The relative importance of critical opalescence versus absorption in chemically reactive critical systems in the light of comments by Morrison and by Wheeler and Petschek is discussed.

A recent prediction that the temperature derivative of the ion concentration in isobutyric acid, water solutions should diverge at the critical solution point with an exponent $\frac{(\delta -1)}{\delta }$ is in error because the author fails to properly account for the variables held constant in taking the derivative, and in the path of approach to the critical point. The correct behavior is a standard weak divergence governed by the exponent $\alpha$.

The formation of ${\mathrm{Na}}^{+}$ and ${\mathrm{Na}}_2^{+}$ in sodium vapor (∼${10}^{11}$-${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$) containing laser-selected Rydberg atoms $(15\mathrm{}<~n<~35)$ has been studied. At low atom densities photoionization, collisional ionization, and associative ionization dominate, but more-complicated mechanisms contribute to the ion yield at higher densities.

High-resolution energy spectra of electrons detached from ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ upon collisions with He and Ar have been measured at and near ${\theta }_L=0\mathrm{}°$. The measurements reveal some structure previously unseen in the high-energy peak at $v_e=v_i$. This structure, along with changes in the spectra for ${\theta }_L>0\mathrm{}°$, leads to the assumption that the peak at $v_e=v_i$ is due to the superposition of a very sharp peak on a broader peak. This very sharp peak has been stripped from the data and transformed to the rest frame of the incident ion. The ejection probabilities for the very-low-energy group of electrons (≲ 0.10 eV) were found to possess forward-backward symmetry and to be insensitive to the target gas and velocity of the ion.

Doubly excited states of atoms in an intense laser field can be described approximately within the framework of an adiabatic separation method with the use of hyperspherical coordinates. Under suitable conditions, depending on the zero-field atomic spectrum and on the laser field, new laser-induced two-electron resonances appear.

The photon linear polarization in the elementary process of electron-electron bremsstrahlung was measured in a triple coincidence experiment for coplanar geometry. An electron beam of 300 keV was used incident on a carbon target. For the present parameters the radiation was found to be completely polarized with the electric vector in the reaction plane. This is in agreement with a theoretical calculation within the experimental error.

We report on the first application of microwave optical polarization spectroscopy to a short-lived excited electronic state of a diatomic molecule. Microwave-induced rotational transitions in the $A^1\Sigma ,v=1\mathrm{}$ state of BaO were observed via changes in the polarization of a laser-beam pumping optical transitions in the $A^1\Sigma -X^1\Sigma \mathrm{}(1,0)\mathrm{}$ band. Compared with other microwave optical double-resonance experiments the new technique requires lower laser intensities and yields remarkably reduced linewidths.

We report the excitation of highly excited $6d_{\frac{5}{2}\left(\frac{3}{2}\right)}\mathrm{nd}\left(\mathrm{ns}\right)\mathrm{}$ states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound $6snd\left(\mathrm{ns}\right)\mathrm{}$ Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the ${\left(6\mathrm{}{d}_{\frac{5}{2}}n{d}_j\right)}_{J=4\mathrm{}}$ states exhibit a sharp enhancement in the autoionization rate beyond $n=25\mathrm{}$ which is accompanied by the onset of ejection of very-low-energy electrons (< 0.02 eV). We observe that the ${6d_{\frac{5}{2}}n{d}_j)}_{J=4\mathrm{}}\mathrm{}(n>\mathrm{}25\mathrm{})\mathrm{}$ states predominantly autoionize to the nearly degenerate $6d_{\left(\frac{3}{2}\right)}$ state of ${\mathrm{Ba}}^{+}$. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.

We present a novel application of Monte Carlo sampling techniques for the direct evaluation of the interfacial tension that is applicable over a wide range of system sizes and temperatures. The results for the two-dimensional Ising model with system size up to 32 × 32 for temperature at and below $T_c$ have been analyzed within scaling theory. An accurate estimate for the surface-tension amplitude in excellent agreement with Onsager's exact result is obtained.

The quenching cross sections of highly excited Rb $ns\left(25\mathrm{}\le n\le 45\right)$ and Rb $nd\left(23\mathrm{}\le n\le 44\right)$ states by CO, a weak polar molecule, are measured, and these experimental data are compared to theoretical predictions where the main process responsible for the quenching is taken to be the Rydberg-electron-CO interaction. It is shown that the long-range Rydberg-electron-molecular-dipole interaction is often dominant leading mainly to $n$-changing and, to a lesser extent, to Rydberg ionization via quasiresonant molecular rotational transitions. It is also shown that the Rydberg-$e^{-}$ —molecular-quadrupole interaction is negligible and the short-range interaction leading to $l$-mixing processes is less efficient. The agreement between experimental and theoretical values is quite satisfactory.

The geometrical effect on the measurement of stopping powers for 7-MeV protons, which was a hitherto-not-noticed effect until our previous experiment, has been studied more systematically with 7-MeV protons by changing the target thickness for Be, Al, Cu, Ag, and Ta. An improved experimental arrangement has been used to prevent the edge-scattered protons from affecting the energy-loss measurement. For all targets, the observed energy losses have been confirmed to increase with increasing emergence angle. It has also been found that this effect has a strong dependence on the thickness of the target. The dependence of this effect on the target atomic number cannot be judged properly from the present experiment. Angular distributions of protons due to multiple scattering agree fairly well with the predictions of Molière's theory. The comparison between our geometrical effect and the angular dependence of the energy loss observed for low-energy heavy ions has been discussed.

A joint theoretical-experimental study of the Thomas peak in electron capture by several-MeV protons on hydrogen and on He is presented. The folding of theoretical cross sections with the experimental angular resolution, required to compare theory to experiment, is explained and illustrated. Strong-potential Born calculations, with and without the full peaking approximation, are presented and compared to observation. Also compared are recent continuum-distorted-wave calculations of Salin and Rivarola.

The linear-algebraic—optical-potential method is applied to low-energy ${}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\Pi _g^{}{}_{}{}^{}e+{\mathrm{N}}_2$ scattering in order to understand the nature of the correlation effects in these two symmetries. The calculations clearly show that the two symmetries are dominated by quite different physical effects. The ${}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ scattering is sensitive to long-range dipole distortions of the ${\mathrm{N}}_2$ core, while ${}_{}{}^2{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ symmetry is critically dependent on short-range correlations which conserve the ${\Sigma }_g^{+}$ symmetry of the ${\mathrm{N}}_2$ core electrons. Quantitative results which support these conclusions are presented and implications of the calculations on the construction and use of model potentials and polarized orbitals are briefly discussed.

We have measured the static electric dipole polarizability of ground state ${}_{ 49}{}^{115}{}_{}{}^{}\mathrm{In}5s^{2}5p\left({}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ with a small (∼9%) admixture of metastable $5s^{2}5p\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$. Three different methods were used: (a) $E-H$ gradient balance, (b) comparison of deflection in an inhomogeneous electric field with an alkalimetal-atom "standard," and (c) a deflection analysis using a computer program with no adjustable parameters except the polarizability itself. All methods agree to within 11%. Our weighted final result is (10.18±1.20)×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^3$. This is in very close agreement to a recent computation by Liberman and Zangwill, using fully relativistic wave functions and including electron correlation.

With use of variational wave functions, oscillator strengths have been calculated for all the allowed transitions between the states $n{}_{}{}^1{}_{}{}^{}S,n{}_{}{}^1{}_{}{}^{}P,n{}_{}{}^1{}_{}{}^{}D,n{}_{}{}^3{}_{}{}^{}S,n{}_{}{}^3{}_{}{}^{}P$, and $n{}_{}{}^3{}_{}{}^{}D$,$n\le 9$, in neutral helium, with an estimated accuracy of better than 1% for most of the transitions and better than 0.1% for about a third of the transitions. The error estimate is based on numerical convergence as the number of expansion terms in the wave function is increased. By use of rapidly converging trial functions consisting of two sets of Hylleraas-type basis functions differing in the values of nonlinear parameters, the expansion lengths required are kept moderate (at most 140 terms). A discussion is given about the completeness of the Hylleraas-type basis.

We have determined accurate values for the energy of the $6snf{}_{}{}^1{}_{}{}^{}F_3^{}{}_{}{}^{},6\mathrm{snf}{}_{}{}^3{}_{}{}^{}F_2^{}{}_{}{}^{}$ and $6snf{}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}$ states of Ba I with principal quantum number $n$ ranging from 13 to 45, using laser—atomic-beam spectroscopy. The experimental level energies for the thus-far-unknown ${}_{}{}^1{}_{}{}^{}F_3^{}{}_{}{}^{}$ Rydberg series compare well with predicted values from hyperfine-structure measurements in the $6snf{}_{}{}^3{}_{}{}^{}F_{2,3}^{}{}_{}{}^{}$ series for $n>\mathrm{}30\mathrm{}$ and from Stark-effect studies in the $6snd{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ series.