Table of contents

A brief historical survey is given of the status of ion implantation around 1960. The new features added to the field by the LSS theory are discussed explicitly, and further developments and applications are described briefly. The paper concludes with a few remarks on possible future developments.

The history of experimental and theoretical evidence for Z₁³ ("Barkas") and Z₁⁴ ("Bloch") corrections to the Bethe stopping formula is outlined. It is found that Lindhard's theory of the Barkas effect and Bloch's original theory for the Z₁⁴ term describe experimental evidence from hydrogen, helium, and lithium stopping in a satisfactory manner. It is not clear to how high Z₁ values the theoretical corrections may be extended as experiments with heavier ions involve the use of partially stripped ions. Although effective charges may be deduced in a consistent manner from stopping data for such ions by means of the theory of higher-order corrections, interpretation of the results obtained is not straightforward.

The systematics of present transient field data are briefly reviewed. The contribution to the field from bound polarized s-electrons and scattering of polarized electrons are discussed. The transient field is compared to measured K-vacancy fractions for light ions in solids, and a mechanism for polarizing the bound electrons of the ions is suggested. This mechanism predicts that the transient magnetic field might decrease for velocities higher than ∼ 10 ν₀. Examples of the utilization of the strong transient magnetic field in nuclear g-factor experiments are presented by recent results from measurements of g-factors for high-spin rotational states in deformed nuclei.

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A brief history of the events leading to the discovery of "channeling" is presented, with special emphasis on the importance of Jens Lindhard's comprehensive theoretical framework. The basic concepts underlying both the channeling phenomenon and its application to various solid state problems are outlined. Finally, two currently exciting examples of such applications are discussed in some detail in order to illustrate the scope of the technique.

The use of channeling/ion scattering as a surface structure probe is described. It is shown that this technique is the simplest example of the influence of atomic ordering on ion trajectories in solids.

A theoretical description of planar-channeling radiation is given, with special emphasis on the scattering processes contributing to the radiation linewidth. Owing to coherence between scattering in the initial and final states for the radiation process, the contribution to the linewidth from processes with small momentum transfer (e.g., plasmon excitation) is strongly suppressed, and the linewidth is dominated by thermal interband scattering. Also contributions from nonsystematic reflections and from finite crystal thickness are evaluated. Experimentally, planarchanneling radiation has been studied for 4 MeV electrons in nickel, at different target temperatures, and the radiation energies and linewidths confirm the predictions. The influence of correlation of vibrations is revealed by a variation of the linewidth with the angle to a major axis in the plane. The radiation intensity as a function of incidence angle to the plane is strongly modified by multiple interband scattering, and a comparison between measured and calculated intensities serves as a further check on the theoretical treatment.

Energy-loss measurements for 2-15 GeV positive and negative protons, kaons, pions, and electrons transmitted through thin Si and Ge crystals acting as intrinsic detectors are discussed. For the random direction, the results are compared with the relativistic Bethe-Bloch stopping theory with and without restrictions. The strong influence of density effect is included through the Sternheimer calculations. The most probable energy loss clearly saturates at the "Fermi plateau", which is completely constant up to the maximum γ value (2 × 10⁴) investigated here. The spectral distribution of energy losses is considerably wider than the Landau distributions, especially for very thin (∼ 100 μm) targets but is in good agreement with straggling calculations based on photoabsorption cross sections. For axial and planar directions in the crystals, positive particles show considerably reduced energy loss as compared to random, and the results are in good agreement with the Esbensen and Golovchenko calculations. Axially channeled negative particles show an increase in energy loss by as much as 20%, as compared to random. The influence of relativistic effects on channeling phenomena is introduced through a short discussion of the first electron/positron experiments, where the basic features of the effects were explored, and the results are compared to yields from close-encounter processes for heavy GeV particles. The experimental results have been compared to calculated-yield curves based on the Lindhard channeling model, and very good agreement is found over the enormous range of five to six orders of magnitude in momenta. The strong influence of channeling on small-angle scattering for GeV particles is considered through a discussion of multiple scattering, dechanneling, and the so-called "doughnut effect". The latter, in which the correlated scattering from atomic rows results in ring-shaped intensity distributions on the exit side, is found to exist for angles of incidence far beyond the critical angle, ψ₁. This shows that the continuum description in the GeV region can be used for angles of incidence large compared to ψ₁. Radiation from 2-55 GeV/c planar-channeled electrons and positrons has been measured and compared to classical calculations based on harmonic and more realistic anharmonic potentials and for cases, where the dipolarity condition is not satisfied. For positrons, it is found that for certain incident momenta between 1 and 10 GeV/c, the channeling radiation is practically independent of transverse energy, resulting in very sharp peaks in the spectra. This offers the possibility of making strong, nearly monoenergetic γ sources in the energy range of 1-100 MeV. Finally, a few applications of high-energy channeling in investigations of inner-shell excitations and δ-ray yields are discussed, together with some recent results on the use of channeling for bending of GeV beams.

Talk presented at the symposium honouring Jens Lindhard's 60th Birthday, February 25-26, 1982, Aarhus, Denmark.

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Exact singularity free models for a spherically symmetric, non-static fluid of uniform density, but non-uniform pressure are investigated. It is found that the model is conformally flat and the pressure gradient is non-positive. The rate of change of (1/2π) × (circumference) as measured by an observer riding in a shell of matter is increasing throughout the sphere. If this "velocity" is smaller than the velocity of light, the surface of an oscillating or a bouncing model does not penetrate the Schwarzschild radius. For a model which continually contracts this "velocity" is equal to or greater than the velocity of light before or when the outer boundary of the material penetrates the Schwarzschild radius. The necessary and sufficient criterium for the interior solution to be matched to the Schwarzschild exterior solution is given. Apparent horizons exist if and only if the surface is inside the Schwarzschild surface, then there exist in fact two horizons: The absolute Schwarzschild surface and an apparent horizon in the interior of the fluid matter. Some models reminding us of phenomena of astrophysical interest are given (Quasars, supernova explosion, contraction of a white dwarf forming a neutron star, vibrating neutron stars).

Some features of Ag/Br reactions at 1.4 GeV/c incident momentum are studied by means of the emulsion technique. The distributions of the number of charged particles/event are present.

The angular distributions indicate some anisotropic process in the nuclear matter, possibly shock-waves, and some back-to-back emission.

The problem of resonant interaction between an electromagnetic radiation field and a vibronic system, such as a molecule or a localized electronphonon system, is considered. The vibronic coupling is assumed to result from displaced and distorted potential energy surfaces for the vibrational motion in the electronic quantum states involved in the resonant interaction with the radiation field. Furthermore, the vibrational energy relaxation is taken into account without assumptions about Markovian-type relaxation. In order to study correlation properties of higher than second order in the electric field strength of the light emitted a method of calculation of the expectation value of an arbitrary product combination of effective time evolution operators of the vibronic system is developed. The calculations are performed by utilizing Feynman's disentangling and operator ordering techniques and yield closed solutions in the low-temperature limit. The theory is used for studying the intensity and the normally ordered intensity correlation function of the resonance fluorescence light from a two-atom molecule excited by a relatively weak laser field, which undergoes phase fluctuations.

It is shown that the vibronic coupling can give rise to an anticorrelation effect which is substantially stronger than in the case of resonance fluorescence from an atom. For the case of small vibrational damping a general formula which directly connects the asymptotic long-time behaviour of the normalized intensity correlation function with the vibrational relaxation in the molecule is deduced. It should be convenient for the determination of very small vibrational decay rates.

Semi-empirical molecular orbital theory is explained in terms of the exact solution to the electronic Schrödinger equation. The necessity is pointed out for a parametrization which simulates the effect of a large orbital basis. The non-uniqueness of parameters is discussed using results obtained by Davidson and Ponec. The present work is meant as an introduction to an accompanying paper on the HAM/4 parametrization of atomic energies.

The 3p-3d transitions of the manganese-like ions Y¹⁴⁺, Zr¹⁵⁺, Nb¹⁶⁺, Mo¹⁷⁺, Ru²⁰⁺, Rh²¹⁺ and Ag²²⁺ have been studied in low inductance vacuum sparks. Isoelectronic regularities and theoretical calculations of wavelengths and line strengths supported the classification of 272 lines. The interpretation of 43 levels of 3d⁷ and 98 levels of 3p⁵3d⁸ by means of the Slater-Condon theory led to fitted energy parameters in generalized least-squares procedures involving the 8 elements. Magnetic M1 dipole transitions within the ground configuration 3d⁷ are predicted for diagnostics in low density plasmas.

We compare the electron densities and Hartree potentials in the local density and the Hartree-Fock approximations to the corresponding quantities obtained from more accurate correlated wavefunctions. The comparison is made for a number of two-electron atoms, Li, and for Be. The Hartree-Fock approximation is more accurate than the local density approximation within the 1s shell and for the spin polarization in Li, while the local density approximation is slightly better than the Hartree-Fock approximation for charge densities in the 2s shell. The inaccuracy of the Hartree-Fock and local density approximations to the Hartree potential is substantially smaller than the inaccuracy of the local density approximation to the ground-state exchange-correlation potential.

In an empirical study of 2200 atomic configurations from H up to Kr (Z = 36) it is shown that the atomic energies can be calculated using Slater's method. The condition is that the shielding efficiencies have the form α - β/ξ_μ where ξ_μ is the orbital exponent of the shielded electron. The average error is then 0.1 eV.

It is shown that Slater's energy expression can be used to introduce the Trees correction to the conventional expressions for multiplet splitting.

In an effort to explain these results the Hartree-Fock energy expression, to which the pair-correlation energies have been added, is rearranged so that the one-center terms are identical to them in the Slater method which was used in the empirical work. It appears that β in the shielding efficiency represents the pair-correlation energy and that the Trees parameter α can be expressed as a difference of pair-correlation energies.

The extension of this work to molecular calculations is briefly mentioned.

The spectrum of S II from a sliding spark discharge has been recorded from 9650 Å to 550 Å. The wavelengths of 573 lines in the air region and 294 lines in the vacuum region were measured. Energy levels have been established or revised in the following configurations: 3p⁴, 4s-7s, 4p-5p, 3d-7d, 4f-7f and 5g. Most of these configurations have been studied by means of ab initio calculations or least-squares fits. The ionization limit 3s²3p²³P₂ 189 066 ± 2 cm^-1 was calculated from f and g levels.

An investigation of very low frequency (ω ≪ ν_thi/R) magnetic fluctuations in a two-fluid model is carried out. This shows that such modes do not make a significant contribution to cross field transport. Test-particle diffusion due to an ensemble of quasi-static fluctuations is discussed. It is found that there are significant differences in the low collisionality limit between periodic and infinite systems. Necessary conditions for field line ergodicity due to equilibrium constraints are derived. A novel asymptotic magnetic surface construction is used to derive bounds on test-particle diffusion, where all relevant time-scales are taken into account.

A model based on two characteristic quantities, an electron energy gap and a group number, connects the thermodynamic properties of superconducting metals with the thermal excitations and the exitations caused by electromagnetic radiation and by tunneling electrons.