The higher-order relativistic contributions to the Zeeman effect in triplet states in helium and heliumlike ions have been calculated to order ${\alpha }^3{\mu }_{B}H$.

We have calculated electron screening corrections in muonic Ba and Pb using true relativistic Hartree-Fock electron densities. We find that the error due to the usual local Slater approximation to the exchange potential is small, on the order of 1-2% of the total screening correction in those cases where the screening is significant.

We give the exact matrix elements (i.e., with retardation taken into account exactly) for the electromagnetic interaction for relativistic hydrogenic atoms in terms of integrals of the atomic radial functions. The present paper represents an extension of earlier results for the nonrelativistic case. The matrix elements can be used to obtain the exact selection rules and transition probabilities per unit time for the emission of a photon.

Direct precession measurements of the muonium atomic system have been made in argon gas at low pressures of 1290 and 2280 Torr. At the higher pressure the probability of muonium formation was found to be (85 ± 9)%. This lower limit for the rate of formation is in agreement with that predicted by Firsov and Byakov on the basis of muonium energy thresholds.

The conservation of orbital angular momentum has the following consequence for Scott-effect experiments: The torque exerted on the outer cylinder is equal in size but opposite in direction to the torque on the inner cylinder. This is true for each of the two contributions to the torque, namely, the bulk and slip contribution.

We calculate binding energies for heliumlike ions of variable dimensionality ($D$) with the wave functions $A=e^{(-\alpha R_1-\beta R_2)}+e^{(-\beta R_1-\alpha R_2)}$ and $B=A(1+c{R}_{12})$. The binding energy decreases with increasing $D$. Functions $A$ and $B$ predict "critical binding dimensionalities" at $D=3.99\mathrm{} \mathrm{and} 4.89$, respectively, above which there is no binding in the hydride anion. The exact ground-state binding energy at $D=5\mathrm{}$ is shown to be equal to that of the doubly excited $2p^2$ ${}_{}{}^3{}_{}{}^{}P_{}^e{}_{}{}^{}$ state into in three dimensions. By "dimensionality scaling" of atomic units the $D=1\mathrm{}$ atom is transformed the $\delta$-function model for which exact energies are known. In the infinite dimensional limit, function $A$ predicts no exchange contribution to binding for nuclear charge $Z\ge \surd 2$, with $\alpha \ne \beta$ only for $Z<\mathrm{}\surd 2$.

An absorption-cell technique is utilized to determine the rate constants for the C${\mathrm{O}}_2$ ${02}^0$0-${10}^0$0 relaxation in C${\mathrm{O}}_2$ and binary-gas mixtures of C${\mathrm{O}}_2$-He and C${\mathrm{O}}_2$-${\mathrm{N}}_2$. The experiment consists of injecting a ∼2-nsec 10.4-μm pulse through the absorption cell—saturating the C${\mathrm{O}}_2$ ${10}^0$0 to ${00}^0$1 transition, and monitoring the subsequent molecular re-equilibrations by recording the temporal intensity of a spatially overlapping probe beam in the 9.4-μm band connecting the ${02}^0$0 and ${00}^0$1 states. The measured rate constants are $k_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}{\mathrm{O}}_2}=3\mathrm{}\pm 1$, $k_{\mathrm{C}{\mathrm{O}}_2-\mathrm{He}}=0.8\mathrm{}\pm 0.3$, and $k_{\mathrm{C}{\mathrm{O}}_2-{\mathrm{N}}_2}=3\mathrm{}\pm 1$, all in units of ${10}^5$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{Torr}}^{-1\mathrm{}}$. The experimental approach also yields information on the thermalization of other processes having slower rates, and these are discussed qualitatively. Finally, a comparison is made with other related experimental investigations and two calculations which predict values for $k_{\mathrm{C}{\mathrm{O}}_2-\mathrm{C}{\mathrm{O}}_2}$ appreciably different from those measured.

The temperature dependence of dc electric-field-induced optical second-harmonic generation has been measured for C${\mathrm{H}}_4$, C${\mathrm{H}}_3$F, C${\mathrm{H}}_2$${\mathrm{F}}_2$, CH${\mathrm{F}}_3$, C${\mathrm{F}}_4$, CCl${\mathrm{F}}_3$, and CBr${\mathrm{F}}_3$. For each molecule, experiments yield the second-order polarizability $\overline{\chi }(-2\mathrm{}\omega ; \omega , \omega )$ and the third-order polarizability $\overline{\chi }(-2\mathrm{}\omega ; 0, \omega , \omega )$. Both independent nonzero spatial components of each are measured. The bond-additivity model is shown to work well (±15%) for third-order polarizabilities but poorly (discrepancies of about a factor of two) for second-order polarizabilities.

The exact Green's function for a harmonic oscillator interacting with an oscillatory external electric field in the dipole approximation is used to evaluate the transition probabilities in closed form. The case of a particle in crossed electric and magnetic fields is also considered.

The $L_2-L_{3}X$ Coster-Kronig transition probability for $Z=63, 94, \mathrm{and} 96$ has been measured by the $L$-x-ray—$K$-x-ray coincidence technique. The value measured in ${}_{63}{}^{}{}_{}{}^{}\mathrm{Eu}$ was corrected for the presence of an unresolved $L\eta$ x-ray component in the $L\alpha$ x-ray group. The values reported are for ${}_{63}{}^{}{}_{}{}^{}\mathrm{Eu}$, $f_{23}=0.172\mathrm{}\pm 0.015$, for ${}_{94}{}^{}{}_{}{}^{}\mathrm{Pu}$, $f_{23}=0.233\mathrm{}\pm 0.015$, and for ${}_{96}{}^{}{}_{}{}^{}\mathrm{Cm}$, $f_{23}=0.226\mathrm{}\pm 0.017$.

Angular correlations between $K$ and $L$ x rays in Ta obtained from the electron-capture decay of ${}_{}{}^{181}{}_{}{}^{}\mathrm{W}$ were measured using two different experimental systems. High-resolution Si(Li) x-ray detectors were used to detect $L$ x rays. $K$ x rays were detected with a high-resolution Ge(Li) spectrometer in one experiment and a NaI(T1) crystal in the other experiment. The adopted values for the $K{\alpha }_1-Ll$ $K{\alpha }_1-L{\alpha }_{1+2}$, and $K{\alpha }_1-L\beta$ cascades are $A_{22}=(24\mathrm{}\pm 3)\times {10}^{-2\mathrm{}}$, $A_{22}=(3.2\mathrm{}\pm 0.2)\times {10}^{-2\mathrm{}}$, and $A_{22}=(3.4\mathrm{}\pm 0.9)\times {10}^{-2\mathrm{}}$, respectively. The results for the $K{\alpha }_1-L\alpha$ cascade agree with theoretical predictions but the results for the $K{\alpha }_1-Ll$ and $K{\alpha }_1-L\beta$ cascade are slightly lower than predicted.

The 115-750-nm radiation produced in collisions of 1.5-25-keV ${\mathrm{H}}^{+}$ and H with ${\mathrm{O}}_2$ has been studied in an atomic-beam experiment under thin-target conditions. For both ion and neutral impact, the ($b{}_{}{}^4{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}-a{}_{}{}^4{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$) first-negative ($1N$) and ($A{}_{}{}^2{}_{}{}^{}\Pi _u^{}{}_{}{}^{}-X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$) second-negative ($2N$) band systems of ${\mathrm{O}}_2^{+}$ are the most intense spectral features. Also observed are several emissions from excited states of O and ${\mathrm{O}}^{+}$. Relative cross sections for individual bands of the ${\mathrm{O}}_2^{+}$ $1N$ system, obtained by deconvolution of overlapped features in several vibrational sequences, were used to show that the vibrational population $P\left(v^{\prime }\right)$ for ${\mathrm{O}}_2^{+}$ $b{}_{}{}^4{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}$ formed in both ${\mathrm{H}}^{+}$ and H impact is in good agreement with the predictions of a simple Franck-Condon mechanism over the 2-25-keV energy range. Total cross sections for formation of ${\mathrm{O}}_2^{+}$ $b{}_{}{}^4{}_{}{}^{}\Sigma _g^{-}{}_{}{}^{}$ in ${\mathrm{H}}^{+}$ and H impact, determined from the latter results by normalization to previous proton-impact data, are in good agreement with values calculated via application of the semiempirical procedure developed recently by Edgar et al. For both projectiles the semiempirical approach gives a good prediction of the energy dependence and an adequate prediction of the magnitude of the cross sections for formation of ${\mathrm{O}}_2^{+}$ $A{}_{}{}^2{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ determined by integration of measured $2N$ band intensities.

The problem of extraction of cross-section information from thick-target x-ray yields is considered for the case of heavy projectiles having internal states. In particular, an analysis is carried out for the situation in which the projectile must carry an inner-shell vacancy into the collision in order to produce excitation of the target atom. It is found that the usual thick-target yield analysis, involving one derivative of the yield with respect to incident projectile energy, does not apply in this case. Explicit expressions are derived for obtaining cross-section information from the yield data.

The method of distribution among quantum states of exact classical energy transfer (the DECENT model) for vibrational excitation in molecular collisions, in which three-dimensional classical trajectories are used to evaluate the quantum vibrational transition probabilities, is applied to ${\mathrm{H}}^{+}$ + ${\mathrm{H}}_2$ and ${\mathrm{D}}^{+}$ + ${\mathrm{H}}_2$ collisions at relative kinetic energies between 35 and 1000 eV. We compare the results with the experimental data of Herrero and Doering. Taking into account the scattering-angle discrimination in the experimental measurements, we find good agreement with experiment for the shapes of the energy dependence of the calculated total cross sections for individual final vibrational states and for the cross-section ratios. The absolute magnitudes of the calculated vibrational-excitation cross sections at high energies are about a factor of 2 larger than the experimental values, probably because of electronically nonadiabatic behavior ignored in the calculation, but a systematic discrepancy exists at lower energies which cannot be completely reconciled with the available experimental information.

For comparison with our recent group-theoretical predictions of configuration-mixing coefficients, we report extensive configuration-interaction calculations for doubly excited states in the helium isoelectronic sequence below the $N=2\mathrm{}$ ($S, P, D$ states), $N=3\mathrm{}$ ($S, P, D, F, G$ states), $N=4\mathrm{}$ ($S, P, D$ states), and $N=5\mathrm{}$ (${}_{}{}^1{}_{}{}^{}P_{}^{\mathrm{o}}{}_{}{}^{}$ states) ionization thresholds. Two new quantum numbers label the Rydberg series, and are used to predict three "selection rules" for dipole excitation and autoionization processes which agree with experiment. New autoionization widths are reported for the helium states and confirm our group-theoretically predicted selection rules. Our width of 0.151 eV for the ${}_{}{}^1{}_{}{}^{}P_{}^{\mathrm{o}}{}_{}{}^{}$ state at 62.92 eV is in agreement with the recent experimental value 0.132 ± 0.014 eV. Calculations of helium energies and autoionization widths are strongly affected by "avoided crossings" of the doubly excited states as $Z$ is varied continuously. The new quantum numbers project out states in ${\mathrm{H}}^{-}$ which correspond to observed ${}_{}{}^1{}_{}{}^{}P_{}^{\mathrm{o}}{}_{}{}^{}$ shape resonances above the $N=2\mathrm{} \mathrm{and} 3$ ionization thresholds.

The production of electronically excited iodine atoms is observed by time-resolved atomic absorption spectroscopy following the dissociation of molecular iodine excited by a tunable dye laser to the $B{}_{}{}^3{}_{}{}^{}\Pi _{0_u^{+}}^{}{}_{}{}^{}$ state above and below the thermochemical dissociation limit. The quantum efficiency is independent of excitation wavelength above the dissociation limit. Substantial yields of $\mathrm{I}\left(5\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ are found for excitation as much as $4.5kT$ below the dissociation limit.

A measurement, carried out with a curved-crystal spectrometer, of the $K$ x rays of thulium emitted in the $K$ capture of ${}_{}{}^{169}{}_{}{}^{}\mathrm{Yb}$ has revealed the existence of a weak line which may be identified as the $K{\alpha }_1$ (Tm) hypersatellite line. The energy shift amounts to 1.04 ± 0.01 keV and the intensity of the hypersatellite line relative to the intensity of the $K{\alpha }_1$ (Tm) line is (7.6 ± 3.8) × ${10}^{-5\mathrm{}}$. The value of the energy shift agrees nicely with the shifts measured in other isotopes; the relative intensity is roughly in agreement with the value that may be expected on the basis of experimental data for other isotopes.

The static electric dipole polarizabilities of barium and strontium atoms have been measured using an electric deflection technique. The results are normalized to the polarizability of lithium by comparing deflections of the alkaline-earth beams to deflections of a lithium beam passing through an inhomogeneous electrostatic field. The measured polarizabilities are (39.7 ± 3.2) × ${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^3$ for barium and (27.6 ± 2.2) × ${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^3$ for strontium.

Many-body perturbation theory (MBPT) is applied to the electronic structure of the sodium hydride molecule using a basis set of Slater-type orbitals. Both a $V^N$ and a $V^{N-1\mathrm{}}$ potential are considered for determining the virtual orbitals, but these are shown to give nearly equivalent results when all pairlike diagrams are summed through third order in the MBPT expansion using shifted denominators. A value of -0.2544 hartrees is obtained for the correlation energy, which is ∼59% of the experimental value. A clear separation into intrashell and intershell correlation energy effects is observed.

The 12-50-MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ beam-induced $K$ and $L$ x rays of Ag and the $L$ x rays of Au from thin targets were studied with a Si(Li) detector. Absolute $K$ and $L$ x-ray production cross sections were derived from measured x-ray yields and compared to available theoretical calculations for inner-shell Coulomb ionization cross sections. Inclusion of Coulomb repulsion, binding, and ${\left(\frac{Z_1}{Z_2}\right)}^3$ effects in the plane-wave Born-approximation calculations gives good agreement with experimental values of ${\sigma }_{\mathrm{KX}}$ for Ag. The agreement between theory and experiment for $L$-shell ionization is not as good, possibly owing to the effects of multiple ionization on the fluorescence yield, although the energy dependence of ${\sigma }_{\mathrm{LX}}$ for Au is well reproduced. The energies of the $\mathrm{Ag} K \alpha$ and $K \beta$ and $\mathrm{Au} L \alpha$, $L \beta$, and $L \gamma$ lines were observed to change with beam energy as did the intensity ratios of these lines. HFS calculations for the $\mathrm{Ag} K \alpha$ and $K \beta$ lines indicate that the observed shifts are probably associated with multiple $M$-, not $L$-, shell ionization.