We report results of a fully ab initio treatment of electronic excitation in a polyatomic molecule by electron impact. We have calculated total and differential cross sections for exciting the two lowest (n→${\pi }^{\mathrm{*}}$${)}^1$,3${\mathrm{A}}_2$ excited states of ${\mathrm{H}}_2$CO for collision energies between 5 and 20 eV. The calculations were carried out in a three-state close-coupling approximation using the complex Kohn variational method. The absence of any backward peaking in the cross sections calculated for the ${}_{}{}^3{}_{}{}^{}\mathrm{A}_2^{}{}_{}{}^{}$ state is reminiscent of a rigorous selection rule for ${\Sigma }^{+}$-${\Sigma }^{\mathrm{-}}$ transitions in linear molecules. However, with no experimental studies available for comparison, the present results must be regarded as predictive.

Isotropic scattering functions and Compton profiles together with their directional components for several directions relevant to the molecular structure of ${\mathrm{C}}_2$${\mathrm{H}}_2$ and ${\mathrm{C}}_2$${\mathrm{H}}_4$ have been evaluated for ab initio self-consistent field and configuration-interaction wave functions. The internally folded density (reciprocal form factor) B(r) is calculated and discussed as are various momentum expectation values. Comparison is made with available experimental and other theoretical results.

We have investigated the connection between small-angle multiple scattering and charge-changing processes for fast atomic and molecular ions traversing ultrathin solid targets. Results for the atomic ions show a strong target-thickness dependence of the measured final-charge-state distributions. There is also an accompanying dependence of the multiple-scattering widths upon the final charge state of the ions. These results demonstrate conditions far from equilibrium for these processes. A Monte Carlo simulation of particle motion in the foil is described that successfully accounts for these observations. The same simulation is applied to describe the multiple scattering of diatomic molecular ions and to compute the spread of the internuclear velocity after Coulomb explosion. It is found that the Coulomb repulsion tends to reduce multiple-scattering components along the internuclear axis. Consequences for Coulomb-explosion studies of molecular structures are discussed.

We demonstrate a procedure to obtain absolute differential cross sections for electron scattering by excited atoms. Specifically we report on electron scattering by ground-state and laser-excited 3 ${}_{}{}^2{}_{}{}^{}P_{3/2}^{}{}_{}{}^{}$, F=3 sodium using an electron and photon double-recoil technique. Measured differential cross sections are obtained with 3-eV electrons in the angular range from 25° to 40°. Agreement of the ground-state scattering data with a four-state exchange close-coupling calculation [D. L. Moores and D. W. Norcross, J. Phys. B 5, 1482 (1972)] is to within experimental error. Cross sections for excited-state atoms are four to ten times larger than those for ground-state atoms. There are at present no theoretical data for comparison.

The electron degradation spectrum plays a key role in describing the electron slowing-down processes in matter. The validity of a method to obtain the electron degradation spectrum approximately, i.e., the continuous-slowing-down approximation with full accounting of electron production, is examined. Numerical results derived with this method are compared with the direct solution of the Spencer-Fano equation for argon using the same set of electron-collision cross sections. The method is effective at electron kinetic energies much higher than the first ionization threshold but fails at low electron kinetic energies even when the kinetic energy of an incident electron is high. Reasons for this failure are discussed in detail.

The effects of external electric fields on Δn=0 dielectronic recombination (DR) rate coefficients for ${\mathrm{O}}^{\mathrm{q}+}$, q=1,...,5 are calculated using a configuration-average distorted-wave approach. Field-ionization effects lead to a decrease in the DR rate coefficient while field-mixing effects lead to an increase in the DR rate coefficient. Overall, strong fields produce a significant decrease in the DR rate coefficient at the peak temperature, i.e., by over a factor of 10 for several oxygen ions. The configuration-average distorted-wave results are also compared with the Burgess general formula II [A. Burgess (private communication)] that includes both field-mixing and ionization effects. It is found that this formula roughly models these effects, and some guidelines as to the range of validity of this simple-to-use formula are developed. In the limit of zero external electric fields, the distorted-wave approach is further compared with recent sophisticated configuration-mixing intermediate-coupling calculations as well as with the Burgess general formula I [A. Burgess, Astrophys. J. 141, 1588 (1965)].

Results of an electron-spectroscopy study of two-electron detachment (ionization) and excitation processes in ${\mathrm{S}}^{\mathrm{-}}$ collisions with He, Ne, Ar, Kr, ${\mathrm{H}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{N}}_2$ are presented. The electron spectra indicate that singly excited autoionizing states of S ${(}^2$D nl and ${}_{}{}^2{}_{}{}^{}\mathrm{P}$ nl series) are populated in these collisions, and this should lead to substantial ${\mathrm{S}}^{+}$ production. Previously unknown autoionizing states of the ${}_{}{}^2{}_{}{}^{}(_{}^2{}_{}{}^{}$P)nl series are identified. In the case of the ${\mathrm{O}}_2$ target, charge exchange to the lowest ${\mathrm{O}}_2^{\mathrm{-}}$ ${(}^2$${\Pi }_g$) shape resonance is observed.

We have measured heavy-ion-induced ($Z_P$=2,10,18,36,54; 15 keV/u≤$E_P$/$M_P$≤600 keV/u secondary-electron (SE) yields from sputter-cleaned entrance (${\gamma }_B$) and exit surfaces (${\gamma }_F$) of thin solid foils (C, Al, Ti, Ni, and Cu; d≊1000 Å) in ultrahigh vacuum (p${=10\mathrm{}}^{\mathrm{-}7}$ Pa). A pronounced increase of the forward to backward SE yield ratio R=${\gamma }_F$/${\gamma }_B$ with increasing $Z_P$ is observed. The SE yield to energy-loss ratio ${\Lambda }^{\mathrm{*}}$=γ/$S_e$ has been found to be smaller for heavy ions (HI) than for light ions (H and He); i.e., ${\Lambda }^{\mathrm{*}}$(HI)<${\Lambda }^{\mathrm{*}}$(He)<${\Lambda }^{\mathrm{*}}$(H). Also, at low projectile velocities ($v_P^2$<50 keV/u), the value of ${\Lambda }^{\mathrm{*}}$ increases with decreasing $v_P$. The velocity and projectile dependence of both R and ${\Lambda }^{\mathrm{*}}$ can be described within simple extensions of Schou’s SE emission transport theory and a semiempirical Sternglass-type model introduced by Koschar and co-workers as caused by nonequilibrium projectile energy losses $S_e^{\mathrm{*}}$ near the surfaces. The near-surface energy losses are reduced compared to tabulated bulk energy loss values $S_e$ both for forward and backward emission under the assumption of a proportionality between SE yields and dE/dx. The $Z_P$-dependent reduction factors, i.e., the ratios $S_e^{\mathrm{*}}$/$S_e$, as well as material parameters Λ=γ/$S_e^{\mathrm{*}}$, are deduced from the SE yield measurements. Nevertheless, a rough overall proportionality γ∼dE/dx over four decades of both forward and backward secondary-electron yields γ and electronic energy losses dE/dx in a wide range of projectile velocities (15 keV/u ≤$E_P$/$M_P$≤16 MeV/u) and projectile nuclear charges $Z_P$ (1≤$Z_P$≤92) is found.

The scattering theory implied by the close-coupling equations is studied using a Lippmann-Schwinger formalism. The new results derived can be summarized as follows: An alternative form of the equations that ensures there are no spurious solutions in the scattering region can be constructed, and moreover there is an infinite number of such forms. The Neumann- (perturbation-) series expansion diverges in general for most energies for both the old and new forms. The Born limit nevertheless holds and can be recovered by appropriate rearrangement of the Neumann series. The original integral formulation may give convergent scattering amplitudes despite the lack of uniqueness of the solutions. The conditions under which this happens are examined.

In this paper a quantum electrodynamical theory of cooperative Raman scattering in fluids is developed. The process is one in which pairs of molecules undergo concerted Raman transitions via an intermolecular energy transfer mechanism. The formalism is also extended to chromophore pairs in which intramolecular energy transfer is involved. Using the Power-Zienau-Woolley multipolar Hamiltonian, intensity equations of the Kramers-Heisenberg type are derived which are valid over all regions of molecular separation. In the near zone it is illustrated how the intensity of cooperative scattering is simply that obtained by consideration of the instantaneous response of one molecule to the scalar field generated by another. Within this regime it is shown that the rate of scattering has the familiar $R^{\mathrm{-}6}$ dependence on intermolecular separation R associated with interacting induced dipoles. In the far zone it is demonstrated that the fully retarded and causal interaction between molecules is mediated through virtual photon coupling, and that this leads to a result for the intensity that dies off with $R^{\mathrm{-}2}$. This long-range-interaction contribution to the overall scattering intensity is generally ignored in collisional treatments of intermolecular interactions, yet is likely to be of more significance than higher-order multipole terms in near-zone calculations. Finally it is shown how bands in a Raman spectrum due to cooperative scattering effects may be identified by their unique pressure characteristics, or by observation of light scattering that occurs outside the interaction volume of the laser beam. In the case where the scattering occurs between molecules or chromophores held in a fixed orientation with respect to one another, it is demonstrated how detection may be facilitated through the manifestation of a differential scattering effect.

Hyperfine structures of atomic lanthanum at sub-Doppler resolution have been recorded using diode-laser-initiated resonance-ionization and mass-spectrometric detection. Spectra were recorded for two diode-laser-pumped transitions, ${}_{}{}^2{}_{}{}^{}\mathrm{D}_{3/2}^{}{}_{}{}^{}$${\mathrm{-}}^4$F${\mathrm{°}}_{3/2}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{D}_{5/2}^{}{}_{}{}^{}$${\mathrm{-}}^4$F${\mathrm{°}}_{3/2}$, at 753.9 and 819.0 nm, respectively. The linewidths observed were as narrow as 165 MHz, substantially below the expected Doppler width of 1 GHz for the 1700-K sample. Atomic hyperfine structure constants for the 13 13260-${\mathrm{cm}}^{\mathrm{-}1}$ ${}_{}{}^4{}_{}{}^{}\mathrm{°}_{3/2}^{}{}_{}{}^{}$ state were determined from the spectra: A=-350±4 MHz and B=54±12 MHz. Isotopically selective resonance ionization was achieved, even though a broadband dye laser was involved in the overall ionization process.

In this paper we examine the x-ray spectrum corresponding to the atomic and nuclear bremsstrahlung (AB and NB) produced by heavy ions in collisions with atoms. K-shell AB double- and single-differential and integrated cross sections are evaluated in first-order perturbation theory. Retardation effects are also considered in first order. Universal shape functions are obtained in closed form that give us a comprehensive way of plotting the available experimental results. The theory provides a good estimation of the data and allows us to predict most of the parameters of interest. We also derive a scaling law for the NB process beyond the dipole approximation. The degree of linear polarization of both processes is also studied. We have found that within the range studied the polarization takes positive and negative values for AB and NB, respectively. This feature produces an abrupt change of sign around the crossing photon energies where NB takes over AB as the dominant mechanism.

We have carried out experimental and theoretical investigations of the Hanle effect in the ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$${\to }^3$${\mathrm{P}}_1$ transition at λ=555.6 nm in ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$, emphasizing the dependence of the Hanle signal on the intensity and on the statistical parameters of a well-characterized phase-diffusing optical field. The phase and frequency fluctuations imposed on the laser beam by a system of modulators are described by two parameters corresponding to the bandwidth β and the spectral density b of the noise spectrum. The shape of the laser power spectrum is continuously variable between nearly Gaussian (b/β≫1) and nearly Lorentzian (b/β≪1). Measurements are made with a well-collimated Yb atomic beam crossed at right angles by a linearly polarized laser beam. In accord with theoretical predictions, the full width at half maximum (FWHM) of the Hanle effect coherence dip is found to increase as the square root of the laser intensity for monochromatic excitation. For a given intensity, the FWHM decreases as the laser bandwidth increases. With the same laser intensity and spectral width, the FWHM of the Hanle coherence dip increases as the laser spectral profile approaches the Gaussian limit, particularly when β/2π is within an order of magnitude of the natural width of the atomic line. The Hanle signal shapes agree well with theoretical calculations using a Brownian motion phase-diffusion model of the laser field. For large β, it suffices to apply substitution rules for simple phase diffusion to the density-matrix equations.

The radiative Auger emission (RAE) from 0.94–6.25-MeV/u ${}_{16}{}^{}{}_{}{}^{}\mathrm{S}_{}^{13+}{}_{}{}^{}$ (lithiumlike) projectiles excited in collisions with He target atoms has been measured. For these highly stripped ions the intensity of RAE photons relative to Kα x-ray emission is enhanced by about a factor of five compared with theoretical calculations and an earlier experimental measurement for S ions with few electron vacancies. The enhancement of RAE for ${\mathrm{S}}^{13+}$ is qualitatively similar to results reported previously for lithiumlike ${}_{23}{}^{}{}_{}{}^{}\mathrm{V}_{}^{20+}{}_{}{}^{}$; however, some differences between S and V are evident.

Near-threshold photodetachment processes are considered for a simple model system that represents a negative ion that supports an excited state close to the threshold of a single-electron continuum. The excited state is assumed to be autodetaching if it lies above the threshold. Expressions for the discrete-state–discrete-state matrix elements of the Green’s operator are derived and are used to obtain a solution of the time-dependent Schrödinger equation which is exact within the model. The weak-field resonance profile is presented, and the importance of terms that are neglected in the usual pole approximation is discussed. The time development of the populations and shifts of threshold are discussed for stronger fields. Population trapping through threshold shifts and through interference effects are considered. The possibility of meeting both trapping conditions simultaneously is discussed.

Contrary to general expectations it is shown that for ions in an axially symmetric trap the component of the center-of-mass motion orthogonal to the symmetry axis and laser direction is not cooled. For slightly asymmetric traps the cooling of this component is typically much slower than that of the other degrees of freedom.

We study the spin-exchange frequency shift of the cryogenic hydrogen maser for B≠0. A general expression is derived in terms of populations of ground-state hyperfine levels. The coefficients in this expression are calculated in the degenerate-internal-states approximation, as well as to first order in the hyperfine plus Zeeman splitting. Numerical results are compared with rigorous coupled-channel calculations. Some implications are pointed out for the frequency stability of the H maser in a magnetic field.

A single ${\mathrm{Ba}}^{+}$ atom was confined in a radio-frequency ion trap and cooled by near-resonant laser light. Quantum jumps into and out of the metastable 5d ${}_{}{}^2{}_{}{}^{}D_{5/2}^{}{}_{}{}^{}$ level were observed that followed the expected exponential distribution in dark periods to good agreement. Measurement of quantum-jump distributions together with careful measurements of the absolute partial pressures of all residual gas species enabled accurate measurements of the quenched 5d ${}_{}{}^2{}_{}{}^{}D_{5/2}^{}{}_{}{}^{}$ lifetime as a function of quenching gas pressure. Measurements of quenching were observed at pressures where the mean collision rate was on the order of 1 ${\mathrm{s}}^{\mathrm{-}1}$. The results yielded quenching rate constants for the metastable level for a series of gases that typically make up the residual gas environment of ultra-high-vacuum systems (${\mathrm{H}}_2$, He, ${\mathrm{CH}}_4$, ${\mathrm{H}}_2$O, CO, ${\mathrm{N}}_2$, Ar, and ${\mathrm{CO}}_2$) together with an improved value of the $5^2$${\mathrm{D}}_{5/2}$ radiative lifetime of $t_0$=34.5±3.5 s. The above quenching rate constants were then compared with classical ion-molecule collision theory. It was found that the quenching rates for molecular gases were comparable to the classical collision rates, while the rates for atomic gases were considerably lower.

The presence of a magnetic field gradient in a sample cell containing spin-polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ gas will cause an increased transverse relaxation rate. The origin of this relaxation mechanism is the loss of phase coherence of the atoms due to the fluctuating magnetic field seen by the atoms as they diffuse throughout the cell. The Redfield theory of relaxation due to randomly fluctuating magnetic fields is applied to yield expressions for the transverse relaxation rate for spherical and cylindrical sample cells. Experiments performed with an experimental ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ free-precession magnetometer give results in agreement with the theory.

A prototype mechanism for producing amplitude-squeezed states, based on the so-called ‘‘photon fractioning’’ procedure, is investigated. The coherent state is connected with a quasinumber eigenstate through an intermediate density matrix involving multiphoton processes. The enhancement in the phase fluctuations and the simultaneous reduction of the noise in the photon number result from two competing limits; the ‘‘bright limit,’’ which is semiclassical and requires highly excited states, and the photon fractioning limit, which is pure quantum. Asymptotical evaluations of the number probability distribution and of the Q function show that the mixed states are very close to the exact number-phase minimum-uncertainty states.