Experimental evidence for the existence of optically biaxial lyotropic nematics is presented. The first experimental measurements of the optical uniaxial birefringence $\Delta n$ versus the temperature in a discotic nematic are reported; they show a discontinuity of $\Delta n$ at the discotic-calamitic phase transition.

It is noted that, in a manner similar to that of the electrostatic-convective-cell mode, the zero-frequency magnetostatic mode in an electron-hole plasma can cause anomalous particle diffusion across the external magnetic field.

Differential cross sections for the $2s-3s$ excitation of the lithium atom by electron and positron impact are calculated using a two-potential-modified Born approximation at incident particle energies of 60 and 200 eV. The results are compared with the available experimental data.

The resonant dielectronic recombination cross section is estimated for a ${\mathrm{Cl}}^{7+}$ target interacting with incident electrons in the 10 ∼ 20-rydberg range. Cross sections averaged over energy bins of size $\Delta E=0.1\mathrm{}$ Ry are approximately (1-3) × ${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$.

The ultradense plasma appearing as a result of the radiative collaspe and the radiative cooling of two counterstreaming relativistic electron and positron beams is considered on the basis of quantum Fermi statistics. There are two different limiting cases (quasiclassical and ultraquantum) depending on the value of the relativistic factor $\gamma$. Winterberg's estimate for the density, based on Heisenberg's uncertainty principle, corresponds to the ultraquantum limit. In the quasiclassical case the compression is limited by the Pauli-Fermi principle, and the density of the plasma increases more sharply with the current (as $I^{\frac{3}{2}}$ for $I<<\frac{I_A}{\beta \gamma }$ and $I^3$ for $I>>\frac{I_A}{\beta \gamma }$, $I_A=17\mathrm{}$ kA being the Alf$\stackrel{\mathrm{`}}{\mathrm{v}}$en current).

In order to secure a proper $p$-type behavior of the radial Slater orbitals $r^n\exp (-\frac{\eta r}{2})$ for the two $p$ electrons near origin in the $1s2p2p$ configuration, the fixed $n$ power must always be chosen greater than or equal to unity. Unfortunately, this restriction has not been applied to both $p$ electrons in all 30 basic products of the 1967 trial wave function for the even-parity ${}_{}{}^4{}_{}{}^{}P$ states, only correct for the one $p$ electron in the case of the ${}_{}{}^4{}_{}{}^{}P_{}^0{}_{}{}^{}$ states. According to this restriction only 14 basic products used are allowed. The remaining 16 basic products are not allowed and therefore have been omitted in the present investigation. This error has made the 1967 calculated total energies too low. Corrective results of the total energy using the allowed correlated 14-term wave function are given in this Communication. The present result for ${\mathrm{He}}^{-}1s2p^2{}_{}{}^4{}_{}{}^{}P$ indicates a position 148 meV in the continuum above the $\mathrm{He}2{}_{}{}^3{}_{}{}^{}P$ threshold, in good agreement with recent theoretical calculations.

We have made a comprehensive study of charge-changing cross sections in ${\mathrm{N}}_2$ of Pb and Xe ions at velocities near 4 × ${10}^9$ cm/sec. We determine the velocity, ionization state, and $Z$ dependence of the capture and ionization cross sections and make an extensive comparison with theory.

Pseudopotential molecular-structure calculations have been used to obtain the low-lying interaction energies for Na${\mathrm{H}}^{+}$. The wave functions were used to calculate accurate radial and rotational coupling matrix elements. Scattering calculations which include electron translational factors were performed using up to eight coupled channels for laboratory energies 0.1 to 10 keV. Electron capture from ground-state $\mathrm{Na}3s$ yields cross sections in the ${10}^{-15\mathrm{}}$-${\mathrm{cm}}^2$ range of which the dominant products are $\mathrm{H}2s$ and $\mathrm{H}2p$. Electron capture from excited $\mathrm{Na}3p$ does not show an enhanced cross section relative to capture from the ground state even though the energy gap $\Delta E(R=\infty )$ to the dominant electron-capture channel is reduced from 1.74 to 0.36 eV.

Numerical calculations of the $2E1$ and $M1\mathrm{}$ decay rates of the $2s_{\frac{1}{2}}$ metastable states of the hydrogen isoelectronic sequence are presented. The $2E1$ rates are found to be in good agreement with recent calculations of Goldman and Drake, but substantially different from the earlier numerical values of Johnson. Effects of nuclear finite size on the $2E1$ rates are found to be insignificant, whereas finite-size effects reduce the $M1\mathrm{}$ rate by about 1.1% at $Z=92\mathrm{}$.

This Communication describes a novel spectroscopic technique for studying high-lying atomic states. The approach uses two cw narrow-band stabilized dye lasers configured in a four-wave-mixing geometry. We demonstrate that the measurements are Doppler-free and the method is capable of high resolution. Using this technique we examine the complex dephasing parameter ${\gamma }_{\mathrm{ij}}$ associated with buffer-gas collisions in atomic sodium. Rates for pressure-induced broadening and resonance shifts are reported for neon.

The stability of the internal kind mode in a finite $\beta$ model confinement configuration is investigated using the ideal magnetohydrodynamic energy principle. We demonstrate analytically that the mode becomes stable at sufficiently high $\beta$, independent of its toroidal wave number. A numerical estimate of the "second" marginally stable point is given.

The scaled decay rate ${\Gamma }^{*}[=\frac{{\Gamma }^c}{\overline{R}{k}_B}\frac{T{k}^3}{\eta }\mathrm{}(T\left)\right]$ of the system polydimethylsiloxane-diethyl carbonate, where $\eta \mathrm{}\left(T\right)\mathrm{}$ is the shear viscosity, $k$ the transfer wave vector, and $\overline{R}$ a universal constant, is in agreement with the mode-coupling theory in the range of $0.2\lesssim k\xi \lesssim 3$, and in disagreement with the renormalization-group calculation of critical dynamics with $\overline{R}=1.2\mathrm{}{\left(6\mathrm{}\pi \right)}^{-1\mathrm{}}$. The nonlinear shear viscosity due to shear gradients is in agreement with the theoretical prediction by Oxtoby.

We show that the time-averaged density matrix which involves the solution of any Liouville equation with a specified initial condition is also the steady-state solution of an appropriately constructed Bloch equation taken in the weak-damping limit. This leads to new explicit expressions for the time-averaged density matrix which, unlike expressions in general use, give a completely analytic relation between the time average and the initial condition and contain no numerically determined eigenvector components. We apply this approach to driven multilevel systems to find new expressions for the time-averaged level populations in a three-level system. A strong initial-condition dependence is exhibited even for an equally spaced resonantly driven system. Analytic forms specifically showing a Stark-shifted multiphoton resonance and the associated line shape are also derived.

A number of spectrum lines arising from magnetic dipole transitions in the $n=3\mathrm{}$ shell ground configurations of molybdenum and zirconium ions have been identified. These lines are particularly suitable for spectroscopic diagnostics in tokamak-type plasmas in the 500-1500-eV temperature range.

Laser-induced breakdown of argon and other gases at a wavelength of 0.35 μm has been measured in the pressure range 200 to 2000 Torr. The results of short-pulse (${\tau }_p=0.4\mathrm{}$ ns) and long-pulse (${\tau }_p=500\mathrm{}$ ns) measurements in argon indicate that at $p=1\mathrm{}$ atm the breakdown threshold $I_{\mathrm{th}}$ scales as ${\tau }_p^{-0.7\mathrm{}}$. Practically complete absorption of the laser beam by the broken-down plasma has been observed at the highest pressures studied. A model for breakdown in Ar is described which gives satisfactory agreement with the data.

Kramers's original modeling of the thermally activated escape rate over a barrier is generalized for Brownian-motion theory with long-time memory.

New exact solutions to the time-dependent Schrödinger equation are presented and the equivalence of two methods of arriving at these solutions shown.