A connection is made between classical transport theory and the usual description of collisional processes in laser spectroscopy. In classical transport theory, collisional processes are described in terms of either transport coefficients or collision integrals. In analyzing the influence of collisions on laser spectroscopic line shapes, collisions are often described in terms of collision kernels. Two sets of equations are obtained relating the collision integrals to the collision kernels. While these two sets of equations are equivalent for any physically realistic kernel, they need not be equivalent if one carries out calculations using phenomenological kernels. If the two methods give similar collision integrals for a phenomenological collision kernel, it may serve as a justification for the use of that kernel. We show that the two methods do give very similar results for the Keilson-Storer kernel but give dramatically different results for a ‘‘difference’’ kernel (a kernel that is a function of the difference between the initial and final velocity). The calculations, carried out for a low-density binary gas mixture, provide a link between classical transport theory and the collision kernels commonly used in analyzing experiments in laser spectroscopy. Implications of the results to experimental situations of current interest, such as light-induced drift, are explored.

We have measured the electron energy spectrum from 0.5-MeV ${\mathrm{H}}^{\mathrm{-}}$-He collisions at ${\mathrm{theta}}_L$=0° for single-electron detachment. This process selection was made by requiring a time correlation between the detection of an electron and the detection of a neutral H atom. It was found that this time-correlated electron spectrum was, within experimental error, the same as the electron spectrum taken without any time-correlation requirement. We have also investigated the ‘‘very sharp peak’’ seen in previous experiments near ${\mathrm{theta}}_L$=0° and extracted the angular distributions of these electrons in the projectile frame. The angular distributions were found to be isotropic in the projectile frame. Previous comparisons between theory and experiment are discussed.

The absolute cross section for removal of Rydberg electrons from Na Rydberg atoms by impact of singly charged ions was measured. ${\mathrm{Ne}}^{+}$, ${\mathrm{Ar}}^{+}$, and ${\mathrm{Xe}}^{+}$, accelerated to energies 1–5 keV, bombarded a Na atomic-beam target that was laser excited to nd states, n=30–45. Electron loss, the sum of ionization and charge transfer, was detected by collecting impact-produced ${\mathrm{Na}}^{+}$ from the target after 1–3 μs exposure to the projectile beam. The velocity dependence of the cross section was measured for n=40 with projectile velocities ṽ=0.8–3.7 times the Bohr orbital velocity of the target atoms. The n dependence was measured at reduced velocity ṽ=2.0 for n=30–45. A study of field ionization of higher collisionally excited states was made, and important corresponding corrections were applied to the data. The effects of collisional l change in the target and of l mixing due to applied, pulsed electric fields were noted. The absolute unnormalized cross section for n=40 agreed with classical-trajectory calculations and with the classical binary-encounter approximation. The classically predicted n scaling of the cross section at fixed reduced velocity was directly demonstrated, and a scaling exponent 4.08±0.07 between n=1 and 45 was found. The results may be compared with ${\mathrm{H}}^{+}$+H(1s) collisions at projectile energies 16–340 keV.

In this paper, theoretical results are given for proton ionization of a p state of argon. If relativistic effects can be ignored and the scattered proton is not observed, the density matrix describing the p hole for a given ejected-electron energy and angle is fully described by five independent parameters. In addition to the (absolute) doubly differential cross section, there are four (relative) alignment and orientation parameters which, in the case of the 2p shell, can be determined experimentally by observing in coincidence both the ejected electron and the light emitted in the 3s→2p decay of ${\mathrm{Ar}}^{+}$. In the present paper distorted-wave Born results are presented for these five parameters for 300-keV incident protons and ejected-electron energies in the range of 30–300 eV.

In recent articles, we pointed out the fundamental difference between the molecular treatment of processes involving a multicharged ion and hydrogen or helium atoms, which is the (formal) autoionizing character of the molecular channels, and we reported a (new) implementation of the Feshbach method to calculate the molecular energies and couplings. In the present work we use the wave functions calculated with this Feshbach method for the ${\mathrm{BeHe}}^{4+}$ quasimolecule, introduce a common translation factor in the formalism, and calculate the single and double charge-exchange cross sections in ${\mathrm{Be}}^{4+}$+He(1$s^2$) collisions for impact energies 0.2–20 keV/amu. The mechanisms of the processes are discussed in detail.

A mathematically transparent derivation of the multiple-scattering equations valid for a general non-muffin-tin potential, as applied to clusters of atoms with and without a surrounding outer sphere, is presented. These equations are shown to be a natural generalization of the analogous equations valid for muffin-tin potentials. An expression for the photoabsorption and electron scattering cross section in the framework of the multiple-scattering theory valid for a general potential is derived for what may be the first time, providing the necessary generalization for the similar expression valid in the muffin-tin case. A connection with the Green-function approach to the problem is also established via a generalized optical theorem.

Various momentum-space properties of ${\mathrm{N}}_2$ in its ground electronic state (X ${\mathrm{}}^1$${\Sigma }_g^{+}$) are computed from a wave function substantially more accurate than those previously used for such purposes. The wave function was obtained by first-order configuration interaction (CI) relative to a full valence multiconfiguration self-consistent-field reference wave function constructed from a large basis set of Slater-type functions. The calculated isotropic Compton profile agrees best with 160-keV γ-ray experiments and with a previous CI calculation. The calculated values of 〈$p^k$〉 (-2≤k≤4) agree well with values obtained from 25-keV electron-impact experiments. The value and second derivative of the spherically averaged momentum density at p=0 are not quite as sensitive to details of electron correlation as had been thought previously. Anisotropic properties such as directional Compton profiles parallel and perpendicular to the bond and the J(0,γ) surface are also calculated; however, calculations with a larger basis set will be required to obtain converged values of these anisotropic properties.

Collision strengths are presented for all transitions between the states (including fine structure) 3$s^2$ ${}^1$$S_0$, 3s3p(${}^3$$P_0$,1,2,${}^1$P ${}_1{}^{}{}_{}{}^{}$), 3$p^2$(${}^3$$P_0$,1,2, ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$,${}^1$$S_0$), 3s3d(${}^3$$D_1$,2,3,${}^1$D ${}_2{}^{}{}_{}{}^{}$), and 3s4s(${}^3$$S_1$,${}^1$$S_0$) in Mg-like ions S v, Ar vii, Ca ix, Cr xiii, and Ni xvii. The calculations are carried out in LS coupling using the distorted-wave approximation with a ten-state eigenfunction expansion. Relativistic effects in the target states are taken into account using the Breit-Pauli Hamiltonian, and the term-coupling coefficients thereby obtained are employed to accomplish the transformation of the scattering matrices to intermediate coupling. Atomic effects involving autoionizing resonances in some transitions are considered, as well as the contribution of high partial waves to the transition 3$s^2$ ${}^1$$S_0$–3s3d ${}^1$$D_2$. The problem of bound-channel correlation-type functions in the eigenfunction expansion is discussed in some detail.

The semiclassical coupled-channel resonance-energy quantization is studied in detail for the case of predissociating states. Applications are presented for a model MgH system, where full resonance spectra in the predissociation region have been computed. Variation of the interaction strength $A_{\mathrm{int}}$ allows the study of correlations of diabatic resonances ($A_{\mathrm{int}=0\mathrm{}}$) with the adiabatic limit ($A_{\mathrm{int}}$ large). Several novel features, including limiting states with zero lifetime, are discussed.

A beam-foil study of core-excited states of B iii in the wavelength region 36–200 nm is reported. Fourteen new lines are identified in the ${}_{}{}^4{}_{}{}^{}\mathrm{L}$ system and four in the core-excited ${}_{}{}^2{}_{}{}^{}\mathrm{L}$ system, confirming the recent theoretical predictions of Chung et al. The wavelength accuracy has been improved for most of the transitions already observed between doubly excited states. The previously unknown energies of the 1s2p4s ${\mathrm{}}^4$P ° and [(1s2p${)\mathrm{}}^3$P5f${]\mathrm{}}^2$F terms have been determined. Lifetime results for the [(1s2p)${\mathrm{}}^3$Pnd]${\mathrm{}}^2$D?(de (n=3,4) and for the [(1s2p)${}^3$Pnf)]${}^2$F (n=4,5) terms are reported. The autoionization width of the (1s2$p^2$)${}^2$D state has been measured by analyzing the profile of the broadened (1s2$p^2$)${}^2$D-[(1s2p)${\mathrm{}}^3$P3d ]${\mathrm{}}^2$D° line. The measured width of 44±4 meV is in good agreement with the value calculated by Davis and Chung.

The production of entropy is analyzed for the stopping of a beam of atoms by momentum transfer from a counterpropagating laser beam. Expressions are given for the rate of entropy production in the photon beam and in the atomic beam. In the latter case, the kinetic entropy rate is computed from the compression of the velocity distribution function of the atoms under the radiation pressure. A thermodynamic analysis yields large irreversibilities in this process, because the total rate of entropy production due to absorption and emission of the photons is several orders of magnitude larger than the rate of entropy reduction in the atomic beam.

The Feynman path-integral method is applied to solve the problem of a H atom in an electric field. In accordance with the midpoint philosophy, the propagators are symmetrized in every time interval and, via several transformations, one of which is the Langer modification, the Green’s function is calculated in parabolic coordinates, decomposed into partial propagators, and expressed in terms of two one-dimensional Green’s functions. The perturbation method being no more valid for high excited levels, the spectrum is given, according to the WKB method, as a solution of a system of two elliptic integral equations. The exact spectrum of the H atom is obtained for a zero electric field.

Calculations of atomic parameters for dielectronic satellite lines of hydrogenlike chromium have been performed in the intermediate-coupling scheme with the inclusion of effects of configuration interaction. The doubly excited intermediate resonance states considered are ‖2l nl’ SLJ〉, n=2, 3, and 4 with all allowed values of l and l’. The Hartree-Fock-Slater atomic model is used to calculate transition rates. X-ray and Auger transition energies are corrected for relativistic effects. Radiative and radiationless transition rates, fluorescence yields, nonradiative branching ratio, and lifetime of these doubly excited states are reported. For the wavelengths of a few strong satellite lines, effects of self-energy and vacuum polarization are included in our calculation. Dielectronic recombination rate coefficients are presented as a function of electron temperature.

The absolute excitation rate coefficients for Cu xiii and Cu xvii optically allowed transitions have been measured using the Texas Experimental Tokamak (TEXT) tokamak. Cu is injected by the laser-ablation method at a time when the plasma has attained steady-state temperature and density profiles. The absolute intensities of the magnetic-dipole-forbidden transitions within the Cu xiii ${3\mathrm{s}}^2$3$p^5$ and the Cu xvii ${3\mathrm{s}}^2$3p ground configurations are measured. The intensities of optically allowed vacuum-ultraviolet lines within the 3$p^4$3d-3$p^5$ transition array of Cu xiii and within the 3d-3p transition array of Cu xvii are measured with a 2.2-m grazing incidence spectrometer. This instrument is radiometrically calibrated in situ by using the branching-ratio technique. The absolute excitation rate coefficients are deduced from these measurements, assuming coronal equilibrium, optically thin line emission, and statistical population within the ground configuration. The experimental rate coefficients for these transition arrays are found to be 2.84×${10}^{\mathrm{-}8}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$ for Cu xiii at an electron temperature of 127 eV and 3.5×${10}^{\mathrm{-}8}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$ for Cu xvii at an electron temperature of 240 eV. These results are in good agreement with excitation rate coefficients computed in a distorted-wave approximation. The estimated uncertainty in the experimental values is ±70%.

Subshell photoionization parameters for photoionization from the 4f, 5p, and 5s subshells of atomic ytterbium have been calculated using the relativistic random-phase approximation (RRPA). Interference effects from interchannel coupling of 21 relativistic dipole channels have been included. Interesting differences in the results for 4f parameters relative to a previous eight-channel RRPA calculation have been observed and are attributed to strong effects coming from coupling with the 5p channels.

We have investigated the dynamic Stark effect in a J=0→1→0 three-level ladder system in an atomic beam of natural barium. An intense, actively stabilized cw dye laser was tuned to resonance between the upper two levels, and a similar laser was scanned through resonance with the transition connecting the lower two levels. Fluorescence arising from transitions out of the middle and upper levels was separately recorded as a function of probe laser frequency. This is the third and final paper in a series, of which the first two papers investigated theoretically and experimentally the ‘‘weak-probe’’ case, where the intensity of the probe laser was sufficiently low for the interaction between the atoms and the probe laser to be linear. In this paper we present an experimental investigation of the ‘‘strong-probe’’ case, where both lasers interact nonlinearly with the atoms, and we compare the results with the predictions of a theoretical model based on the optical Bloch equations.

Photoelectron spectroscopy with monochromatic synchrotron radiation (100–115 eV) is used to study the silicon 2p excitation and ionization in the silane molecule. Valence main bands and satellites are observed when ${\mathrm{SiH}}_4$ is excited below the Si 2p edge and off resonance. Auger lines are seen for excitation energies above the edge. The excitation of a core electron into the lowest antibonding molecular orbital results in dramatic changes in the spectrum. In addition to autoionization and resonant Auger processes, which lead to ${\mathrm{SiH}}_4$ with one-hole and two-hole–one-particle (or multiple-hole) excited configurations, respectively, an unexpected relaxation pathway due to the purely repulsive character of the core-excited state is proposed to explain the existence of very low binding energy resonating lines. It consists of a very fast dissociation into ${\mathrm{SiH}}_3$${\mathrm{}}^{\mathrm{*}}$+H followed by autoionization of the core-excited ${\mathrm{SiH}}_3$${\mathrm{}}^{\mathrm{*}}$ fragments.

We present theoretical electron-impact-excitation cross sections for several transitions in ${\mathrm{Be}}^{+}$, and fluorescence radiation polarizations for the ${\mathrm{Be}}^{+}$ resonance doublet. The projectile-electron energy varies from 0.3–2.0 Ry. A five-state close-coupling approximation is used. The target model is the most sophisticated employed in any scattering calculation to date, and yields oscillator strengths for several transitions in ${\mathrm{Be}}^{+}$ that are the most accurate available. Our results do not, however, improve upon previous work as regards the notable discrepancies between calculations and experimental measurements for excitation of the resonance transition. This indicates that high rigor in the treatment of short-range core-correlation effects is not required for an accurate description of these processes, and therefore either that other, heretofore ignored, effects must be taken into account in the theory or that other measurements are necessary. An accurate measurement of the hyperfine structure of the j=(3/2) fine-structure level of the doublet would be of particular and decisive importance.

We present a theory and calculations of transitions between autoionizing states in He. A ${}_{}{}^1{}_{}{}^{}\mathrm{P}$ resonance is assumed excited by a weak source while a strong laser couples it to other autoionizing states. We calculate the total ionization probability, the line shape, and their dependence on the intensity of the laser. Possible experimental verification is also assessed.

Collisions that produce a change in the total angular momentum and quenching processes involving the triplet z ${}_{}{}^3{}_{}{}^{}°$ titanium excited level and noble gases have been studied by laser perturbation and time-resolved spectroscopy. Titanium atoms are produced in a hollow-cathode discharge, and the analysis of resonance and sensitized fluorescence light decays enable us to determine total-angular-momentum-changing (J-changing) and quenching cross sections induced by collisions with helium, neon, and argon atoms. The thermally averaged J-changing cross section (z ${}_{}{}^3{}_{}{}^{}F$ °${\mathrm{}}_i$→z ${}_{}{}^3{}_{}{}^{}F$ °${\mathrm{}}_j$) in units of ${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$ are σ${¯}_{4\to 2}$=1.7±0.5, 0.7±0.06, 3.1±1; σ${¯}_{4\to 3}$=11±2, 1.4±0.1, 11±1; σ${¯}_{3\to 2}$=7±2, 2.2±0.3, 14±2, respectively, for helium, neon, and argon colliding partners. As for alkali-metal–noble-gas collisions, a pronounced minimum of the J-changing cross sections is obtained for the titanium (z ${}_{}{}^3{}_{}{}^{}F$ °)–neon collisions. Radiative destruction probabilities of the z ${}_{}{}^3{}_{}{}^{}F$ °${\mathrm{}}_j$ (j=4,3,2) titanium sublevels have been measured also, and good agreement with accepted values is found.