Influence of atom-atom collisions on the collisional-radiative ionization and recombination coefficients of hydrogen plasmas

The collisional-radiative recombination and ionization coefficients have been calculated for hydrogen plasmas of low ionization degree. The method consists in solving a coupled system of rate equations which not only accounts for the electronic but also for the atomic collision processes.     

Influence of Laser radiation on the collisional-radiative recombination and ionization coefficients

It is shown that irradiation of a plasma by a Laser beam lowers the collisional-radiative coefficient for recombination (α) and increases the one for ionization (S) compared to unirradiated plasmas. Numerical values ofα andS are given for a hydrogen plasma irradiated by CO₂, Ruby, and Ar-Ion Lasers. The calculations are based on a collisional-radiative model in which photoionization and stimulated recombination intervene. Stimulated recombination has an important effect and can lead to saturation of free-bound transitions at high power density of the Laser beam as long as multiphoton ionization effects and plasma heating by inverse bremsstrahlung remain negligibly small.     

Collisional-radiative volume recombination and ionization coefficients for quasi-stationary helium plasmas

Collisional-radiative recombination and ionization coefficients, α andS, have been calculated on the basis of a non-hydrogen-like collisional-radiative model for atomic helium. The singlet and triplet systems have been considered as two individual systems coupled to each other through a set of elementary processes for collisions and radiation. Auto-ionization states have not been taken into account. Therefore the computations of the coefficient α have been limited to the range 125≦T _e [°K]≦64000. (AboveT _e=64 000 °K the auto-ionizing states considerably influence the recombination rates (di-electronic recombination).) The results show that the α- andS-coefficients for helium are more sensitive to radiative trapping than those for hydrogen under comparable absorption conditions. Concerning the α-values: For low electron densities one obtains approximately α_helium≈2α_hydrogen, whereas at high electron densities one finds approximately α_helium≈1/2α_hydrogen (to 1/10 α_hydrogen at high electron temperatures). Collisional-radiative ionization coefficients for atomic helium have not yet been published in the past. In the present paper one finds for the first time a complete set ofS-coefficients for different degrees of radiative trapping.     

Saha decrements and collisional-radiative recombination and ionization coefficients for a nonequilibrium oxygen plasma

Using a time-dependent approach, a collisional-radiative model is developed for an oxygen plasma. Effective recombination and ionization coefficients are calculated for two specific cases, optically thin and optically thick in bound-bound u.v. radiation. The steady-state population densities for neutral and ionic states are presented in terms of their deviations from Saha equilibrium predictions. These results show the approach of the plasma to a state of local thermodynamic equilibrium (LTE) as a function of electron temperature and density. The computations are for temperatures between 1.0 and 3.0 eV and electron densities between 10¹⁶ and 10¹⁹ cm^-3.     

The collisional-radiative model and laser-induced excitation transfer in helium plasmas: Role of associative ionization

The effect of associative ionization processes on temporal enhancement and decay of fluorescence from helium levels following laser-induced selective excitation is investigated through a collisional-radiative model. In low-temperature moderate-pressure helium plasmas the associative ionization processes significantly affect the population densities; the effect is more pronounced in n = 3 than in n = 4 levels.     

On Drawin's formula for ionization in atom-atom collisions

Drawin's semiclassical formula for ionization in collisions of neutral atoms or molecules is examined. Both, from an analysis of its derivation and from a comparison with additional experimental data, it appears that considerable errors can be expected in general applications of this formula.     

Collisional-radiative ionization and recombination coefficients for quasi-stationary homogeneous hydrogen and hydrogenic ion plasmas

The recombination and ionization coefficients have been re-calculated on the basis of the collisional-radiative model. The numerical results obtained deviate from those published byBates, Kingston, andMcWhirter. The deviations reach a factor of about four for the recombination and a factor ten for the ionization coefficients. The smallest deviations occur in the recombination coefficients when the plasma is assumed to be optically thin in all transitions. The conditions for the applicability of the collisional-radiative model are discussed.     

Transport coefficients of helium and argon-helium plasmas

Calculated values of the viscosity, thermal conductivity, and electrical conductivity of helium, and mixtures of argon and helium, at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the mixtures. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30000 K. The results are compared with those of previously published studies. Significant discrepancies are found; these are attributed to the improved values of the collision integrals used here in calculating the transport coefficients     

Collisional-radiative volume recombination and ionization coefficients for He+-He2+-e plasmas

Coefficients have been calculated for collisional-radiative recombination of doubly ionized helium atoms with electrons and collisional-radiative ionization of singly into doubly ionized helium atoms due to electronic collisions and radiative absorption. Up to one hundred coupled equations have been solved which corresponds to a level system in which levels up to principal quantum number one hundred have been taken into account. The numerical results are presented in form of curves for a large number of parameters corresponding to very different experimental situations. Comparison of our values with those experimentally determined by Mosburg and by Gippiuset al. is made.     

Binary and recoil collisions in strong field double ionization of helium

We have investigated the correlated momentum distribution of both electrons from nonsequential double ionization of helium in a 800 nm, 4.5 x 10(14 W/cm2 laser field. Using very high resolution coincidence techniques, we find a so-far unobserved fingerlike structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process. We identify features corresponding to the binary and recoil lobe in field-free (e,2e) collisions. This interpretation is supported by analyzing ab initio solutions of a fully correlated three-dimensional helium model.     

Characterization of non-lorentzian line shapes in atom-atom collisions

Summary Two different characterizations of the spectral line shape in the core region of resonance lines of atoms perturbed by other atoms in terms of 1) its moments about the origin and 2) its frequency moments are given. Simple expressions relating the line width and the asymmetry parameter of these collision-broadened lines with two of these moments are obtained. These expressions might lead to a new experimental determination of the average time of duration of atom-atom collisions since the involved moments are measurable.

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Riassunto Si danno due diverse caratterizzazioni della forma di linea spettrale nella regione del nocciolo di lince di risonanza di atomi perturbati da altriatomi in base a 1) i suoi impulsi intorno all'origine e 2) i suoi impulsi di frequenza. Si sono ottenute semplici espressioni che connettono l'ampiezza di linea e il parametro di assimmetria di queste linee allargate dalla collisione con due di questi impulsi. Queste espressioni potrebbero portare ad una nuova determinazione sperimentale della durata media delle collisioni atomoatomo poichè gli impulsi coinvolti sono misurabili.

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Резюме Приводятся две различньіе характеристикя Формы спектралпьной нинии в области резнансных линий атомов, возбужденных другими в терминах 1) моментов относиелъхо начала и 2) частотхых моментов. Получаются простые выраженыя, связывающие ширихну линии и параметр асимметрии для зтих лихий, уширенных за счет соудалелй, с двумя моментами. Эти выражения моГут прывести к новому экспенменталнъному опреденению зредхей продолжитености соударений атома с атомом, так как фигунируюшие в выражениях моменты являются измеримыми.

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To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Influence of electron correlations on double-capture process in proton helium collisions

The first-order correct-boundary Coulomb–Born distorted-wave approximation is used to study the double-electron capture by protons from the ground-state helium atoms at intermediate and high impact energies. The differential double capture cross sections are obtained as a function of the projectile scattering angle and the total cross sections as a function of the impact energy. In the considered range of impact energy, our calculation shows that although the results are not so sensitive to the static inter-electronic correlations in the initial channel, the strong final-state correlations have a large effect on the magnitudes of the double capture cross sections. The calculated differential and integral cross sections are compared with their available experimental values. The comparison shows a good agreement between the present calculations and the measurements. The comparison of the integral cross sections shows that the present approach is compatible with other theories.     

Influence of the helium autoionization structures on the single/double ionization signal

Calculations of intense field (around 10¹⁶ W/cm²) single- and double-ionization processes in helium at XUV wavelengths are presented. The laser wavelength is chosen near the | 2s2p ¹ P autoionization structure and the dynamics are explored. Single and double ionization yields, as well as the photoelectron energy spectrum for photon energies around the autoionization structure are calculated. In the case of a pulse of few femtoseconds duration, no significant enhancement of the double ionization yield has been found in tuning the photon frequency around the peak of the resonance. It is also shown that in the case of a long pulse (and hence narrow compared with the relevant autoionization width), the branching ratio of double to single ionization yield can be relatively enhanced by tuning to the absorption minimum of the resonance. Received 19 February 2002 / Received in final form 2 May 2002 Published online 19 July 2002     

A collisional-radiative model for helium and its application to a discharge plasma

For the collisional-radiative model applied to helium, two alternative formulations are presented in which non-linear process such as ionization due to metastable-metastable collisions may be included. The atomic data, including dielectronic,recombination, employed in the calculations are summarized. The collisional-radiative calculation is applied to a positive-column plasma for which both the nonlinear processes and radiation trapping are taken into account. The calculated population densities of excited levels for a low-pressure and low-current discharge plasma show good agreement with a measurement made using the hook method. The population density distribution among excited levels is interpreted in terms of excitation-ionization mechanisms in the plasma. This leads to a grouping of the excited levels into two phases for the discharge plasma considered: the corona phase applies to the lower-lying levels and the quasi-saturation phase to the higher-lying levels. The importance of the ladder-like excitation-ionization mechanism is noted for this class of plasmas.     

Influence of the collisional recombination on the electrostatic fluctuation spectrum in an helium plasma

The collisional recombination, in the afterglow of a dense plasma, is regarded as a source process for an overpopulation of the high energy tail of the electron velocity distribution function. The perturbation of the distribution function leads to an important enhancement of the fluctuations of the electrostatic field in a narrow range near the plasma frequencyω _p .     

Ionization and recombination in plasmas

Ionization and recombination in a plasma are considered. They give rise to a system of kinetic equations with cubic source terms. An existence theorem is proved for such a system.     

Heating of laser-induced plasmas in helium

This letter reports several observations on the heating of laser-produced plasmas in He. Agreement of a qualitative nature is noted between experiments and published theoretical predictions. The possibility of an instability in the transfer of energy from the radiation field to the electrons in the breakdown region is pointed out.