By using an electron-energy analyzer adapted on a high-voltage electron microscope, energy-loss straggling of relativistic electrons (0.3-1.2 MeV) through 1- to 8.5-μm-thick aluminum foils has been investigated in the energy range 0-4000 eV. The energy resolution of the analysis is 3 eV. The experimental values of the most probable energy loss $\Delta E_p$ and of the full width at half maximum $\Delta E_{\frac{1}{2}}$ are interpreted by taking account of the effects of all-order terms in the Landau theory. This latter is revisited by introducing the relativistic Bethe cross section associated with all kinds of excitation processes including plasmons; it is observed that plasmon influence is negligible in this thickness and energy range.

Three-body neutral stabilized recombination has been studied in a pulsed discharge afterglow apparatus. The rate coefficient of the He stabilized recombination reaction ${\mathrm{Cs}}^{+}+e+\mathrm{He}\to \mathrm{Cs}+\mathrm{He}$ at 625°K is deduced to be less than ${10}^{-28\mathrm{}}$ ${\mathrm{cm}}^6$${\sec }^{-1\mathrm{}}$, with the most probable value being 4×${10}^{-29\mathrm{}}$ ${\mathrm{cm}}^6$${\sec }^{-1\mathrm{}}$. Since this value is much lower than that obtained for the recombination of ${\mathrm{He}}_2^{+}$ by the same mechanism, it suggests that the efficiency of the three-body neutral stabilized recombination process depends sensitively upon the atomic or molecular nature of the recombining ion.

The impact-parameter dependences of the ionization probability of the carbon and aluminum $K$ shells by proton impact have been measured in a proton-x-ray coincidence experiment. Influences of the fluorescence yield and of the multiple scattering of the projectiles inside the target foils on the evaluation of the ionization probabilities have been investigated. The experimental results are compared with existing theories. Whereas the aluminum data can be explained by Coulomb ionization if adiabatic relaxation of the $1s$ state is taken into account, the experimental results for carbon exceed the theoretical values, especially at small impact parameters. Several possible reasons for this result are discussed.

Transitions between adiabatic surfaces are described semiclassically via trajectories propagating about their complex intersection point. This intersection point is a branch point of the potential function and a simple pole of the nonadiabatic coupling. Thus transitions may be described without explicit introduction of the nonadiabatic coupling. This structure is independent of the total number of states, but is most easily shown for two-state systems. A thrėe-state model of the ${\mathrm{He}}^{+}$ + Ne collision system is solved by treating the three-state interaction as a sequence of two-state interactions. The results are in excellent agreement with accurate quantum calculations.

The fractional population of ${\mathrm{Si}}^{12+}$ ions in the ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ metastable state which is produced by the passage of 15-60-MeV Si ions through thin C and Au foils has been determined by measuring the x-ray yield from the spontaneous $M1\mathrm{}$ decay of this state. This fraction (∼ 24% for the thickest C foils and highest bombarding energies) increases linearly with increasing Si energy and is highly dependent on the foil thickness and material. This information has been used to obtain absolute cross sections for Si $K$ x-ray production in ${\mathrm{Si}}^{12+}\left(1s2s{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\right)\to \mathrm{He}$ collisions. From the results it is concluded that $K$ x-ray emission predominantly occurs following $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ excitation at the higher energies (∼ 2 MeV/amu) and following outer-shell electron capture at the lower energies (∼ 0.5 MeV/amu).

The combined use of laser ablation and selective excitation śpectroscopy is proposed as a new approach to the convenient and accurate measurement of atomic parameters. Measurement of the radiative lifetimes for the three resonance transitions in chromium served as an initial test of the technique. For the transitions at 425.4, 427.5, and 429.0 nm, our measurements indicate lifetimes of 31.2, 32.2, and 31.4 nsec, respectively, with an estimated uncertainty of 1 nsec. These are in good agreement with the values previously obtained by other techniques.

We have observed the selective ionization of the degenerate $\left|m_j\right|$ levels of $s$, $p$, and $d$ states of sodium in the range of $n$ from 15 to 20 and have shown that atoms produced in these low-field (∼ 10 V/cm) states pass adiabatically from low-field $\left|m_j\right|$ to intermediate-field $\left|m_l\right|$ states. Experiments on the Stark manifold states in intermediate fields (∼ 1 kV/cm) and high fields (∼ 4 kV/cm) show that the $\left|m_l\right|=0\mathrm{} \mathrm{and} 1$ states pass adiabatically from intermediate- to high-field states and that the passage of the $\left|m_l\right|=2\mathrm{}$ states is very nearly adiabatic. These findings suggest a general approach for predicting the threshold fields for ionization of atomic Rydberg states.

The relativistic random-phase approximation is used to calculate the excitation energies and absorption oscillator strengths for the $3s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-3s3p{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$, -$3s4p{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$, -$3s5p{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$, and -$3s3p{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ transitions of selected elements of the magnesium isoelectronic sequence through nuclear charge $Z=92\mathrm{}$. The results are compared with previous theoretical and experimental values. The data plotted as a function of $Z^{-1\mathrm{}}$ enable us to study deviations from the predictions of the nonrelativistic $Z$-expansion theory.

Cross sections and angular distributions for the photoionization of the rare gases have been calculated using a modified Hartree-Fock equation for the continuum orbital and a multiconfiguration Hartree-Fock description of the ground state. The modification of the Hartree-Fock equation for the continuum orbital is produced by virtual pair excitations of the ionic core analogous to those incorporated in the ground state. The modification reduces to the addition of two terms to the standard $V^{N-1\mathrm{}\left(\mathrm{LS}\right)\mathrm{}}$ potential when small terms are neglected. The addition of this final-state correlation effect gives improved agreement with experimental cross sections as compared to a previous calculation which included only ground-state correlation effects.

Cross sections for the photoionization of ${\mathrm{H}}_2(X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}, v_i)$ in vibrational levels $v_i=0-14\mathrm{}$ are determined for a range of photon energies by a two-center treatment in which the derived electronic matrix elements are averaged over the initial and final vibrational states of the molecule and the molecular ion. Very good agreement with available measurements for $v_i=0\mathrm{}$ is obtained. Cross sections for the full Franck-Condon array of accessible transitions ${\mathrm{H}}_2\left(v_i\right)\to {\mathrm{H}}_2^{+}\left(v_f\right)$ are also presented. In the energy range considered, when the $2p{\sigma }_u$ dissociative state of ${\mathrm{H}}_2^{+}$ is inaccessible, substantial contributions (up to 50% for $v_i=6-10\mathrm{}$) to the photoionization originate from ${\mathrm{H}}^{+}$ atomic ions which are formed via transitions to the vibrational continuum associated with the $1s{\sigma }_g$ state of ${\mathrm{H}}_2^{+}$. Single-center and fixed-nuclei approximations which permit great simplification to the present treatment are also fully investigated.

For the case in which laser radiation interacts with two levels of an atom, it is shown that there can be gain, even when there is no population inversion, because of recoil effects. The effect of recoil on certain precision measurements is also shown.

Using a laser-excitation, optical-detection scheme, we have observed microwave transitions from the $\mathrm{nd}$ to the $\mathrm{nf}$ and $\mathrm{ng}$ orbital levels in lithium states with principal quantum number $n$ from 7 to 11. These measurements yield precise values for the $d$, $f$, and $g$ state fine-structure intervals as well as $d-f$ and $d-g$ energy separations. Within the accuracy of these measurements, the fine structures are found to be hydrogenic. The dipole polarizability of the ${\mathrm{Li}}^{+}$ core is determined from the $d-f$ and $d-g$ energy splittings. The $d-f$ intervals are compared to pseudopotential calculations.

The problem of correcting experimental Compton profiles for the effects of multiple scattering is shown to be equivalent to obtaining the solution to a nonlinear operator equation. Two methods of solving the equation are presented. One, analogous to the Newton-Raphson technique for algebraic equations, leads to a new solution to the problem. Another more efficient approach, based upon the idea of successive approximation, is seen to be equivalent to the method of Felsteiner, Pattison et al., though it fails to converge if the fraction of multiple scattering is too great. The double-scattering operator is derived and examined in some detail. Extensions to higher-order scattering are indicated.

The operator formalism developed in the previous paper is used to derive upper and lower bounds on the maximum error associated with multiple-scattering corrections to experimental Compton profiles. In some cases it may be possible to use these bounds to deduce the amount of multiple scattering in the experimental profile.

Oscillator strengths and excitation energies are computed for electric dipole transitions connecting valence-shell states of Mgi and of Cai. The method used is the variational solution of two-particle Bethe-Goldstone equations for both initial and final states of each transition, with a common frozen core determined by a matrix Hartree-Fock calculation. Results are compared with available theoretical and experimental data, and are found to be in excellent agreement with previous results for the two resonance transitions. For Mg, the computed length and velocity oscillator strengths are closer together for most transitions considered than are previous theoretical values, and fall within their range. For Ca, the computed results confirm conclusions about valence-shell even-parity states inferred from analysis of recently observed two-photon ionization spectra.

Three-photon transitions induced by a dichromatic C${\mathrm{O}}_2$ laser operating on the $P\left(10\right)\mathrm{}$ and $P\left(20\right)\mathrm{}$ lines of the 961-${\mathrm{cm}}^{-1\mathrm{}}$ band have been observed in a helium plasma. Laser photons interact resonantly with the $4{}_{}{}^3{}_{}{}^{}S$ and $4{}_{}{}^3{}_{}{}^{}P$ helium levels, resulting in new spectral lines which flank the 4713-Å, $2{}_{}{}^3{}_{}{}^{}P-4\mathrm{}{}_{}{}^3{}_{}{}^{}S$ He line. The multiple photon transition rates are calculated from a numerical diagonalization of the interaction Hamiltonian and are shown to be in agreement with experimental results.

This paper presents the results of a calculation on the photoionization of Ne and Ar isoelectronic sequences based on a recently developed many-body theory of atomic transitions. Within this theoretical approach, one is able to concentrate attention on studies of the systematics of various many-body effects and of their implications for the photoionization process along isoelectronic sequences. The transition-matrix elements obtained with different degrees of approximation are given in detail. The photoionization cross sections and the asymmetry parameter representing the photoelectron angular distribution are also given. In addition, this study indicates that the usual effective ionization mechanism for the neutral atom, namely, the direct ionization of the outer-shell electron at relatively low energies, becomes ineffective for the ion systems as the degree of ionization increases along the isoelectronic sequence.

Second- and third-order nonlinear optical polarizabilities for a number of halogenated methane molecules—C${\mathrm{Cl}}_4$, CH${\mathrm{Cl}}_3$, C${\mathrm{H}}_2$${\mathrm{Cl}}_2$, C${\mathrm{H}}_3$Cl, C${\mathrm{H}}_4$, C${\mathrm{Cl}}_2$${\mathrm{F}}_2$, and C${\mathrm{Cl}}_3$F—are derived from measurements of the temperature dependence of dc-electric-field-induced second-harmonic generation in the gas phase. Such data are now available for all twelve molecules: $C{X}_n{Y}_{4-n}$ with $X, Y=\mathrm{F},\mathrm{C}\mathrm{l},\mathrm{H}$. While the bond additivity approximation provides an excellent fit to the third-order polarizability data for this set of molecules, the fit for the second-order polarizabilities is poor. This poor fit can be understood on the basis of bond-bond interactions, and inclusion of an estimate of these contributions greatly improves the consistency of fit to the second-order polarizability data.

Trace additions of several gases are known to significantly effect CO-laser power and efficiency. Since small concentrations of additives are generally involved, these additives are recognized as predominantly influencing the characteristics of the laser plasma. Oxygen has been an additive receiving substantial attention in past measurements. In this paper we develop a consistent interpretation for all the observed oxygen effects both from our laboratories and elsewhere. Oxygen in small concentrations will be shown to improve laser efficiency and power through its effect on the electron-ion recombination process. At higher oxygen additions, laser output is degraded as a result of vibrational-translation relaxation by oxygen atoms. In addition, carbon formation and removal, plasma heating and cooling effects, heterogeneous loss processes, and the influence of polymer ions will be discussed through model calculations.

This paper is concerned with the scattering of electromagnetic radiation in an inhomogeneous polarizable medium. In particular, we are interested in scatterers whose positions are correlated and whose characteristic size is on the order of or less than a wavelength. The central problem is that of determining the electric and magnetic fields acting on a given scatterer in the medium. This is the effective wave that polarizes the scatterer, and its calculation is complicated by what are generally referred to as local field effects. Assuming that the scattering is weak, i.e., that the system deviates only slightly from being homogeneous, we are able to treat local field effects through second order in perturbation theory, and to derive explicit expressions for the scattered effective field and the dispersion relation satisfied by the coherent wave. The scattering cross section is derived and related to the attenuation of the coherent wave in an infinite medium. In addition, the diffractive effects due to scattering from a finite medium are discussed.