Electrons trapped by their image potential on the surface of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ can escape into the gas by thermal ionization. I have calculated the rate of ionization, assuming that the electron motion parallel to the liquid surface is described by the collective motion of an electron crystal. The first electrons to leave the surface extract their ionization energy from the kinetic energy of those that remain behind. This adiabatic cooling effect rapidly lowers the electron temperature to a small fraction of the liquid temperature. The ionization rate is substantially less for this electron-crystal model than for the free-electron model of the electron motion parallel to the surface. The rate also depends on the density of surface electrons.

It is demonstrated that in a Penning collision of optically oriented metastable helium atoms with an impurity metal atom such as barium, both the longitudinal and transverse components of spin angular momentum are conserved. This results in a polarization of the ion which may be detected by optical emission in the case of excited ion levels or absorption for the ground states. A series of experiments is described which demonstrates the spin dependence of the Penning-collision process. The magnetic resonance of excited-state ions may be observed directly; from the resonance-linewidth radiative lifetimes may be measured.

It is now well established that current and density may be used as coordinates in describing a nonrelativistic many-body system. The current and density operators form a closed algebra as they obey a set of equal-time commutation relations. A particular choice of the algebra containing only the longitudinal component of the current describes a fluid in a simple way. For Bose systems, we show that there exists a special choice of this longitudinal current which is an irreducible representation of the algebra, but expressed in two different ways leading to the formulations of Bogoliubov and Zubarev (BZ) and that of Sunakawa and his coworkers (S). This demonstration proves the formal equivalence of the two formulations. The BZ formalism leads to a non-Hermitian Hamiltonian for which use of a certain mathematical technique recently proposed by us yields completely divergence-free results for Bose systems. A temperature-dependent matrix Green's-function theory is developed and the self-energy matrix calculated. From this, we also deduce the structure factor in a straightforward way. These results are applied here to compute numerically the excitation spectrum of the superfluid liquid helium using the experimental structure factor as the only input into the computation. Comparison of this with other calculations and with experiment is discussed in detail in this paper. The problem of a charged Bose gas is considered as a testing ground for all theories of Bose fluids since certain exact results are known for this system. We have applied our method using the BZ Hamiltonian to this problem and we are in complete agreement with the exact results. It thus appears that the current-algebra approach can be quite successfully applied in elucidating properties of interacting Bose systems.

The energy loss of a beam of cesium or lithium ions traversing a near-thermal equilibrium cesium plasma has been measured as a function of plasma density at ion-beam energies of 35-150 eV. The plasma electron-ion temperature ranged from 2100 to 2500 °K, and the charged-particle density ranged from (0.1 to 8.0) × ${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The measured energy loss is found to agree well with theoretical predictions.

The radiative correction to the decay $2S_{\frac{1}{2}}\to 1S_{\frac{1}{2}}+1\mathrm{}\gamma$ in hydrogenlike atoms is calculated to relative order $\alpha$. All terms of order $\alpha \ln \alpha$ and $\alpha$ are found to cancel. The cancellation of the order-$\alpha$ terms does not occur in other relativistic $M1\mathrm{}$ transitions in hydrogenlike atoms.

The frictional force derived from the stopping power of an electron gas is used in the classical equation of motion for the negative muons. We calculate the energy spectrum of the captured muons and the angular momentum distribution of muons at the energy of the electronic $K$ orbit. The resulting $P\left(l\right)\mathrm{}$ distribution closely resembles the statistical $2l+1$ distribution.

We have detected previously unobserved momentum-transfer distributions generated by rotationally inelastic molecular collisions of C${\mathrm{O}}_2$ and ${\mathrm{H}}_2$ involving only the 00${}_{}{}^0{}_{}{}^{}1$ state.

The paper of Penney, concerning the role of "heat-exchange" torques in the "$\lambda$-point anomaly" of Pellam, is examined. It is shown, using only the equations and assumptions of Penney's paper, that the heat-exchange torque reduces to simply a torque produced by the inertia of counterflow. His calculations of the sense and temperature dependence of the torque are shown to be in error. It is shown in the light of these results that little effect on the "$\lambda$-point anomaly" would be expected for an assumed approximately isotropic room-temperature radiation.

A phase transition in a system of interacting two-level atoms and an electric field can be explained semiclassically at least semiquantitatively. The optical Bloch equations for two-level atoms interacting through a longitudinal field are solved exactly to support the thermodynamic result.

Rydberg states of He of the configuration $\left(1s nl\right){}_{}{}^{1,3}{}_{}{}^{}D$, ${}_{}{}^{1,3}{}_{}{}^{}F$ for $n=3-7\mathrm{}$ have been calculated using polarized-orbital and related approximations, notably the extended-polarization approximation. The singlet states are superior in the full polarized-orbital approximation whereas the triplet cases are better in the extended-polarization approximation. Reasons for this behavior are given. Comparison is also made with recent diagrammatic calculations of Chang and Poe. A comment on some recent polarized-orbital calculations of photoionization is included.

It is shown that a quantized vortex ring in superfluid ${\mathrm{He}}^4$, moving freely towards a flat plate, gives to it an impulse equal to the fluid impulse of the ring, provided only that the radius of the plate is larger than the initial vortex radius but smaller than the distance to the source of the ring. This is in agreement with experiment, in contradiction to two previous theoretical analyses.

High-resolution (full width at half-maximum ∼ 3 eV) neon $K$ x-ray spectra obtained from 80-MeV argon-ion bombardment are presented for two narrow ranges of the projectile charge state ($q=\sim 6,\sim 14$). Production of the various satellite lines is found to depend on the projectile charge state. When the projectile is ${\mathrm{Ar}}^{14+}$, the target is at least 6 times ionized, and a significant fraction (> 20%) of the observed radiation comes from hydrogenlike and heliumlike ions. We comment on the reliability of vacancy-production cross sections deduced from measured x-ray intensities.

The contribution to the positronium ground-state hyperfine structure due to the fourth-order vacuum polarization correction to the annihilation diagram is computed. The updated theoretical value is one standard deviation above experiment.

The procedure for obtaining scattering phase shifts using a discrete Laguerre-type basis set developed by Heller, Rescigno, and Reinhardt is generalized so as to be applicable with different discrete bases. As an illustration, $s$- and $p$-wave phase shifts are computed for the static hydrogen potential using three-dimensional harmonic-oscillator wave functions.

Recent calculations of Thorson and co-workers for ${\mathrm{H}}^{+}$ + H ionization mechanisms and the Briggs-Macek scaling law for $K$-vacancy production in symmetric heavy-ion collisions are used to show the following: (1) If Fano-Lichten-type electron promotion is inhibited because of the absence of a $2p$ vacancy, i.e., if $Z>\mathrm{}10\mathrm{}$, $K$ vacancies in symmetric collisions are produced mainly by $2p\sigma$ electron excitation to vacant bound and unbound molecular-orbital states. (2) The direct $1s\sigma$ excitation cross section to vacant bound and unbound states can be obtained experimentally. The relationship between atomic and molecular-orbital Coulomb excitation cross sections is discussed.

Upper and lower variational bounds on the transition amplitudes of the impact-parameter model for ion-atom scattering were recently reported. A comparison of these bounds with those given by Spruch leads to an improved first-order variational bound on the second-order error term in the variational principle recently given. In addition, the second-order variational bound obtained by Spruch from an apparently different second-order error term also follows from this second-order error term. Finally, as Spruch has suggested, the second-order bound is simply given as one-half the first-order bound squared for those amplitudes corresponding to a rearrangement from state $n$ of one arrangement channel to state $n$ of another arrangement channel.