Inelastic collisional processes in fine-structure-state mixing have been studied by selective stepwise excitation and observation of the resulting fluorescence during collisions of excited rubidium $6{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ state atoms with various noble-gas atoms. It is shown that for intermediately excited rubidium atoms the fine-structure-state mixing cross sections are generally increasing from helium to xexon. The cross sections for the process $\mathrm{Rb}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}\right)+A\left(n{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)\to \mathrm{Rb}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\right)+A\left(n{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+\Delta E$ have the values ${\sigma }_{\mathrm{fs}}^{*}=2.4\mathrm{}\pm 0.9, 3.1\pm 1.2, 2.6\pm 1.0, 5.8\pm 2.2, \mathrm{and} 9.3\pm 5.6$ in units of ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$, where $A$ denotes in sequence any one of the noble gas atoms He, Ne, Ar, Kr, and Xe, respectively. Transfer cross sections for the transfer of excited Rb$6{}_{}{}^2{}_{}{}^{}D$-state atoms out of the doublet were also determined and are about an order of magnitude smaller than the fine-structure-mixing cross sections.

We report our measurements of the cross sections ${\sigma }_{5S}$ and ${\sigma }_{4D}$ for the electronic-energy-transfer process $\mathrm{Na}\mathrm{}\left(3P\right)+\mathrm{Na}\mathrm{}\left(3P\right)\mathrm{}\to \mathrm{Na}\mathrm{}(5S,4D)+\mathrm{Na}\mathrm{}\left(3S\right)\mathrm{}$. To obtain these cross sections we have measured the fluorescence intensity for the transitions $5S\to 3P$, $4D\to 3P$, and $3P\to 3S$ in sodium vapor excited with a cw dye laser. The study of this electronic energy transfer is complicated and there is a large disagreement (≳${10}^4$) among reported measurements. We believe we have avoided most of the difficulties of previous experiments. Our results are ${\sigma }_{5S}=(2.0\mathrm{}\pm 0.7)\times {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ and ${\sigma }_{4D}=(3.2\mathrm{}\pm 1.1)\times {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$, and they are compared with the results of other experiments.

In elastic atom-atom collisions resonances can be formed even for zero angular momentum, provided that the potential has a barrier. It is shown that their contribution in the cross section is much larger than expected from their energy width. This is due to an unusually large broadening effect in the angular momentum variable which only occurs for zero-angular-momentum resonances. The broadening effect in the partial waves is analyzed, and it is shown how such resonances affect the cross sections.

We study a class of spectra $a(J^{\prime }\leftarrow J)$ of rotational transitions of linear and spherical-top rotators. Examples are the cross sections for photoabsorption, photoionization, and collisions in the sudden approximation. Both the sum $a(J+\Delta J\leftarrow J)+a(J-\Delta J\leftarrow J)$ and the transition-energy—weighted sum $\Delta E(J+\Delta J\leftarrow J)a(J+\Delta J\leftarrow J)+\Delta E(J-\Delta J\leftarrow J)a(J-\Delta J\leftarrow J)$ are nearly independent of $J$ at high $J$, the $J$-dependent correction terms being of the order of $J^{-2\mathrm{}}$ for linear rotators and much smaller for spherical tops. The ratio $\frac{a(J+\Delta J\leftarrow J)}{a(J-\Delta J\leftarrow J)}$ is $1+c \frac{\Delta J}{J}+O\left(J^{-2\mathrm{}}\right)$, where $c=1\mathrm{}$ for linear rotators and $c=2\mathrm{}$ for spherical tops. The anisotropy parameter $\beta (J^{\prime }\leftarrow J)$ for the angular distribution of photoelectrons satisfies $\beta (J\pm \Delta J\leftarrow J)={\beta }_{\infty }[1+c_{\beta }{J}^{-2\mathrm{}}+O(J^{-3\mathrm{}}\left)\right]$ and $\frac{\beta (J+\Delta J\leftarrow J)}{\beta (J-\Delta J\leftarrow J)}=1+O\left(J^{-3\mathrm{}}\right)$ for linear rotators, where ${\beta }_{\infty }$ and $c_{\beta }$ are constants independent of $J$. The $\beta$ for spherical tops show much weaker dependence on $J$.

Resonant electron scattering from the ${\mathrm{N}}_2$ molecule is treated within the framework of Feshbach's projection-operator formalism. The problem of nuclear motion in the complex, nonlocal, and energy-dependent potential of the resonance state is solved with the use of basis-set expansion methods. The width function $\Gamma \mathrm{}(E,R)\mathrm{}$ of the 2.3-eV shape resonance is taken from previous ab initio calculations A. U. Hazi, T. N. Rescigno, and M. Kurilla, [Phys. Rev. A 23, 1089 (1981)]. The potential-energy curve $V_1\mathrm{}\left(R\right)\mathrm{}$ of the discrete electronic state giving rise to the resonance is adjusted to obtain quantitative agreement with experiment for the $v=0\mathrm{}\to 1$ vibrational excitation channel. Calculated cross sections are reported for many vibrational excitation channels including scattering from vibrationally excited target molecules and are compared with experiment. The results are in good agreement with experiment as far as experimental data are available. The information on the 2.3-eV shape resonance in ${\mathrm{N}}_2$ obtained in this way (potential-energy curve, width function, complex pole of the $S$ matrix, phase shift, etc.) is discussed and compared to the results of other ab initio and empirical calculations. Particular emphasis is laid on the assessment of the accuracy of the local complex potential approximation (boomerang model). It is shown that the local approximation is excellent for the 2.3-eV resonance in ${\mathrm{N}}_2$ as far as the calculation of vibrationally inelastic cross sections is concerned. The local complex potential emerging from the present study differs significantly from the potential obtained empirically by Dubé and Herzenberg [Phys. Rev. A 20 194 (1979)], although the calculated cross sections are virtually identical for a large number of channels. We discuss the precision with which a local complex potential can be defined.

We discuss a method for systematically correcting results obtained using variational expressions in scattering theory. The approach taken is to compute a sequence of Padé approximants of the form [$\frac{N}{N}$] for the error in an initial variational estimate obtained using a basis-set expansion. The relationship between the Padé-approximant approach and the iterative Schwinger method for correcting variational estimates is also examined. We discuss a large class of general variational expressions to which the Padé-approximant approach can be applied. The variational expressions considered include those for the wave function, for photoionization transition matrix elements, as well as for scattering matrix ($K$-matrix) elements. We have applied this approach to the $5\sigma$ photoionization of CO using the frozen-core Hartree-Fock and fixed-nuclei approximations. We find that the Padé-approximant method converges rapidly and reliably. Both total photoionization cross sections and photoelectron angular distributions from threshold to 40 eV are presented and compared to previous experimental and theoretical results. We find major quantitative discrepancies between the present results for the total cross section and previous theoretical results.

The photoionization of an atom is examined in the presence of a foreign atom. We find that a two-photon radiative collision process induces large cross sections for electron continuum-continuum scattering. The enhancement is caused by an intensity-induced collisional shift which, when used to cancel out the collisional dephasing between the initial and final scattering states at wide range of internuclear separations, enhances the electron continuum-continuum scattering process.

Measurements are reported on the multiple-scattering widths for 1—4-MeV ${\mathrm{N}}^{+}$ projectiles in thin carbon targets as a function of the charge state of the outgoing ion. Evidence is found for a significant enhancement of the angular widths for $K$-vacancy—bearing emergent ions. A quantitative model is proposed demonstrating how the "memory" of $K$-vacancy—producing collisions can give rise to such an enhancement despite apparent charge-state equilibration.

The question concerning the existence of a long-lived doublet $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ state has been addressed by measuring the lifetime and energies of autodetaching electrons from 20—120-keV ${\mathrm{He}}^{-}$-ion beams formed by charge exchange in Ca vapor—a vapor conducive with $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ formation. The measured lifetime and electron energy levels agree closely with accepted values for components of the $\left({}_{}{}^4{}_{}{}^{}P_J^o{}_{}{}^{}\right){\mathrm{He}}^{-}$ states. No other structures were detected in the electron energy spectrum encompassing the anticipated energy level of the autodetached electron from the $\left({}_{}{}^2{}_{}{}^{}P_{}^e{}_{}{}^{}\right){\mathrm{He}}^{-}$ state.

The x-ray transitions ($n=2\mathrm{}\to n=1\mathrm{}$) emitted by fast hydrogenlike and heliumlike argon ions have been studied. The absolute energy of the Lyman $\alpha$ lines of hydrogenlike ions has been measured and the value of the ($1s$) Lamb shift of argon evaluated for the first time. The ${}_{}{}^3{}_{}{}^{}P_{1,2}^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ transitions of heliumlike argon have also been studied at very high precision and their energies compared to multiconfiguration Dirac-Fock calculations. The energies of Lyman $\alpha$ lines are of 3323.2±0.5 eV ($\mathrm{Ly}{\alpha }_1$) and 3318.1±0.5 eV ($\mathrm{Ly}{\alpha }_2$), and those of $n=2\mathrm{}\to n=1\mathrm{}$ transitions for heliumlike argon are of 3123.6±0.25 eV (${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$), 3126.4±0.4 eV (${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$), and 3139.6±0.25 eV (${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$).

By using saddle-point technique, 12 autoionizing and two metastable autoionizing states of B III are calculated. To obtain high accuracy relativistic effects and mass polarization effect are included in the computation. Spin-orbit, spin—other-orbit, and spin-spin effects are also considered. Using the B IV $\left(1\mathrm{}{s}^2\right){}_{}{}^1{}_{}{}^{}S$ result of Pekeris the corresponding Auger energies are tabulated. These data are then used to recalibrate the reported boron electron spectra of Rødbro, Bruch, and Bisgaard by about 0.15 eV. The calculated energies enable us to make seven new identifications in the boron Auger spectra. The fine-structure calculation also reveals many inverted $J$ levels among these core-excited states.

For a hydrogen atom in a uniform magnetic field in the range $B\approx 2.35\times {10}^8-2.35\mathrm{}\times {10}^9$ G we have calculated the energies and widths of resonant states which can autoionize by decay of a Landau excitation. The autoionization widths are quite large with some lifetimes as short as ${10}^{-15\mathrm{}}$ sec. The widths decrease with increasing azimuthal quantum number $\mathrm{}\left|m\right|\mathrm{}$, hydrogenic quantum number, or magnetic field strength. With decreasing field strength the lower resonances approach the ionization threshold which they cross with finite width and a pseudoresonance structure of finite width persists below threshold in the quantum defects of the bound states. Because of the finite widths, there are no approximate level crossings as these resonances cross the ionization threshold.

We describe a new coaxial laser-ion-beam photodetachment spectrometer designed to measure high-resolution photodetachment spectra of negative ions, and the measurement of ${\mathrm{C}}_2^{-}$ autodetachment rates with it. The apparatus resolution of 10 MHz is sufficient to resolve linewidth and line shape of autodetaching resonances in ${\mathrm{C}}_2^{-}$. Autodetachment rates of the $B{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state have been measured for a variety of rotational levels with $v^{\prime }=6, 7, 8, \mathrm{and} 9$. The observed rates vary between ${10}^7$ and ${10}^{10}$ ${\mathrm{s}}^{-1\mathrm{}}$, depending on vibrational and rotational state. Autodetachment of high vibrational levels ($v^{\prime }=19\mathrm{}$) of the $A{}_{}{}^2{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state has been seen to be slower than ${10}^8$ ${\mathrm{s}}^{-1\mathrm{}}$. The physical processes giving rise to these results are discussed in some detail.

Momentary depletion of the ${\mathrm{Xe}}_2^{+}$ (or ${\mathrm{Kr}}_2^{+}$) $1{\left(\frac{1}{2}\right)}_u$ population is observed by photodissociating the molecule at 351.1, 337.1, or 307.9 nm [on the $1{\left(\frac{1}{2}\right)}_u\to 2{\left(\frac{1}{2}\right)}_g$ band] and monitoring the temporally resolved emission from the dissociative-recombination-fed neutral Xe line at 828.0 nm ($6p{\left[\frac{1}{2}\right]}_0\to 6s{\left[\frac{3}{2}\right]}_1$). Near total suppression (>90%) of the Xe ($6p\to 6s$) fluorescence is obtained for laser intensities of 50 MW ${\mathrm{cm}}^{-2\mathrm{}}$. Measurements of the absolute electron density confirm that this effect is due to depletion of the dimer-ion concentration rather than photoionization of the $6p{\left[\frac{1}{2}\right]}_0$ state. However, photoionization of the Xe ($6s^{\prime }$ or $6p$) levels is observed and the cross section is measured to be 2.3×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at 351 nm. These experiments demonstrate a means for studying the collisional kinetics of rare-gas dimer ions in real time and in the presence of large background gas pressures.

The spectrum of zinc was photographed in the region 500-2000 Å on 6.65- and 10.7-m normal-incidence spectrographs. The sources used were a triggered spark and a sliding spark. The new observations have led to the establishment of all 38 levels of the $3d^{7}4s$ configuration and 17 unknown levels of the $3d^{7}4p$ configuration of Zn V. The Hartree-Fock and least-squares calculations support the analysis. Four hundred and forty seven additional lines have been classified in this spectrum.

The radiation emitted by the radiative charge transfer ${\mathrm{He}}^{+}$+Ne→He+${\mathrm{Ne}}^{+}$+$h\nu$ has been observed with the use of a selected-ion drift apparatus in conjunction with spectroscopic instrumentation. The two observed emission features with wavelengths around 4100 and 4240 Å evidently arise from transitions in the short-lived He${\mathrm{Ne}}^{+}$ excited molecular ion which is formed in the collision. Mass-spectrometric observations of the product ions indicated that the reaction occurred with a rate coefficient of 1±0.3×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^3$/sec at a gas temperature of 300 K. In addition to charge transfer, a second associative reaction channel producing He${\mathrm{Ne}}^{+}$ ions is found to have an effective rate coefficient of about 2×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^3$/sec.

Collisional relaxation and transfer of coherence following sudden excitation of a multiplet of the type $(j_1,j_2;j)=(1,\frac{1}{2};\frac{3}{2} or \frac{1}{2}$) is analyzed theoretically in an attempt to predict the fluorescent intensity as a function of time following excitation. Resonant, pulsed, circularly polarized exciting radiation is considered, with the subsequent fluorescence in each of the three orthogonal optical polarizations being studied. In this type of multiplet the intensity operators form a number of different irreducible tensors, some of which are degenerate with respect to spherical tensor order. These are collisionally mixed, with the result that collisional transfer, as well as relaxation, of coherence occurs, together with a resulting transfer of photobeats between different optical polarizations in fluorescence. As an example, a novel and somewhat surprising result is the prediction of a photobeat collisionally transferred from initially excited ${\sigma }_{-}$ fluorescent polarization to ${\sigma }_{+}$ fluorescence. With depth of oscillation as large as a few percent, it exists only at higher pressures, where its amplitude varies as the inverse of pressure. Its lifetime is relatively very long, being proportional to pressure, while the photobeat frequency is twice that associated with the energy separation of the excited multiplet.

The quantal theory of field-free associative ionization and Penning ionization, due to O'Malley, Bieniek, and others, is generalized to include the closely analogous laser-induced chemi-ionization processes $A+B+\hslash \omega \to A{B}^{+}{e}^{-}$, $A+B^{+}+e^{-}$. The principal limitations of the theory are its reliance upon the two-state approximation and its neglect of Born-Oppenheimer terms. Formulas are derived for several scattering cross sections descriptive of the energy spectra and angular distributions of the reaction products. These cross sections vary with the direction of polarization of the laser radiation. The polarization dependences of the heavy-particle ($A+B^{+}$) energy-angle double-differential cross section and of the associated heavy-particle (or photoelectron) energy distribution assume simple cosinesquared forms provided that there is a large number of contributing partial waves. Numerical calculations of these cross sections should be no more difficult to perform than for comparable field-free processes.

High-resolution electron-beam-excited $M_{4,5}{N}_{4,5}{N}_{4,5}$ Auger-electron spectra of Cs have been measured and analyzed both from atomic Cs and molecular CsI vapors. The observed main fine structure is reproduced fairly well with the mixed-coupling calculations based on the closed-shell approximation. Some extra structure exists, which is stronger in the spectrum of molecular CsI than in the spectrum of atomic Cs. This structure is investigated by carrying out Dirac-Fock calculations for the energies of the Auger transitions locating the outermost electron either on the $6s$, $5d$, or $4f$ orbital.

With the help of Ge(Li) detectors, elastic and Compton scattering cross sections of copper, tin, and lead were determined in the double-shadow-cone geometry from 4.51° to 12.05° in the case of 1.1732- and 1.3325-MeV $\gamma$ rays of ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$. A new method was developed for the accurate measurement of cross sections. The measured elastic scattering cross sections are determined predominantly by Rayleigh scattering from bound atomic electrons. The nuclear Thomson and the Delbrück scattering amplitudes have also been considered in the interpretation of data. The relativistic modified form-factor theory of Rayleigh scattering leads to good agreement with the data for $x<\mathrm{}12\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$, where $x=\frac{sin\left(\frac{\theta }{2}\right)}{\lambda }$, and where $\theta$ is the scattering angle and $\lambda$ the photon wavelength. The measured Compton scattering cross sections for $x>\mathrm{}4.5\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$ are in agreement with those calculated on the basis of tabulated nonrelativistic incoherent scattering functions, although there are deviations at smaller $x$ values.