A theory of sound propagation and attenuation near the cholesteric–twisted-grain-boundary-A phase transition in liquid crystals is presented. A mode-coupling formalism is used to investigate the behavior of the viscosities at the transition. Pretransitional effects are predicted in both the speed of sound and ultrasonic attenuation. The speed of sound is found to exhibit an isotropic dip, and the attenuation coefficient is anisotropic only in the prefactors, having isotropic critical exponents. These results are in qualitative agreement with experiment.

A disordered nematic surface layer, with negative orientational order or biaxial order, is shown to result from interaction with substrates which favor random planar alignment. We propose a simple interaction Hamiltonian which yields predictions in excellent agreement with measured pretransitional effects for 4-n-pentyl-4’-cyanobiphenyl on clean glass and unrubbed n-methylaminopropyltri- methoxysilane substrates. When the dimensionless interaction strength is larger than a critical value, a pretransitional biaxial boundary layer is predicted for a temperature range which is bounded above by surface biaxial-uniaxial and biaxial-biaxial transitions, and below by a complete wetting transition of the boundary layer.

Perturbation theory up to eighth order is tested taking the sum of Fock operators as ${\mathit{H}}_0$ and using a space of configuration-state functions defined as excitations from a set of reference configurations. The energies obtained in this way are approximations to the exact many-body perturbation theory energies. The method is easy to implement in high orders and the central processing unit time requirements are proportional to the number of reference configurations times ${\mathit{n}}^6$ where n is the number of orbitals. Potential-energy surfaces generated by this approach are almost parallel to the surfaces generated by the full configuration-interaction method. Test calculations on the potential-energy surface of the water molecule in three different basis sets are presented as an illustration, and comparison is made with the results of the multireference and full configuration-interaction methods. The convergence of the perturbation series strongly depends on the one-electron basis set used.

A method is presented allowing the determination of the three principal values (${\mathrm{\varepsilon }}_1$,${\mathrm{\varepsilon }}_2$,${\mathrm{\varepsilon }}_3$) of the dielectric tensor for a chiral smectic-C liquid crystal in the case that the material has an uncompensated helix. Hence it is applicable for essentially all single substances and therefore suitable for developing correlations between molecular structure and dielectric properties of the smectic-${\mathit{C}}^{\mathrm{*}}$ materials. The method requires two samples and uses three different measurement geometries. The first sample has the smectic layers parallel to glass plates and is also used for the determination of the tilt angle θ needed in the evaluation of (${\mathrm{\varepsilon }}_1$,${\mathrm{\varepsilon }}_2$,${\mathrm{\varepsilon }}_3$). The two other measurements are made on a sample with the smectic layers essentially perpendicular to the glass plates, allowing chevron or uniformly tilted layer structure in the sample. One measurement is taken in the presence of the helix and another in the nonhelical (unwound) state obtained by applying a bias field. Measurements have been performed on one ester compound exhibiting ${\mathit{A}}^{\mathrm{*}}$ and ${\mathit{C}}^{\mathrm{*}}$ phases. The dielectric tensor components have been calculated and are presented as functions of temperature and frequency.

The gelation of a silicon alkoxide has been investigated by ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ NMR. The degree of reaction, which is given by the analysis of the magic-angle spinning spectra, is shown to saturate at the degree of condensation c≊0.84 at long times. The gel fraction is obtained from the decomposition of the static spectra into a solidlike component arising from the infinite static network (the gel) and a liquidlike component coming from the coexisting finite mobile polymers (the sol). The time variation of the gel fraction has been measured from the gel time ${\mathit{t}}_{\mathit{g}}$ to 4${\mathit{t}}_{\mathit{g}}$; it can be reproduced by the classical kinetics theory of gelation.

Tilt domains are dynamical defects of cellular growth fronts consisting of a group of asymmetric cells traveling laterally along the front. We study the dynamics of these defects in thin-film directional solidification of the ${\mathrm{CBr}}_4$-${\mathrm{C}}_2$${\mathrm{Cl}}_6$ eutectic alloy. We show that the sweeping of the growth front by tilt domains, at a given growth velocity V, brings it into a well-defined ‘‘dynamically selected’’ state. Once this state is reached, the tilt domains travel with a constant width. A sudden increase (decrease) of the growth velocity transforms constant-width tilt domains to growing (shrinking) ones. This behavior is in complete agreement with the theoretical predictions made by Coullet et al. [Phys. Rev. Lett. 63, 1954 (1989)] and Caroli et al. [J. Phys. I (Paris) 2, 281 (1992)]. We study the V dependence of the dynamically selected wavelength. It does not follow the ${\mathit{V}}^{\mathrm{-}1/2}$ law, contrary to the ‘‘selected’’ wavelength of lamellar eutectics in the range of velocities and thermal gradients that we use. Above a velocity ${\mathit{V}}_{\mathrm{cr}}$, the dynamically selected basic state is unstable against the lamellar-extinction instability. We describe some dynamical patterns resulting from the interplay between the latter instabiilty and the tilt instability.

Binary mixtures of 4-n-undecyloxy-(4’-isothiocyanatophenyl)-benzoate (11.O.NCS) and 4-n-decyloxyphenyl-4’-(4’’-cyanobenzyloxy)-benzoate (10OPCBOB) exhibit a closed reentrant nematic (${\mathit{N}}_{\mathit{r}}$) region—a ‘‘nematic lake’’—between X=51 and 79, where X is the mole percent of 10OPCBOB. A high-resolution ac calorimetric study has been carried out on six mixtures with this composition range to characterize the smectic-${\mathit{A}}_{\mathit{d}}$ (Sm-${\mathit{A}}_{\mathit{d}}$)–${\mathit{N}}_{\mathit{r}}$ and ${\mathit{N}}_{\mathit{r}–}$smectic-${\mathit{A}}_1$ (Sm-${\mathit{A}}_1$) transitions plus one mixture (X=80.7) that undergoes a direct Sm-${\mathit{A}}_{\mathit{d}–}$Sm-${\mathit{A}}_1$ transition. Of special interest is the ${\mathit{N}}_{\mathit{r}–}$Sm-${\mathit{A}}_1$ transition, which exhibits three-dimensional XY critical heat-capacity behavior at X=53.8 and 64.5, shows crossover behavior with nonuniversal effective exponents α≃0.26 for X=74.5 and α≃0.31 for X=76.4, and seems to approach tricritical behavior near the entry to the lake. The presence of such a tricritical point is consistent with theoretical predictions of the multicritical topology near a Sm-${\mathit{A}}_{\mathit{d}–}$${\mathit{N}}_{\mathit{r}–}$Sm-${\mathit{A}}_1$ point.

We study the dynamics of phase separation in models with different forms for the free-energy functional and address the question of dynamical universality. In large-scale computer simulations carried out in two dimensions, we find that for a model of polymer mixtures the growth exponent is the same as that found in models for binary alloys. Although the growth exponent is universal, we find, quite surprisingly, that the scaling function for the polymer mixtures is somewhat different than the binary-alloy scaling function.

Properties of a ternary system with fixed oil and water components, but an amphiphile of varying strength, are calculated by means of a lattice model and the Landau-Ginzburg free energy it generates. Using the lattice model, we calculate the variation of the amphiphile concentration at the base and apex of the three-phase triangle from the tricritical to the four-phase point, as well as the oil-water interfacial tension and interfacial profiles. These profiles are combined with the Landau-Ginzburg free energy to determine, for the same range of amphiphiles, the bending moduli that describe the nature of the surfactant sheet separating oil and water within the microemulsion.

Results are presented for two-dimensional invasion-percolation simulations into percolating clusters. The fractal dimensions of the invading clusters are calculated for different occupation fractions. If the percolating cluster is above the percolation threshold, the fractal dimension is 1.81. On the other hand, if the percolating cluster is at the percolation threshold, the fractal dimension of the invading cluster is approximately 1.37. At threshold the invading cluster is formed exclusively of links and dead ends. If the dead ends are suppressed, the fractal dimension of the object is virtually 1.20, which is close to the fractal dimension of the minimum path between two points connected by the backbone.

The lifetime of 10–100-keV ${\mathrm{Ca}}^{\mathrm{-}}$ beams in a heavy-ion storage ring was measured to be ∼490 μs. A combination of experimental and theoretical evidence strongly suggests that the observed decay is associated with blackbody-radiation-induced photodetachment of ${\mathrm{Ca}}^{\mathrm{-}}$(4${\mathit{s}}^2$4p ${}_{}{}^2{}_{}{}^{}$P) ground-state ions at room temperature. Blackbody-radiation effects may also explain the previously reported existence of a long-lived (290±100 μs) metastable ${\mathrm{Ca}}^{\mathrm{-}}$(4s4${\mathit{p}}^2$ ${}_{}{}^4{}_{}{}^{}$P) ion [D. Hanstorp et al., Phys. Rev. Lett. 63, 368 (1989)].

Fast hydrogen atoms with an energy of 25 keV are scattered off a clean and atomically flat W(100) surface under grazing angles of incidence. Switching of an electric field directed normal to the scattering plane allows the separation of angular distributions of scattered projectiles leaving the surface as neutral hydrogen atoms and as protons, respectively. We observe a shift for protons towards smaller angles of scattering in direct comparison with neutral atoms. Our experiments give evidence for an effect of image potential and charge exchange on the trajectory of scattered projectiles. The analysis of data allows one to deduce information on charge exchange in atom-surface interactions.

A double-slit interference of ultracold neon atoms in the 1${\mathit{s}}_3$ metastable state is studied. The interferometer is aligned vertically, and because of gravity acceleration the atom changes its velocity as much as 2 m/s during the passage of the interference. The interference fringes are observed for the initial atomic velocity between 0 and 2 m/s.

By applying an all-order, complex eigenvalue polyelectronic theory, applicable to ground or excited states, we have obtained electric field-dependent energy shifts for the Li 1s2${\mathit{s}}^2$ ${}_{}{}^2{}_{}{}^{}$S and 1${\mathit{s}}^2$2p ${}_{}{}^2{}_{}{}^{}$P° states for field strengths in the range 0.0–0.02 a.u. These results are then used in conjunction with the appropriate expressions for the scalar and tensor polarizabilities (${\mathrm{\alpha }}_0$,${\mathrm{\alpha }}_2$) and hyperpolarizabilities (${\gamma }_0$,${\gamma }_2$) of P states, to obtain these quantities for the Li 1${\mathit{s}}^2$2p ${}_{}{}^2{}_{}{}^{}$P° state. Whereas the linear anisotropy is negligible (${\mathrm{\alpha }}_0$=20.11 A${\mathrm{̊}}^3$, ${\mathrm{\alpha }}_2$=0.019 A${\mathrm{̊}}^3$), the nonlinear one is huge (${\gamma }_0$=2.42×${10}^5$ A${\mathrm{̊}}^6$, ${\gamma }_2$=-1.53×${10}^5$ A${\mathrm{̊}}^6$), a fact implying observable anisotropic effects in strong laser fields. The difference between the ${\mathrm{\alpha }}_0$ of the ${}_{}{}^2{}_{}{}^{}$P° and ${}_{}{}^2{}_{}{}^{}$S states is δ${\mathrm{\alpha }}_0$=4.93 A${\mathrm{̊}}^3$, in fairly good agreement with the recent measurement (δ${\mathrm{\alpha }}_0$=5.51 A${\mathrm{̊}}^3$) of Hunter et al. [Phys. Rev. A 44, 6140 (1991)].

The wavelength of the 6${\mathit{f}}_{7/2}$→3${\mathit{d}}_{5/2}$d transition in several nickel-like ions has been measured on the Livermore electron-beam ion trap with a precision as high as 27 ppm in order to assess their utility in a resonant photopumping scheme to achieve x-ray lasing below 80 Å. An interpolation of the results is presented, which provides an estimate of the transition energies for ions between Dy (Z=66) and U (Z=92). The closest line coincidence was found for the 3d-6f transition in nickel-like Hf, which differs by 1.2 mÅ from the wavelength of the Ly-${\mathrm{\alpha }}_2$ transition in hydrogenic C1, and for the 3d-6f transition in T1, which differs by 0.8 mÅ from Ly-${\mathrm{\alpha }}_1$ in hydrogenic Ca.

An atomic transition that has been made transparent by applying an additional electromagnetic field exhibits a rapidly varying refractive index with zero group velocity dispersion at line center. A 10-cm-long Pb vapor cell at an atom density of 7×${10}^{15}$ atoms/${\mathrm{cm}}^3$ and probed on its 283-nm resonance transition has a calculated optical delay of 83 ns [(c/${\mathit{V}}_{\mathit{G}}$)=250].

Time-delayed four-wave mixing, using incoherent laser pulses 5 ns long and 350 Å wide, has been used to measure the optical phase relaxation of a large molecule (nile blue) in solution at 300 and 4.5 K. In the latter case an asymmetric response as well as a quantum beat (signaling a vibronic mode at 583 ${\mathrm{cm}}^{\mathrm{-}1}$) is observed. Its magnitude and shape gives its electron-vibron coupling factor S=0.79 together with the homogeneous relaxation time ${\mathit{T}}_2$=43.7 fs and the inhomogeneous relaxation time ${\mathit{T}}_2^{\mathrm{*}}$=13.3 fs (full width of 940 ${\mathrm{cm}}^{\mathrm{-}1}$). Supporting evidence for the large value of S is presented.

Ionic fragments of ${\mathrm{BF}}_3$ produced by the B 1s→${\mathrm{a}}_2^{\mathrm{″}}$ photoexcitation have been mass analyzed in coincidence with the resonance spectator B KVV Auger electrons. Dissociation from the ${\mathrm{BF}}_3^{+}$ states with one outer-valence hole produces only ${\mathrm{BF}}_2^{+}$, while dissociation from the ${\mathrm{BF}}_3^{+}$ states with two outer-valence holes and one electron excited to the ${\mathrm{a}}_2^{\mathrm{″}}$ orbital produces predominantly ${\mathrm{B}}^{+}$. Dissociation producing ${\mathrm{B}}^{+}$ or ${\mathrm{F}}^{+}$ from the ${\mathrm{BF}}_3^{+}$ states with one outer-valence hole, one inner-valence hole, and one excited electron occurs in competition with the autoionization.

The jump-rate problem in a periodic potential is exactly solved in the whole damping range by a Fourier analysis of the energy width of the first-Brillouin-zone dynamic structure factor, numerically calculated by the matrix-continued-fraction method. An analytical solution of the escape problem from a bimetastable potential is obtained and its relationship with the solution of the periodic problem is discussed.

We study the multicritical behavior of a class of two-dimensional ice-type vertex models on different lattices using renormalization-group theory. The models are classified by an integer m, with m=2 corresponding to the known square lattice case. For m>2, the specific-heat exponent is a=(m-2)/(m-1) with an upper critical dimensional confluent (lnt${)}^{1/2}$ divergence for m=3. The nature of the transition is similar to the mth-order multicritical point, yet the exponents are not those known from c<1 conformal invariance. The models are anisotropic with ${\nu }_{\mathrm{\parallel }}$=1 and ${\nu }_{\mathrm{\perp }}$=12. A few special features of the models are discussed.