We have developed a theory for the effect of magnetic-field inhomogeneities on the spin relaxation of gases in cells with negligible relaxation at the walls. There is a characteristic pressure $p^{\mathrm{*}}$ at which the time ${\tau }_d$ required for an atom to diffuse across the cell is equal to the time ${\tau }_l$ required for the spin to precess by one radian in the mean magnetic field. For ‘‘high pressures,’’ p≫$p^{\mathrm{*}}$, the longitudinal spin-relaxation time $T_1$ is inversely proportional to the pressure. This is the classic pressure dependence discussed in the literature. The new results reported in this paper are that at ‘‘low pressures,’’ p≪$p^{\mathrm{*}}$, the pressure dependence changes and the longitudinal relaxation time becomes directly proportional to the pressure; that is, motional narrowing occurs. We show that the transverse relaxation time $T_2$ will ordinarily be proportional to the pressure at both low and high pressures, but with different coefficients. There is also a small, pressure-dependent shift of the Larmor frequency associated with the field inhomogeneity.

The effects of autoionizing resonances on the electron-impact excitation rates for n=2 to n=2 intrashell transitions in Be-like ions have been studied for six ions with atomic numbers Z=30, 34, 36, 42, 47, and 54. The resonance contributions to the excitation rates were calculated in the isolated-resonance approximation. Detailed Auger and radiative rates were evaluated in the multiconfiguration Dirac-Fock model. Results are compared with background rates from the Coulomb-Born-exchange approximation. Autoionizing resonance have been found to enhance the electron-impact excitation rates by a factor of 2–4 for the electric-dipole-forbidden transitions, while for dipole-allowed transitions, the resonance effects amount to only a few percent of these rates.

Elastic differential, integral, and momentum-transfer cross sections are reported for the scattering of electrons by water molecules in the energy range of 100–1000 eV. A parameter-free model optical potential which is the sum of three spherical terms, namely the static, exchange, and polarization forces, is constructed from near–Hartree-Fock one-center-expansion water wave functions. The total optical potential is then treated exactly in a partial-wave analysis using the variable-phase approach to yield the scattering phase shifts. We employ several versions of parameter-free polarization and exchange potentials. We find that the present calculated differential cross sections reproduce all the important features (such as forward peaking, dip at middle angles, and enhanced backward scattering) observed in recent experiments. Quantitatively, the present results are in very good agreement with experiment and better than earlier calculations.

A systematic theoretical study of alignment and orientation of the excited H(2p) orbital in H+He collisions in the energy range from 0.8 to 8 keV is carried out with use of a molecular-orbital representation. The present model takes the active electron in the H atom explicitly into account, while treating the He atom as a frozen core. Calculated alignment and orientation agree very well with recent measurements, implying that the dominant excitation mechanism for H(2p) excitation is the strong X ${}_{}{}^2{}_{}{}^{}A$ ${}_{}{}^2{}_{}{}^{}\Sigma$ radial coupling for all energies studied. Hence, the integral alignment $A_{20}$ is negative and slowly varying with energy. Present partial excitation cross sections for H(2s) and H(2p) agree well with the corresponding measurements, except at energies below 1–2 keV.

Cross sections for projectile ionization have been measured for hydrogenlike oxygen and fluorine ions incident on a molecular-hydrogen target over a projectile energy range of 0.5–2.5 MeV/amu. The experimental cross sections are compared to the plane-wave Born approximation (PWBA) and to the Glauber-approximation cross sections all of which were calculated for atomic hydrogen and multiplied by 2. The PWBA calculations have a projectile energy dependence similar to the measured cross sections but slightly underestimate them. The Glauber approximation also underestimates the measured projectile-ionization cross sections when the hydrogen target electrons are neglected, while it overestimates the measured cross sections when the effects of the hydrogen target electrons are included. The measured projectile-ionization cross sections for hydrogenlike ions incident on molecular hydrogen are approximately a factor of 2 smaller than previously reported projectile-ionization cross sections for hydrogenlike ions incident on helium. No cross sections are available for atomic hydrogen in this velocity and ion-charge regime.

An experimental study of Čerenkov line radiation is reviewed. The Čerenkov line radiation is emitted in a gas by electrons from a ${}_{}{}^{90}{}_{}{}^{}\mathrm{Sr}$ β source and measured by a coincidence technique. Its major characteristics were studied experimentally. It is discovered that the radiation has a line spectrum, its angular distribution is obviously anisotropic, it is plane polarized, and it has a short emitting time.

The theory of atomic associative ionization (AI) presented here is a many-channel treatment incorporating radial Born-Oppenheimer and Coriolis couplings among the reactant channels as well as analogous couplings among the ionized, final states. Careful attention is paid to characterizing the basis sets used to represent the pre- and post-collisional states, especially with regard to the internal and relative angular momenta of the fragments. Particularly noteworthy in this context is our use of a total angular momentum representation. This representation is excellently adapted to the derivation of selection rules needed in the analysis of AI experiments involving atoms excited into oriented hyperfine states by means of polarized lasers. Selection rules previously obtained by heuristic means are derived more rigorously here. Finally, explicit and compact formulas are constructed for the lowest-order (two-state) approximation to the AI scattering amplitude and for the first-order Born-Oppenheimer corrections as well.

Si LVV and Cl LVV Auger spectra of ${\mathrm{SiCl}}_4$ have been studied in the gas phase with the use of different photon energies from synchrotron radiation or an electron beam, either to ionize 2p electrons or to excite them selectively to unfilled molecular or Rydberg orbitals. In the case of resonance excitation, the kinetic energies of the main Auger peaks have been found to depend clearly on the antibonding molecular orbital to which the 2p electrons have been excited. For the excitation at the first strong resonance maximum, the Si LVV kinetic energies have been found to be 5.2 eV higher than the energies of the corresponding peaks in the normal Auger spectrum. This shift compares with 7.5 eV for the equivalent excitation in ${\mathrm{SiF}}_4$. In the case of Cl, the spectator Auger spectra show very high intensity and broad structureless peaks. Valence-band intensities do not vary greatly at either the Si 2p or Cl 2p edges, showing that the spectator Auger process is the major decay mechanism for the resonantly excited core hole.

We report a calculation of the basic structure parameters of 3$p^4$${(}^3$P)ns Rydberg spectra of the chlorine isoelectronic ions from Ar ii to Zn xiv as functions of effective nuclear charge by using the eigenchannel quantum-defect theory. The results indicate that the channel interaction gradually diminishes along the isoelectronic sequence. This behavior can be interpreted as the competition between electrostatic and spin-orbit interactions along the sequence. A great amount of basic atomic data can be evaluated with these parameters as input.

The Cu Kα emission spectrum was measured with a novel channeled monolithic double-crystal spectrometer. The wavelength of the Cu K${\alpha }_2$ line was determined to be λ(Cu K${\alpha }_2$)=1.544 425 6 Å (1.2 ppm). A small shoulder was detected on the low-energy side of the K${\alpha }_2$ peak at a shift of 2.2 eV. To an accuracy lower than 1%, no other structure was detected. The width and asymmetry parameters obtained for the K${\alpha }_2$ peak are in good agreement with previous measurements. The K${\alpha }_2$ line shape is analyzed within the framework of the radiative-Auger and many-body-excitation theories of x-ray emission. A well-defined lifetime width of the K${\alpha }_2$ line is obtained, yielding a K-level width of 1.56±0.04 eV, in excellent agreement with theoretical and semiempirical predictions. The applicability of the radiative-Auger, multivacancy, and many-body theories to the measured data is discussed.

The ionization energies of the four-electron isoelectronic series are calculated by both g-Hartree and Dirac-Hartree-Fock Rayleigh-Schrödinger (Mo?ller-Plesset) second-order perturbation theory.

We have observed the acceleration of 100-keV Ne atoms by the radiation pressure of a laser field, tuned into resonance with the fast-moving atoms. The acceleration was 0.12 V/μsec in agreement with an analytic expression of the light-pressure-induced modification of the atomic phase-space distribution function. Representing the first stage of laser cooling of a fast beam, the observed acceleration shows the potential for laser cooling in heavy-ion storage rings.

Two highly noise-correlated light beams are generated by nondegenerate parametric down-conversion. We report the production of sub-Poissonian light in one of the beams by using negative feedback (to the pump) from the other. This nonclassical light source has a postdetection Fano factor of 0.78 over a limited frequency range near 50 Hz with an intensity of about 60 pW.

Comparison between theoretical calculations, using Dirac-Slater (DS) and Dirac-Kohn-Sham (DKS) potentials, and the x-ray interferometer experiments on Si for Ag Kα¯ and Mo Kα¯ energies indicates that the DS potential is better than the DKS potential in the dispersion-relation calculation of the real part of the anomalous x-ray atomic scattering factor f’. The difference between DS and experimental results suggests that the solid-state effect on f’ (with respect to DS) is ∼3–5 % of the value of f’. However, the 3–5 % difference between theory and experiments may not have any significance after taking the experimental accuracy of these experiments into account. To have a better knowledge of the solid-state effect, experiments with higher accuracy are needed.

We analyze schemes for performing quantum nondemolition (QND) measurements in optical cavities. We consider three schemes: (1) measurement of a quadrature phase amplitude using a parametric process, (2) measurement of a quadrature phase amplitude using the optical Kerr effect in a nonlinear fiber, and (3) measurement of the photon number also using the Kerr effect in a fiber. We show that in the second scheme an enhancement of the QND effect may be obtained by making the cavity finesse for the signal larger than that for the probe.

The structure of the virtual transverse-photon cloud surrounding a ground-state hydrogen atom is considered in the presence of a uniform static electric field, which is capable of polarizing the atom. It is shown that this virtual cloud is distorted with respect to the spherically symmetric shape which is obtained when no static electric field is present. The distortion is carried by new components of the cloud, arising from the contributions of magnetic-dipole and electric-quadrupole virtual photons. These new contributions are shown to possess an overall cylindrical symmetry around the direction of the static field, and to be present also at large distances from the atom. The possibility of detecting these contributions experimentally is briefly discussed.

The theoretical basis of a mechanism for squeezing through a frequency jump of a harmonic oscillator is examined. It is found that a related squeezing transformation exists, but its interpretation in terms of a frequency change is not possible. A reinterpretation of the results in terms of a scaling transformation in coordinate or momentum variables is given. Moreover, a sudden jump is not essential, and squeezing can evolve continuously. The equations of motion for such a process are obtained and the interaction responsible for squeezing is obtained.

The influence of electron density gradients on optical guiding in free-electron lasers (FEL’s) is considered. We study a model problem of a FEL which employs a sheet electron beam in the absence of a cavity or a waveguide. We calculate the gain and the rate of optical guiding by solving for the eigenvalues and the actual eigenmodes of the system. Similar to what has been found by Moore for a cylindrical beam [Nucl. Instrum. Methods A239, 19 (1985)], we find that two parameters characterize the interaction: a coupling parameter and a detuning parameter. We solve the problem for two density profiles, a uniform density profile and a triangular-shaped density profile. For large and small values of the coupling parameter we obtain the results analytically. For intermediate values of the coupling parameter we obtain numerical results. When the coupling parameter is small, diffraction is large. The gain and the wave profile are then similar for the two density profiles. When the coupling parameter is large, optical guiding is large. The gain for the triangular-shaped beam is then larger by $2^{1/3}$ than the gain for the uniform beam. When the coupling parameter is large, the wave profile for the uniform density beam converges to a certain profile confined to the beam volume. For the triangular-shaped beam the wave profile becomes more and more concentrated near the midplane of the sheet beam. For the solution of the problem we derive an energy integral that determines the domain in the complex plane where nonreal eigenvalues are allowed. We also show the existence of an accumulation point of nonreal eigenvalues in certain cases.

A multilevel quantum system interacting with an intense laser field is shown to exhibit many invariants, or constants of evolution, under various conditions. Our results also apply to other similar physical problems.

Fluorescent photons emitted from an atom will interfere with a classical field only if the atom is not in a pure excited state. This is a strictly quantum-mechanical condition, which can be tested, in principle, by allowing the light emitted from an atom undergoing Rabi oscillations to interfere with the coherent pumping field. The theory of this process is discussed.