An experiment to detect the existence of 1/f fluctuations in α-particle decay rates has been performed. No evidence for such fluctuations has been found.

It is shown how to calculate any generalized fractal dimension of the Feigenbaum attractor from the trajectory scaling function. The Hausdorff dimension is determined.

We report a remeasurement of the Rydberg constant via a single-photon determination of the Balmer-$\alpha$ wavelength. The new value reflects the redefinition of the meter, a reexamination of corrections, and improvements of the experimental apparatus. The result is $R=109737.31569\mathrm{}\left(7\right)\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, where $c=299792458\mathrm{}$ m/s by definition. This result does not significantly disagree with the preceding measurement by Amin et al., significantly disagrees with the measurement by Hildum et al., and agrees very well with a recent measurement by Biraben and Julien.

It is demonstrated theoretically for arbitrary diatomic molecules (and atoms) and verified numerically with a twentieth-order perturbation study of the Stark effect for the ground state of ${\mathrm{H}}_2^{+}$ that high-order nonadiabatic electric polarizabilities in each order result from the delicate partial balancing of larger opposing perturbational shifts in their kinetic, nuclear-potential, and field-potential components, where the relative magnitudes and signs of these component shifts can be precisely determined by general a priori relationships. This method of analysis, based upon a combination of the Stark virial, Hellmann-Feynman, and remainder theorems, and implemented via the perturbational-variational Rayleigh-Ritz formalism, provides a novel mechanism for studying in detail the physical origins of molecular polarizabilities.

Ionization of excited helium atoms in $n{}_{}{}^1{}_{}{}^{}P\left(14\mathrm{}\le n\le 16\right)$ states due to collisions with the polar molecules N${\mathrm{H}}_3$ has been studied in a thermal energy crossed-beam experiment. A large ionization signal from these low Rydberg states is observed even though there is insufficient energy available from rotational $\Delta J=-1\mathrm{}$, $\Delta K=0\mathrm{}$ relaxation transitions of the molecule. In order to explain our observations, large $\left|\Delta J\right|$ transitions must be postulated.

Coherent transient (echo) techniques have been employed in a high-precision study of collisional velocity thermalization in a sample of $\mathrm{}\left(6s6p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$, excited-state, ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$ atoms perturbed by He or Ar. This work probes the comparative response of aligned and oriented atoms to both depolarizing and to weak, nondepolarizing and/or velocity-changing collisions. By direct inversion of our time-domain echo relaxation data, we also provide a model-independent determination of velocity-change spectra (collision kernels) for nondepolarizing collisions.

Electron-impact ionization cross sections for laser-excited ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}(\dots 5p^{6}6s6p;{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{},M=-1)$ and cascade-populated ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}(\dots 5p^{6}6s5d;{}_{}{}^1{}_{}{}^{}D+{}_{}{}^3{}_{}{}^{}D)$ atoms were measured in the threshold to 10 eV energy range. The peak cross sections for the excited species are about a factor of 2 larger than that for ground-state Ba. In addition, it was demonstrated that ionization from individual magnetic substates of various hyperfine levels can be studied. The ionization cross sections in the case of ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}\left({}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}\right)$ were found to be equal for the $M=0\mathrm{}$ and for the $M=\pm 1$ sublevels within the experimental error limit.

We have performed a fixed-nuclei two-state coupled-channel calculation for the photoionization of ${\mathrm{N}}_2$ leading to the ${\left(2\mathrm{}{\sigma }_u\right)}^{-1\mathrm{}} B{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ and ${\left(3\mathrm{}{\sigma }_g\right)}^{-1\mathrm{}} X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ states of ${\mathrm{N}}_2^{+}$, using the Padé-approximant $\tilde{C}$-functional formalism. We have found quantitative agreement with vibrationally unresolved experimental data for the photoelectron asymmetry parameter. Of particular interest is the good agreement of the asymmetry parameter in the ${\left(2\mathrm{}{\sigma }_u\right)}^{-1\mathrm{}}$ channel in the photon energy range of 25-35 eV where there are strong channel-coupling effects due to the shape resonance in the ${\left(3\mathrm{}{\sigma }_g\right)}^{-1\mathrm{}}$ channel.

The photoionization of the $7s$ subshell of Ra has been investigated using a 20-channel relativistic random-phase-approximation calculation. The results show that in addition to the "natural" Cooper minimum, there are two others induced by interchannel coupling with the $6p$ and $5d$ photoionization channels. This result has important consequences for photoelectron angular distributions as well as excited-state photoionization for high-$Z$ atoms.

Short-pulse (100-psec) soft-x-ray excitation from a laser-produced plasma and fast optical detection have been used to study multielectron photoionization from the $3p^6$ neutral ground state to the ten terms of the $3p^{4}4p$ configuration of ${\mathrm{Ar}}^{+}$. Good agreement between measured populations and those obtained from an excitation model based on energy-dependent cross sections calculated with a relativistic Hartree-Fock atomic physics code using configuration interaction techniques was obtained.

X-ray photoabsorption measurements of simultaneous two-electron transition in krypton were carried out using synchrotron radiation. A full spectrum involving all $1s+nl$ electrons was detected. Nonrelativistic Hartree-Fock and, in particular, relativistic, Dirac-Fock energy-level calculations are in good agreement with the data, as are those derived from the optical data of Rb in the $z+1\mathrm{}$ approximation. The cross sections relative to that of the single $1\mathit{s}$ ionization are found to decrease with shell number, from 2.2% for $1\mathit{s}4\mathit{p}$ to 0.058% for $1\mathit{s}2\mathit{s}$ i.e., slower than predicted theoretically. No indications are found for a preferred Coster-Kronig decay mode over the radiative one for the $1\mathit{s}\mathit{n}\mathit{s}$ levels.

New observations are reported of tunable vacuum ultraviolet light generated in the forbidden region to the red of the xenon $6s^{\prime }{\left[\frac{1}{2}\right]}_1^0$ state due to the introduction of a second, resonant laser field. Striking observations of double-resonant ionization signals on the blue side of the xenon $6s^{\prime }$ level are also reported. Both sets of data are interpreted in terms of the changes in phase matching and the nonlinear susceptibility induced in the medium by the second laser. These changes are successfully analyzed in terms of the ac Stark effect in the three-photon level due to the second laser field.

Saffman-Taylor fingers with a relative width much smaller than the classical limit $\lambda =0.5\mathrm{}$ are found when a small isolated bubble is located at their tip. These solutions are members of a family found by Saffman and Taylor neglecting superficial tension. Recent theories have shown that when capillary forces are taken into account an unphysical cusplike singularity would appear at the tip of all the fingers with $\lambda <0.5\mathrm{}$. Conversely, here the replacement of the tip by a small bubble makes these solutions possible. At large velocity these fingers show dendritic instability.

Theoretical problems related to the Brochard-Leger wall in liquid crystals have been further explained. It has been shown that there exists a critical region in liquid crystals beyond which the Brochard-Leger wall does not exist. The relaxation behavior of the wall has been discussed, and the relaxation time has been calculated. The speed of the Brochard-Leger wall has been investigated for the case for which the tilt angle of the external field is equal to a critical angle.

A particle with a nonconservative force quadratic in the velocity is described by an appropriate Lagrangian. The particle can be canonically quantized, but ambiguities concerning the ordering of operators arise. These ambiguities are eliminated if the Feynman path-integral procedure is used to quantize the particle.

We analyze all the time-ordered two-photon-exchange Feynman diagrams for electron-atom scattering in the Coulomb gauge. This includes the exchange of Coulomb as well as transverse photons. At threshold energy, we recover the two-photon exchange potential previously obtained by Feinberg and Sucher using covariant field-dispersion-theory technique. The present calculation is an analog of the classical work of Casimir and Polder on the two-photon exchange between a pair of neutral atoms and bridges a gap between the covariant field-dispersion-theory calculation and other nonrelativistic calculations. The Casimir effect is manifested in the first nonadiabatic potential which changes from a $R^{\mathrm{-}6}$ dependence to a $R^{\mathrm{-}7}$ dependence at asymptotic distances (R>a/α). We calculate terms up to the order of (${\mathrm{hlambda}}_e$/R${)}^2$ or, equivalently, of order (a/R${)}^4$ compared to the classical electric dipole polarization potential, and include the lowest-order energy dependence beyond the threshold. The neglected terms are of order ${\alpha }^2$ or smaller at asymptotic distances. The present results are valid also for positron-atom scattering.

The equilibrium properties of ${\mathrm{Na}}_2$${\mathrm{Cl}}_2$ and ${\mathrm{Na}}_2$Cl in both the neutral and the singly ionized state are evaluated by the Hartree-Fock method in a pseudopotential approach. Specific attention is given to ionization processes, in relation to observations on the relative abundance of ${\mathrm{Na}}_{\mathrm{n}}$${\mathrm{Cl}}_{\mathrm{n}\mathrm{-}1}$${\mathrm{}}^{+}$ and ${\mathrm{Na}}_{\mathrm{n}}$${\mathrm{Cl}}_{\mathrm{n}}$${\mathrm{}}^{+}$ clusters in mass spectra. Ionization is shown to be followed in both systems by symmetry breaking, leading to an equilibrium linear structure for ${\mathrm{Na}}_2$${\mathrm{Cl}}^{+}$ and to a triangular shape for the ionized dimer. The latter is very weakly bound against dissociation into ${\mathrm{Na}}_2$${\mathrm{Cl}}^{+}$ and a chlorine atom. These results provide a natural explanation for mass spectroscopy data on clusters, showing that chlorine-deficient ionized aggregates are more frequently observed than stoichiometric ones, and are also in broad agreement with experimental data on dissociation energies, ionization potentials, and equilibrium structural parameters.

Near a surface an atom acquires a permanent electric dipole. When we take into account the presence of neighbors, three processes simultaneously contribute to the reduction of its dipole: a dipolar, a quadrupolar, and a quantum one. These depolarization processes of an adatom by its neighbors are particularly important for large coverage of the surface. We discuss them and give the expression for the dipole moment induced by such effects on an adatom. We then express the formulas with respect to the dipolar and quadrupolar polarizability and a nonlinear coefficient η, which leads us to propose numerical estimates of these effects for hydrogen and inert gases. For realistic distances of the adatom from the surface and from its neighbor, we show that the dipolar term has the greatest influence and that the quadrupolar-induced term always has a negligible influence on the neighboring adatom.