Cross sections for the ejection of electrons, differential in the angle and energy of emission, were measured for proton collisions with two molecular gases, oxygen and carbon dioxide, over the primary energy range of 7.5–150 keV and an angular range of 10° to 160°. The energy distributions, obtained by integration over the angle, were fitted by an analytical model. A discrepancy in the angular distributions compared to those of Gibson and Reid [J. Phys. E 17, 1227 (1984); J. Phys. B 19, 3265 (1986); Radiat. Res. 112, 418 (1987); Australian Atomic Energy Commission Report No. AAEC/E659, 1987 (unpublished)] is discussed. At energies up to 50 or 100 keV, the angular distributions were found to be largely independent of the ejected electron energy and very similar for different targets.

Cross sections for single and double ionization of helium resulting from pure ionization and from electron transfer (capture or loss) are reported for 25–1000-keV/u hydrogen-atom impact. The single-target ionization data are compared with existing first Born calculations. Good agreement is found above 200 keV, and inadequacies for specific channels at lower energies are identified. Double-target ionization is shown to occur more frequently for ${\mathrm{H}}^0$ impact than for fast electron or proton impact. It is also demonstrated that, for impact energies greater than 150 keV, double ionization of helium results from a single projectile-target interaction.

We have determined absolute densities of vibrationally excited hydrogen molecules with ν’’ up to 5 and atoms in a discharge, using resonance-enhanced multiphoton ionization. All measurements were done as a function of discharge voltage, discharge current, and gas pressure. The occupation of ν’’=3–5 is strongly superthermal. This effect is more pronounced at higher discharge currents. Scaling laws for the dependence of the density of various particles on the discharge parameters are presented. The analysis shows that vibrationally highly excited molecules are predominantly produced by primary electron excitation and deexcited on each wall collision. H-atom densities have also been determined by resonance-enhanced multiphoton ionization. The atoms are produced by dissociation of molecules by primary electrons, dissociation of molecules on the filaments, and collisions between positive ions and electrons and destroyed by recombination on the walls. Our analysis shows that the recombination coefficient of atoms on the walls is ≊0.1.

This paper reports measurements and calculations of absolute differential cross sections for ${\mathrm{H}}^{+}$-He charge-transfer and direct scattering. Charge-transfer measurements have been obtained at 5.0 keV laboratory energy over the laboratory angular range 0.02°–1.0°, while direct scattering has been observed at 0.5, 1.5, and 5.0 keV laboratory energy over a closely corresponding angular range. Calculations are reported for 0.5, 1.5, and 5.0 keV charge-transfer and direct scattering. The measured cross sections are in good agreement with those derived from fully quantum-mechanical molecular-orbital close-coupling calculations. Direct scattering cross sections are also found to be in agreement with single-potential calculations using directly summed Jeffreys-Wentzel-Kramers-Brillouin phase shifts derived from proposed ${\mathrm{H}}^{+}$-He ground-state interaction potentials. The cross sections exhibit significant structure over the range of angles and energies studied. The measured cross sections have been integrated over the experimental angular range providing absolute integral cross sections for comparison with theoretical results and previously measured total cross sections.

Kinetic emission of secondary electrons (SE) in high-energy ion-solid collisions is strongly correlated to the strength and gradient of the electronic stopping power of the projectile at the entrance and the exit surfaces of the solid. We have measured the total, forward, and backward SE yields of ${\mathrm{S}}^{\mathrm{q}+}$ (3.9 MeV/u) and ${\mathrm{C}}^{\mathrm{q}+}$ (1 MeV/u) passing through carbon foils as a function of the initial charge state q (S, 10–16; C, 2–6) and of the target thickness ρx (3–180 μg/${\mathrm{cm}}^2$). The strong increase of the total and backward SE yields with q is discussed in the framework of preequilibrium stopping and effective charge of the projectile ion at the entrance of the solid surface. It reveals shell effects of structured projectiles.

A classical phase-space model of the hydrogen molecule is presented and applied to the study of the electron-capture and -ionization processes in collisions of fully stripped ions with ${\mathrm{H}}_2$ at intermediate impact energies and charge states from 1 to 10. The model is based on the independent-electron and the impact-parameter approximations. The electron–impinging-ion and electron–target-nuclei interactions are exactly taken into account. The interaction between the electrons is approximated by model potentials. The calculated total cross sections for production of free electrons and capture of one electron are in good agreement with different experimental data. The ratio between the capture cross sections from molecular and atomic hydrogen targets is also analyzed and compared with available empirical scalings. It is found that this ratio varies from a value less than 1 at low impact energy to 4 at higher energies. The reasons for these differences are discussed. A comparison is made between the capture cross sections for different orientations of the hydrogen molecule.

Differential cross sections for the electron-impact excitation of ions are calculated and compared along the He-like isoelectronic sequence. A distorted-wave method is used to obtain the cross sections for the transitions 1 ${}_{}{}^1{}_{}{}^{}2$ ${}_{}{}^1{}_{}{}^{}S$ or 2 ${}_{}{}^1{}_{}{}^{}P$ in ${\mathrm{Li}}^{+}$, ${\mathrm{O}}^{6+}$, and ${\mathrm{Si}}^{12+}$. To complete the systematic study, a comparison is also made to the cross sections for neutral He and in the limit Z→∞ (Z being the nuclear charge). Dependence on the collision energy and comparisons to the cross sections obtained by other methods are discussed in detail. For the 2 ${}_{}{}^1{}_{}{}^{}P$ excitation, an apparent generalized oscillator strength is derived for the ions using the present differential cross section. Their behavior is very different from the corresponding values of He.

The electron-stimulated ${\mathrm{O}}^{\mathrm{-}}$ desorption yield from condensed ${\mathrm{O}}_2$ exhibits three peaks around 7, 8.5, and 13 eV, while that from ${\mathrm{O}}_2$ gas shows only the 7-eV peak. The 7-eV peak is known to arise from the ${\mathrm{O}}_2^{\mathrm{-}}$(1${\mathrm{\pi }}_{\mathrm{u}}^{\mathrm{-}1}$1${\mathrm{\pi }}_{\mathit{g}}^2$${)}^2$${\mathrm{\Pi }}_{\mathit{u}}$ compound state. In this paper the 8.5- and 13-eV peaks are shown to arise from the ${\mathrm{O}}_2^{\mathrm{-}}$(3${\mathrm{\sigma }}_{\mathit{g}}^{\mathrm{-}1}$1${\mathrm{\pi }}_{\mathit{g}}^2$${)}^2$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ and ${\mathrm{O}}_2^{\mathrm{-}}$(2${\mathrm{\sigma }}_{\mathrm{u}}^{\mathrm{-}1}$1${\mathrm{\pi }}_{\mathrm{g}}^2$${)}^2$${\mathrm{\Sigma }}_{\mathrm{u}}^{+}$ compound states, respectively. The absence of the 8.5- and 13-eV peaks in the gas-phase spectrum presents the first experimental evidence for a forbidden electron attachment, which is due to the ${\sigma }^{\mathrm{-}}$ selection rule [H. Sambe and D. E. Ramaker, Chem. Phys. Lett. 139, 386 (1987)].

Elastic scattering of positrons from argon has been calculated for incident-positron energies of up to 300 eV. The target atom is represented by a frozen core in a continuum relativistic Hartree-Fock calculation with dipole- and quadrupole-polarization corrections. The cutoff parameter $r_0$ is fitted to reproduce measured values of the scattering length. Scattering lengths near -4 a.u. give good agreement of the calculations with recent measurements of total, differential, and momentum-transfer cross sections below 3 eV. Relativistic and positron-polarization effects are found to be much smaller than for electrons. The relativistic effects are dominated by the relativistic contraction of the target atom.

Calculations of the photodetachment cross section of ${\mathrm{F}}_2^{\mathrm{-}}$ in the 3${\sigma }_u$ channel are reported in both the static-exchange and the first Born approximations. A high-energy resonance feature is found in the static-exchange result, which is evidently not associated with tunneling through an angular momentum barrier. The behavior of this feature with varying internuclear distance is explored, and the possible effects of channel coupling on this resonance are discussed.

Scattering of charged particles at high velocity from diatomic molecules is formulated in the impact-parameter treatment using a Fourier transform of the amplitude in the quantum-mechanical treatment. Probabilities and total cross sections are evaluated for electron capture for a fixed orientation of the intermolecular axis. Total cross sections, integrated over projectile impact parameters, for capture from ${\mathrm{H}}_2$ predict variations of a factor of 2 or more for various orientations of the molecular axis. Capture from heteronuclear diatomic molecules is also considered.

The theory of the nonrelativistic potential scattering in the presence of a strong laser field is extended to consider a fluctuating radiation field, having a chaotic non-Markovian statistics. The statistical properties are assumed on the electric field and, carrying on a suitable approximation on the characteristic time scales entering the process, the stochastic properties of the vector potential are derived. As a consequence of the assumed non-Markovian statistics, the field spectrum is non-Lorentzian, and its role on the temporal coherence of the field is discussed. Scattering linewidths and line shapes, as well as coherence factors as functions of intensity, are calculated and discussed in the context of recent experimental results obtained with multimode fields.

The photoionization cross section is analyzed as a sum of resonance contributions plus background. The contribution of each resonance is proportional to the square of a complex transition-dipole matrix element divided by a complex energy denominator and has a natural asymmetric line shape that is the superposition of a dispersive and an absorptive Lorentzian component.

We report tunable-laser photodetachment of ${\mathrm{F}}^{\mathrm{-}}$ and ${\mathrm{Br}}^{\mathrm{-}}$ ions near the first threshold corresponding to the atomic ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{3/2}^{}{}_{}{}^{}$ ground state. Using a high-resolution pulsed uv laser calibrated with molecular iodine absorption and doubly crossed laser and ion beam technique, we determine the electron affinities of fluorine and bromine with high accuracy. These determinations account for the hyperfine structure of the atomic ground states of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ and ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$${,}^{81}$Br. The threshold values obtained for the transition between the ion ground state ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ and the atomic ground hyperfine state ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{3/2}^{}{}_{}{}^{}$ are 27 432.440(25) ${\mathrm{cm}}^{\mathrm{-}1}$ or 3.401 190(4) eV for fluorine and 27 129.170(15) ${\mathrm{cm}}^{\mathrm{-}1}$ or 3.363 590(3) eV for bromine, and the isotope shift of ${\mathrm{Br}}^{\mathrm{-}}$, ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$ is found to be below our experimental resolution.

The resonance lines of berylliumlike and lithiumlike molybdenum have been observed in high-temperature Joint European Torus tokamak discharges into which molybdenum was introduced using the laser blow-off method. The measured transition wavelengths are 2$s^{2\mathrm{}1}$$S_0$–2s2p ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ 49.904±0.03 Å, 2$s^2$ ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$–2s2p ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ 137.787±0.03 Å for Mo xxxix, and 2s ${}_{}{}^2{}_{}{}^{}\mathrm{S}_{1/2}^{}{}_{}{}^{}$–2p ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{1/2}^{}{}_{}{}^{}$ 143.998±0.02 Å, 2s ${}_{}{}^2{}_{}{}^{}\mathrm{S}_{1/2}^{}{}_{}{}^{}$–2p ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{3/2}^{}{}_{}{}^{}$ 58.499±0.02 Å for Mo xl. For the berylliumlike ion the results are compared with the semiempirical predictions by Edlén [Phys. Scr. 28, 51 (1983)], while the results for lithiumlike Mo are compared with the calculations by Johnson, Blundell, and Sapirstein [Phys. Rev. A 37, 2764 (1988)].

Spectral line broadening due to resonance absorption and induced emission is investigated for general spectral distributions of the radiation field using the frequency-dependent relaxation operator method. An analytic self-consistent solution is found for the case of equal spectral profiles of field and radiative cross sections. The often-assumed Gaussian distribution of the field in saturated amplifiers is shown to yield symmetrical doublet structure for the radiative cross sections if the induced rates are larger than or comparable to the Gaussian width. A similar calculation was therefore performed for double Gaussian field spectra that reproduces such doublet structure for the cross sections in the case of narrowly spaced Gaussians. Self-consistent solutions of gain and line-broadening equations are accordingly expected to favor the sidebands, and at very high powers and gains amplified-spontaneous-emission lines should generally split into two lines of equal intensity separated by an averaged Rabi frequency. Since the cross-section profiles vary along the amplifier, the overall gain may be diminished, although the local cross sections have enhanced maxima at high powers.

We determined the absolute partial photoionization cross section σ and the angular distribution parameter β of the 3d, 3p, and 3s photoelectrons in atomic manganese for 65≤hν≤250 eV. The total photoionization cross section is in satisfactory accord with the single-particle Hartree-Slater calculation and in very good agreement with a relativistic time-dependent local-density-approximation calculation. Binding energies and intensity ratios of the 3$s^{\mathrm{-}1}$ and 3$p^{\mathrm{-}1}$ multiplet lines were obtained. Five lines were identified within the 3$p^{\mathrm{-}1}$ multiplet, and the measured spacings are close to those found in solid ${\mathrm{MnF}}_2$. Similarly, the spacings in the 3$s^{\mathrm{-}1}$ multiplet are equal to the ones measured in solid ${\mathrm{MnF}}_2$. There is good accord between our results for the angular-distribution parameter of the 3$p^{\mathrm{-}1}$ ${(}^7$P) line and those of Malutzki et al. [J. Phys. B 18, 1735 (1985)]. The intensity ratios of ${}_{}{}^5{}_{}{}^{}\mathrm{P}$(1) to ${}_{}{}^7{}_{}{}^{}\mathrm{P}$ and ${}_{}{}^5{}_{}{}^{}\mathrm{S}$(1) to ${}_{}{}^7{}_{}{}^{}\mathrm{S}$ were seen to increase slowly somewhat above their thresholds. More rapid variations of all multiplet ratios were found within about 30 eV of threshold.

Relative transition probabilities of 29 lines belonging to the 3s-3p, 3s-4p, 3p-3s’’, and 2s2$p^4$-2$s^2$2$p^2$4p transition arrays of N i have been measured in emission with a wall-stabilized arc and were normalized to an absolute scale utilizing two recent lifetime results. Our experiment was undertaken to help resolve some long-standing discrepancies in the nitrogen data. Our results are usually slightly larger than two earlier emission measurements but are in closer agreement with recent calculations. The total uncertainties of our individual transition-probability values are estimated not to exceed a range from ±12 to ±15%, the larger uncertainties occurring for the very weak lines.

We analyze the effect of a dc electric field on classical chaos in one-dimensional hydrogen in a microwave field in the n nonmixing regime and also in the inter-n-mixing regime where significant dc field-induced ionization occurs. We study the ac field-induced nonlinear classical resonances, the threshold of chaos, and the number of states trapped in the resonances. In the strong-n-mixing and ionizing regime (unclamping dc field), we find the chaotic dynamics depend sharply on the dc field and the number of states trapped in the resonances, allowing the system to undergo a transition from a regime of classical behavior to a regime of uniquely quantum behavior as the dc field is changed. We show that ionization by classical chaos competes favorably with ionization by tunneling in the transition region, and that tunneling allows very sensitive spectroscopy of this region.

We investigate the effect of intense, low-frequency fields on the structure of atomic hydrogen (prepared in the presence of an external dc electric field). The em field (of amplitude $E_0$ and frequency ω) interacts with a low-lying (n in the range of 2–7) two-level system of ‘‘one-dimensional’’ hydrogen [transition frequency ${\omega }_0$=(N+1)ω and transition dipole moment μ]. The field couples strongly to the permanent dipole moments of the two levels ($d_1$ and $d_2$), creating a large number of equally spaced sidebands (spacing equal to ω) that share μ among themselves. The distribution of the oscillator strength depends on the universal parameter ($d_2$-$d_1$)$E_0$/ω. We determined the transition moment ${\mu }_N$ of the Nth sideband for a variety of ladder and off-ladder systems. Our results indicate that at sufficiently high intensities that are comparable to the threshold of classical chaos of the many-level hydrogen-atom system, off-ladder sidebands become as strong as ladder sidebands, thus breaking the one dimensionality of the system.