for these quantities as a function of the particle phonon coupling, intersite transfer probability, particle energies, and lattice properties. The temperature dependence of the rate coefficients is analyzed numerically. If tunneling is minimal and a change of site can occur only after the phonon transfer promotes the particle in a state located above the intersite barrier, an Arrhenius temperature dependence of the diffusion coefficient is observed. The form of the functional dependence of the pre-exponential factor on temperature changes with anharmonicity, Einstein frequency, and phonon particle coupling. If the lattice is harmonic the activation energy is equal to the intersite barrier plus the energy of the distortion of the lattice upon chemisorption. If anharmonicity is important the activation energy is close or equal to the intersite barrier. If tunneling is important non-Arrhenius temperature dependence may be observed.

Measurements of excitation rate coefficients for singlet and triplet transitions of neon VII

Laboratory measurements of excitation rate coefficients for eighteen transitions in neon VII are presented. These were done by measuring the intensities of the corresponding spectral lines and at the same time determining the electron temperature by a laser scattering technique. The results are considered to be accurate to about a factor two absolutely and about +or-50% relatively. Within this accuracy they are found to be in agreement with the theoretical values due to Eissner except for X(2s2p 3P0-2s3d 3D) which is six times smaller than the theoretical value.