The close-coupled theory of atomic collisions in the presence of a radiation field may be used to calculate the distribution of final atomic states which results from absorption of polarized light during a collision. The theory applies equally well to optical collisions (line broadening) and to radiative collisions (laser-induced collisional energy transfer). For an optical collision the detuning $\omega -{\omega }_{\infty }$ is restricted to be larger than either the Rabi frequency or the widths due to natural, Doppler, or pressure broadening. The radiation field is assumed to be weak enough that the transition probabilities are linear in field intensity. The molecular picture is emphasized in which the wave function is expanded in a basis of field-free molecular states and the Hamiltonian is blocked in accordance with molecular quantum numbers. The quantities needed to predict experimental observables are reduced radiative scattering $S$-matrix elements $s_{\omega }$ in the asymptotic Hund's case-(e) representation. The theory of product orientation and alignment is equivalent to that which has been developed for molecular photodissociation. It requires coherent sums of $s_{\omega }$ matrix elements from different transition branches, that is, for $P$-, $Q$-, and $R$-type transitions corresponding to changes of -1, 0, and +1 in total molecular angular momentum. On the other hand, the total absorption coefficient and the branching ratio to different final fine-structure states irrespective of orientation or alignment depend on incoherent sums of the same matrix elements.

The problem of photoelectric detection of light is reexamined when the light is polychromatic and not strictly stationary, and when the frequency response of the detector has to be taken into account. It is shown that, whereas the one-photoelectron detection probability can always be expressed in a familiar form involving the expected light intensity, the same is not true of the twofold detection probability. In general, the photodetector does not count photons in any precise sense.

Discrete transitions are observed in an ${\mathrm{OH}}^{+}$-ion beam which lead to predissociation into ${\mathrm{O}}^{+}$+H. The transitions are assigned to the $A{}_{}{}^3{}_{}{}^{}\Pi \leftarrow X{}_{}{}^3{}_{}{}^{}\Sigma _{}^{-}{}_{}{}^{}$ system leading to quasibound levels supported by the $\Omega =2\mathrm{}$ and $\Omega =1\mathrm{}$ substates of $A{}_{}{}^3{}_{}{}^{}\Pi$. The observed predissociation manifests the action of longrange dynamic coupling in the ${\mathrm{O}}^{+}$+H, O+${\mathrm{H}}^{+}$ charge-transfer channels. Improved values for the bond energy of ${\mathrm{OH}}^{+}$ and the ionization potential of OH are obtained.

Kinetic energies of the Auger transitions are calculated with the multiconfiguration Dirac-Fock method for selected elements for which reliable experimental free-atom data are available. A critical comparison between calculated and experimental data is presented.

The $4d^9\to 5s^{0}5p^6$, $4d^9\to 5s^{1}5p^5$, and $4d^9\to 5s^{2}5p^4$ Auger transitions of xenon have been studied both experimentally and theoretically. Final states of these transitions are found to be affected by strong electron correlation effects. Results from Auger-electron spectra are compared with the corresponding results of optical spectroscopy and with the theoretical estimates based on Dirac-Fock multiconfiguration computations.

General properties of the semiclassical time propagator are given for molecules in interaction with intense laser fields, i.e., for molecular systems advanced by a periodic time-dependent operator. The operator expression of Floquet's theorem is used to unify various approaches in the literature and to emphasize that the problem can be reduced to a time-independent one with an effective quasienergy operator, if evolution is only considered at multiples of the optical period. The Magnus expansion (through third order) is used to obtain an explicit expression for the quasienergy operator. The recursive-residue-generation method, devised to evaluate transition amplitudes for systems with a very large number of coupled states, is developed in detail and applied to the model problem (with over 3 × ${10}^3$ states) of an anharmonic oscillator, dipole coupled to the laser and also linearly coupled to a multimode harmonic bath.

An analytical nonperturbative theory of a two-mode laser with coupled transitions is presented. The gain medium is modeled as a homogeneously broadened $\Lambda$-type three-level system. Stable stationary solutions of such a two-mode laser are studied with particular attention paid to the two-photon-induced atomic coherence effects. Our analysis shows that the nonradiative collisional decay of atomic coherence affects substantially the extent of mode competition. In particular, the coexistence domain of the two modes increases with a faster decay of the quadrupole atomic coherence that also leads to an increase in the power extraction efficiency.

It is pointed out that a method recently described by Hillery and Zubairy can be generalized to yield the multitime correlation function in any order for an interacting quantum field in the form of a phase-space average. The correlations are expressible as $c$-number integrals with the help of the coherent-state propagator, but the associated phase-space density is complex.

Recent investigations on the ultimate sensitivity of gravitational-wave detectors have led to a new measurement strategy known as "back-action evasion" which allows one to measure precisely one component of an oscillator's motion without disturbing that component. This detection strategy can be carried out on electromagnetic field modes as well. Here a scheme, using a degenerate four-wave mixer, is presented for carrying out back-action-evading measurements on optical field modes.

The output intensity of a hybrid bistable system with a delay in the feedback loop is known to undergo regular and irregular self-pulsing. With the help of high-resolution power spectra, we have analyzed numerically the transition of this system into chaos for several different values of the delay time, and found evidence for the existence of an infinite sequence of period-doubling bifurcations, followed by a reverse sequence in which the periodic components of the spectrum are superimposed to a continuous noisy structure. We have studied the effect of external Gaussian random noise and verified, as expected, the occurrence of bifurcation gaps. In the vicinity of the self-pulsing threshold, more than three independent frequency components coexist without evidence for the appearance of a strange attractor.

Highly charged submicrometer plastic spheres suspended in water at low ionic strength will order spontaneously into bcc crystals or polycrystals. A simple linear shear orients and disorders these crystals by forcing (110) planes to stack normal to the shear gradient and to slide relative to each other with a <111> direction parallel to the solvent flow. In this paper we analyze in detail the disordering and flow processes occurring beyond the intrinsic elastic limit of the bcc crystal. We are led to a model in which the flow of a colloidal crystal is interpreted as a fundamentally different process from that found in atomic crystals. In the colloidal crystal the coupling of particle motion to the background fluid forces a homogeneous flow, where every layer is in motion relative to its neighboring layers. In contrast, the plastic flow in an atomic solid is defect mediated flow. At the lowest applied stress, the local bcc order in the colloidal crystal exhibits shear strains both parallel and perpendicular to the direction of the applied stress. The magnitude of these deformations is estimated using the configurational energy for bcc and distorted bcc crystals, assuming a screened Coulomb pair interaction between colloidal particles. As the applied stress is increased, the intrinsic elastic limit of the crystal is exceeded and the crystal begins to flow with adjacent layers executing an oscillatory path governed by the balance of viscous and screened Coulomb forces. The path takes the structure from the ${\mathrm{bcc}}_1$ and ${\mathrm{bcc}}_2$ twins observed at zero shear to a distorted two-dimensional hcp structure at moderate shear rates, with a loss of interlayer registration as the shear is increased. This theoretical model is consistent with other experimental observations, as well.

A model is presented for the melting and freezing behavior of finite clusters. The model has the property—and suggests that some real clusters may so behave—that the rigid, solidlike form is the only thermodynamically stable form below a "freezing" temperature $T_f$, that the solidlike form may coexist with a nonrigid, liquidlike form within a sharply-bounded range of temperatures between $T_f$ and a higher "melting" temperature $T_m$, and that above $T_m$, only the liquidlike form of cluster is thermodynamically stable. The temperatures $T_f$ and $T_m$ are functions of $N$, the number of particles in the cluster; it is suggested that the traditional melting point is the common temperature to which $T_f\mathrm{}\left(N\right)\mathrm{}$ and $T_m\mathrm{}\left(N\right)\mathrm{}$ converge as $N\to \infty$.

A convergent method of approximating thermodynamic Green functions is outlined briefly. The method constructs a sequence of approximants which converges independently of the strength of the Hamiltonian's coupling constants. Two new concepts associated with the approximants are introduced: the resolving power of the approximation, and conditional creation (annihilation) operators. These ideas are illustrated on an exactly soluble model and a numerical example. A convergent expression for the scattering rate in a field theory is also derived.

Several effective-range-theory parameters for the Lennard-Jones 12-6 potential needed in helium equation-of-state calculations are determined analytically. Though the results are not expressed in terms of known functions, we show how they may be computed at machine precision. The power expansion of these parameters in terms of the coupling constant of the attractive part of the potential is also determined through the first fourteen terms.

The freezing process has been studied by means of quasielastic light scattering at the solid-liquid interface of growing salol crystals. In the range of the investigated growth velocities, $0.2 \frac{\mu \mathrm{m}}{\mathrm{s}}\le v_k\le 0.8 \frac{\mu \mathrm{m}}{\mathrm{s}}$, we observe two different dynamical processes at the interface, which can be distinguished by the scattered light. In a first process intense Rayleigh scattering sets in if a critical growth velocity is exceeded, which for growth along the [010] axis is in the range of $0.2 \frac{\mu \mathrm{m}}{\mathrm{s}}\le v_{\mathrm{crit}}\le 0.8 \frac{\mu \mathrm{m}}{\mathrm{s}}$. Intensity measurements suggest that the scattering arises from fluctuations in an interface layer of a thickness of the order of 1μm. The spectrum of the scattered light can be fitted by a single Lorenzian with a linewidth $\Gamma =D_i{q}^2$ ($\overrightarrow{\mathrm{q}}$ is the scattering vector). The thus obtained value of the diffusion constant, $D_i=(1.0\mathrm{}\pm 0.25)\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{-1\mathrm{}}$, is by 6 orders of magnitude smaller than the thermal diffusivity of liquid salol. In a second process the light is scattered by a propagating line-grating-like structure, which we interpret in terms of almost equidistant steps of growth spirals originating at screw dislocations. The step spacing $d$ was determined from the angular distribution of the scattered intensity, and the step velocity $v_s$ from the Doppler shift. For a growth velocity $v_k=0.5\mathrm{}$ μm/s perpendicular to the (001) facet typical values are $d=0.4\mathrm{}$ μm and $v_s=40\mathrm{}$ μm/s. For the step height $h=\frac{v_{k}d}{v_s}$ we obtain values of the order of 2.5 lattice constants. The product $v_{s}d$ is independent of $v_k$ and has a value of (1.8±0.4)×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{-1\mathrm{}}$, whereas $v_s$ is proportional to the square root of $v_k$. An interpretation of the data is presented.

A renormalization-group analysis is carried out of the long-time behavior of random walks in an environment with a positionally random local drift force. It is argued that, independent of the strength of the disorder, the mean-square displacement, $〈x^2\mathrm{}\left(t\right)\mathrm{}〉$, is linear in time (i.e., diffusive) for dimensions $d\gtrsim 2$. In two dimensions, universal $\frac{t}{\ln t}$ corrections are found and for $d=2-\epsilon$, the behavior is subdiffusive with $〈x^2\mathrm{}\left(t\right)\mathrm{}〉\sim t^{1-{\epsilon }^2}$.

High-resolution calorimetric studies have been carried out near the smectic-$A_1$—smectic-$\tilde{A}$ phase transition of 4-heptylpheny-4-(4-nitrobenzoyloxy)benzoate (${\mathrm{DB}7\mathrm{NO}}_2$). The transition was found to be nearly second order with about 10 mK thermal hysteresis. The data with $|T-T_c|>\mathrm{}30\mathrm{}$ mK can be fitted with a power-law divergence with critical exponents $\alpha ={\alpha }^{\prime }=0.37\mathrm{}\pm 0.02$ and a ratio of critical amplitudes $\frac{A}{A^{\prime }}=0.26\mathrm{}\pm 0.03$. Here $T_c$ is the transition temperature.

The initial decay rate of the dynamical-scattering factor for dilute polymer solutions is calculated with the aid of the renormalization-group theory, which has given reasonable results for transport coefficients. The fundamental kinetic equation for a polymer chain is reconsidered critically and the necessity of the $\epsilon$ expansion is explained.

Electron-ion recombination in a weakly ionized molecular gas has been studied by the application of molecular-dynamics and Monte Carlo simulations. Plasma screening effects have been investigated for electron densities exceeding ${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Screening can reduce the recombination rate coefficient $\alpha$ by as much as one half, for $n_e={10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$, before $\alpha$ increase again with $n_e$ due to electron-stabilized three-body collisional radiative recombination. In a plasma with an applied electric field, $\alpha$ is found to decrease approximately proportional to ${\left(\frac{E}{N}\right)}^{-1\mathrm{}}$ for $\frac{E}{N}$ greater than a threshold value.

A new analytic expansion scheme to study the self-avoiding walk and critical phenomena is proposed. The scheme is illustrated by the calculation of the density distribution function of the end-to-end distance of a self-avoiding walk. The new scheme is less systematic but more flexible than the ordinary $\epsilon$-expansion method, and can give results with at least the same order of accuracy of the $\epsilon$-expansion results. It is possible to apply the new scheme to an analytical study of tricritical-critical crossover behavior.