We introduce a discrete model for DNA that takes into account the information about specific base sequences along the double helix. We use this model to study nonlinear wave dynamics of the T7${\mathit{A}}_1$ DNA promoter. As results we show the existence in the promoter of a dynamically active region in which static solitons acquire finite velocities, which contrasts with regions where solitons simply remain static. Furthermore, when they pass through this region moving solitons are accelerated, decelerated, or reflected, depending on their initial velocities. The possibility that these dynamical effects play a role in the mechanism of genetic activation is suggested.

The correlation exponent is widely utilized in experimental situations as an estimate of an attractor’s fractal dimension. However, in the presence of noise, the slope of the correlation integral may increase gradually, biasing dimension estimates. We propose a nonparametric statistical procedure for distinguishing the attractor from the noise process.

A number of years ago, Feynman and Hibbs derived an insightful expression for a quasiclassical effective potential in order to estimate the free energy for a quantum-mechanical system that obeys Boltzmann statistics. In the present paper, it is demonstrated how these ideas can be extended to calculate an approximate equilibrium average of any general coordinate-dependent operator. The central feature of this analysis is the replacement of the operator by an effective Gaussian-averaged quasiclassical function.

Recent attempts to generalize the classical Onsager theory of nematic ordering to finite-density systems of finite-length hard convex bodies are related and compared. It is pointed out that, although good results can be obtained in three dimensions (3D), in two dimensions (2D) the underlying factorization approximation of the radial and angular variables always implies a second-order isotropic-nematic transition instead of the crossover from a weakly first-order transition to a continuous (Kosterlitz-Thouless) transition as seen in the simulations. The quantitative agreement with the simulations is also much poorer in 2D than in 3D. On the contrary, for large spatial dimensions these theories become exact.

The liquid-bcc and bcc-fcc transitions for systems interacting through the inverse-fourth-power potential have been investigated using a density-functional approach similar to that introduced by Curtin and Ashcroft [Phys. Rev. Lett. 56, 2775 (1986)] for Lennard-Jones fluids. We show that, in contrast to the direct application of weighted-density-functional theories reported recently, the present approach predicts a stable bcc phase close to melting as well as a bcc-fcc transition at lower temperatures, as suggested by computer simulations.

Simple shear and various simple shear-free flows with constant traceless velocity gradient are simulated at the microscopic level for an atomic fluid at a single state point. The dependence of the viscosity upon the strain rate is obtained for each specific flow and analyzed on the basis of the retarded motion expansion of the nonequilibrium pressure tensor. Within the investigated range of strain rate, the pressure and the internal energy follow a linear behavior in terms of the second scalar invariant of the strain-rate tensor which, as expected by symmetry, is common to all flows.

The second-order impact shifts in hydrogen obtained from the Baranger formalism are compared with those from a kinetic theory approach. The resulting Δn=0 contributions to the shift from the two theories are shown to be identical, except for the neglect of electron-electron correlations in the Baranger formalism. It is also shown that some care is required in taking the classical limit for the perturbing electrons, or else the shift from Δn=0 interactions vanishes.

We consider the interaction between long-wavelength ion-acoustic and electron-plasma surface waves on a semi-infinite plasma. It then turns out that an ion-acoustic solitary wave can be accelerated when the amplitude of the electron-plasma surface wave varies in time.

An estimate for the higher-order binding correction to the Lamb shift in the n=4 state in ${\mathrm{He}}^{+}$ is given, based on a new calculation. The result is in agreement with a recent measurement by Majumder and Pipkin [Phys. Rev. Lett. 63, 372 (1989)].

We have investigated deuteron-deuteron fusion induced by impact of (${\mathrm{D}}_2$O${)}_{\mathit{n}}^{+}$ and (${\mathrm{H}}_2$O${)}_{\mathit{n}}^{+}$ clusters with n=1–150 on deuterated polyethylene targets at energies of 135–225 keV. Both the energy dependence and magnitude of the fusion yield measured with (${\mathrm{D}}_2$O${)}_{115}^{+}$ clusters confirm the results of Beuhler et al. [J. Phys. Chem. 94, 7665 (1990)]. Furthermore, we have observed that the size dependence of the (${\mathrm{D}}_2$O${)}_{\mathit{n}}^{+}$ fusion yields measured at the impact energy of 225 keV for small (n<10) clusters follow the ‘‘thermonuclear’’ model proposed by Carraro et al. [Phys. Rev. A 42, 1379 (1990)] rather than the thick-target model. For ${\mathrm{H}}_2$${\mathrm{O}}^{+}$ and (${\mathrm{H}}_2$O${)}_2^{+}$ at 225 keV, the yields roughly agree with the ‘‘knock-on’’ model by Carraro et al. No fusion event was observed for (${\mathrm{H}}_2$O${)}_{\mathit{n}}^{+}$ clusters with n=4–50; however, n=115 clusters produced an observable fusion rate. The ratio between fusion rates of (${\mathrm{H}}_2$O${)}_{115}^{+}$ and (${\mathrm{D}}_2$O${)}_{115}^{+}$ is 5%±2%, in close agreement with the result, ∼5%, measured by Beuhler et al. at 300 keV.

A closed-form expression is derived for angular-resolved vibrational tranistion probabilities of diatomic molecules in electron scattering using a simple two-center spectator model. The vibration is approximated by a harmonic oscillator and the final rotational states are summed. A vibrational factorization formula relating transition probabilities for initially vibrationally excited molecules to those out of the vibrational ground state is presented. The results are compared with recent calculations and experiments for highly vibrationally excited ${\mathrm{Na}}_2$ at collision energies of 150 eV.

Recoil ions produced in 100-MeV collisions of ${\mathrm{Si}}^{8+}$ with ${\mathrm{CO}}_2$ have been studied by energy-discriminated mass spectrometry. Kinetic-energy distribution measurements of atomic recoil ions formed from very highly charged ${\mathrm{CO}}_2$ molecular ions show that, whereas ${\mathrm{CO}}_2^{+}$ and ${\mathrm{CO}}_2^{2+}$ yield essentially a thermal distribution, atomic C and O ions possess large kinetic energies; O ions possess much larger kinetic energies than corresponding C ions, as expected from multiply charged, linear ${\mathrm{CO}}_2$. The relatively large energies imparted to C ions are explained in terms of a sequential Coulomb explosion model.

A possible mechanism of surface light-induced drift (SLID) under velocity-selective excitation arising from the roughness of the cell wall surface is discussed here both qualitatively and quantitatively. Due to the backscattering from the facets of roughness SLID can arise even when scattering is completely locally diffusive. It provides significant widening of the scope of real physical situations where SLID can occur. This mechanism can basically explain the unexpected sign of SLID observed in recent experiments [Phys. Rev. Lett. 59, 447 (1987); Phys. Rev. A 42, 6471 (1990)]. As a method to study gas-surface interactions, SLID should be combined with some technique providing study of the surface topography.

We have carried out a measurement of absolute interaction cross sections for inelastic collisions of electrons with free size-selected neutral sodium clusters. Electron-induced depletion of the supersonic cluster beam exhibits an increase at impact energies lower than 0.5 eV, and is essentially constant at higher energies. The inelastic interaction range, determined from the measured cross sections, increases with cluster size and is considerably greater than the cluster dimensions. The dominant physical processes are electron attachment and collision-induced fragmentation.

We present results of a numerical integration of the time-dependent Schrödinger equation for hydrogen irradiated by a linearly polarized high-frequency Gaussian pulse, with the atom initially in a state with principal quantum number equal to 3, 4, or 5. We show that stabilization against ionization commences when the ponderomotive and photon energies are roughly equal, and we give a simple explanation of this effect based on momentum-energy considerations. The effect should be observable if the atom is initially in a state with high magnetic quantum number m, and if the energy bandwidth of the pulse is smaller than the binding energy of the atom.

A classical approach is used to study the mechanism of the recently discovered suppression of ionization in superintense fields. We show that most features of this suppression occur in a classical context.

We study atomic-beam deflection by adiabatic passage between Zeeman ground levels via Raman transitions induced by counterpropagating ${\mathrm{\sigma }}^{\pm }$-polarized lasers. We show that complete population transfer between the ground states can be achieved, which corresponds to the scattering of the atomic wave packet into a single final momentum state by absorption and induced emission of laser photons. Although the lasers can be resonant, the excited state(s) are never populated during the adiabatic transfer, which suppresses the effects of spontaneous emission and preserves the coherence of the atomic wave function. This scheme has attractive features as a beam splitter and mirror for atomic interferometry.

The effect of multiplicative colored noise on the stabilization of a bistable system is studied numerically. In particular, numerical simulation of a much discussed theoretical study carried out by Graham and Schenzle [Phys. Rev. A 26, 1676 (1982)] is presented. To corroborate this study, analog simulation studies have been undertaken to date which concluded that there are substantial limitations of the theory. Subsequently, to commensurate with those numerical studies, a variant theoretical interpretation was also made. On the other hand, it is also discussed in the literature that the inherent presence of weak additive stochastic forces in analog-simulation measurements may have prevented the substantiation of the theory. With the present status thus being inconclusive, in this Rapid Communication digital-simulation results are presented that provide a variant perspective for the theoretical study. Importantly, the study also sheds new light on the old controversies: interpretation of Stratonovich and Ito calculi and adiabatic elimination. It is also pointed out that contrary to existing notion in the literature—that digital simulation is biased on the algorithm selected (Ito or Stratonovich)—one algorithm is capable of distinguishing the two interpretations. And this is possible due to the inclusion of inertia in the formulation by Graham and Schenzle.

The phase transition of directed polymers in a random potential has been studied by both weak-disorder expansions and numerical simulations. In dimension d=2+1, no evidence for a finite-temperature phase transition was found. Instead, the crossover length ${\mathit{t}}^{\mathrm{*}}$ is very large at high temperature T and has been found to vary as ln${\mathit{t}}^{\mathrm{*}}$∼${\mathit{T}}^2$. In d=3+1, transfer-matrix studies show the existence of a phase transition and some of the critical exponents have been determined.

The template (or knot holder) is extracted from the low-order unstable periodic orbits in the experimental time series of a chaotic NMR laser. The analysis indicates that the topological organization of the chaotic motion is governed by a horseshoe with a global torsion of zero in the parameter region examined.