Cross sections for the ionization of n of N electrons with equal single-electron ionization probability P are considered. When both N and the projectile charge q are large, the cross sections for single and double ionization are both found to be approximately linear in q at 1 MeV/amu. The ratio of double-to-single-ionization cross sections is independent of q. Moreover, first-order perturbation theory for the single-electron ionization probability P, which varies as $q^2$, is found to be applicable due to the damping of contributions with large P caused by factors of (1-P${)}^{N\mathrm{-}n}$. For large P there are differences between the inclusive probability P and the probability NP commonly used for a target with N electrons. Both of these probabilities differ significantly from the exclusive probability NP(1-P${)}^{N\mathrm{-}1}$ for the ionization of only one electron. For large N and large q, the exclusive ionization probabilities for removing exactly n of the N electrons tend to be concentrated in somewhat separate ranges of impact parameters b, defining impact-parameter ‘‘windows.’’ The windows which we obtain using the quantum-mechanical semiclassical-Coulomb-approximation (SCA) probabilities are similar to those using classical Monte Carlo calculations. Model calculations, based on analytic fits to the SCA probabilities, are used to obtain approximate analytic expressions for single- and double-ionization cross sections and for the impact-parameter windows. Results of this simple model are in reasonable agreement with measured cross sections for single and multiple ionization of neon atoms by projectiles of charge q varying from 1 to 13 at a velocity corresponding to 1 MeV/amu.

The cross sections for excitation of hydrogen atoms from the n=1 and n=2 states are calculated for incident energies between 12.1 and 12.75 eV. The calculations are made variationally, and employ a large pseudostate basis. Energies and widths of resonances are obtained.

The single- and double-ionization cross sections of ${\mathrm{H}}_2$ produced by fast-electron bombardment have been measured. The electrons had selected energies between 750 and 3500 keV/amu. The results are compared to previously reported data from this laboratory for equivelocity protons on ${\mathrm{H}}_2$. The ratio R of double-to-single-ionization cross sections is about a factor of 2 greater for electrons than for protons. The difference occurs because of differences in both the single- and double-ionization cross sections. At 750 keV/amu, the single-ionization cross section produced by electrons is 0.83±0.04 times that produced by protons. At the same velocity, the double-ionization cross section produced by electrons is 2.25±0.35 times that produced by protons. The results from the ${\mathrm{H}}_2$ target are compared to results from a helium target, and marked similarities are observed.

Scattering of 59.5-keV γ rays from Zn, Cd, and Pb foils at angles from 3.5° to 30° has been measured. The influence of Bragg scattering on the determination of differential Rayleigh cross sections has been investigated. It has been shown that interference effects occur in polycrystalline targets for angles ≤20° and some care has to be taken in the measurement of Rayleigh cross sections for momentum transfer less than 2 A${̊}^{\mathrm{-}1}$.

This paper discusses a rigorous relativistic treatment of the differential cross section for Compton scattering from bound electron states in the impulse approximation. Starting with general relativistic central-field Hartree-Fock wave functions, a simple expression is found which is valid for all completely occupied shells. This expression is then compared with the more approximate expression developed by R. Ribberfors [Phys. Rev. B 12, 2067 (1975)]. The Compton profile is a well-defined concept in nonrelativistic theory, but it is not categorically applicable in relativistic theory. Therefore this paper also discusses the circumstances in which the concept of a Compton profile is well defined also in a relativistic context.

A study of elastic scattering of electrons and positrons by helium in its ground as well as its metastable states has been done using a model-potential approach. Reliable static, polarization, and exchange potentials are used to describe the scattering process. Elastic differential and total cross sections are obtained in a wide range of incident projectile energy, viz., from nearly very low energy to 1000 eV. Our results are compared with available experimental data and other theoretical results.

The method proposed by Yates [Chem. Phys. Lett. 25, 480 (1974)] has been extended to the case of 1s→2s,2p excitation of a hydrogen atom by electron impact. An alternative analytical procedure used by Berestetskii, Lifshitz, and Pitaevskii [Quantum Electrodynamics (Pergamon, Oxford, 1982), p. 539] has, however, been adopted for the evaluation of the third term of the Glauber series. In the process Yates’s expression for the third Glauber term for electron–hydrogen-atom elastic scattering has been reproduced. The differential scattering cross section for the excitation process has been calculated as suggested by Yates. This cross section is found to be in reasonable agreement in the low-angle region with the exact Glauber results reported by Tai et al. [Phys. Rev. A 1, 1819 (1970)] and by Gien [Phys. Rev. A 20, 1457 (1979)].

An algebraic variational method, based on application of the Kohn principle to the T matrix, is formulated for multichannel scattering problems and applied to the case of low-energy electron-molecule collisions. The method requires only Hamiltonian matrix elements and is anomaly-free. The present formulation is noteworthy in that it requires no bound-free nor free-free exchange matrix elements. The new technique is illustrated by application to $e^{\mathrm{-}}$+${\mathrm{H}}_2$ scattering in both the static-exchange and the two-state approximations. The method is found to be both efficient and accurate.

The cross section for electron-impact excitation of the resonance transition (2s-2p) of ${\mathrm{Be}}^{+}$ has been calculated in a variety of models at low incident-electron energies. Both five-state (2s,2p,3s,3p,3d) and nine-state (2s–3d,4s¯,4p¯,4d¯,4f¯) close-coupling (with and without polarization potentials) calculations have been completed. The results, while in good agreement with previous calculations, do not resolve all the long-standing discrepancies between theory and experiment. There are still significant discrepancies with the experimental cross section for the 2s-2p transition. However, previous calculations of the polarization of the resonance fluorescence used an expression for the scattering amplitude that was inappropriate for ions. The present results for the polarization use the correct expression, and are in good agreement with experiment. Calculations using the same approach for other quantities, specifically binding energies and resonance parameters, show small but significant improvements when compared with experiment.

Simple expressions that include intermediate coupling mixing effects are given for the cross sections and rate coefficients for populating the singly excited levels of He-like ions through the process of inner-shell ionization of Li-like ions by electron impact. Illustrative numerical values for the rate coefficients for inner-shell ionization of several Li-like ions are given and compared with the rate coefficients for populating the same upper levels by the alternative mechanism of electron impact excitation from the ground level of the He-like ions. For the temperatures at which the abundance of the He-like stage of ionization is a maximum, the inner-shell ionization rate coefficients increase relative to the excitation rate coefficients as the nuclear charge number Z increases. It appears that for all but very low Z, inner-shell ionization is an important mechanism for populating at least some of the excited levels of He-like ions. It is especially important for populating the (1$s_{1/2}$2$s_{1/2}$${)}_1$ or 1s2s ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ level.

An analytic representation of the full Green’s function including bound states, resonances, and remaining contributions has been obtained for a class of dilatation analytic potentials, including the superimposed Coulomb potential. It is demonstrated how to obtain the locations and residues of the poles of the Green’s function as well as the associated generalized spectral density. For a model potential which has a barrier and decreases exponentially at infinity we have found a certain deflation property of the generalized spectral density. A qualitative explanation of this phenomenon is suggested. This constitutes the motivation for an approximation that explicitly shows a decomposition of the (real) continuum, corresponding to scattering data, into resonances and background contributions. The present representation is also shown to incorporate the appropriate pole-background interferences. Numerical residue strings are computed and analyzed. Results for the Coulomb potential plus the above-mentioned model potential are reported and compared with the previous non-Coulomb case. A similar deflation effect is seen to occur, as well as basically the same pole- and residue-string behavior. The relevance of the present analysis in relation to recently planned experiments with electron-cooled beams of highly charged ions is briefly discussed.

We report the first experimental evidence of variable collision-broadening parameters within the hyperfine multiplet of a rotational line. A similar behavior is observed in the Stark pattern of one hyperfine line. All the experimental results, obtained on ${\mathrm{CH}}_3$I, are explained in terms of a theoretical model accounting for line coupling. The same approach is successful in interpreting the literature data for the Stark lines of ${\mathrm{CH}}_3$Br and previous null results for other molecules.

We report on muonic Lyman-series x-ray intensities measured in gaseous argon and in ${\mathrm{K}}^{+}$, ${\mathrm{Cl}}^{\mathrm{-}}$, ${\mathrm{Na}}^{+}$, and ${\mathrm{F}}^{\mathrm{-}}$ in the compounds KF and NaCl. The analysis of their structures, including neon, shows that both the spatial symmetry and the binding energy of the electrons of outer shells determine the angular momentum distribution of captured muons. The conclusions are in agreement with the modified model of ‘‘large mesic molecules.’’

Detailed experiments confirm the narrowing of ac-Stark-broadened 2+2 multiphoton-ionization spectra of nitric oxide (NO) at thermal equilibrium in a cell at pressures on the order of 1 Torr. Experiments with a supersonic beam of NO at comparable densities exhibit much less narrowing. The possibility that the apparent narrowing of lines from the thermal cell is an artifact associated with space charge is examined and found to be inconsistent with our results. However, these experiments do show that the narrowing of the lines in the room-temperature cell is due to collisions, rather than being a density effect. Additional confirmation of the ac Stark mechanism for the asymmetric broadening of the lines is provided by comparison of line characteristics under excitation by linearly and circularly polarized light.

Partial photoionization cross sections and angular-distribution asymmetry parameters have been determined for the Xe 4d and ‘‘4p’’ subshells in the photon-energy region of the 4d Cooper minimum (160 eV to 270 eV, to 520 eV for the 4d asymmetry parameter). The Cooper minimum is observed as a distinct cross-section minimum in the 4d photoionization channel. The 4d angular-distribution results are in excellent agreement with Dirac-Fock and relativistic random-phase-approximation calculations and with previous measurements. Effects of interchannel coupling are evident in the ‘‘4p’’ results. Both the partial cross section and the angular distribution for the ‘‘4p’’ photoelectron peak track the 4d cross section and angular distribution as functions of photon energy. A similar result is observed for the summed intensity and the angular distribution of the 4$d^{\mathrm{-}1}$5$p^{\mathrm{-}1}$np satellites of the 4d main line, which is probably due to the effects of electron correlation between the 4d and the satellite photoionization channels.

The hyperpolarizability of neon (${\gamma }_{\mathrm{Ne}}$) is determined by an electric-field-induced second-harmonic-generation (ESHG) experiment at several wavelengths in the visible region. The deviation from Kleinman symmetry is also measured. The dispersion curve obtained from this experiment allows one to critically assess the previous nonlinear-optical measurements for Ne and allows a reliable estimate of the static value of ${\gamma }_{\mathrm{Ne}}$. The bounds obtained for the static ${\gamma }_{\mathrm{Ne}}$ are 75 a.u. <${\gamma }_{\mathrm{Ne}<93\mathrm{}}$ a.u. The results of recent ab initio calculations are considered in the light of this revised estimate for ${\gamma }_{\mathrm{Ne}}$. .AE

Pump-and-probe methods with time resolution were used to excite fluorescence from the ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }$, ${}_{}{}^3{}_{}{}^{}{(}^1\mathit{S})$+Zn${(}^3$P)], ${}_{}{}^1{}_{}{}^{}{(}^1\mathit{S})$+Zn${(}^1$P)], and ${}_{}{}^1{}_{}{}^{}{(}^1\mathit{P})$+Zn${(}^1$S)] states of the HgZn excimers produced by laser pulses in a quartz cell containing a mixture of Hg and Zn vapors. An analysis of the resulting fluorescence and excitation spectra yielded vibrational constants and permitted the assignments of the bands to specific upper and lower states on the potential-energy diagram.

The dependence of the electronic absorption spectrum of a dye molecule on the time of delay between the excitation of the molecule by a pump-laser pulse and the probing of the absorption by a test-laser pulse is studied. The effect of dephasing of the molecular transition arising from the interaction with dissipative systems is described by a stochastic modulation of the electronic transition frequency. It is shown that in the case when the two laser pulses are short compared with the correlation time of the frequency modulation and when the spectral linewidth of the pump-laser light is smaller than the bandwidth of the equilibrium absorption spectrum, hole burning appears although the molecules have equal mean transition frequencies. This effect arises from non-Markovian correlations between the dephasings of the molecule during the actions of the pump- and the test-laser pulse. With increasing delay time the holes are smoothed out and disappear when the delay time exceeds the correlation time. Such effects were found in recent femtosecond absorption experiments by Shank et al. [in Ultrafast Phenomena V, dited by G. R. Fleming and A. E. Siegman (Springer, Berlin, 1986), p. 179].

A microscopic theory for radiative lifetimes and energy transfer rates in molecular aggregates is developed. Using an effective Hamiltonian, we show that the radiative decay rate of isolated small aggregates with N molecules is Nγ, where γ is the radiative rate of a single molecule. This linear dependence on N saturates when the aggregate size becomes comparable with the wavelength of light associated with the optical transition, and for large N the rate attains a limiting value independent of N. Intermolecular dephasing processes are incorporated by introducing a thermal bath and solving for the time evolution of the density matrix with use of an effective Liouville operator. Intermolecular dephasing destroys the coherent interaction of the aggregate with the radiation field and the rate changes from Nγ to γ as the dephasing rate increases. A coherence length which depends on the strength of intermolecular interactions and on the dephasing rate is defined and calculated.

We generalize the traditional Maxwell-Bloch theory of a ring laser and analyze the dynamical role of transverse effects. With the help of a suitable extension of the uniform-field limit we perform an essentially analytical study of the steady-state and linear-stability properties of a ring resonator containing a homogeneously broadened active medium. In this context we incorporate the effects of curved reflecting surfaces, the possible lack of transverse uniformity of the pump, and the detailed structure of the cavity modes. In the uniform-field limit we prove that the laser steady state is of the single-mode variety and that lack of stability can be induced in this field configuration even only a few percent above the threshold for laser action. Thus our main claim is that transverse effects can be responsible for low threshold instabilities. This explanation is more plausible to explain the observed pulsations in many lasers than what is available in terms of the traditional plane-wave theories.