Collision-induced dipole transitions and collisional broadening in the wing of $\mathrm{Cs}\mathrm{}\left(5D\right)\mathrm{}$ due to mixing with $\mathrm{Cs}\mathrm{}\left(7S\right)\mathrm{}$ and $\mathrm{Cs}\mathrm{}\left(6P\right)\mathrm{}$ are studied using two-photon resonance ionization spectroscopy. From the structure of the line shape, we measured the van der Waals well of the ground state without performing a temperature study or using the calculated potential curves, and from the absolute yield we measured the induced-dipole oscillator strength as a function of the internuclear separations at large separations. The results are compared with theoretical calculations.

The infrared vibrational-rotational spectrum of the ${\mathrm{H}}_2$${\mathrm{D}}^{+}$ molecular ion has been observed. Nine resonances between 1800 and 2000 ${\mathrm{cm}}^{-1\mathrm{}}$ have been measured to a precision of 0.0005 ${\mathrm{cm}}^{-1\mathrm{}}$ or 0.3 ppm (at 68% confidence level). This triatomic molecular ion is the simplest asymmetric top in nature. The quantum numbers of the observed transitions have not yet been unambiguously identified. This species is of great astrophysical interest in view of its important role in the chemistry of molecular clouds in the interstellar medium.

Millimeter-wave radiation in the band from 53 to 73 GHz has been generated by means of a Smith-Purcell free-electron laser. Power levels of approximately 100 mW were observed with beam currents of 90 to 100 mA; the threshold currents ranged from 35 to 82 mA. The electronic tuning transconductance and the linewidth were also measured, yielding $\frac{\mathrm{df}}{\mathrm{dV}}=0.25\mathrm{}$ MHz/V and $\Delta f<~0.4$ MHz, respectively. The measured results agree well with predictions based on a linear theory.

We present a model for a Smith-Purcell free-electron laser, including nonlinear effects, that can be solved in closed form. The model is shown to obey the pendulum equation. The average electron efficiency calculated by means of this model is compared with that obtained from an exact numerical calculation and from a simpler "phase-space" model for a wide range of physical parameters.

Energy shifts of atomic inner-shell vacancy states caused by interaction with Auger and Coster-Kronig continua are considered. Numerical calculations for the $L$ shell show that $L_2$- and $L_3$-subshell-energy shifts due to this effect are negligible, while the $L_1$-level shift is substantial ($\sim -4\mathrm{}$ eV for $30<~Z<~47$). This [$2s$] vacancy-state shift is dominated by the interaction with $\mathrm{}\left[\mathrm{LM}\right]\mathrm{}\epsilon l$ Coster-Kronig continua and brings ab initio self-consistent-field calculations into satisfactory agreement with experiment, for atoms lighter than Sn. For atomic numbers above $Z\cong 50$, discrepancies remain that may be results of errors in quantum-electrodynamic corrections.

The ability of existing nonadjustable local-exchange approximations to represent interelectronic exchange is tested by comparing valence-electron energies and average radii in Rb and Ag as determined in two different approaches: a relativistic-model-potential approach and one employing the polarizable frozen-core relativistic Hartree-Fock method. The calculations are identical in these two approaches except for the treatment of exchange. Two approximations originally developed for electron-atom-scattering processes are found to be the most accurate. A new approximation based on the free-electron-gas model is also discussed.

We study the Fokker-Planck equation for the distribution in velocity of a test particle uniformly distributed through a weakly coupled classical fluid. The diffusion tensor, described first by Landau and by Chandrasekhar, is a complicated function of velocity. We discuss the manner in which the distribution function approaches, in time, its final Maxwellian form. The angle-averaged ($l=0\mathrm{}$) and the ($l=1\mathrm{}$) components are particularly interesting, the latter governing the autocorrelation function for velocity. Both relax as $t^a\exp (-t^b)$. Our analysis is based upon R. E. Langer's method of comparison equations. The two-dimensional case is remarkably like the three-dimensional.

The relativistic random-phase approximation has been used to obtain the oscillator strength for the resonance transition of mercury. Two calculations were performed; in one the $6s$ valence-electron correlation is included, and in the other, core polarization. The result obtained by including the core-polarization contribution of the $5d$ electrons is in agreement with experiment.

A variationally optimized effective-potential model for atoms and ions is applied to the study of energies and intensities of shake-up states accompanying inner-shell ionization of Ne, Kr, Ar, and Xe. It is found that the energies are well described by the theory, and that the relative intensities are in qualitative agreement with experiment.

Hartree-Fock (HF) theory is generalized so as to apply to nondeterminantal densities, while retaining the fundamental HF restriction to one-body dynamics. Time development in conventional time-dependent Hartree-Fock and its extension are identical to classical Hamiltonian dynamics.

An alternative to the Gilbert construction is presented. It is shown that for any nonnegative, normalized density an arbitrary number of orthonormal orbitals can be constructed with squares which sum, with minimal restrictions on the occupation numbers, to the given density. In three dimensions substantial freedom remains in the choice of orbitals within the basic scheme. The kinetic-energy density obtained includes the Weizsäcker term and another term which, in simple cases, is proportional to the cube of the density.

Momentum-space calculations exhibit two kinds of advantages over position space: First, the numerical solution of Hartree-Fock equation is feasible without expansion of the wave functions in a particular basis. Equations only exhibit one avoidable singularity even for the multicenter case. Several mathematical techniques are presented, including standard fast Fourier-transform (FFT) techniques and numerical calculation of the involved convolutions. Second, momentum representation contributes in an original way to a better understanding of several physical problems arising in quantum chemistry. The two-body density matrix involving the electronic correlation are examined in both position and momentum space. If an expansion in Gaussian functions is used, momentum space renders feasible the obtainment of a multidimensional fully correlated wave function, starting from the Hartree-Fock solution.

$K$-shell ionization probabilities were measured by the particle-x-ray coincidence technique for 7-MeV protons of 15 to 39 fm impact parameter on 24 targets from Cr to Bi. For 22 of these targets, total $K$-shell ionization cross sections are also reported. Additional $K$-shell ionization probabilities were measured for 7-, 10-, and 12-MeV protons on Mo, Cd, and Sn, at impact parameters down to 2 fm. While the total $K$-shell cross sections closely follow relativistic semiclassical (RSCA) calculations supplemented by minor binding-plus-polarization corrections, significant discrepancies of up to 70% occur for the $K$-shell ionization probabilities at small impact parameters, particularly for the targets from Cr to Zr. They can be traced back to the use of hydrogenic wave functions in the RSCA calculation, whereas nuclear recoil may account only for a few percent of the discrepancies. The data on both $K$-shell total cross sections and ionization probabilities exhibit the pattern characteristic for small distortions of the $K$-electron binding energy by the projectile charge: adiabatic binding for ${\xi }_K<1\mathrm{}$ and nonadiabatic polarization for ${\xi }_K>1\mathrm{}$. The perturbed-stationary-state approximation somewhat underestimates these distortion effects.

Using techniques similar to those previously employed, we apply the eikonal approximation to the evaluation of the cross section for electron capture from an arbitrary $\mathrm{nlm}$ shell of a hydrogenic target atom into an arbitrary $n^{\prime }{l}^{\prime }{m}^{\prime }$ of a fast hydrogenic projectile. The results are obtained in exact analytical closed form. Numerical results are presented for the case ${\mathrm{H}}^{+}+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}\to \mathrm{H}\left(n^{\prime }{l}^{\prime }{m}^{\prime }\right)+{\mathrm{H}}^{+}$ when $n^{\prime }=2\mathrm{} \mathrm{and} 3$. Comparison is made with the corresponding Oppenheimer-Brinkman-Kramers (OBK) results.

The scattering cross section of gaseous argon as a function of electron energy (2-300 meV) has been redetermined from the temperature dependence of the thermal electron mobility ${\mu }_{\mathrm{th}}$. The cross section at $\epsilon <10\mathrm{}$ meV is larger than previously reported. The threshold drift velocity above which electron heating occurs is $v_d^{\mathrm{thr}}\approx 100$ m/s in the low-density gas at 121 K; the ratio of $v_d^{\mathrm{thr}}$ to the speed of sound is $\frac{v_d^{\mathrm{thr}}}{c}\approx 0.5$, characteristic of energy loss by elastic collisions. In the dense gas at $\frac{n}{n_c}\gtrsim 0.2$, where $n_c$ is the critical density: (1) the value of $n{\mu }_{\mathrm{th}}$ increases; (2) the maximum in the plot of $\mu n$ against field strength $\frac{E}{n}$ shifts to lower $\frac{E}{n}$; (3) the temperature coefficient of ${\mu }_{\mathrm{th}}$ at constant density increases. (1) is due to the mutual screening of the attractive, long-range scattering interactions; (2) is due to (1) and the constant "saturation" drift velocity; (3) is due to quasilocalization of the electrons. Quasilocalization or enhanced scattering in the coexistence vapor and liquid is significant at $0.2\lesssim \frac{n}{n_c}\lesssim 1.6$, and maximizes near $\frac{n}{n_c}=0.6\mathrm{}$. Quasilocalization occurs to a smaller extent in argon than in xenon at the same $\frac{n}{n_c}$ and $\frac{T}{T_c}$. The low-energy wing of the Ramsauer-Townsend effect is obliterated by screening at $n\ge 1.0\times {10}^{22}$ molecule/${\mathrm{cm}}^3$ in both argon and xenon, which corresponds to $\frac{n}{n_c}=1.2\mathrm{}$ in the former and 2.0 in the latter. The maximum in ${\mu }_{\mathrm{th}}$ occurs at 1.2 × ${10}^{22}$ molecule/${\mathrm{cm}}^3$ in both liquids, corresponding to $\frac{n}{n_c}=1.5\mathrm{} \mathrm{and} 2.4$, respectively. The magnitude of the maximum in ${\mu }_{\mathrm{th}}$ is reasonably interpreted by the Lekner zero-scattering-length model, but it is not yet possible to explain quantitatively the density at which the maximum occurs. The drift velocities at high fields, $\frac{E}{n}>5\mathrm{}$ mTd ($5\times {10}^{-20\mathrm{}}$ V ${\mathrm{cm}}^2$/molecule) little affected by density up to $\frac{n}{n_c}=1.6\mathrm{}$ in argon. At higher density the drift velocities increase. Relatively large densities are required to affect the behavior at these fields because the scattering cross section in the vicinity of the Ramsauer-Townsend minimum is low (∼1 × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$).

Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with helium, neon, and argon atoms in the energy range from 15 to 800 eV using a beam-transmission technique. These measurements reveal a merging of the positron and electron cross-section curves for helium at energies above 200 eV while the available theories predict this merging to occur at considerably higher energies. For neon and argon the positron and electron total-cross-section curves are slowly approaching each other at the highest energies. The present experimental approach is analyzed with regard to the discrimination against small-angle forward elastic scattering, and estimates are made of other potential errors in the measured total cross sections. The present results are used to test the zero-energy sum rule, obtained from forward dispersion relations, and it is found that these data are consistent with prior measurements in that the sum rule is found to fail for electron scattering and to be valid for positron scattering.

A microwave-afterglow mass-spectrometer apparatus has been used to determine the dependence on electron temperature $T_e$ of the recombination coefficient of electrons with the dimer ion $\mathrm{N}_2^{}{}_{}{}^{+}$·${\mathrm{N}}_2$. We find $\alpha =(1.4\mathrm{}\pm 0.2)\times {10}^{-6\mathrm{}}{\left[T_e\frac{\left(\mathrm{K}\right)}{300}\right]}^{-0.41\mathrm{}}$ ${\mathrm{cm}}^3$/s over the range $300\le T_e\le 5600$ K, and $T_{+}=T_n=300\mathrm{}$ K. These results are quite similar to those found for the C${\mathrm{O}}^{+}$·CO dimer ion but quite different from the $T^{-1\mathrm{}}$ dependence inferred from previous measurements in nitrogen in which the ions were not mass identified.