The dynamic storage and loss shear moduli, G’(ω) and G’’(ω), have been measured with the multiple lumped resonator instrument for a tetraethylorthosilicate reaction as it proceeds through the macroscopic gel point. For five frequencies from 104 to 6200 Hz, lnG’(ω) vs time shows a sharp increase during the sol portion of the reaction prior to formation of a gel. The loss tangent shows a frequency-dependent maximum prior to gelation and then falls to 1. The power-law dependence on frequency for G’ and G’’ varies from 0.9 to 0.7 and 1.5 to 0.7, respectively, during the reaction.

An equation describing ballooning modes localized to a magnetic field line embedded in rotating plasma is derived and analyzed. On a typical ballooning growth time these modes may be completely stabilized by shear in the flow, in a way which appears to be simple to achieve experimentally. Their long-time behavior is more complicated.

Absolute K-shell ionization cross sections have been measured for thin targets of Si, P, S, K, Ca, Zn, and Ga using carbon ions between 1.0 and 6.4 MeV and protons of 1 and 2 MeV. The dependence of x-ray production cross sections on target thickness was determined. The experimental results are compared to the semiclassical approximation [Laegsgaard, Andersen, and Lund in 3 Proceedings of the Tenth International Conference on the Physics of Electron and Atomic Collisions, Paris, 1977, edited by G. Watel (North-Holland, Amsterdam 1977)], to the theory for direct Coulomb ionization of the 1sσ molecular orbital [Montenegro and Sigaud, J. Phys. B. 18, 299 (1985)], to the perturbed stationary-state approximation with energy-loss, Coulomb, and relativistic corrections (ECPSSR) [Brandt and Lapicki, Phys. Rev. A 23, 1717 (1981)], and to the modification of the ECPSSR approximation (MECPSSR) [Benka, Geretschläger, and Paul, J. Phys. (Paris) Suppl. 12, C9-251 (1987)]. The results for carbon ions are also compared to the statistical molecular orbital theory of inner-shell ionization for symmetric or nearly symmetric atomic collisions [Mittelman and Wilets, Phys. Rev. 154, 12 (1967)].

The proton results agree with empirical reference cross sections [Paul, Nucl. Instrum. Methods 169, 249 (1980)]. Also, MECPSSR and ECPSSR provide best overall agreement for protons. For carbon ions the MECPSSR theory predicts the experimental data best. For the lightest targets, however, the MECPSSR underestimates the measured ionization cross sections appreciably for scaled velocities 0.4≤ξ≤0.8. We found that in this ξ range Pauli excitation via interacting level crossing contributes strongly to the measured ionization cross section. The sum of Mittelman-Wilets predictions and MECPSSR predictions then reproduces the measured cross sections satisfactorily. This result indicates that the direct Coulomb ionization of atomic orbitals, the Pauli excitation, and the direct Coulomb ionization again, but now of molecular orbitals, are the significant K-shell ionization mechanisms for carbon ions (atomic number $Z_1$=6) on targets (atomic number $Z_2$) with $Z_1$/$Z_2$∼0.4, for high ion velocities, intermediate velocities, and the lowest velocities, respectively.

The dynamics of core ionization by electron impact is investigated through the measurement of the triply differential cross section of the C σ1s orbital in the molecule ${\mathrm{C}}_2$${\mathrm{H}}_2$. The (e,2e) experiments have been performed under asymmetric conditions and at small scattering angles, with a scattered electron energy of 1500 eV and low energies of the ejected electrons (9.6 and 41.0 eV). The measured angular distributions are characterized by large-size recoil lobes, breaking of the symmetry around the momentum-transfer direction, and unusual deviations of the maxima of the recoil peaks towards smaller deflection angles. In the (e,2e) energy spectrum a shift is observed in the position of the C σ1s peak with respect to the expected value as measured by x-ray photoelectron spectroscopy. The amplitude of the shift amounts to 0.46±0.23 eV at 9.6 eV excess energy, and it is too large to be explained only in terms of postcollision interactions.

Absolute cross sections for electron-impact ionization of ${\mathrm{Ti}}^{5+}$ have been measured from below threshold to 1500-eV collision energies. Distorted-wave calculations including direct ionization and excitation autoionization from the 3$s^2$3$p^5$ ground state and from the 3$s^2$3$p^4$3d excited configurations have been performed. Comparison of experiment and theory indicates that about 33% of the incident ions were in metastable configurations. Ionization rate coefficients and fitting parameters derived from the experimental results are also presented.

Using the hyperradial adiabatic expansion method, dμ+t collisions at thermal energies are studied in a two-state approximation. Analytic expressions for the lowest two hyperradial adiabatic potentials and their nonadiabatic correction terms are derived to order 1/$R^4$ for large values of hyperradius R. .AE

Differential cross sections for single and multiple ionization of Ne and Ar by protons have been measured at impact energies of 3 and 6 MeV, and at proton scattering angles between 0.1 and 0.9 mrad. The recoil ions formed in the collision processes were extracted from the collision region and detected in coincidence with the scattered protons. Their charge states were determined by their flight times. The experimental differential cross sections for single and multiple ionization are qualitatively explained by a simple model that takes into account the contributions of scattering from the electrons and from the target nucleus. The model indicates that ionization by hard proton-electron collisions remains the largest contributor to single and multiple ionization of both Ne and Ar for projectile scattering angles larger than 0.1 mrad. The ratio of double-to-single ionization cross sections for Ne and Ar increases with increasing proton scattering angle, suggesting an increasing importance of inner-shell ionization at small impact parameters.

An improved Born approximation for charge exchange is derived, based on a canonical transformation of the Fock Hamiltonian, leading to a novel decomposition into unperturbed and interaction terms. The generator is constructed so as to yield a manifestly post-prior symmetric transition amplitude and to include both bound-state–continuum orthogonalization and post-prior interaction-amplitude interference corrections. Resultant differential cross sections for positronium formation in the reaction $e^{+}$+H(1s)→Ps(1s)+${\mathrm{H}}^{+}$ are presented for comparison between the post, prior, and their average with and without interference. The total cross sections, which are calculated in the Ore gap, show reasonable agreement with the Kohn variational-principle results of Humberston [Adv. At. Mol. Phys. 22, 1 (1986)]. For the higher energies, our results are close to the classical trajectory Monte Carlo and distorted-wave methods.

Close-coupling calculations are carried out using the R-matrix method for the excitation of a number of states of boronlike carbon, nitrogen, and oxygen by electron impact. The ten lowest LS states of ${\mathrm{C}}^{+}$ and the eight lowest states of ${\mathrm{N}}^{2+}$ and ${\mathrm{O}}^{3+}$ are considered in the eigenfunction expansions. Including fine structure the collision strengths are calculated for all transitions among the states 2${\mathit{s}}^2$2p${(}^2$${\mathrm{P}}_{1/2,{3/2}^{\mathrm{o}}}$), 2s${2\mathrm{p}}^2$${(}^4$${\mathrm{P}}_{1/2,3/2,5/2}$${,}^2$${\mathit{D}}_{3/2,5/2}$${,}^2$S ${}_{1/2}{}^{}{}_{}{}^{}$${,}^2$${\mathrm{P}}_{1/2}$,3/2), 2$p^3$${(}^4$$S_{3/2}^o$${,}^2$${\mathrm{D}}_3$ ${}_2{}^{}{}_{}{}^{}$,${5/2}^o$${,}^2$${\mathrm{P}}_{1/2}$${,}_{32}^{\mathrm{o}}$), 2$s^2$3s${(}^2$$S_{1/2}$), and 2$s^2$3p${(}^2$$P_{1/2}$,${3/2}^{\mathrm{o}}$) for ${\mathrm{C}}^{+}$ and among all states of ${\mathrm{N}}^{2+}$ and ${\mathrm{O}}^{3+}$ excluding the n=3 configurations. Resonance structures in the collision strengths are delineated at a large number of energies and are found to have a substantial effect. A number of selected results are presented, in particular for the fine-structure transition ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{1/2}^{\mathrm{o}}{}_{}{}^{}$${\mathrm{-}}^2$${\mathrm{P}}_{3/2}^{\mathrm{o}}$ and the resonance transition ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{}^{\mathrm{o}}{}_{}{}^{}$${\mathrm{-}}^2$D in all three ions.

The numerical multiconfiguration Hartree-Fock method is used to calculate the photoionization cross section for the Cs 7d excited state. The electron correlation and the dynamical core-polarization effects, which are very important for photoionization-cross-section calculations of the cesium atom, have been taken into account in an ab initio manner through the configuration-interaction procedure. A minimum has been found in the total photoionization cross section. The calculation of the photoionization angular-distribution asymmetry parameter is proved to be an excellent test to determine the existence and location of the minimum in the cross section. The length and velocity forms of the cross section and the asymmetry parameter are found to be in excellent agreement over the entire incident photon energy range considered, suggesting that converged results may be obtained in this interval with the present approach. The results obtained are compared with the recent experimental measurements of the absolute photoionization cross section and are found to be in good agreement.

We have performed a fully quantum-mechanical calculation of the partial $L_2$-hole ${\mathrm{Ar}}^{+}$ yield from photoionization of Ar at energies just above the $L_2$ threshold. The result is in excellent agreement with a recent synchrotron-radiation experiment by Eberhardt et al. [Phys. Rev. A 38, 3808 (1988)], illustrating the importance of the Coulomb interaction between the reaction products in influencing the photoionization cross section and Auger rate near threshold.

We report integral and differential cross sections for the electron-impact excitation of the b ${}_{}{}^3{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ state of CO using the Schwinger multichannel formulation. The calculations were carried out using a two-state approximation, with the incident electron energies in the range from 10.66 to 20 eV. We find four peaks in the inelastic cross section, centered at approximately 10.87, 11.6, 13.4, and 16.2 eV. The origins of the peak structures are discussed in terms of a partial-wave analysis, and comparison is made with experiment and theory where appropriate. We attribute the two sharp, low-energy (10.87 and 11.6 eV) peaks to the decay of ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ core-excited Rydberg resonance states of ${\mathrm{CO}}^{\mathrm{-}}$. The two higher-energy peaks are broad and less well defined. The 13.4-eV peak is not clearly identifiable as a resonance, whereas the 16.2-eV peak in the inelastic cross section occurs as a result of a ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ shape resonance in the elastic channel of the $b^3$${\Sigma }^{+}$ state. The magnitudes of the cross sections indicate that the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ symmetry is dominant for all four peaks. In all the resonances, we find more than one partial wave contributing at energies slightly shifted with respect to each other. The present calculation represents the first ab initio study of core-excited Rydberg resonances in electron-molecule scattering.

A program has been developed for making calculations of fully relativistic distorted-wave cross sections for excitation of highly charged ions to specific magnetic sublevels $M_f$ by a beam of electrons. The approach uses the same Dirac-Fock-Slater potential for all electrons and is expected to be accurate for nuclear charge number Z≳2.5N, where N is the number of bound electrons per ions. To our knowledge there is at present no other fully relativistic program available with which we could compare results for this type of cross section. However, the present results for He-like, Li-like, and Ne-like iron and for Ne-like molybdenum are compared with other partially relativistic results, and generally good agreement is obtained for these ions. It is expected that the present program will be useful for comparison with and interpretation of experimental results for very highly charged ions, such as those obtained in electron-beam ion-trap experiments at Livermore [R. E. Marrs et al., Phys. Rev. Lett. 60, 1715 (1988)].

The strong-potential Born approximation has been formulated to include effects of elastic scattering in initial and final states. We use the standard distorted-wave theory and the eikonal approximation to derive closed-form expressions for scattering wave functions. Our results show that lowest-order theories must account for off-energy-shell electron propagation in the strong potential as well as for elastic scattering in initial and final states. Capture from inner shells and radiative electron capture are computed to illustrate the theory.

A technique is proposed to improve the squeezing of the mechanical noise in a back-action evasion measurement. The scheme is based on the use of a parametric multipump system and, in the limit of many pump components, approaches a stroboscopic measurement. A simple model of the back-action evasion effect for this scheme is developed. Experimental evidence of a significant increase in the squeezing with respect to the two-pump scheme and in accordance with the model is reported.

Extension of the kinetic theory of stopping, in the Oddershede-Sabin (OS) [At. Data Nucl. Data Tables 31, 275 (1984)] orbitally decomposed form, from gas phase to films and crystals generates the need for orbital mean excitation energies calculated within the local-spin-density approximation (LSDA) to density-functional theory (DFT), the preeminent theoretical model for such extended systems. In LSDA, the orbitals and orbital energies used in the construction of the electron density ρ have no standing for estimation or calculation of excitation energies. As an alternative we propose an orbital density generalization of the local plasma approximation (LPA) of Lindhard and Scharff [K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 27, No. 15 (1953)] that we test by calculation of mean excitation energies $I_k$ for each atomic orbital k within the LSDA for all atoms with Z<37. Stopping cross sections for a representative sample (about half) of these elements have been calculated using these $I_k$ values. The results do not differ substantially from those of OS (who used $I_k$ values from Dehmer, Inokuti, and Saxon [Phys. Rev. A 12, 102 (1975)]; Inokuti, Baer, and Dehmer [ibid. 17, 1229 (1978)]; Inokuti, Dehmer, Baer, and Hanson [ibid. 23, 95 (1981)]), with the discrepancy generally less than 15%.

Comparison of the LPA and OS orbital mean excitation energies exhibits a number of striking systematic features, including (1) the orbital $I_k$’s corresponding to the orbital lying highest in energy generally differ by less than 40% between the two calculations; (2) the difference at lower energies can be as much as a factor of 3; (3) the ratio $I_k^{\mathrm{OS}/I_k^{\mathrm{LPA}}}$ shows, within columns of the Periodic Table, a monotonically decreasing variation as a function of Z for the highest-energy orbital but a smooth variation as a function of Z for the lower-energy orbitals. There seem to be characteristic patterns for each different orbital. Although the two methods give very different $I_k$’s for core orbitals, there is little effect on the resulting stopping cross sections because the valence orbital $I_k$ dominates the stopping for low projectile energies, the energy range of interest here. We give a brief analysis of the sensitivity of the stopping to shifts in $I_k$ values.

A hyperfine-structure study of the strongly perturbed 3$d^{10}$5p ${}_{}{}^2{}_{}{}^{}P_{3/2}^{}{}_{}{}^{}$ state of neutral copper was performed using pulsed level-crossing spectroscopy. Excitation was accomplished at the short wavelength of 202 nm, where intense laser pulses were obtained using frequency tripling of dye laser radiation. For ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ we obtained a=61.7(9) MHz, b=4.9(7) MHz, and τ=25.5(10) ns for the magnetic dipole and the electric quadrupole interaction constants and the lifetime, respectively. A comparison with theoretical calculations based on the multiconfiguration Hartree-Fock method is made. A discussion of the usefulness of level-crossing, quantum-beat, and radio-frequency techniques for high-resolution spectroscopy at wavelengths in the uv and vacuum-uv region is presented.

Excitation spectra of CdNe and CdAr van der Waals molecules were produced in a molecular beam employing free-jet supersonic expansion, crossed with a pulsed dye-laser beam. Bands arising from A ${}_{}{}^3{}_{}{}^{}$$0^{+}$←X ${}_{}{}^1{}_{}{}^{}$$0^{+}$ and B ${}_{}{}^3{}_{}{}^{}$1←X ${}_{}{}^1{}_{}{}^{}$$0^{+}$ transitions between vibrational levels were analyzed, yielding spectroscopic constants for the three states. Computer simulations of the bands combined with the application of the London dispersion relation led to values for $r_e$, the equilibrium internuclear separations. The effective lifetimes of the CdAr${(}^3$$0^{+}$) and CdAr${(}^3$1) molecules in various vibrational states were also determined and were found to be affected by quenching collisions.

New or improved photoabsorption spectra of gas-phase silicon molecules Si$X_4$ (X=H,D,F,Cl,Br,${\mathrm{CH}}_3$,${\mathrm{C}}_2$Hd5,${\mathrm{OCH}}_3$, ${\mathrm{OC}}_2$${\mathrm{H}}_5$) have been measured near the silicon 1s edge in the 1830–1900-eV photon energy range. On the basis of configuration-interaction calculations performed on the core-equivalent species, we show that for ${\mathrm{SiH}}_4$ and ${\mathrm{SiF}}_4$ the two lowest resonances are due to excitation of the Si 1s electron into unoccupied orbitals, both having a mixed-valence Rydberg character. For all other systems, there is a unique intense transition into a pure low-lying valence orbital, and all Rydberg states have a negligible oscillator strength. Vibronic coupling effects are found in the intensity of a symmetry-forbidden valence transition, but only for X=F,Cl,Br. In addition, we interpret multiplet features found in the immediate vicinity above the ionization edge, as double core-valence vacancy-excited states. Possible overlap of such states with shape resonance in Si$X_4$ (X=F,${\mathrm{OCH}}_3$,${\mathrm{OC}}_2$${\mathrm{H}}_5$) is discussed.

Previously, we have computed the induced-dipole-moment surface of collisional ${\mathrm{H}}_2$-He complexes from first principles using highly correlated wave functions. These data were used to calculate the collision-induced absorption spectrum of the fundamental band [L. Frommhold and W. Meyer, Phys. Rev. A 35, 632 (1987)]. The present study examines the dependence of radial induced-dipole transition moments and the associated spectra on the rotational states involved, a dependence that was suppressed in the previous study. Whereas the neglect of the rotational quantum numbers j,j’ of the initial and final state of ${\mathrm{H}}_2$ is justified for the purely rototranslational spectra of ${\mathrm{H}}_2$-He, we find significant variations with j,j’ of the radial induced-dipole transition elements and the induced spectra for the fundamental band of ${\mathrm{H}}_2$. Similar effects are expected for other molecular bands and systems of interest, especially those involving hydrogen molecules.