The Ar+${\mathrm{D}}_2$ collision is studied in an energy range from 0.750 to 3.00 keV. At small angles the collision is primarily electronically elastic, but electronically inelastic processes are found to increase in importance with increasing scattering angle and become dominant. The electronically elastic results are compared to the predictions of Sigmund’s scaling law, which suggests that the reduced energy loss f depends only on the reduced scattering angle τ=Eθ. At sufficiently small values of θ the data do obey this scaling, but at larger values of θ, which fall outside the postulates of Sigmund’s theorem, they do not. However, a generalization of the usual definition of f is presented which does retain the scaling property even at the larger values of θ. We also find that there is an interesting and systematic development of f(τ) in the sequence of collision systems He+${\mathrm{D}}_2$, Ne+${\mathrm{D}}_2$, Ar+${\mathrm{D}}_2$, in which vibrorotational excitation manifests itself at smaller and smaller values of τ as heavier rare-gas projectiles are used.

The distorted-wave approximation and similar weak-coupling methods are frequently used to calculate electron collision cross sections for studies of astrophysical and laboratory plasmas. Autoionizing resonances neglected in these approximations can significantly enhance the calculated rates for collisional excitation of some transitions in highly charged ions. We investigated the effects of such resonances on the excitation rates for n=2 to n=2 transitions in fluorinelike Fe, Se, Mo, Ag, Xe, and Eu. The background excitation rates were calculated using a relativistic distorted-wave approximation, and the detailed Auger and radiative rates were calculated using a multiconfiguration Dirac-Fock model. The largest effect is on the electric-dipole-forbidden transitions. The enhancement diminishes with increasing atomic number for all transitions.

After ten years of nonsuccessful experimental searches for a ‘‘density effect’’ in inner-shell excitation, a recent experiment has shown pronounced moderation—although not full saturation—in K-vacancy production in aluminum and copper foils at ultrarelativistic impact energies. In the present paper a comprehensive theoretical investigation of atomic K-shell excitation by charged GeV projectiles is presented with special emphasis on the influence of target polarization. The gradual onset of polarization and ‘‘transition radiation’’ emitted near the target incidence surface is shown to compensate partly the reduction due to the density effect. Inner-shell excitation yields depend on the depth penetrated into the target. To lowest order, this depth dependence may be described by means of a simple relation for the equivalent photon pulse responsible for the distant collision contribution. In the comprehensive treatment, the target response is examined in detail, a priori splitting between real and virtual photons is avoided, and influence of actual target geometry (tilt and finite thickness) is considered. Upon inclusion of a relatively small intensity component, the simple relation produces K-shell excitation yields, which are accurate to the percent level at any sample thickness for target atomic numbers above ten. The theory developed explains all existing experimental data.

The excitation cross sections and the alignment of the lowest autoionizing states, ${\mathrm{np}}^5$(n+1)$s^2$, of Na, K, Rb, and Cs, induced by impact of various projectiles, have been calculated. The alignment by projectiles heavier than electrons is seen to exhibit a rich structure as a function of impact energy and it is anticipated that its overall behavior should be observable by experiment. Good agreement is found with the available data on sodium.

Complex-basis-function techniques are used to implement the equations of the random-phase approximation (RPA) in matrix form. This approach allows the direct extraction of total photoionization cross sections from a finite series of (complex) transition energies and oscillator strengths. The RPA is an effective means for including electron correlation effects on the photoionization of closed-shell atoms and molecules. The procedure demonstrated here provides a rigorous way of solving the RPA equations for continuum photoabsorption without resorting to numerical integration. The results of calculations on He and ${\mathrm{N}}_2$ are presented. Correlation effects are found to significantly influence the threshold behavior of the ${\mathrm{N}}_2$ photoionization cross sections.

A new method for calculating moments of atomic transition arrays is described. It is based on the collective vector method described in earlier publications. In this new approach a single collective state vector is generated from a single parent state vector encompassing the entire parent basis. The amplitudes of the basis vectors comprising the parent state vector are randomized. Thus it is representative of the entire parent manifold. We show that a statistical estimate of the transition moments of the array is given by a suitable combination of moments of the matrix element of the E1 (electric dipole) operator between the representative parent state vector and the collective state vector derived from it. Although the detailed characteristics of the dispersion in these statistical results are determined by the detailed characteristics of the Hamiltonian and the model space, we find that overall the dispersion decreases inversely as the square root of the dimension of the model space. This is in keeping with results obtained earlier for the Hamiltonian moments of nuclear systems and means, speaking broadly, the larger the problem the more accurate the method.

The value of the $L_2$-$L_3$ Coster-Kronig transition probability ($f_{23}$) in Pb has been determined by multiparameter K versus L x-ray coincidence measurements using high-energy-resolution germanium and silicon detectors. The Pb x rays were obtained from a radioactive source of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Bi}$. A new method of data analysis to determine the contribution of K${\alpha }_1$ x rays to the K${\alpha }_2$ x-ray peak is described. Precise determination of this contribution is crucial to accurate measurement of $f_{23}$. The value obtained, $f_{23}$=0.112±0.001, is slightly lower than predicted by the latest theoretical calculations. A trend toward deviations from theory in the same direction appears to be confirmed by recently measured values of $f_{23}$ in other medium- and high-Z atoms.

The angular distribution of electrons ejected in photoionization of nonlinear molecules is resolved into contributions with different angular momentum transfer ${\mathrm{l}}_{\mathrm{t}}$=l-${\mathrm{l}}_{\mathrm{r}}$. It is found that, unlike for atomic and linear molecular targets, the photoionization amplitude needs to be characterized not only by a value of $l_t$ but also by its associated projection ‖$m_t$‖≤$l_t$. Expressions for ionization amplitudes, cross sections, and asymmetry parameters corresponding to parity-favored and parity-unfavored transitions have been derived. Of the p and d partial waves, which were recently shown to be necessary and sufficient for representing the continuum $t_2$ orbital of the ejected electron in order to study the hitherto observed angle-resolved photoelectron spectrum of the $a_1$ orbital in certain $T_d$ molecules, all the amplitudes corresponding to parity-unfavored transitions associated with l=1 are found to vanish identically, while for the d wave both parity-favored and parity-unfavored values of $l_t$ make nonzero contributions. The ratio δ¯ of the parity-unfavored to parity-favored cross sections has been deduced from the experimental measurements as a function of photon energy from ionization threshold to higher values. The analysis reveals that the variations observed in the angular distribution asymmetry as a function of photoelectron energy for ionization in the 4$a_1$ orbital of ${\mathrm{CF}}_4$, 6$a_1$ orbital of ${\mathrm{CCl}}_4$, and 7$a_1$ orbital of ${\mathrm{SiCl}}_4$ are due primarily to the dynamical ratio δ¯, because the averaged asymmetry 〈β${¯}_f$〉 for parity-favored transitions always stays close to two and varies little with photon energy.

We present experimental results on core-level photoemission of atomic lithium, leading to both main-line and satellite states of the ${\mathrm{Li}}^{+}$ ion (1snl where n=2–5). We compare the measured 1s2s ${\mathrm{}}^1$,3S main-line cross sections with recent relaxed Hartree-Fock calculations and with previous experimental results, finding reasonable agreement in both cases. For the n=3 satellite, our results disagree with earlier photoemission work which was complicated by additional peaks at 52 eV kinetic energy. We discuss possible explanations for these extra peaks. For the doubly excited state 1s(3s3p) at 71.14 eV, we present total cross-section results in good agreement with previous photoabsorption measurements. The qualitative differences among our partial cross-section profiles are discussed, and the phase for the total cross section is rationalized by estimating the signs of pertinent matrix elements.

Natural radiative lifetimes of ytterbium in the 6snd ${}^1$$D_2$ (n=10–29), 6snd ${}^3$$D_2$ (n=10–20) Rydberg sequences have been measured using stepwise excitation and a fluorescence-decay monitoring method. The lifetimes are perturbed by the 4$f^{13}$5d6s6p configuration near n=11 and 27, except when the $n^{\mathrm{*}3}$ dependence of the lifetime values appeared. The lifetimes of the perturbing 5d6s6p states were also measured.

Many-body perturbation theory has been used to calculate the photoionization cross section of neutral yttrium from threshold at 6.52–100.0 eV. The resonance structure due to 4p excitations to 4d states has also been calculated. Hartree-Fock results are also presented for comparison.

We have developed an integral-equation method for treating multiphoton ionization of atomic systems irradiated by intense fields. We have applied this method to the calculation of partial rates for multiphoton detachment of ${\mathrm{H}}^{\mathrm{-}}$ (into specific continuum channels) by a laser operating at frequencies of 0.2 and 0.5 eV. Nonperturbative features are seen at intensities above about 5×${10}^{10}$ W/${\mathrm{cm}}^2$. We have also applied the method to hydrogen, to the determination of the field-induced width in the ground-state energy (the total ionization rate).

We model the response of a molecular medium to an intense pulse of light. We begin by reducing the Maxwell-Bloch equations for a multilevel molecular model system to a set of simplified propagation equations. We thereby obtain expressions for the frequency, temperature, and field intensity dependence of the ground and excited electronic state absorption cross sections, and the emission cross section, as well as the temperature dependence of the decay rate of the excited state. The phenomena of saturation, excited-state absorption, optically induced dichroism and birefringence, vibronic energy transfer, induced molecular alignment, decay of the matter-field interaction energy, and rotational diffusion are considered within the general formalism for describing the propagation of an intense pulse of light in the molecular medium. We find that the cross section for absorption (emission) of a photon is not identical to the cross section for molecular excitation (deexcitation) via the matter-field interaction. This is explained in terms of an energy balance between the matter, radiation, the matter-radiation interaction, and the bath responsible for the broadening of the molecular transitions. The inequality of these cross sections determines whether the system supplies or draws energy from the thermal bath. The rise in the vibrational temperature of molecules subjected to an intense short pulse is shown to significantly affect propagation of the pulse. The alignment of the molecular system resulting from saturation of the ground-state transition by a polarized laser pulse and the effects of the orientational dependence of the matter-field interaction including effects of rotational diffusion are determined. As an example of the formalism we consider pulse propagation in a Rhodamine 6G solution. We fit the parameters of the model to determine the frequency dependence of the ground- and excited-state absorption cross sections and the emission cross section and compare with experimentally determined cross sections. We describe the propagation of a pulse whose temporal width is comparable to the rotational diffusion time of the molecules in the dye solution and calculate polarization pulse probe signals.

A nonlinear formulation of the free-electron-laser amplifier with a linearly polarized wiggler magnetic field is used to study harmonic generation. Substantial emission is found to occur at the harmonics for a cold beam; however, the harmonics are far more sensitive to beam thermal effects than is the fundamental.

The free-electron laser is described by the self-consistent solution of the coupled system of the Lorentz force equation for the electrons and the Maxwell equation for the laser field. We show that, near saturation and at both low- and high-gain regime, the evolution of each longitudinal mode in the optical cavity can be written as an integral equation. We present numerical solutions of the integral equations for a free-electron-laser oscillator.

We generalize our earlier formalism to treat the effects of both the finite-bandwidth excitations and finite arbitrary detunings, on the fluorescent spectra and second-order intensity correlation functions in optical double resonance. It is assumed that the bandwidth arises from the phase and/or amplitude fluctuations in the fields driving a three-level atom in cascade configuration. Under the conditions that one or both of these fields are intense, secular approximation and the theory of multiplicative stochastic processes are invoked to derive a Markovian master equation for the atomic-density operator averaged over both phase and amplitude. The quantum regression theorem is used to derive analytical expressions for the fluorescent spectra and the second-order intensity correlation functions which explicitly display the effects due to detunings and fluctuations.

A partial differential equation for the probability amplitude in the spherical phase space is derived to describe the nonlinear evolution of a free-electron laser in the Compton regime. A perturbative solution up to an indefinite high order of the electron quantum recoil is obtained. It is used to calculate the laser output, in terms of average photon number, as a function of the wiggler length.

Quantitative comparisons between experiment and theory for the steady-state characteristics in optical bistability with two-state atoms are reported. Nonresonant conditions for both the cavity and the atomic medium are investigated in two different cavity geometries (ring and standing wave). The experiment fulfills to a very good degree the conditions that have been widely employed in theoretical models that consider a collection of two-state atoms interacting with a single mode of the electromagnetic field in a cavity.

We have experimentally studied the output emission of a short ${\mathrm{CH}}_3$F metallic waveguide laser pumped by an injection-locked ${\mathrm{CO}}_2$ transversely excited atmosphere (TEA) laser. The output is found to be extremely unpredictable when lasing occurs in both the excited and ground vibrational states on two seperate transitions (496 and 452 μm, respectively). The dynamics and sensitivity to initial conditions are explained in terms of two Lorenz lasers with pump coupled population inversions.

We derive three-particle higher-order sum rules in two dimensions in analogy with similar rules existing in three dimensions. We employ these rules to study the Planck-Larkin structure of the third cluster integral in the presence of three-particle processes like rearrangement scattering and breakup. In particular, in three dimensions we also show the cancellation between bound-state and continuum contributions at the Efimov point. As a byproduct we obtain a Wigner-Kirkwood-type expansion for the third virial coefficient in two and three dimensions.