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Cataloging in Publication Data Fontana, Peter R. Atomic radiative processes. (Pure and applied physics ;) Incluoes oinliographical references and inoex. l.
Table of contents
Block diagram of the coincidence circuit. Signal from the ion detector channel 1. Signal from the electron detector channel 2. The spectrum for double O-shell ionization is shown in the inset of the figure together with the thresholds for formation of the 5s 2 5p 4 , 5s 1 5p 5 and 5s 0 5p 6 states. Our data are plotted together with a few values obtained by Cairns et al. Our results on electron—atom ionization were the first of its type and corresponded well with those of photo-ionization by real and big synchrotron devices, but our apparatus was much faster and easier to operate.
Our device was a sort of model synchrotron and in fact was considered to be the first table-top synchrotron. Collision processes between fast heavy atoms and ions can be simply described by the interactions between relatively fast protons and alpha particles with neutral atoms. Besides the normal excitations and ionizations which are analogous to what happens in electron—atom collisions, an extra phenomenon occurs, named charge exchange.
The best way to describe both types of phenomena is in treating the three particles involved, viz the point charge projectile, the target atom, and the electron with one Hamiltonian. It is one closed system in which kinetic energy of the projectile is transferred into electronic excitation energy.
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The impact parameter treatment has proven very useful, see Bates [26]. It gave a semiclassical description of the collision process, with the external motions classically and the internal motions quantum mechanically. Due to the heavy mass of the proton or alpha particle, the kinetic energy of the projectile is much bigger than the electronic excitations concerned. Therefore, the trajectory of the projectile is considered rectilinear during the whole collision event. The projectile keeps constant velocity, approximately. For kinetic energies E far above the threshold, we can apply the Dirac condition, which assumes that the most dominant transition is from the initial to the final state i.
Replacing t by z u , where u is the velocity, one gets. Therefore, it will depend on. This is the Massey Criterion. For large values of u, we see. This type of behavior has been studied by Hasted [28,29] who measured total cross-sections for exchange between various kinds of ions and neutral targets. Differential cross-sections, not only velocity dependent but also as a function of the scattering angle, have been measured by Morgan and Everhart [30] and by Kessel and Everhart [31]. Advances in this field were made by measuring electron capture by multiply charged ions. It attracted attention of many physicists in various fields of physics such as astrophysics, plasma physics, controlled thermonuclear fusion research, and X-ray laser production.
For single electron capture, these reactions may lead to population inversion and are of importance in several schemes for the production of XUV and soft X-ray lasers. However, in these collisions, non-radiative i. Measurements of these non-radiative processes by Winter et al. These results were interpreted by them to be the result of capture ionization, i. The measurements of Winter et al. However, data on the energy spectrum of the electrons are still needed to investigate these phenomena in more detail.
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Woerlee et al. The spectrum consists of a continuous background on which peaks are superimposed. The bars in Fig. This shift is equal to the kinematical shift, which would be expected, when the corresponding electrons are emitted by the projectile. Therefore, we concluded that the peaks originate from auto-ionizing states in the projectile, which decay after the collision has taken place. Since no photoabsorption data exist on the auto-ionizing states of multiply charged neon ions, we tried to calculate energy levels of doubly excited neon ions with a single configuration HF method.
In order to determine the energies of the various levels, we included the electrostatic energy splitting due to the core electrons, see El-Sherbini and Farrag [38]. The energy splitting caused by the excited electrons is small and was not taken into account. Further developments in this field were done by El-Sherbini et al. The study shows strongly rising total capture excitation cross-sections and shifts in the post-collision projectile excited-state distributions to higher n levels with the increase in the target atomic number. These studies indicate that single electron charge transfer into excited states of the product ion is the most important inelastic process.
These results are extremely valuable for the developments of controlled thermonuclear fusion reactors see El-Sherbini [41].
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The experimental results were explained qualitatively by considering the MO correlation diagram Fig. It was often found that the cross-section for excitation decreases with the increase in the number of intermediate transitions required in order to reach the excited state. The results have been of particular importance in evaluating theoretical models and have provided a valuable check of the range of validity of existing theories.
Diabatic MO correlation diagram for Ar—He system. In the field of atomic collisions, as we noticed in the previous sections, much attention was paid to the excitation of noble gas atoms. A systematic study of the excitation process requires the knowledge of accurate dipole transition probabilities for spontaneous emission between the various configurations of the ions. Laser physics and astrophysics are other branches, which have stimulated more accurate atomic line strengths and transition probabilities calculations.
Garstang [43,44] performed the first intermediate coupling calculations for Ne II. On this basis, Wiese et al. However, the previously tabulated line strengths were in need of revision. In his work, Luyken [46,47] performed new calculations of line strengths and transition probabilities for Ne II and Ar II where specific configuration interactions were investigated and some effective operators were included.
The results showed that the agreement with the experimental data was improved as compared with the earlier calculations. The transition probability between two states with summation indices i and j refer to the upper and lower level, respectively, is given by. The line strength is given by El-Sherbini [48].
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R l ru r and R l rl r are the one electron radial wavefunctions in the two different states. The parametric potential method was used to calculate the radial part of the wave function [51] , while the method of least squares fit of energy levels [52] was applied in obtaining the angular part of the wave function. The results obtained in intermediate coupling showed a much better agreement with the experimental data than those using pure LS-coupling wave functions.
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Further improvements in the atomic structure calculations of Kr II were obtained by El-Sherbini and Farrag [38] when including configuration interaction effects. Taking into account, configuration-interaction effects in the calculations showed that some observed energy levels of the 5p 4 5d configuration were not correctly designated. A strong interaction between the 5p 4 5d and 5s5p 6 configurations was also reported. Moreover, the calculated energies of the 6s and 5d levels were improved considerably by introducing configuration interactions into the calculations.
The presence of strong configuration interaction between the 4s4p 6 , 4p 4 4d, and 4p 4 5s configurations in singly ionized krypton [38] makes it difficult to perform accurate calculations for the energies, pumping rates, and lifetimes of levels in these configurations. Therefore, it was important to improve upon the previous calculations, see El-Sherbini [54,55] , on the low lying 4p 4 4d and 4p 4 5s laser levels in this ion. The results show that some of these levels are metastable. They also suggest a two-step excitation from the ground state of the ion to the 4p 4 5p level involving some intermediate metastable states as a possible laser excitation mechanism.
Further developments in the field of atomic structure calculations were done by the studies of excitation of electrons in atomic isoelectronic sequences [57—59].
These studies are essential not only for better understanding of atomic structure and ionizing phenomena, but also they provide new laser lines which could be extended into the X-ray spectral region [60,61]. This in turn will help in the development of X-ray laser devices. Once X-ray lasers become reliable, efficient, and economical, they will have several important applications. First and foremost, their short wave lengths, coherence, and extreme brightness should allow the exploration of living structures much smaller than one can see with optical methods. They will also have important applications in high resolution atomic spectroscopy, diagnostics of high density plasmas, radiation chemistry, photolithography, metallurgy, crystallography, medical radiology, and holographic imaging.
Shortly after the demonstration of the first soft X-ray amplification in neon-isoelectronic selenium by Mathews et al. Progress toward the development of soft X-ray lasers with several plasma-ion media of different isoelectronic sequences was achieved at many laboratories [65,66]. A soft X-ray laser transitions in the Be-isoelectronic sequence were proposed by Krishnan and Trebes [67]. Lasing in the soft X-ray region is then possible on 4p—3d and 4f—3d singlet and triplet transitions.
Short wave length laser calculations in the beryllium sequence were done by Feldman et al. They calculated gain at a number of different temperatures and electron densities for the 3p—3s laser transition in the highly charged ions of Be-sequence. Al-Rabban [69] has extended both the work of Krishnan and Trebes [67] and Feldman et al. She carried out an ab initio multi-configuration Hartree—Fock calculations of energy levels, atomic oscillator strengths, and radiative lifetimes for singly and doubly excited states in Be I and Be-like ions.
Configuration interaction effects between the various configurations were included using the computer program code CIV3 described by Hibbert [70]. In this code, the N-electron energies and eigenfunctions are obtained by diagonalizing the Hamiltonian matrix, which may have quite large dimensions.
The choice for the spatial radial part of the single particle wave functions is based on expansions in Slater-type orbitals [71] :. Investigations of the possibilities of obtaining population inversion and laser emission could be achieved by calculating the level population of the excited states. These calculations were done by the group of atomic physics at the Physics Department of the Faculty of Science — Cairo University, solving the coupled rate equations [72].
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Vriens and Smeets [74] gave empirical formulas for the calculation of rate coefficient in hydrogen atom. Their work was extended by Allam [75] to be valid for atoms with one electron outside a closed shell and also for two-electron atoms ions. Allam [75] adopted the method of Palumb and Elton [76] for modeling plasmas of helium-like and carbon-like ions, and he has developed a computer program CRMOC in order to calculate excitation and de-excitation rate coefficients for two-electron system.
In his program which was developed for collisional radiative model calculations, the principal quantum numbers of the excited states were replaced by effective quantum numbers. Using the above theoretical schemes, the atomic physics group was able to extensively investigate the possibility of X-ray laser emission in several isoelectronic systems, see for example Figs. The studies include helium isoelectronic sequence [77] , beryllium isoelectronic sequence [69,78] , boron isoelectronic sequence [79—81] , carbon isoelectronic sequence [82] , sodium isoelectronic sequence [83—85] , magnesium isoelectronic sequence [86—88] , aluminum isoelectronic sequence [89] , silicon isoelectronic sequence [90—92] , sulfur isoelectronic sequence [93] , potassium isoelectronic sequence [94] , scandium isoelectronic sequence [95] , and nickel isoelectronic sequence [96].
The reported stimulated emission transitions in these ions indicate that some of the transitions are promising and could lead to progress toward the development of XUV and Soft X-Ray lasers. Laser-induced breakdown spectroscopy is a form of optical atomic emission spectroscopy [97]. It is a technique based on utilizing light emitted from plasma generated via interaction of a high power lasers with matter solids, liquids or gases.