We have performed experimental and theoretical studies of the deflection of a sodium atomic beam by a laser tuned to the 3 ${\mathrm{}}^2$$S_{1/2}$,F=2–3 ${\mathrm{}}^2$$P_{3/2}$,F=3 transition. The intensity profile of the deflected beam was measured at different laser power levels and for different laser polarizations. Calculations using a constant force derived from a two-level atom yield deflections which are too large. The results of more realistic simulation calculations agree very well with the measured deflection. Those calculations incorporate the atomic-beam spatial and velocity distributions as well as the laser frequency distribution and polarization state. Optical pumping, loss of atoms to the nonresonant F=1 ground state, and Doppler shifting of the atoms out of resonance are all considered, as also are the statistics and angular distributions of the spontaneous photons.

Resonant charge transfer in alkali-ion–alkali-atom collisions is investigated by using the molecular-orbital expansion method incorporating the use of electron translation factors. Molecular wave functions and eigenenergies are obtained by the pseudopotential method. Molecular properties, $R_e$, $D_e$, and ${\omega }_e$, obtained in the present calculation are in good accord with other recent theoretical results, as well as spectroscopic measurements. Three-state close-coupling calculations reproduce the positions of the maxima and minima in the oscillatory structure seen experimentally in the resonant-charge-transfer cross sections for the ${\mathrm{Li}}_2$${\mathrm{}}^{+}$ and ${\mathrm{Cs}}_2$${\mathrm{}}^{+}$ systems. The magnitude of the total cross sections and their velocity dependence are in agreement with experimental measurements.

For the study of induced dipole moments, the collisional He-Ar complex is treated as a molecule in self-consistent-field (SCF) and size-consistent coupled-electron-pair approximation (CEPA) calculations. The basis superposition error is effectively avoided by using nonorthogonal local orbital sets. The results suggest that the SCF plus approximate dispersion dipoles commonly considered may be at best in accidental agreement with experiment because of fortuitous cancellation: for He-Ar, the hitherto neglected intra-atomic correlation increases the exchange dipole by as much as 30% while the dispersion dipole (which has the opposite polarity of the exchange dipole) is almost 3 times larger than a perturbation estimate of the leading term has indicated. Collision-induced absorption spectra are computed on the basis of the ab initio dipole-moment function, and compared with Bosomworth and Gush’s celebrated measurement. We observe close agreement at frequencies where the absorption is strong. Only at the high and low ends of the measured frequency range does the measurement fall below theory by up to 20%, possibly because of the greater experimental uncertainties associated with weak absorption. Other measurements at relatively high densities have resulted in integrated two-body absorption intensities in reasonable agreement with our binary theory.

Partial-wave analysis is applied to a parametrized pseudostate excitation model of high-energy electron-atom scattering. Consistency checks are carried out between asymptotic distorted-wave calculations (for coupled differential equations), second-Born-approximation scattering amplitude calculations, and partial-wave second-Born-approximation calculations. Closure formulas for partial-wave amplitude sums are derived for a static model potential and for the second-Born-approximation amplitude due to the asymptotic dipole excitation potential. Calculations using these closure formulas in $e^{\mathrm{-}}$+H and $e^{\mathrm{-}}$+Ar models at 15 keV show cusplike forward elastic scattering peaks, confirming recent exact second-Born-approximation results for an $e^{\mathrm{-}}$+H pseudostate model. Using parameters appropriate to ground-state rare-gas atoms, the computed forward peaks are much too small to account for recent experimental observations. The theory indicates that these structures increase in magnitude rapidly with atomic radius, suggesting that the observed strong forward peaks may arise from excited atoms or ions in the electron beam path.

The R-matrix method is used to calculate the contributions of excitation autoionization to electron-impact ionization in Mg-like ${\mathrm{Al}}^{+}$, ${\mathrm{S}}^{4+}$, ${\mathrm{Cl}}^{5+}$, and ${\mathrm{Ar}}^{6+}$ ions. The ground 3$s^2$ ${\mathrm{}}^1$S state together with the autoionizing states arising from the 2$p^5$3$s^2$3p, 2$p^5$3$s^2$3d, 2$p^5$3$s^2$4s, 2s2$p^6$3$s^2$3p, 2s2$p^6$3$s^2$3d, and 2s2$p^6$3$s^2$4s configurations are included in the R-matrix expansion. Large numbers of bound terms of appropriate symmetry are included in the expansion to account for the resonant-excitation–double-autoionization process. The total ionization cross sections are obtained by combining the cross sections for inner-shell excitation with direct-ionization cross sections. Our calculation shows an abrupt and significant rise in the cross sections, in agreement with recent crossed-beam measurements of Howald et al. The importance of indirect processes over the direct process increases with increasing ionic charge along the sequence.

With the use of a previously developed close-coupling theory, and simple models for the energy gap and propagator that arise in that theory, calculations are made of the properties of the survival probability for the negative ions and of the energy spectrum of detached electrons. Special attention is given to interference effects that might be seen under favorable circumstances.

Total cross sections are reported for electron transfer in collisions between protons and ${\mathrm{Li}}^{2+}$ ions at proton energies 17.5–200 keV relative to the ${\mathrm{Li}}^{2+}$ ion and for ionization at energies 50–200 keV. A coupled-state, Sturmian approach has been taken. For electron transfer, a detailed, basis-convergence study has been carried out. Larger-basis results have also been obtained for electron transfer in p-${\mathrm{He}}^{+}$ collisions, confirming the accuracy of results reported previously with smaller bases, and p-${\mathrm{He}}^{+}$ ionization cross sections are also reported for the first time with a Sturmian basis. For the p-${\mathrm{Li}}^{2+}$ processes, results are compared with the only previously existing pseudostate results (one-and-a-half-center results), and, for electron transfer, the energy range of existing results is extended. There do not appear to be any experimental results for p-${\mathrm{Li}}^{2+}$ collisions. A detailed comparison with the other theoretical results and with experimental results is carried out for the p-${\mathrm{He}}^{+}$ processes; except for some discrepancies at the extrema of the energy range, the results are in general accord.

The cross sections for direct charge transfer of ${\mathrm{He}}^{+}$ ions in low-energy collisions with neon atoms are calculated in a diabatic formulation for several choices of the origin of coordinates. It is shown numerically that the omission of electronic translation factors creates serious uncertainties at intermediate energies. The differences in the cross sections diminish as the collision energies increase to 900 eV where they attain a value of 1×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$. Direct charge transfer is more efficient than radiative charge transfer at energies above about 25 eV.

The three-body system is analyzed in relation to the calculation of atomic scattering cross sections. A method is presented to generate the initial electronic conditions for the classical-trajectory Monte Carlo method in the case where the active electron is subject to non-Coulomb interactions. The method is then applied to study the collisions of ${\mathrm{H}}^{+}$ with He and ${\mathrm{Li}}^{+}$ targets in the intermediate- to high-energy range. Single-electron capture and single-ionization total cross sections are presented for both collision systems. In the case of He targets, total cross sections for double ionization and singly differential cross sections for free-electron production are also calculated. Cross sections and initial electronic distributions are obtained with both Coulomb and model interactions and compared. Good agreement is found between theoretical and experimental results, except for the double ionization of He.

The high-resolution $N_4$,5OO normal Auger spectrum and resonantly excited Auger and autoionization spectra of Xe have been investigated using synchrotron radiation to ionize or excite the 4d levels. Excitation of the 4d electrons to the 4$d^9$5$s^2$5$p^6$6p resonance states results in the decay processes with a 6p spectator or participator electron. Besides the previously reported strong resonance Auger peaks, the whole resonance Auger spectrum has been studied in detail. Energies and intensity distributions of different processes are calculated and compared with experiment.

The $M_4$,5NN normal and resonance Auger spectra of Kr have been measured using different photon energies to ionize or excite the 3d levels. Excitation of the 3d electrons to the 3$d^9$4$s^2$4$p^6$5p resonance states results in decay with a 5p spectator or participator electron. The spectra are analyzed by comparing them with the calculated profiles. The calculations take into account the presence of a spectator electron, and the correlation effects in the case of a 4s hole state.

We calculate, including correlation, relativistic, and radiative effects, the Auger energies associated with the ${\mathrm{KL}}_2$,3${\mathrm{L}}_2$,3${(\mathrm{}}^3$P,${\mathrm{}}^1$ $D_2$,${\mathrm{}}^1$$S_0$) transitions in Mg to be, respectively, 1171.25, 1166.9, 1161.9 eV. The experimental values are, respectively, 1171.0 (1170.85), 1167.1 (1167.0), and 1161.9 (1161.75) eV. For the ${\mathrm{KL}}_2$,3${\mathrm{L}}_2$,3${(\mathrm{}}^1$${\mathrm{D}}_2$) transition in Ca, we obtain 3275.8 eV and for the Be ${\mathrm{KL}}_1$$L_1$${(\mathrm{}}^1$$S_0$) transition, 96.21 eV, in good agreement with the experimental value of 96.11 eV. Predictions are also made for O and F KLL Auger energies.

Excitations of the Xe 4d subshell to discrete np and continuum εf states have been studied by photoemission. The resonant photoelectron spectra provide the first direct evidence that the emission of continuously distributed shakeoff electrons is one of the dominant decay modes of the 4d→np resonances. The same effect is proposed to account for the missing 4d intensity in the 4d→εf shape resonance compared to the absorption cross section. The fraction of direct recombination in the decay of the 4d→np resonances was determined quantitatively by partial cross-section and branching-ratio measurements of the 5s and 5p main lines in the photon-energy range from 60 to 70 eV. The modified Fano theory for partial cross sections was applied to nearly Lorentzian-type profiles in order to derive partial decay widths. The decay into the 5s and 5p photoemission channels amounts to less than 0.5% of the total decay rate. The 5$p_{3/2}$:5$p_{1/2}$ branching ratio was measured over a broad range (17–140 eV) in order to test details of relativistic random-phase approximation calculations concerning intershell correlations between the 4d and 5p subshells.

Polarized and depolarized rotational Raman spectra of gaseous ${\mathrm{H}}_2$S have been measured at room temperature and different pressures, together with the Q branch of the ${\nu }_1$ band used as an internal standard. Isotropic and anisotropic spectra are found to be affected by ‘‘collision’’-induced effects which are not completely explainable in terms of the first-order dipole–induced-dipole (DID) mechanism. Integrated intensities, depolarization ratios, and spectral shapes are compared with theoretical predictions in order to discriminate between the different scattering mechanisms, which are accounted for in the framework of the multipolar and higher-order polarizability series expansion. Higher-order polarizabilities are considered for induced effects only, the laser power being fairly low. At the density considered, the anisotropic integrated intensity is due to ordinary rotational and induced contributions, the first-order DID one accounting for most of the measured intensity. Other induced contributions appear in the frequency range 100–450 ${\mathrm{cm}}^{\mathrm{-}1}$, the DID scattering being confined to low frequency. Their origin can be understood by analyzing the measured isotropic spectra. The rotational induced contribution involving the dipole-quadrupole polarizability tensor (αA contribution) is mainly responsible for the broad isotropic spectrum extending up to 450 ${\mathrm{cm}}^{\mathrm{-}1}$, whereas an induced contribution involving the first hyperpolarizability is negligible. It has been possible to estimate $A_{123}$,$1^2$+${\mathrm{A}}_{123}$,$3^2$ =3 A${̊}^8$.

At the highest frequencies (300–450 ${\mathrm{cm}}^{\mathrm{-}1}$), the depolarization ratios and the comparison between experimental and theoretical αA spectra indicate the presence of other induced contributions which are likely due to the gradient of the quadrupolar field and to higher-order multipolar polarizabilities. The point-scatterer approximation of the multipolar series expansion is shown to reproduce the main spectral features. At low frequency, i.e., below 150 ${\mathrm{cm}}^{\mathrm{-}1}$, a sharp peak is found to be superimposed on the αA isotropic spectrum. The peak is likely due to higher-order DID effects, and to induced contributions involving second hyperpolarizability and long-range dispersion forces. Its origin is very similar to that of the isotropic spectrum of noble gases.

Three-photon ionization spectra of Ba were studied which result from two-photon resonances between the 6$s^2$ ${\mathrm{}}^1$$S_0$ ground state and the 6snd J=2,6s(n+1)s ${\mathrm{}}^1$$S_0$ (16≤n<40) Rydberg states and 5d7d ${\mathrm{}}^1$$D_2$,${\mathrm{}}^3$$P_0$,2 perturber states. A tunable laser with field strength ${10}^{\mathrm{-}\left(4\mathrm{}\mathrm{-}5\right)}$ a.u. was used to drive the excitation in a low-density atomic beam (n≲3×${10}^8$ ${\mathrm{cm}}^{\mathrm{-}3}$). Resonant line shapes were systematically studied as a function of light polarization and intensity. Studies with linearly polarized light find all resonant line shapes manifest the same peak shift and asymmetry which is traced to the ac Stark shift of the ground state. The resonant peak height, shift, and linewidth scale linearly with light intensity which is accounted for with a rate-equation formulation.

For the case of circular polarization, a distinct minimum of ionization occurs between each J=2 doublet of the 6snd series. One level (generally ${}_{}{}^3{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$) is significantly broadened in comparison to the laser linewidth (0.1 ${\mathrm{cm}}^{\mathrm{-}1}$) with increasing intensity while its sister level remains narrow (∼0.2 ${\mathrm{cm}}^{\mathrm{-}1}$). Further, light shifts of the ${}_{}{}^3{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$ states are generally ≳1 ${\mathrm{cm}}^{\mathrm{-}1}$ to the red while shifts of ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$ states are negligible. This behavior switches for the n=27 doublet. Lastly, the dynamic shifts are not quadratic with the field strength. These effects are attributed in part to cancellation due to channel interference and a two-photon Raman process which mixes each J=2 doublet. These processes act in conjunction with the ac Stark effect to produce the observed profile behavior. The shifts and coupling of the excited states are attributed to the influence of the 6pnd $J^{\pi }$${=3\mathrm{}}^{\mathrm{-}}$ autoionization series and are shown to be sensitive to the singlet-triplet mixing coefficient β of the J=2 doublets. A perturbation analysis is introduced which accounts for the salient features of the data, although it does not include bandwidth and saturation effects. Calculations of light shifts of the 6snd J=2 levels based on this analysis require β be modified from previous assignments based on hfs measurements for n in the vicinity of the 5d7d ${\mathrm{}}^1$$D_2$ perturber.

We study the influence of many-electron effects in multiphoton ionization within the framework of diagrammatic many-body perturbation theory. We renormalize the electron-dipole coupling by summing to infinite order both many-electron interactions using the random-phase approximation and higher-order intensity terms. We introduce an effective intensity, which takes into account the screening of the field by the electrons and represents the intensity really seen by an electron. The theory is applied to a calculation of the two-photon one-electron ionization rate of helium in the weak-field limit, using a local-density approximation one-electron basis set. The influence of many-electron effects strongly depends on the field frequency. The two-photon ionization rate of helium is lowered at low frequency (by a factor up to 1.4) and increased at high frequency when the photon energy is above the ionization threshold. Finally, the importance of many-electron effects in multiphoton ionization (e.g., regarding inner-shell ionization) is qualitatively discussed in connection with experiments.

We analyze the transient probe spectra that arise when a three-level atom is strongly driven by an optical field nearly resonant with one transition and is probed by a step-function weak field on a coupled transition. We pay particular attention to the effect of imbalances in the initial dressed-state populations on the observed spectra. As recently pointed out, such imbalances may be induced by sudden changes in the phase of the driving field. We find that transient probe spectra may differ dramatically for different initial atomic conditions. We follow the temporal relaxation of these novel probe spectra to their steady-state form. Our calculations do not include the effect of inhomogeneous broadening and are hence appropriate to studies of collimated atomic beams.

Two laser beams at frequencies ${\nu }_1$ and ${\nu }_2$ are used to produce nonlinear scattering (wave mixing) in ${\mathrm{I}}_2$ vapor at frequency 2${\nu }_1$+${\nu }_2$. The scattering results from near-resonant allowed transitions in ${\mathrm{I}}_2$ molecules and is observed to be anomalously strong when both laser beam frequencies are detuned in the same direction from frequencies corresponding to resonance in the free molecule. The strong scattering is interpreted in terms of a resonantly enhanced ac Stark shift caused by a configuration interaction between the ‘‘dressed molecular levels.’’ By tuning the two pulses simultaneously, a tuning range of approximately 2 Å (50 ${\mathrm{cm}}^{\mathrm{-}1}$) is achieved in the mixed wave. The scattering process should therefore have practical application in the efficient generation of tunable coherent radiation in the ultraviolet and vacuum-ultraviolet range of the spectrum. Some of the spectroscopic properties of the three-quantum state at E∼53 000 ${\mathrm{cm}}^{\mathrm{-}1}$ are studied.

We have experimentally and theoretically investigated the behavior of Na atoms under the combined influence of resonant laser light and a magnetic field. Linearly polarized light was tuned to the (3s) ${\mathrm{}}^2$$S_{1/2}$ (F=2)→(3p) ${\mathrm{}}^2$$P_{1/2}$ (F’=1) component of the $D_1$ line. The intensity of fluorescent light was observed as a function of the laser intensity for various strengths of the magnetic field B. Trapping in the $M_F$=±2 levels of the ground state could be observed to be of decreasing importance for an increasing component of B perpendicular to the laser polarization ε^. For laser intensities increasing beyond certain critical values, however, the fluorescence decreased and trapping seemed to become again important. For a quantitative explanation of the observed behavior we describe the system by generalized rate equations for the density matrix. We find that stimulated absorption and emission processes effectively inhibit the Larmor precession, thereby enforcing the trapping effect. Precession can only take place at sufficiently low light intensities.

The n-photon rotational line strength is studied for diatomic molecules and for transitions between two states which both belong to Hund’s case-(b) coupling, as well as for transitions between a state with Hund’s case-(a) coupling and a state with Hund’s case-(b) coupling. We suppose that all the absorbed photons have the same energy and the same polarization state which can be linear or circular. Explicit forms for the rotational line factors of any spin multiplicity are presented and selection rules are obtained from angular momentum conservation arguments. The number and labeling of the n-photon rotational branches have also been considered.