The Lamb shift of a hydrogenic ion can be deduced from the anisotropy in the angular distribution of the $2s_{\frac{1}{2}}-1\mathrm{}{s}_{\frac{1}{2}}$ electric field quenching radiation. The accuracy of our previous anisotropy measurement for deuterium is improved to about ± 150 ppm. The derived Lamb shift is (1059.36±0.16) MHz. The sources of error are carefully analyzed and the prospects for further improvements in the accuracy are discussed.

The highly sensitive method of infrared-radio-frequency double resonance inside a laser cavity has been applied to the observations of "pure" nuclear quadrupole resonances of gaseous C${\mathrm{H}}_3$I corresponding to the transitions $\Delta J=0\mathrm{}$ and $\Delta F=\pm 1$. Radio-frequency signals with large signal-to-noise ratios (∼ ${10}^4$) have been observed for many C${\mathrm{C}}_2$ and ${\mathrm{N}}_2$O laser lines. The characteristic pattern of the radio-frequency spectrum specific to each rovibrational level has enabled us to assign the rotational levels, the infrared coincidences, and the associated far-infrared laser transitions unambiguously. From the radio-frequency spectrum the accurate vibrational and rotational dependences of the nuclear-quadrupole coupling constant and the values of the spin-rotation coupling constants have been determined. Because of the high sensitivity of the method, it was possible to observe collision-induced radio-frequency resonances as satellites to main line. This effect not only increases the number of observable quadrupole resonances, but also provides information on the detailed mechanism of rotational energy transfer. This method of observing "pure" quadrupole spectra of gases is applicable to any polar symmetric-top molecule with hyperfine structure which has rotational levels with double parity.

The method of level-crossing spectroscopy has been employed to measure the hyperfine interaction constants and the lifetimes of the $2{}_{}{}^2{}_{}{}^{}P$ and $3{}_{}{}^2{}_{}{}^{}P$ states of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$. The parameters for the $P_{\frac{3}{2}}$ states were obtained from a conventional level-crossing experiment; it was also possible to measure the magnetic dipole hyperfine structure constant $A$ for $P_{\frac{1}{2}}$ states by using a newer method (similar to level crossing) called the "decoupling" method. With these techniques, it was possible to measure $A$ for the $2{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$, $3{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$, and $3{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ states of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$. These measurements, together with the measurement of $B$ for the $3{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ state, made it possible to infer the nuclear quadrupole moment of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$. In order to check the consistency of the $A$ values obtained from the decoupling runs, data were taken (using the decoupling method) for $A$ for the $2{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ and $3{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ states of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$. In addition to the hyperfine data, the lifetimes for the $2{}_{}{}^2{}_{}{}^{}P$ and $3{}_{}{}^2{}_{}{}^{}P$ states were also obtained. The results are: $A_{\frac{3}{2}}\left({}_{}{}^7{}_{}{}^{}\mathrm{Li}2{}_{}{}^2{}_{}{}^{}P\right)=-2.95\mathrm{}\left(4\right)\mathrm{}$ MHz, $A_{\frac{3}{2}}\left({}_{}{}^7{}_{}{}^{}\mathrm{Li}3{}_{}{}^2{}_{}{}^{}P\right)=-1.036\mathrm{}\left(16\right)\mathrm{}$ MHz, $A_{\frac{1}{2}}\left({}_{}{}^7{}_{}{}^{}\mathrm{Li}3{}_{}{}^2{}_{}{}^{}P\right)=13.7\mathrm{}\left(12\right)\mathrm{}$ MHz, $A_{\frac{1}{2}}\left({}_{}{}^6{}_{}{}^{}\mathrm{Li}3{}_{}{}^2{}_{}{}^{}P\right)=5.3\mathrm{}\left(4\right)\mathrm{}$ MHz, $A_{\frac{1}{2}}\left({}_{}{}^6{}_{}{}^{}\mathrm{Li}2{}_{}{}^2{}_{}{}^{}P\right)=17.8\mathrm{}\left(3\right)\mathrm{}$ MHz, $B\left({}_{}{}^7{}_{}{}^{}\mathrm{Li}3{}_{}{}^2{}_{}{}^{}P\right)=-0.094\mathrm{}\left(10\right)\mathrm{}$ MHz, $\tau \left(2\mathrm{}{}_{}{}^2{}_{}{}^{}P\right)=26.4\mathrm{}\left(8\right)\mathrm{}$ nsec, $\tau \left(3\mathrm{}{}_{}{}^2{}_{}{}^{}P\right)=203\mathrm{}\left(8\right)\mathrm{}$ nsec, and $Q\left({}_{}{}^7{}_{}{}^{}\mathrm{Li}\right)=-59\mathrm{}\left(8\right)\mathrm{}$ mb.

Theoretical calculations by Macek and others have shown that the ${\mathrm{H}}^{-}$ photoabsorption cross section should be affected by a narrow shape resonance at 1129.5 Å. This resonance is expected to cause a noticeable 1.5-Å-wide structure in the usual vacuum-ultraviolet emission spectrum of a thermal hydrogen plasma, composed of Lyman line wings, H and ${\mathrm{H}}^{-}$ continua, and molecular-${\mathrm{H}}_2$ emission lines. The present paper describes the very thorough investigation of a steady-state local-thermodynamic-equilibrium hydrogen-arc plasma in this spectral region. The arc is operated at atmospheric pressure and its axis temperature ranges from 11 300 to 13 200 K. In contrast to some earlier similar experiments, there is a clear indication of the resonance at exactly the predicted wavelength. The recorded spectra demonstrate that Macek's theory certainly overestimates the strength of the resonance. The reduced oscillator strength of Broad and Reinhardt's more recent calculations can be confirmed, but with half the peak height and twice the width predicted by this theory. This is considered to be an indication for pressure broadening of the resonance in the plasma.

A treatment is given of two-photon ionization in which particular attention is paid to the influence of the bandwidth of the ionizing light on the ionization rate. The model adopted for the atom is a common one, including two bound levels and a continuum. The model of the laser is general enough to allow short-term temporal fluctuations of amplitude or phase, and these fluctuations give rise to the laser bandwidth. It is found that the fluctuations have the most interesting effects on the ionization rate when the laser coherence time is shorter than the atomic memory time, as could be expected. In addition to laser bandwidth, we consider the influence on the ionization rate of the detuning of the laser from the intermediate-state resonance, the laser power, and the finite lifetime of the intermediate state. Comparisons with related earlier calculations are made and similarities and differences are pointed out.

The fluorescent spectrum of the Na $D$ lines, pressure broadened by Xe and Kr, has been measured for noble-gas densities of 2×${10}^{19}$-3×${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$; at the lower density, the lines are isolated while, at the higher, they are severely blended. The spectra are obtained in normalized intensity units allowing the nonbinary behavior of the line wing intensity to be clearly observed. At the lower density the broadening is well characterized by isolated binary interactions; at the higher density multiple-perturber interactions dominate. Nonlinearities in the pressure dependence of shifts, widths, and satellite shape are reported.

A frequency domain technique is demonstrated which permits the determination of the homogeneous linewidth and relaxation rates of inhomogeneously broadened transitions in crystals, liquids, and glasses. The density-matrix formalism is employed to explain the origins of the intensity-dependent dichroism and birefringence observed in these experiments and to extract the relevant relaxation times. The samples initially studied were liquid solutions of the organic dyes malachite green and 1,3′ -diethyl-2,2′ quinolylthiacarbocyanine iodide. The ground-state recovery times were found to be 1.2±0.1 and 3.4±0.4 ps, respectively, for the two dyes in water solution. The transverse relaxation times were estimated to be less than 0.02 ps. The present technique is compared to resonant Rayleigh-type three-wave mixing and to time-resolved spectroscopic techniques.

We report measurements of frequency broadening due to self-phase-modulation (SPM) in optical fibers. The use of single-mode silica-core fibers and mode-locked argon-laser pulses leads to the simplest and cleanest measurements yet made of SPM. The qualitative features of the frequency spectrum are in good agreement with theoretical expectations. The experiment provides an independent measurement of $n_2$, the self-focusing coefficient. The results also point to some simple and useful techniques based on fibers for the measurement and analysis of mode-locked laser pulses.

An argument is given which rules out super-radiant phase transitions in which the atomic states involved are connected by electric-dipole matrix elements. The argument is based on gauge invariance, and is valid even in the presence of direct interatomic interactions. Its relevance is demonstrated by presenting a number of two-level models exhibiting super-radiant phase transitions which are not forbidden by previous sum-rule arguments. As a preliminary to the argument, the nature of the super-radiant phase transition is discussed, and a simple model is given which reproduces its main features. It is argued, finally, that in at least one case in which our argument fails a super-radiant phase transition is possible in principle.

A simple method for observing coherent optical-transient phenomena is described. Effects such as photon echoes and free induction decay (FID) should allow studies of the dynamic properties of atoms, molecules, and solids on a time scale ranging from milliseconds to ∼ 50 ps. The frequency of a cw dye laser is switched with an intracavity electro-optic modulator, producing in an external sample coherent transients which are detected in the forward beam. Measurements of long dephasing times are possible, as in FID, because the laser can oscillate indefinitely at the new frequency following a step-function switching pulse. Measurements of short decay times are also practical because the switching time is not restricted by the laser gain or cavity ringing time. In addition, the dye's dephasing time of a few picoseconds or less is too rapid to interfere with these observations. Since the pulse sequence can be preselected, the entire class of coherent optical transients is accessible. The method thus incorporates all of the advantages inherent in Stark switching, including heterodyne detection and high sensitivity, without being restricted to Stark-tunable systems. Laser frequency switching is discussed also in terms of a theoretical model which exposes the time-dependent properties of a phase-modulated cavity mode. Dephasing studies have been carried out thus far in atomic and molecular gases and low-temperature solids containing inorganic impurity ions or large organic molecules. The utility of the FID effect is emphasized where precise dephasing times are obtained from time-frequency Fourier transformation of FID signals using a digital computer. The first quantitative and detailed test of FID theory is provided by these experimental techniques.

We have studied the collision-induced light-scattering spectrum of fluid methane at several densities between 75 and 780 amagat. The results indicate that only two- and three-body contributions are significant up to densities much higher than found in previous work. The two- and three-body intensities are found to have rather similar frequency dependences.

A modified Boltzmann equation is considered where the collision integral is replaced by the sum of several simple relaxation-time terms which describe the tendency of the system to relax towards local equilibrium under the influence of different competing interparticle-collision processes. It is argued that consistency requires one to impose on each relaxation mechanism separately the appropriate conservation laws. The consequences of these conditions for the spectrum of collective modes are discussed. It is shown that only if the relaxation times are constants will the local equilibrium functions have a simple physical meaning.

The standard photoionization-avalanche mechanism, though it presents a basically satisfactory picture of cathode-directed streamer propagation, is shown to contain basic inconsistencies regarding the charge distribution at the streamer tip and the field that it produces. Specifically, the compact unstructured charge distribution postulated in standard theory is incompatible with continuous unidirectional propagation of streamers, especially when the space-charge field is large compared with applied field. A computer-simulation study of streamer-tip behavior is made to resolve these inconsistencies. The program calculates effects of electron drift, electron avalanching, space-charge-generated fields and photoionization in cylindrical coordinates for a matrix of cells extending in both the radial and axial directions. The program is used to compute the evolution in time of trial distributions of the streamer-tip space charge. Beginning with a concentrated spherical charge configuration, the simulation reveals a rapid redistribution of the charge into the form of a thin, hollow, bluntly pointed sheath. The sheath is filled with a conducting neutral plasma and the field within the sheath is nearly zero. When such a sheath configuration is used as the initial distribution, the simulation shows that it propagates forward without essential change of form. The sheath configuration is shown to be free from the inconsistencies inherent in the assumption of a compact charge distribution.

Taking the point of view that a nematic is an ordinary simple liquid, whose anisotropic properties can be treated in terms of perturbation theory, an equation of state for nematic liquid crystals is derived, based on both anisotropic repulsion between (slightly) nonspherical molecular hard cores and anisotropic attractive forces. The spatial order in the liquid is, in a first approximation, thought to be determined by a hardsphere repulsion. The orientational ordering is considered to result from the effect of the eccentricity of the molecular hard core and the superimposed attractive interaction, both of which are taken into account by means of perturbation theory. A molecular field approximation and, in order to account for short-range orientational correlations, a two-site cluster approximation, are both applied to the orientational molecular coordinates. Expressions for the equilibrium thermodynamics quantities are derived. Extensive numerical model calculations are reported and compared with experimental data for $p$-azoxyanisole (PAA) and the results of Cotter. As a special case an equation of state for the Maier-Saupe model (spherical molecules) is presented. If short-range orientational order is taken into account, quantitative agreement with the experimental data for PAA can be obtained.

Alternating current microcalorimetry was used to measure the specific heat of 4-$n$-pentylphenylthiol-4′-$n$-octyloxybenzoate in the nematic, smectic-$A$, and smectic-$C$ phases. Critical exponents were found to be consistent with the superfluid analogy of de Gennes for the nematic to smectic-$A$ transition, i.e., $\alpha ={\alpha }^{\prime }=0\mathrm{}$ (logarithmic divergence). For the smectic-$C$ to smectic-$A$ transition, the data were consistent with ${\alpha }^{\prime }=-0.54\mathrm{}$ and inconsistent with the superfluid analogy.

The analogy between equilibrium phase transitions and chemical instabilities is studied in detail in the vicinity of the critical point of a nonequilibrium reacting diffusing system characterized by multiple homogeneous steady states. The critical point of such a system is defined and its mathematical properties are discussed. These properties are shown to be essential in the subsequent reduction of the equations of motion near the critical point. The order parameter characterizing the transition is defined, and its equation of motion near the critical point is obtained in the form of a time-dependent Ginzburg-Landau equation. Fluctuations are taken into account phenomenologically using a Langevin-equation approach. Fluctuations originating from diffusion processes are shown not to be important near the critical point. The size of the critical region for chemical instabilities is estimated using an equivalent of the Ginzburg criterion of equilibrium critical phenomena. The reduction method fails within this region. It is concluded that critical points of chemical instabilities can in principle exhibit "nonclassical" criticàl behavior. Systems involving the photothermochemical instability and computer simulations of model systems seem currently to be the best candidates for studying the critical properties of chemical instabilities, and for experimental or numerical tests of the predictions of the present theory.

Starting with a pairwise, spatially and orientationally dependent intermolecular potential constructed to include steric effects, we carry out a systematic solution of the mean-field equation for liquid crystals. A model parameter which is connected to the molecular structure measures the strength of the steric forces. Its inclusion makes possible a semiquantitative comparison of our results to the experimentally obtained phase diagrams of several homologous series. The model predicts phase diagrams similar to that found by Lee et al. Improvements over the latter include (1) the characteristic feature of the smectic $A$ phase, that the director prefers to be perpendicular to the smectic layers, arises naturally from our model; (2) the connection of one of our model parameters with the structure of molecules can now be used to explain the differences in phase-transition properties between homologous series whose molecules are of similar structure but differ in the length of the rigid section.

The cross section for photoionization of potassium atoms in the $4{}_{}{}^2{}_{}{}^{}P$ states has been measured in a triple-crossed-beam experiment.

A recent conclusion of Blum and Burke concerning the singularity structure of the static-exchange amplitude for electron-hydrogen elastic scattering is shown to be incorrect.

A simple physical model is presented which explains the asymmetry recently predicted in the fluorescence spectra from a two-level atom driven slightly off resonance by a partially coherent field. The asymmetry is due to a continuing reinitiation of the transient response of the atom as its dipole moment tries to settle down to a steady-state phase relation with the fluctuating applied field. A Lorentz-model calculation based on this idea is able to give a qualitative explanation of the spectra at all field strengths, and a quantitative explanation of the low-intensity spectra. The predicted spectra are compared with those recently derived by Kimble and Mandel.