Atomic charge-transfer cross sections in the presence of a laser field are calculated using proton-hydrogenatom scattering as an example. Both strong and weak laser limits are obtained. An experiment is suggested to observe directly the modification of the differential cross section by the laser.

The relative optical excitation functions and polarizations of the Ba resonance line (5535 Å) and of the ${\mathrm{Ba}}^{+}$ resonance lines (4554 and 4934 Å) have been measured, using crossed electron and barium-atom beams. The $\mathrm{Ba} 6{}_{}{}^1{}_{}{}^{}P$ excitation function has been normalized to the Bethe theory in the high-energy limit, and the excitation functions of the ionic lines have been measured relative to that of the atomic resonance line. Using spontaneous-emission branching ratios for the ${\mathrm{Ba}}^{+}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}P\right)$ levels, we have also obtained normalized cross sections for these ionizing excitation levels. From a few electron volts above threshold, the ${\mathrm{Ba}}^{+}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}P\right)$ ionizing-excitation cross section has nearly the same energy dependence as has been reported for the total ${\mathrm{Ba}}^{+}$ ionization cross section, but about 1/6 the magnitude. In particular, a broad maximum at ∼22 eV, attributed to core-excited autoionizing levels, apparently contributes an equal fraction to both cross sections. This relationship holds also for the ionizing excitations versus total ionization of Sr. The cross sections for exciting the resonance levels of He, Mg, Ca, Sr, and Ba are also intercompared in reduced units and found to be strikingly similar at collision energies above ∼3 times the threshold energies.

Negative-ion formation cross sections have been determined for dissociative collisions of $\mathrm{H}_2^{}{}_{}{}^{+}$, $\mathrm{H}_3^{}{}_{}{}^{+}$, and $\mathrm{HD}_2^{}{}_{}{}^{+}$ in ${\mathrm{H}}_2$, He, and Xe in the energy region from 40 to 600 keV. In addition, the maximum and equilibrium ${\mathrm{H}}^{-}$ or ${\mathrm{D}}^{-}$ fractions for the three molecular ions in ${\mathrm{H}}_2$ gas were determined. For equivelocity incident $\mathrm{HD}_2^{}{}_{}{}^{+}$ and $\mathrm{H}_3^{}{}_{}{}^{+}$, ${\mathrm{D}}^{-}$ or ${\mathrm{H}}^{-}$ formation cross sections were the same. For $\mathrm{H}_2^{}{}_{}{}^{+}$ the ${\mathrm{H}}^{-}$ formation cross sections were smaller than those of the triatomic incident beams for velocities greater than 3.3 × ${10}^8$ cm/sec. Equilibrium fractions were independent of the incident particle at equal velocities and were 0.024 at 2 × ${10}^8$ cm/sec.

Cross sections for the processes ${\mathrm{H}}^{+}+\mathrm{H}\left(1\mathrm{}s\right)\to {\mathrm{H}}^{+}+{\mathrm{H}}^{*}$($n=2, 3, 4$) have been extracted from the energy-loss spectra of 15- to 200-keV protons by using a spectrum-fitting technique. Absolute cross sections have been obtained by normalization of the spectra to the theoretical cross section through the use of the Born approximation at 200 keV. The cross-section curves are very similar in shape with maxima at 60 keV. The results have been compared with available theoretical calculations. The $n=2$ and $n=3$ cross-section measurements are in very good agreement in curve shape with Glauber-approximation calculations.

Rate coefficients for the excitation of the 4.3-μm bands of ${\mathrm{CO}}_2$ by low-energy electrons in ${\mathrm{CO}}_2$ have been measured using a drift-tube technique. The ${\mathrm{CO}}_2$ density [(1.5 to 7) × ${10}^{17}$ molecules/${\mathrm{cm}}^3$] was chosen to maximize the radiation reaching the detector. Line-by-line transmission calculations were used to take into account the absorption of 4.3-μm radiation. A small fraction of the approximately ${10}^{-8\mathrm{}}$ W of the 4.3-μm radiation produced by the approximately ${10}^{-7\mathrm{}}$-A electron current was incident on an InSb photovoltaic detector. The detector calibration and absorption calculations were checked by measuring the readily calculated excitation coefficients for vibrational excitation of ${\mathrm{N}}_2$ containing a small concentration of ${\mathrm{CO}}_2$. For pure ${\mathrm{CO}}_2$ the number of molecules capable of emitting 4.3-μm radiation produced per cm of electron drift and per ${\mathrm{CO}}_2$ molecule varied from ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ at $\frac{E}{N}=6\mathrm{}\times {10}^{-17\mathrm{}}\mathrm{V} {\mathrm{cm}}^2 \mathrm{to} 5.4\times {10}^{-16\mathrm{}}{\mathrm{cm}}^{-2\mathrm{}}$ at $\frac{E}{N}=4\mathrm{}\times {10}^{-16\mathrm{}}$ V ${\mathrm{cm}}^2$. Here $E$ is the electric field and $N$ is total gas density. The excitation coefficients at lower $\frac{E}{N}$ are much larger than estimated previously. A set of vibrational excitation cross sections is obtained for ${\mathrm{CO}}_2$ which is consistent with the excitation coefficient data and with most of the published electron-beam data.

The final charge states of 0.15- to 1.20-MeV ${\mathrm{Xe}}^{+}$ ions scattered through angles from 1 to 20 deg by single collisions with Xe atoms have been measured. The data show the dependence of multiple $N$- and $M$-vacancy production upon the collision's distances of closest approach, which ranges from approximately 0.1 to 0.5 Å. The final charge states of those ions scattered to particular angles have average values which range from 1.5 to 16.0 in these collisions.

Distributions in energy and angle have been measured for the breakup products created when thin foils are bombarded by energetic beams of He${\mathrm{H}}^{+}$. The resolution in both energy and angle is greatly improved over that previously used in such studies. This permits the observation of several new features. The results can be explained in terms of the electric field experienced by the trailing ion (of the two produced in the molecular breakup) when it finds itself in the polarization "wake" created by its partner.

It is shown that, despite the double-scattering infinities of the differential scattering rates for three particles, the total scattering rates for three particles are unambiguous and finite. These rates are expressed in terms of $T$-matrix elements by means of a well-defined principal value of an integral whose integrand has a double pole. We also derive an expression for the double-scattering rate.

The velocity dependence of the Penning and associative ionization cross sections for the $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)+\mathrm{H}$ interacting system is measured for the relative velocity range 1900-5300 m/sec in a crossed-beam experiment by a time-of-flight technique. We observe that the total ionization cross section decreases monotonically with increasing kinetic energy, and that associative ionization is a minor process, contrary to some theoretical predictions made for the $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)+\mathrm{H}$ system.

Total collision cross-section measurements of 1.25- to 25-keV ${\mathrm{H}}^{+}$, ${\mathrm{H}}^0$, ${\mathrm{H}}^{-}$ and $\mathrm{H}_2^{}{}_{}{}^{+}$ in Xe are reported for the nine processes ${\mathrm{H}}^{+}$ ⇆ ${\mathrm{H}}^0$, ${\mathrm{H}}^{+}$ ⇆ ${\mathrm{H}}^{-}$, ${\mathrm{H}}^0$ ⇆ ${\mathrm{H}}^{-}$, and $\mathrm{H}_2^{}{}_{}{}^{+}$ → [${\mathrm{H}}^{+}$, ${\mathrm{H}}^{-}$, ($\mathrm{H}_2^{}{}_{}{}^0$ and ${\mathrm{H}}^0$)]. Single- and double-electron-stripping cross sections for which the Xe atom does not change charge are deduced by combining present and past results. Comparisons are made with available experimental data and with theoretical values.

Cross sections for ${\mathrm{H}}_2$ production (nondissociative charge transfer) from 5- to 30-keV ${\mathrm{H}}_2^{+}$ collisions with xenon are reported. The measurements were made using a thin-window proportional counter as a particle detector. Using these and other available measurements, we determined the cross section for total dissociation and that for H-atom production. We show that ${\mathrm{H}}_2^{+}$ → H + H is the dominant dissociation process.

The mobility of molecular xenon ions in xenon has been measured in the temperature range 210 to 293 K at neutral-gas densities ranging from 2 × ${10}^{16}$ to 2 × ${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The mobility was observed to be pressure dependent. Evidence is given to show that this pressure dependence arises from the equilibrium reaction ${\mathrm{Xe}}_2^{+}$ + 2Xe⇄ ${\mathrm{Xe}}_3^{+}$ + Xe. The equilibrium constant for this reaction and the zero-field mobilities of ${\mathrm{Xe}}_2^{+}$ and ${\mathrm{Xe}}_3^{+}$ are obtained from an analysis of the data. The heat of dissociation of ${\mathrm{Xe}}_3^{+}$ into ${\mathrm{Xe}}_2^{+}$ + Xe is estimated to be 272 ± 20 meV. A negative temperature and field dependence of the mobility is observed for ${\mathrm{Xe}}_2^{+}$, while that of ${\mathrm{Xe}}_3^{+}$ shows a positive temperature dependence.

Absolute cross sections for charge transfer into several excited levels of sodium have been determined over the energy range 0.1-1 keV. The energy dependence of all cross sections appears to follow the form $\sigma (v,\Delta E)=K{e}^{\frac{-\Delta \mathrm{Ea}}{\hslash v}}$, where $v$ is the ion velocity and $\Delta E(n,l)$ is the "energy defect" for charge transfer into $n$, $l$ excited levels of sodium. This experiment is the first test of the adiabatic hypothesis in which $v$ and $\Delta E$ can be treated in the above expression essentially as independent variables. We find that at constant $v$, relative magnitudes of electron-capture cross sections are governed by the energy defect without regard to principal quantum number or angular momentum of the final state. This functional dependence is discussed in terms of a semiclassical coupled harmonic oscillator model.

Kinetic-energy spectra of long-lived high-Rydberg nitrogen atoms produced by electron impact dissociative excitation were measured using a time-of-flight apparatus. Four principal features were found in the fragment kinetic-energy spectra with peaks at 0.6, 2.4, 3.4, and 6.7 eV. The excitation function exhibited a sharp resonance-like behavior near a threshold of 24.1 eV, corresponding to fragment production of $\mathrm{N}\left({}_{}{}^4{}_{}{}^{}S\right)+{\mathrm{N}}^{**}$, where N** is a high-Rydberg atom with an energy near the ionization limit of N(14.54 eV). The observed features are discussed in terms of the core-ion model of high-Rydberg dissociation. The data are compared with other dissociative excitation and dissociative ionization work. The maximum differentil cross section for the production of energetic long-lived high-Rydberg N atoms was found to be $\frac{d\sigma }{d\Omega }\simeq 9\times {10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{sr}}^{-1\mathrm{}}$ within a factor of 2 at an electron energy of 180 eV and at an angle of 90° with respect to the electron beam. Geophysical implications of these measurements concerning heating of the atmosphere, radiative transfer of resonance radiation, and chemical reactions are mentioned briefly.

With the aim to investigate systematically diabatic correlations, a series of adiabatic molecular-orbital correlation diagrams for quasimolecular systems ranging from Ne + Ne to Xe + Xe is generated from the variable-screening model and subsequently analyzed. The novel feature of the diagrams for both symmetric and asymmetric systems is the appearance of numerous pseudocrossings which hitherto have not been incorporated into the standard qualitative diabatic correlation diagrams. An analysis of these additional crossings leads to the formulation of an empirical, new rule for diabatic correlations, which is stated in two equivalent forms and which should, under certain conditions, replace the rule given by Barat and Lichten. By a study of perturbations which screening effects impose on a Coulombic quasimolecular system, the conditions of validity of the "old" and the "new" rule can be qualitatively clarified. Experimental consequences of the new rule are briefly discussed.

Starting with Bohr's semiclassical stopping-power equation, a set of stopping-power equations has been deduced for both partially and completely stripped heavy ions. Each of these equations is valid in a particular ion-energy region depending upon the ion-medium combination. A simple evaluation of $\int E^{}dE{{\left(\frac{dE}{dx}\right)}_E}^{-1\mathrm{}}$ with the values of ${\left(\frac{dE}{dx}\right)}_E$ from the appropriate stopping-power equation valid in a particular ion-energy region gives the total range of a heavy ion. We have compared the available energy-loss data from other workers with the corresponding calculated values for a variety of heavy ions in different media ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$, and ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ ions in Al; ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ ions in Al and Au; ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ ions in Al; ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ions in Ar and ${\mathrm{N}}_2$; ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ ions in Ar and ${\mathrm{N}}_2$; ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ions in Si; and ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ in oxygen). In general the agreement is very good.

A modified Sternheimer method is used to provide analytic, polynomial solutions to the equations of perturbation theory for multiphoton absorption by hydrogenic ions. The solutions are in the form of asymptotic series whose coefficients can be rapidly computed by means of two-term recursion formulas. These series eventually lead to divergent solutions as a function of photon energy; however, calculations show that accurate two-photon cross sections can be obtained for the entire excitation spectrum and for the ionization continuum to about 1250 Å. Three-photon ionization cross sections are also calculated and these are shown to be accurate to about 2400 Å.

Doppler-free population inversion induced by coherent two-photon resonance is calculated analytically for a two-photon analog of the hyperbolic-secant pulse and numerically for a Gaussian, linearly chirped pulse. The numerical calculations are carried out for a two-photon resonance of a chirped ruby laser line with the $6S_{\frac{1}{2}}\to 9D_{\frac{3}{2}}$ transition in Cs. Complete inversion is found when the fractional change in frequency during the pulse lies between 6 × ${10}^{-6\mathrm{}}$ and 2 × ${10}^{-5\mathrm{}}$. The coherent two-photon resonances are described by optical Bloch equations which are derived from a multiple-time-scale perturbation theory.

The radiative lifetimes of the 17992-,25068-,28857-, 37414-, 40564-, and 44017-${\mathrm{cm}}^{-1\mathrm{}}$ neutral levels and the 30392-${\mathrm{cm}}^{-1\mathrm{}}$ ion level of Yb have been measured by the Hanle method in a fast-flowing He system. The lifetimes (in units of ${10}^{-9\mathrm{}}$ sec) were found to be 820(20), 5.12(0.12), 14.4(0.4), 77.4(6.0), 9.32(0.6), 39.1(3.5), and 5.8(0.6), respectively. In a flowing system with He as a buffer gas the alignment depolarization cross sections with Yb were obtained and are reported here for the first time. They are (in units of ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$) 5.0(1.0), 5.9(1.0), 5.9(1.2), 17.9(2.0), 28.2(3.0), 5.16(4.0), and 7.2(2.5), respectively.

Using two- and three-photon resonance techniques, we have observed $d-g$ and $d-h$ transitions in the $n=13\mathrm{} \mathrm{to} 17$ states of sodium. The results yield accurate values for the quantum defects of the $d$, $f$, $g$, and $h$ states as well as the fine-structure splitting of the levels. Although the $d \mathrm{}(l=2\mathrm{})\mathrm{}$ fine structure is inverted, for $l>\mathrm{}2\mathrm{}$ the fine structures are observed to be hydrogenic.