Cross sections were measured for the production of electrons with kinetic energies from 50 to 150 keV by heavy-ion-atom collisions. Beams of 40- and 70-MeV nickel and 70- and 120-MeV bromine ions were incident on targets of carbon, nickel, potassium bromide, silver, and gold. Electron energies were measured using a steering electron spectrometer capable of 7% transmission and 4-keV resolution. The observed cross sections exceed the plane-wave-Born-approximation predictions by factors of 2 to 10 with deviation becoming more pronounced at high electron energy.

The authors have used accurate theoretical values for the energies of low-lying states of the isoelectronic series of helium, together with $\frac{1}{Z}$-expansion theory, to compute to six-figure accuracy some of the spectral lines to be expected if a quark were to be embedded in the nucleus of a helium atom. These wavelengths may be of interest to searches for quark-He lines in laboratory or astrophysical spectra or for single-atom detection of such quark atoms.

The authors demonstrate that, for excited-core Rydberg states of high angular momentum $l$, a simple relationship exists between the autoionization rate and the quantum defect of the corresponding ground-core state. For many cases, both quantities can be expressed in terms of the dipole polarizability of the core. The autoionization rates of the excited-core Sr ($5\mathrm{pnl}$) states are calculated as an example, and compared to existing experimental data.

Relative cross sections for electron-impact ionization of the rare-gas Van der Waals dimers ${\mathrm{Ar}}_2$, ArKr, ${\mathrm{Kr}}_2$, KrXe, and ${\mathrm{Xe}}_2$ have been measured with a mass spectrometer in the electron energy range from threshold to 180 eV. The following appearance potentials for direct ionization from the corresponding neutral dimers have been found: ${\mathrm{Ar}}_2^{+}$: 15.2±0.2 eV, Ar${\mathrm{Kr}}^{+}$: 14.0±0.2 eV, ${\mathrm{Kr}}_2^{+}$: 13.45±0.3 eV, Kr${\mathrm{Xe}}^{+}$: 12.2±0.2 eV, and ${\mathrm{Xe}}_2^{+}$: 11.75±0.3 eV. These values agree with those obtained in recent photoelectron studies. The appearance potential of ${\mathrm{Ar}}_2^{+}$ formation via associative ionization of argon atoms is found to be 14.6±0.2 eV.

The authors have measured the collisional depopulation cross sections of Na $s$ states from $n=6\mathrm{} \mathrm{to} 11$. The maximum cross sections observed for He, Ar, and Xe, respectively, are ∼20, 20, and 100 ${\mathrm{\AA }}^2$. The cross sections rise from ∼0.1 ${\mathrm{\AA }}^2$ at $n\sim 6$ to a plateau at $n\sim 10$. The small size of the cross sections suggests that the process proceeds via the interaction of the ${\mathrm{Na}}^{+}$ core with the rare-gas atom. Specifically, this short-range interaction produces a transient dipole on the rare-gas atom which efficiently couples Na states of different $n$. This is an entirely different mechanism than that responsible for the collisional $l$ mixing previously reported.

Measurements of the target-thickness dependences of the target $K$ x-ray production yields are reported for 1.7-MeV/amu ${\mathrm{F}}^{q+}$ ($q=6, 8, \mathrm{and} 9$) ions incident upon thin solid targets of elements from Ti to Co. Target $K$ x-ray production cross sections were extracted in the limit of vanishing target thickness. Comparisons of the data to theoretical estimates based upon combinations of direct ionization and electron-transfer processes are presented. Projectile $K$-vacancy production and quenching cross sections were obtained from the three-component model for the target $K$ x-ray data as a function of foil thickness. Comparisons of the projectile parameters to theoretical estimates based upon electron transfer, direct ionization, and excitation processes are presented.

The recently observed "bulge" effect in rotationally inelastic scattering is substantiated by the detection of its isotope shift in K-CO collisions. This effect is a classical rainbow-type phenomenon which has been seen in the energy transfer or recoil-velocity dependence of K-CO and K-${\mathrm{N}}_2$ scattering at large angles and collision energies of about 1 eV. It is caused by the nonspherical bulge of the repulsive equipotentials of the intermolecular force and may be useful as a probe to its anisotropy.

Expressions are derived using the semiclassical Coulomb approximation for probabilities for $2s-2p$ excitation during a collision between a target atom and a proton. Results of calculations are presented. For proton energies above 1 MeV the transition probabilities are less than ∼0.03. Unlike excitation from one atomic shell to another, our intrashell results are largely independent of the target atomic charge $Z_2$ and binding-energy difference $\Delta E_{2s2p}$. The shapes of our impact-parameter distributions, when scaled to the atomic radius, are similar for various targets. However, for a given target, the dependence of the $m=0\mathrm{}$ and $m=\pm 1$ amplitudes on both impact parameter $B$ and $\Delta {E/}_{2s2p}$ is quite different, indicating that the polarization of light emitted in subsequent $2p-1s$ transitions may vary with both $B$ and $\Delta E_{2s2p}$. Furthermore, near $B=0\mathrm{}$ the probabilities are sensitive to $\Delta E_{2s2p}$, which varies with the charge state of the target atom. Consequently, the authors suggest that dependences of collision cross sections and probabilities on the atomic charge state may be investigated by considering impact-parameter dependence or polarization fractions in $2s-2p$ atomic excitation by ion impact.

The cross sections for the formation of $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ metastable atoms in ${\mathrm{H}}^{+}$ collisions with Mg and Ba alkaline-earth-metal vapors have been measured over the energy range 1.5-90 keV. The absolute value of the cross sections has been obtained by normalizing the present results to the cross section for $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ formation in Ar gas at 30 keV. In Ba, the cross section reaches a maximum value of 5.6 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 3 keV. In Mg, the maximum value is 3.2 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ and occurs at 7 keV. The measured cross sections exhibit structure at high energies which is attributed to inner-shell capture. A comparison of the present results with previous $n=6\mathrm{}$ electron-capture data in Mg indicates that the $n^{-3\mathrm{}}$ capture law is not obeyed. An analysis based on data available for 16 different collisions between ${\mathrm{H}}^{+}$ ions and $n{s}^1$ and $n{s}^2$ targets shows that the scaling law for the velocity $v_m$ at which the single-electron-capture cross section is a maximum is not consistent with the Massey adiabatic criterion. Instead, the data are consistent with the relation $v_m\propto {\left|\Delta E_{\infty }\right|}^{\frac{1}{2}}$ where ${\left|\Delta E_{\infty }\right|}^{\frac{1}{2}}$ is the energy defect of the collision. The fraction of the neutral beam in the $2s$ state formed by collisions of ${\mathrm{H}}^{+}$ ions with thin targets of Mg and Ba is also reported. In Ba, the fraction is equal to 0.25 ± 0.05 at 2.0 keV and is rising at low energies. In Mg, the maximum value of the fraction is 0.13 ± 0.03 and occurs at ∼ 7 keV.

Time-of-flight spectra of $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ and $\mathrm{D}\mathrm{}\left(2S\right)\mathrm{}$ fragments from electron-bombardment-dissociated ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD have been observed. Structure in the kinetic-energy distributions below 1.3 eV is interpreted as arising from predissociation of bound vibrational levels. For ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ the primary predissociation process appears to be predissociation of the D state via the $B^{\prime }$ state. Predissociation structure for HD, although more prominent than for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$, is not as easily interpreted. Evidence for the direct dissociation of the $B^{\prime }$ state has also been obtained and the relative probabilities of direct dissociation and predissociation have been estimated.

An analytic atomic independent-particle model potential adjusted to experimental energy levels is used to generate wave functions for the excited states of Bei, Bii, Ciii, Niv, and Ov. Using these wave functions in conjunction with the Born approximation and the $\mathrm{LS}$-coupling scheme, the authors calculate optical oscillator strengths, generalized oscillator strengths, and total cross sections for excitations from the $2s^2\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ ground state. The results are compared with experimental ones and other calculations. Regularities and systematic trends along the isoelectronic sequence are discussed.

The semiquantal treatment of collision processes such as $\mathrm{H}\mathrm{}\left(\mathrm{nl}\right)+\mathrm{H}\left(n_0{l}_0\right)\to \mathrm{H}\left(n^{\prime }{l}^{\prime }\right)+{\mathrm{H}}^{+}+e$ is described. The associated cross sections display systematic trends in the distribution over final angular momentum states in keeping with those previously predicted for $e-\mathrm{H}\mathrm{}\left(\mathrm{nl}\right)\mathrm{}$ inelastic collisions. Deexcitation between close-neighboring levels $n$ and $n^{\prime }$ of the projectile (with simultaneous ionization of the target) is generally more rapid than the endothermic processes involving the reverse transition i$\stackrel{\mathrm{`}}{\mathrm{n}}$ the projectile and, as $n$ is increased, becomes quite competitive with those processes associated with no internal-energy change in the projectile, particularly at the lower impact energies. The cross sections, with appropriate statistical weights, for forward and reverse transitions in the projectile tend to the same high-energy limit.

The energy and angular distributions of electrons ejected from argon are reported for ionization by ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{2+}$ ions in the energy range 0.3-2.0 MeV. Absolute cross sections derived from these measurements illustrate the effect of screening and the contribution to the electron spectra of the projectile electron. Comparison of the cross sections to previous data for equivalent-velocity proton impact provides an indication of the region of applicability of $Z^2$ scaling for differential ionization cross sections. The significance of continuum charge transfer and projectile stripping are discussed with regard to their contribution to the ionization process.

Ionization collisions between two excited hydrogen atoms are investigated theoretically for the case of simultaneous deexcitation of one of the atoms. An improved impulse approximation is employed to evaluate the cross sections and to look into the state and collision-energy dependence of the cross section. A new interesting phenomenon is found in the behavior of the cross sections as a function of collision energy.

The long-range parts of the ($A$,$C{}_{}{}^1{}_{}{}^{}\Sigma _{u,g}^{+}{}_{}{}^{}$ and $a$,$c{}_{}{}^3{}_{}{}^{}\Sigma _{u,g}^{+}{}_{}{}^{}$) excimer potentials of the ${\mathrm{He}}_2$ molecule have been accurately determined from high-resolution differential-cross-section measurements in the relative kinetic-energy range from 18 to 140 meV. The barrier heights and internuclear distances of the intermediate maxima of the $A$ and the $a$ state have been obtained with an accuracy of better than 2-5 meV and 0.1 Å, respectively. The long-range parts of the singlet potentials from the ab initio calculation by Gubermann and Goddard are always 5-20 meV higher than the potential determined from our experiments, but the splitting of the potentials coincides with our results within experimental accuracy for $R\ge 3$ Å. The experimental setup, data evaluation, and the construction of the potential are described in detail. The complicated interference structure of the angular distribution is discussed with the help of the quantal deflection function. Total and excitation-transfer cross sections are calculated and compared to other experimental results, and the similarity to other exchange processes is pointed out.

The lifetime of the first vibrational state of the electronic ground state of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ has been measured to be 29.6±2.5 msec. Measurements were carried out using an electric resonance molecular-beam apparatus utilizing a velocity-selected beam. Estimates of this lifetime based on spectroscopic data and on Rittner's model are in fair agreement with the measured value. A remeasurement of the same lifetime for ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$F is consistent with an earlier measurement which disagrees with detailed calculations of Harrison and Fisher.

Relativistic calculations of radiationless transition rates to $L$-subshell vacancy states in selected atoms with $70\le Z\le 96$ have been performed. The Auger and Coster-Kronig transition probabilities are calculated from perturbation theory, assuming frozen orbitals, in the Dirac-Hartree-Slater approach. Transition rates, fluorescence yields, and Coster-Kronig yields are compared with nonrelativistic theoretical results and with experiment. Relativity is found to affect the $L$-subshell Auger widths by (10-25)% and individual transition rates to certain $j-j$ configurations by as much as 40% at $Z=80\mathrm{}$. The widths of $L_i$ vacancy states and the $L_2$ Coster-Kronig yields $f_{23}$ from these relativistic calculations agree much better with experiment than earlier nonrelativistic theoretical values.

The Rayleigh scattering cross section is measured with a short-duration pulse of laser light in different gases. It depends on the pulse duration, involving a time constant $\tau$, following the expression $\frac{S_{\exp }}{S_{\mathrm{theor}}}=\left(\frac{{\tau }_1}{\tau }\right)(1-e^{-\frac{\Delta t}{\tau }})$, where $S_{\exp }$ and $S_{\mathrm{theor}}$ are the experimental and the theoretical cross sections and ${\tau }_1$ is the classical time constant corresponding to the dipole damping. The time constant $\tau$ was determined with a ruby laser whose pulse half-width was varied between 6 and 200 nsec. Using a tunable dye laser and its nitrogen laser pump, the authors have found that $\tau$ is proportional to ${\lambda }^2$ (square of the wavelength). The variation of $\tau$ for gases of different molecular sizes shows that $\tau$ is proportional to their diameter $a$. The angular distribution of the scattered light has been determined and found to be favored in the forward direction. The constant $\tau$ is proportional to $sin\left(\frac{\theta }{2}\right)$ (where $\theta$ is the scattering angle), i.e., to the momentum transfer. By analogy with a corpuscular collision, dimensional considerations lead to the formula $\tau =0.85\mathrm{}{\left(\frac{\lambda }{{\lambda }_c}\right)}^2\left(\frac{a}{c}\right)sin\left(\frac{\theta }{2}\right)$ (${\lambda }_C$ is the Compton wavelength), which well describes the experimental results.

In this work a more sensitive, multiphoton technique has been introduced for the study of the electronic transitions of alkali-metal-inert-gas excimers. In situ detection of the ionization resulting from multiphoton excitation through resonant excimer states has been accomplished through the inclusion of a space-charge-limited diode in an absorption cell. By using a pulsed tunable dye laser with a linewidth of 0.01 nm, absorption signals from CsAr and CsKr excimers at levels of ${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ have been recorded with signal-to-noise ratios of 40. Relatively low cesium vapor pressures of 0.12 Torr and inert-gas densities ranging from 0.6 to 0.9 amagat were used to minimize interference from overlapping dimer bands. Dispersion curves of the photoionization of CsAr and CsKr over the 430- to 460-nm wavelength range showed single-photon resonances corresponding to the parts of the $6s\sigma \to 6d\sigma$ absorption bands leading to the production of bound $6d{\sigma }_{\frac{1}{2}}$ molecules. The experimental results obtained in this work correlated reasonably well with the available theoretical potential curves.

A simple approximate scheme for treating molecular hyperfine structure is developed by taking account of energy-level clusters. Unitary tableau and frame transformation techniques are reintroduced. Model Hamiltonians for $X{Y}_3$ and $X{Y}_4$ ($X$ spin-zero, $Y$ spin-½) molecules are developed and solved in cluster bases which are appropriate for highly excited rotational states. Two cases emerge: Case (1) for which hyperfine splittings are smaller than the "superfine" cluster splittings and case (2) for which superfine splittings are negligible or zero. The problem of correlating energy levels and states between cases (1) and (2) is solved. Since the $X{Y}_4$ problem in the elementary cluster bases reduces to (2×2) matrices at the worst, the physical interpretation of solutions is not difficult.