The fractional absorption which can be obtained using a single-mode laser with an internal absorption cell is compared to the fractional absorption obtainable with the more conventional system of a laser with an external absorption cell.

Objections are presented to a recent explanation by Hey and Griem of the discrepancy between measured and calculated values for the center of $H_{\alpha }$. Their suggestion that the upper-lower state interference term occurring in the theoretical treatments would be reduced by an accurate consideration of Debye shielding is shown to be unjustified. It is further shown that if the model forming the basis of their arguments is adequately amended and correctly exploited, the well-known results of the unified and impact theories are recovered.

The improved finite-interaction volume model of Voslamber is shown to be inconclusive for the wavelength range where calculations are sensitive to the disputed inelastic interference terms. It is pointed out that calculations which account for perturber correlations without invoking such interaction volume will give additional contributions to all elastic scattering terms. These may have a comparable effect to the inelastic interference terms in the wavelength region near line center.

Using a 38-MeV F beam for vacancy production, we find minimal dependence of the Al $K\alpha$ satellite structure on the total valence electron densities of several different solid environments.

A recent criticism of Henneberger's application of the Kramers transformation suggests that the ionization potentials predicted by this application are valid only at high frequencies even at relatively low intensity. We show that the correct use of the transformed Hamiltonian leads to the same predictions as conventional theory, which uses the minimal coupling Hamiltonian, in the appropriate domain of frequencies. This demands considerations beyond a simple evaluation of the dc term arising from the transformed potential.

When 5- to 30-keV ${\mathrm{H}}_1^{+}$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{H}}_3^{+}$ are incident on Mo, Cu, or stainless-steel targets, a small fraction are backscattered in a neutral excited state. These particles subsequently decay and the emission can be observed as Doppler-broadened spectral lines. Measurements of the total Balmer-$\beta$ line intensity are presented as a function of incoming projectile energy. It is found that ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_3^{+}$ projectiles at energy $E$ behave the same as two or three protons at energy $\frac{E}{2}$ or $\frac{E}{3}$, respectively. The energy dependence of the total intensity down to a projectile energy of 1.67 keV is consistent with a model which includes radiationless deexcitation at the surface.

Absolute transition probabilities of the three most prominent visible BrII lines are measured in emission. Results compare well with Coulomb approximations and with line strengths extrapolated from trends in homologous atoms.

It is pointed out that the reason originally used by Ochkur to justify the need for performing the Ochkur reduction no longer remains valid in the Glauber case.

We extend our proof of the equivalence of the Wilson and Wegner-Houghton generators (previously given for isotropic fixed points and to first order in a perturbation expansion for the critical exponents) to all fixed points which can be described by the corresponding approximate generators. This includes all those fixed points which are to leading order "wave-vector independent." The proof is again to first order in perturbation theory and exploits the properties of the linearized eigenfunctions (eigenoperators) to evaluate the nonlinear terms.

The role of the Born-Oppenheimer approximation in the resonant vibrational excitation of diatomic molecules by electrons is reexamined in the light of a recent paper on the hybrid theory and calculation of $e+{\mathrm{N}}_2$ scattering by Chandra and Temkin. It is pointed out that it is not necessary to resort to a nonadiabatic vibrational close-coupling expansion to reproduce the substructure seen in the resonant vibrational excitation cross section of ${\mathrm{N}}_2$ by electrons. Finally, comment is made on the reasons for the slow convergence of the vibrational close-coupling treatment from a compound-state point of view of the scattering process.

A system of $N$ identical two-level atoms, distributed over an extended region, completely excited initially, and interacting with all modes of radiation field is considered. A rate equation is proposed to describe the evolution of such a system and the transition from initial nondirectional emission to eventual directional emission is reinvestigated. An analytical expression is obtained for the critical point of this transition.

A time-dependent adiabatic formalism is introduced and some of its properties discussed. It is similar but not identical to one proposed earlier by Schulman and Detrano.

We point out that recent calculations of multiphoton bound-bound transition rates are incorrect. The error lies in the angular part of the calculation.

The lowest-order vacuum-polarization potential, known as Uehling potential, is expanded for a spherical charge distribution in a convergent form valid for all distances. The accuracy of this expansion is carefully examined at different distances. The ratios of the vacuum-polarization potentials of orders $\alpha \left(Z\alpha \right)$, ${\alpha }^2\left(Z\alpha \right)$, $\alpha {\left(Z\alpha \right)}^3$, $\alpha {\left(Z\alpha \right)}^5$, and $\alpha {\left(Z\alpha \right)}^7$ to the Coulomb potential for a point nucleus are also calculated and presented in figures for $r\le 3.5{\lambda }_e$. Several simple fitting curves are suggested.

An algebraic derivation of the Jahn-Teller theorem is presented within the framework of the adiabatic approximation. It is shown that two electronic states with nonzero energies cannot be both degenerate and exhibit vanishing forces for the same set of nuclear coordinates. This, in essence, is the general Jahn-Teller theorem. The adiabatic approximation is shown to fail completely in regions where electronic states are degenerate, if at least one of the states possesses nonvanishing forces. The significance of this result is that it demonstrates that there are regions in the nuclear configuration space which cannot be traversed adiabatically.

Four resonant states of ${\mathrm{He}}^{-}$ with all three electrons in the $n=2\mathrm{}$ shell are accessible by electron impact from the He ground state. These resonances have the structure of an electron attached to an autoionizing state ${\mathrm{He}}^{**}$. A discussion is given of the influence of these resonances on electron scattering processes. A theoretical model is proposed that includes the recently postulated post-collision interaction as the second step of a two-step decay process for these resonances. This model is shown to account qualitatively for properties of excitation structures observed in the energy range 57-61 eV, including the apparent selectivity of $L$ values in Rydberg-state excitation. Energies of the ${\mathrm{He}}^{-}$ resonances are computed variationally and are found to be consistent with earlier results for the three lowest resonances. A fourth broad resonance is predicted to occur near 60.4 eV. Experimental evidence for this resonance is discussed.