The stopping cross sections of the three carbon modifications diamond, graphite, and glassy carbon are investigated for carbon projectiles of intermediate velocity. The inverted Doppler-shift attenuation method was used as the experimental technique, and it enabled us to measure the ratios of the three stopping cross sections precisely over a wide energy range. For velocities between 3 and 4 times Bohr’s velocity the stopping cross sections of graphite and glassy carbon are found to be 1.036 and 1.072 times larger than that of diamond, respectively. These differences are attributed to binding effects. To understand these effects, we have evaluated the mean ionization potentials utilizing the local-plasma approximation for the inner-shell electrons and the dielectric response function for the valence electrons. The theoretical ratios calculated by inserting these potentials into the Bethe-Bloch stopping-power formula agree well with our experimental results. Furthermore, we have obtained a value of 53.3±4.1 fs for the lifetime of the first excited state of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ nucleus.

The exact ground-state density n(r) of the Be atom can be generated, at least in principle, from a local one-body potential energy V(r), which must include however an appropriate contribution from exchange and correlation interactions. Here an exact relation between n(r) and this given V(r) is exhibited by making use of the density matrix variational method set up by Dawson and March for two occupied levels in a one-dimensional system.

Comparisons are made between experiment and theory for the bremsstrahlung cross-section differential in photon-emission angle and energy for incident electrons of 50, 75, and 100 keV, for target elements of C, Al, Cu, Y, Ag, Te, Tb, Au, Bi, and U for photons radiated at 90°. The primary results of this work provide a test of the photon-energy dependence of the doubly differential cross section to an accuracy of 3%. Results for the absolute cross section with an accuracy of 22% are also presented.

Measurements of the double-differential cross section (DDCS), as a function of the ejected energy, angle, and primary energy for electron-impact ionization of molecular nitrogen are reported at incident energies of 200, 500, 1000, and 2000 eV. The ejection angle was varied from 30° to 150° in steps of 15°. The cross sections were obtained by use of a crossed-beam apparatus with an effusive gas source and a pulsed electron beam. Scattered and ejected electrons were energy analyzed by time-of-flight analysis from below 2 eV to the primary energy. The relative measurements were placed on an absolute scale by matching the experimental elastic differential cross sections to absolute measurements at each primary energy. Comparisons of the DDCS with previous reported values revealed significant differences. The DDCS were fitted to a Legendre polynomial expansion as a function of the ejection angle. Platzman plot analysis was carried out on the energy distributions determined from the fit coefficients. The total ionization cross sections at these primary energies were deduced from this plot. An autoionization feature at 2.3 eV was observed for the first time in measurements of this nature and has been assigned as due to a Rydberg state converging to the B ${\mathrm{}}^2$${\Sigma }_u^{+}$ ionic state which decays to the X ${\mathrm{}}^2$${\Sigma }_g^{+}$ ground state of ${\mathrm{N}}_2$${\mathrm{}}^{+}$. An analysis of the autoionization lines observed in the present work in the range 0.4–2.5 eV is also presented.

A new apparatus has been constructed for the measurement of absolute partial electron-impact-ionization cross sections of neutral atoms, molecules, and free radicals. A fast neutral beam is prepared by charge-transfer neutralization of a mass-selected ion beam and is ionized as it crosses an electron beam. From careful analysis of the apparatus, the absolute accuracy of measured cross sections is calculated to be ±12%. Combined with statistical errors, this gives about ±15% for the overall accuracy. Measurements of ionization cross sections for the four rare gases He, Ne, Ar, and Kr from 0 to 200 eV agree with the most reliable previous values within ±6%. The measured Xe ionization cross section is 12% greater than the previous best value. Ratios of double- and triple-ionization to single-ionization cross sections for Ar, Kr, and Xe confirm the recent measurements of Stephan, Helm, and Märk [J. Chem. Phys. 73, 3763 (1980)].

Electron-impact-ionization cross sections have been measured for the halogen atoms F, Cl, Br, and I from threshold to 200 eV. The absolute accuracy is ±14%, except for F which is ±20%. The halogen cross sections have shapes similar to those of the neighboring rare gases but are greater in magnitude. Simple semiclassical formulas of electron-impact ionization are able to accurately predict the experimentally determined halogen to rare gas cross-section ratios.

Single- and double-electron-capture cross sections were measured for 1–2-MeV/u bare ${\mathrm{B}}^{5+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{Si}}^{14+}$ projectiles in collisions with helium atoms. The measured double-electron-capture cross sections are 2–3 orders of magnitude smaller than corresponding single-capture cross sections. The single-capture cross sections are well described by continuum-distorted-wave calculations. For ${\mathrm{B}}^{5+}$, the double-capture cross sections at high velocities are in reasonable agreement with scaled Oppenheimer-Brinkman-Kramers calculations, assuming capture of independent electrons. Cross sections for electron capture and loss are also reported for nonbare ions (${\mathrm{O}}^{6+}$, ${\mathrm{O}}^{7+}$, ${\mathrm{Si}}^{8+}$, and ${\mathrm{Si}}^{13+}$). Single-capture cross sections for nonbare ions are about 30% smaller than corresponding cross sections for bare ions with the same ionic charge q. The reported electron-loss cross sections are well described as ionization of an ion by an incident helium nucleus and two free electrons, using theoretical ionization cross sections in the Born approximation.

Total inelastic cross sections are calculated for collisions of ground-state ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$ with fast electrons according to the revised Bethe theory given by Inokuti et al. The asymptotic forms of the total elastic-scattering cross sections in the first Born approximation due to Inokuti and McDowell and including an electron exchange correction are also evaluated for these molecules. For ${\mathrm{H}}_2$, various accurate configuration-interaction wave functions are used to compute the total inelastic and the integral elastic-scattering cross sections. Good agreement between theory and experiment is found for the total cross sections (inelastic and elastic) for high incident energies as well as incident energies near 1 keV. The cross sections are also given for ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$. Since there are no experimental reports for the integral elastic cross section for ${\mathrm{N}}_2$ or ${\mathrm{O}}_2$ at very high incident energies, the comparison is made only for the experimental data at incident energies near 1 keV. It is found that the theoretical results overestimate the experimental data by about 30% for ${\mathrm{N}}_2$. However, the Bethe total inelastic cross sections are in good agreement with experiments. Since the Born-Bethe approximation may not be valid at lower incident energies, more accurate experimental data at very high incident energies are required for valid comparison with the theoretical results reported here.

Energy distributions of electrons ejected in collisions of light ions with Ar atoms were measured as a function of projectile scattering angle. For this purpose the ion-electron coincidence technique was combined with the time-of-flight method for electron momentum analysis. Thus, the simultaneous detection of electrons with energies from a few hundred meV up to some keV and ions at different projectile scattering angles could be performed. The electron analyzer could observe electrons at ejection angles of 40°–140° with respect to the beam axis. A time resolution of less than 400 ps was achieved with a recently developed position-sensitive ion detector. For the first time, absolute double differential yields for electron emission were measured as a function of impact parameter and electron energy for the system ${\mathrm{H}}^{+}$+Ar at collision energies of 30, 100, and 350 keV. Also L-shell ionization yields were determined for 30-, 100-, and 350-keV ${\mathrm{H}}^{+}$ and 100-keV ${\mathrm{He}}^{2+}$ impact on Ar. The results are compared with predictions of the plane-wave Born approximation, an atomic orbital expansion, and the semiclassical approximation. Total L-shell ionization cross sections were deduced and compared to other measurements.

All the available theoretical calculations of the cross section for the ion-pair formation reaction H(1s)+H(1s)→${\mathrm{H}}^{+}$${\mathrm{H}}^{\mathrm{-}}$(1${\mathit{s}}^2$) have been performed using methods that are only valid at high collision energies. They get good agreement with the experiments for impact energies greater than 25 keV, but fail completely at smaller energies. In this work we report the cross section for this reaction at impact energies less than 10 keV, calculated in the framework of the impact-parameter approximation and using the molecular method with a common translation factor.

Electronic excitation of ${\mathrm{N}}_2$ and CO condensed (i.e., physisorbed) on a metallic substrate has been investigated by high-resolution electron-energy-loss (HREEL) and low-energy electron-transmission (LEET) spectroscopies in the incident energy range 0–19 eV. The HREEL results yield a more complete picture of the spin-forbidden vibronic bands of the solids than previously available from photon spectroscopies. All levels are found to be shifted down by a few tens of meV and broadened with respect to the gas-phase values. As expected, Rydberg states are absent from such spectra. The LEET experiments provide a measurement of inelastic transitions near the electronic excitation threshold. By recording the second energy derivative of LEET spectra, transitions having a strong cross section near threshold are identified by the sharp structure they produce. These are the a ${}^3$Π state in CO and, in ${\mathrm{N}}_2$, the A ${}^3$${\Sigma }_u^{+}$ and B ${\mathrm{}}^3$${\Pi }_g$ states and a new previously unobserved vibronic band near 12 eV. The magnitudes of thresholds cross sections in the solid phase are discussed in terms of single-electron–molecule scattering mechanisms.

The oscillator strength of the (6s6p${)\mathrm{}}^3$$P_1$-(6s7s${)\mathrm{}}^1$$S_0$ transition and the lifetime of the (6s7s${)\mathrm{}}^1$$S_0$ state in ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$ have been measured to be (8.0±1.6)×${10}^{\mathrm{-}4}$ and 45.8±1.0 nsec, respectively. The former value was obtained by use of coherent optical techniques (optical nutation and ac Stark splitting), while the latter was obtained by monitoring cascade fluorescence following pulsed excitation.

The response of an electronic system to the creation of double and multiple core vacancies is investigated. Under certain conditions the calculations are considerably simplified and a linked-cluster theorem can be formulated for the corresponding core-hole spectra. Particular attention is paid to the relation of properties of double and multiple core holes to those of the well-studied single core holes.

A previous study of the collision-induced dipole moment has treated the ${\mathrm{H}}_2$-He complex as a molecule in self-consistent-field and size-consistent coupled-electron-pair approximation [Meyer and Frommhold, Phys. Rev. A 34, 2771 (1986)]. Based on that work, the vibrational dipole transition elements 〈v‖$A_{\lambda L}$(R,r)‖v’〉 associated with the fundamental band (v=0→v’=1) are obtained as functions of separation R of the collisional pair for the isotropic and anisotropic overlap induction components (λL=01 and 21), and the quadrupole- and hexadecapole-induced parts (λL=23 and 45). From these induced dipole components and Meyer, Hariharan, and Kutzelnigg’s isotropic part of the ab initio potential surface, we compute in the (forbidden) fundamental band of hydrogen the collision-induced absorption spectra of the collisional complex of hydrogen (${\mathrm{H}}_2$) and helium from an exact quantum formalism. Both the shape of the computed spectral profiles and the theoretical absolute intensity agree closely with existing measurements at temperatures from 18 to 300 K. The fact that these spectra, and presumably the analogous overtone and ‘‘hot’’ (v>0) bands of the ${\mathrm{H}}_2$-He complex which are not known from measurements, can be accurately obtained from basic principles is significant for research related to the atmospheres of the giant planets and late-type stars.

We have developed a new method for determination of atomic transition probabilities based on laser-induced-fluorescence spectroscopy (LIFS). In the method one produces a known population of atoms in the upper level under investigation and relates it to an observed absolute line intensity. We have applied this method to the argon 430.0-nm line (1$s_4$-3$p_8$): In an argon discharge plasma the 1$s_5$-level population and spatial distribution are determined by the self-absorption method combined with LIFS under conditions where the 3$p_8$-level population is much lower than that of the 1$s_5$ level. When intense laser light of 419.1 nm (1$s_5$-3$p_8$) irradiates the plasma and saturates the 3$p_8$-level population, the produced 3$p_8$-level population and its alignment can be determined from the 1$s_5$-level parameters as determined above, by solving the master equation on the basis of broad-line excitation. By comparing the observed absolute fluorescence intensity of the 430.0-nm line with the above population, we have determined the transition probability to be A=(3.94±0.60)×${10}^5$ ${\mathrm{s}}^{\mathrm{-}1}$. We also determined the 3$p_8$-level lifetime by LIFS. Several factors which might affect the measurement are discussed. The result is τ=127±10 ns.

The energies and wave functions of the 1s, 2s, 2p($m_l$=0), and 2p($m_l$=±1) levels of hydrogen in a uniform magnetic field B (${10}^9$ G≤B≤${10}^{12}$ G) are calculated in two cylindrical adiabatic approximations, each of which includes the influence of the Coulomb field on the radial motion of the electron. For the lowest levels of each symmetry [i.e., the 1s, 2p($m_l$=0) and 2p($m_l$=±1) levels] our calculations provide rigorously both upper and lower bounds on the true level energies and binding energies. We present these cylindrical adiabatic upper and lower bounds on the binding energies together with the spherical adiabatic upper and lower bounds of Starace and Webster as well as all superior variational lower bounds known to us in order to provide in one place stringent tests of past (and future) calculations. The very detailed 1984 eigenfunction expansion results of Rösner, Wunner, Herold, and Ruder are found to be consistent with the upper and lower bounds presented.

A practical solution is provided for the problem of using finite-basis-set methods to calculate the asymmetry parameter, β(E), which determines the angular dependence of photoejection in the dipole approximation. This problem has been an outstanding limitation of basis-set approaches to photoionization in general, which have heretofore been able only to produce the total cross section. The procedure proposed here is given in two forms: an evaluation of the differential cross section in terms of the outgoing flux, and an alternative prescription involving matching to the outgoing wave function. In both cases the method of complex basis functions is used to compute the necessary matrix elements of the electronic Green’s function. Numerical results are reported for β(E) for ionization of the 2p state of atomic hydrogen.

Recoil ions emitted at 70° from ${\mathrm{H}}^{+}$-Ar and at 50° and 70° from ${\mathrm{H}}^{+}$-Kr collisions at 20–70 keV were analyzed by an electrostatic spectrometer. Ions selected in this way result from collisions at very small, essentially zero, impact parameters. Charge states up to 6+ were identified in the spectrum. The cross section for each charge state was measured. The data indicate that these close collisions produce a larger fraction of ions in high charge states than more distant collisions.

We have observed a narrow (0.075-${\mathrm{cm}}^{\mathrm{-}1}$) isolated autoionizing level to undergo a stepwide increase in width at an electric field strength ∼2 V/cm. At such low fields, classical field ionization or ‘‘forced autoionization’’ cannot account for this phenomena. We find that the autoionizing level is narrow because of its high angular momentum (J=5). The level is energetically embedded in the Rydberg series converging to a nearby limit. The model we propose suggests that such narrow autoionizing levels will increase in width near a field F (a.u.)=(1/3)$n^5$ for n values of the Rydberg levels which lie nearby in energy. At this field (Ingliss-Teller limit), adjacent n Stark manifolds begin to overlap, resulting in the onset of complete ‘‘l mixing.’’

In the 280-GHz frequency region several rotational transitions of AgBr in four vibrational states (v=0,1,2,3) have been observed. The analysis resulted in extended and independent sets of Dunham parameters $Y_{01}$, $Y_{11}$, $Y_{21}$, $Y_{02}$, and $Y_{03}$ of the four natural isotopic species ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}_{}^{79}{}_{}{}^{}$Br, ${}_{}{}^{109}{}_{}{}^{}\mathrm{Ag}_{}^{79}{}_{}{}^{}$Br, ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}_{}^{81}{}_{}{}^{}$Br, and ${}_{}{}^{109}{}_{}{}^{}\mathrm{Ag}_{}^{81}{}_{}{}^{}$Br. With these microwave data the constants ${\omega }_e$ and ${\omega }_e$$x_e$ were calculated. The parameters of the Dunham potential $a_0$, $a_1$, $a_2$, $a_3$, and $r_e$ are given.