The hydrodynamics of biaxial nematics has been obtained in the presence of an external field $\overrightarrow{H}$. By working with the triad order parameter, we obtain results valid for arbitrary nonsingular textures. We also present the bending energy for biaxial nematics. Our derivation of the hydrodynamics explicitly includes angular momentum conservation. The resulting hydrodynamic equations clearly possess a rotating-equilibrium solution. In an Appendix we discuss extending the theory to include the internal angular momentum of the molecules which constitute the biaxial nematic.

A shock wave generated by an underground nuclear explosion was used to perform impedance-matching experiments for several different materials at pressures from 1 to 7 TPa. The nearly planar shock passed through a 180-mm-diam by 12-mm-thick molybdenum standard and into stacks of 10-mm-thick samples of the following adjacent pairs of materials: LiD-Be, Be-LiD, Al-Mo, Pb-quartz, W-Mo (low density, ${\rho }_0=8.29\mathrm{}$ g ${\mathrm{cm}}^{-3\mathrm{}}$), U-Mo, and Fe. An array of 75 electrical contact pins was used to determine the shape of the shock front and the shock velocities in the standard and in the samples with uncertainties ranging from 1.5 to 2.5%. The measured shock velocity of 27.30 km/s (±1.5%) in the molybdenum standard corresponds to a pressure of 4.95 TPa (±3.5%) based on a theoretical equation of state (EOS) that includes electronic-shell-structure effects. An impedance-matching analysis was performed for each sample stack to obtain Hugoniot data for all samples except those in the U-Mo stack; neutron-induced fission heating in the uranium perturbed the initial state and produced inconclusive results. Predictions, based on the SESAME EOS library, of shock velocities in most samples agree with the experimental results within the uncertainties. For the quartz and low-density molybdenum, the SESAME-predicted shock velocities are too large by 5 to 10%. The following Hugoniot points in pressure ($P$)-particle-velocity ($u$) coordinates were determined for the indicated samples: LiD ($p=0.946\mathrm{}$ TPa, $u=29.39\mathrm{}$ km/s), Be (1.46, 22.69); Be (1.79, 25.42), LiD (1.038, 31.23); Al (2.23, 23.89), Mo (4.03, 16.05); Pb (4.80, 17.84), quartz (1.66, 23.64); W (6.51, 15.10), and Mo (${\rho }_0=8.29\mathrm{}$ g ${\mathrm{cm}}^{-3\mathrm{}}$) (3.68, 18.27). The results for iron have a larger uncertainty but are consistent with the SESAME predictions.

Electronic excitation of nitrogen molecules in collisions of 2.3-keV ${\mathrm{He}}^{+}$ ions has been investigated by means of ion-impact energy-loss spectroscopy. Vibrational band structures in the lowest five singlet and triplet excited states of ${\mathrm{N}}_2$ are resolved with a resolution of 0.13-eV full width at half maximum. The relative intensities of the vibrational band structures agree with the prediction from the Franck-Condon principle. Computer simulation of the observed spectrum suggests the participation of the optically forbidden $B^{\prime }{}_{}{}^3{}_{}{}^{}\Sigma _u^{-}{}_{}{}^{}{}_{\leftarrow }{}^{}{}_{}{}^{}X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ transition into excitation of ${\mathrm{N}}_2$ molecules by the ${\mathrm{He}}^{+}$-ion impact.

The results of an earlier investigation by the authors [Phys. Rev. A 23 2553 (1981)] are extended to study the spectral features of the signals in the optical Hanle effect with regard to the various directions of observations and the polarizations of the emitted and the exciting radiation. The asymmetries in the spectra are found to be critically dependent on each of these parameters.

Radiative lifetimes of 11 levels from the $z{}_{}{}^3{}_{}{}^{}G$, $y{}_{}{}^5{}_{}{}^{}G$, and $y{}_{}{}^3{}_{}{}^{}G$ terms of Zr I are reported. These lifetimes are measured using time-resolved laser-induced fluorescence on a Zr atomic beam.

Experimentally we present high-resolution ac calorimetry measurements on a liquid-crystal compound exhibiting a smectic-$A$—smectic-$C$ ($\mathrm{Sm}A-\mathrm{Sm}C$) phase transition which is shown to be mean-field-like. In the light of this experimental result, a simple and convincing argument, based on the mean-field theory, is given to explain why non-mean-field critical exponents were obtained in many of the studies on the $\mathrm{Sm}A-\mathrm{Sm}C$ transition. The theory not only gives an excellent fit to our data for temperatures below the transition temperature but also provides a qualitative prediction on the size of the deviation from the linear relationship between ($T_c-T$) and the square of the order parameter. The predicted deviation is consistent with x-ray measurements on the order parameter of another liquid-crystal compound.

We have studied the effects of multiplicative noise in the counterflow velocity on the Vinen phenomenological description of vortex-line turbulence in superfluid helium using the Fokker-Planck equation. The principal result is that the noise pushes the critical velocity for the onset of turbulence toward higher values. We discuss a noise-induced phase transition which is not likely to be physically observable in turbulence experiments.

I show that the Bobylev-Krook-Wu solution of the Boltzmann equation violates the inequality ${(-\frac{d}{\mathrm{dt}})}^{n}H\ge 0$ for $n\ge 102$. Thus the conjectured "super-$H$ theorem" does not hold.

Electron-impact-ionization cross sections have been computed for Ar II, K III, Sc V, and Fe X in a distorted-wave Born exchange approximation. For Ar II, the theoretical results are in good agreement with available experimental data. An analytic fit to the data is provided.

We have used nonperturbative quantum-mechanical close-coupled scattering calculations to investigate inelastic atomic collisions induced by strong laser fields. If a partialwave expansion in molecular total angular momentum states is used for the scattering wave function, the selection rule $\Delta J=\pm 1$ for the radiative interaction matrix elements results in an infinite set of close-coupled equations. Model calculations for the total laser-induced inelastic cross section for both beam and homogeneous-gas experiments are carried out for two ${}_{}{}^1{}_{}{}^{}\Sigma$ states coupled by linearly polarized light. A truncated expansion is used with a sufficient number of rotational states to ensure convergence of the cross section. The results show that the exact cross section saturates much more slowly with increasing laser intensity than the cross section calculated for a two-state $J$-conserving model. Angular momentum changes larger than ±1 are possible when the laser intensity is sufficiently high. Thus, a sufficient number of rotational states must be included in the expansion basis to take into account scattering paths that can lead to large changes in angular momentum.

Closed-form analytical time-dependent solutions of the Debye-Smoluchowski equation with a general spherical interaction are obtained for the important cases of geminate and homogeneous recombination by diffusional drift in a gas of liquid medium. A relationship between the time-dependent probability for recombination or escape in geminate recombination and the transient rate for homogeneous recombination is established. An expression for the rate of decay of emitted intensity in fluorescence quenching is also derived.

Recent measurements of the stopping powers of liquid water and water vapor for 2.0-to 5.5-MeV α particles have been analyzed with Bethe-Bloch theory including a Barkas-effect correction term in order to extract mean excitation energies reflecting the two physical states of the target substance. The resulting value of the mean excitation energy of liquid water, 68 eV, exceeds that characteristic of water vapor by about 13%. Although the difference in values is consistent with expectation in both magnitude and direction, the mean excitation energy of liquid water lies some 10% below a recent theoretical prediction.

We calculate in this paper the widths and shifts of many neutral-helium lines in a hot plasma. To evaluate the broadening and shift produced by electronic impacts, a convergent method is applied. The effects of the ions are taken into account by using the quasistatic, adiabatic, and impact approximations. Our theoretical predictions are compared with the experimental results of Wulff, Berg et al., Kelleher, and Diatta.

A neutralized proton beam (averaged velocity $\overline{v}=3.6\mathrm{}\times {10}^8$ cm/s, velocity spread $\Delta v=0.9\mathrm{}\times {10}^8$ cm/s, proton current density ∼ 18 A/${\mathrm{cm}}^2$) is injected into a transverse magnetic field $B_{\perp }$(≤3 kG). It is found that the electric field which satisfies the condition $E=\overline{v}{B}_{\perp }$ value is formed in the beam and the beam propagates across the magnetic field by the polarization drift when the dielectric constant $\epsilon =1+{\left(\frac{{\omega }_{\mathrm{pi}}}{{\omega }_{\mathrm{ci}}}\right)}^2$ of the beam is larger than 100-180. The lowest value of $\epsilon$ for the beam propagation given above is consistent with the theoretical prediction $\epsilon \gg 43$.

Purely-growing magnetostatic modes are shown to exist in a plasma either with contrastreaming electrons or with temperature anisotropy. The relevance of our investigation to the anomalous heat transport in tokamak devices is discussed.

Cross sections for the $1{}_{}{}^1{}_{}{}^{}S\to n{}_{}{}^1{}_{}{}^{}P\mathrm{}(n=2,3,4\mathrm{})\mathrm{}$ excitations of atomic helium by proton impact are calculated in the framework of the Glauber approximation. The calculated cross sections show good agreement with the observed data. Furthermore, the average ration of Glauber cross sections for $3{}_{}{}^1{}_{}{}^{}P$ and $4{}_{}{}^1{}_{}{}^{}P$ excitations at high energies show good agreement with those obtained from other theories and also with the measurements of Hippler and Schartner.

Rigorous lower bounds for all bound-state systems, for the first-gradient corrections to the kinetic and exchange energy functionals, viz., (i) $T_2\left[\rho \right]=\frac{1}{72}\int \frac{{\left|\overrightarrow{\nabla }\rho \right(\overrightarrow{\mathrm{r}}\left)\right|}^2}{\rho \left(\overrightarrow{\mathrm{r}}\right)}d\overrightarrow{\mathrm{r}}\ge \frac{{\pi }^{\frac{4}{3}}{2}^{\frac{2}{3}}}{24N^{\frac{2}{3}}}\int {\rho }^{\frac{5}{3}}\left(\overrightarrow{\mathrm{r}}\right)d\overrightarrow{\mathrm{r}}=\frac{10}{72}{\left[\frac{2}{3}\right]}^{\frac{2}{3}}\frac{T_0\left[\rho \right]}{N^{\frac{2}{3}}}$ and (ii) $\left|K_2\right[\rho \left]\right|=\int \frac{{\left|\overrightarrow{\nabla }\rho \right(\overrightarrow{\mathrm{r}}\left)\right|}^2}{{\rho }^{\frac{4}{3}}\left(\overrightarrow{\mathrm{r}}\right)}d\overrightarrow{\mathrm{r}}\ge \frac{27{\left(\frac{\pi }{2}\right)}^{\frac{4}{3}}}{N^{\frac{2}{3}}}\int {\rho }^{\frac{4}{3}}\left(\overrightarrow{\mathrm{r}}\right)d\overrightarrow{\mathrm{r}}=\frac{3\pi {\left(6\mathrm{}\pi \right)}^{\frac{2}{3}}}{N^{\frac{2}{3}}}\left|K_0\right[\rho \left]\right|$ have been derived [$N$ is the number of electrons and $\rho \left(\overrightarrow{\mathrm{r}}\right)$ is the electron density]. Numerical investigations on these bounds employing Hartree-Fock atomic-electron densities have been carried out. An empirical relationship between the Hartree-Fock $T_2$ and $T_0$ for neutral atoms has also been presented.