C. Cisneros

A time-resolved study of ionization, electron attachment and positive-ion drift in methane

A pulsed Townsend swarm technique has been used to study electron-ionization and electron-attachment processes in methane over the density-normalized electrical field strength, , in the range 1.2-30 fV (120-3000 Td). The time-resolved ionic avalanches were obtained under conditions such that the occurrence of electron-attachment processes was detected unambiguously. The analysis of these avalanche pulses resulted in values of the effective ionization coefficients and positive-ion mobilities over the above range. To the best of our knowledge, no values of positive-ion mobilities had been published before for Td. The electron-attachment effects were observed up to Td.

ABSOLUTE PHOTOIONIZATION CROSS SECTION MEASUREMENTS OF O ii IONS FROM 29.7 TO 46.2 eV

Absolute photoionization cross sections have been measured for a mixture of ground-state and metastable O ii (O + ) ions at photon energies ranging from 29.9 to 46.0 eV (414.7 to 269.5 A ˚ ). All measurements were performed by merging an O + beam with synchrotron radiation from an undulator beam line at the Advanced Light Source (ALS). At a resolution of 17 meV, more than 70 spectral features have been resolved, most of them identified and characterized. These measurements are compared with two independent R-matrix calculations and the data in TOPbase. All three calculations agree within 25% on the direct photoionization cross section, and with the absolute measurements within 40%. Some differences are noted in the predicted resonance positions among the three close-coupling R-matrix calculations, the TOPbase data being the least accurate. The estimated total experimental uncertainty varies from 15% to 20%. Such measurements benchmark theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project. Subject headings: atomic data — atomic processes — methods: laboratory

Photoionization of C2+ ions: time-reversed recombination of C3+ with electrons

We have investigated photoionization (PI) of the 1S ground state and 3Po metastable states of C2+ ions in the photon energy range 40.8-56.9 eV at a resolution of 30 meV. Absolute PI cross sections have been measured using a photon-ion merged beam arrangement at the Advanced Light Source. Detailed calculations using the semi-relativistic Breit-Pauli R-matrix approach suggest a fraction of 40% of metastable ions in the primary beam of the experiment. The present results are discussed in the light of previous electron-C3+-ion photorecombination (PR) studies. As an example, the role of the intermediate C2+(2p4d 1P) resonance in both PI and PR is analysed.

Lifetime of a K-shell vacancy in atomic carbon created by 1s → 2p photoexcitation of C+

Lifetimes for K-shell vacancy states in atomic carbon have been determined by measurement of the natural linewidth of the 1s → 2p photoexcited states of C + ions. The K-shell vacancy states produced by photoionization of atomic carbon are identical to those produced by 1s → 2p photoexcitation of a C + ion: 1s2s 2 2p 22 D, 2 P, and 2 S autoionizing states occur in both cases. These vacancy states stabilize by emission of an electron to produce C 2+ ions. Measurements are reported for the lifetime of the 1s2s 2 2p 22 D, 2 P and 2 S autoionizing states of C + :6 .3± 0.9 fs, 11.2 ± 1.1 fs and 5.9 ± 1.3 fs respectively. Knowledge of such lifetimes is important for comparative studies of the lifetimes of Kshell vacancies in carbon-containing molecules, benchmarking theory, and interpreting satellite x-ray spectra from astrophysical sources such as x-ray binaries. Absolute cross sections were measured for both ground-state and metastable-state ions providing a stringent test of state-of-the-art theoretical calculations. Carbon is ubiquitous in nature and is the building block of life. This atom in its various stages of ionization has relatively few electrons, and is thus amenable to theoretical study. Lifetimes

Valence-shell single photoionization of Kr+ ions: Experiment and theory

Photoionization of Kr

Measurement of ionization and electron transport in methane-argon mixtures

^+

A theoretical and experimental study of the photoionization of AlII

ions was studied in the energy range from 23.3 eV to 39.0 eV at a photon energy resolution of 7.5 meV. Absolute measurements were performed by merging beams of Kr

Photoionization of isoelectronic ions: Mg + and Al 2 +

^+

Polar dissociation of vibrationally de-excited H3+ and D3+ in collision with He

ions and of monochromatized synchrotron undulator radiation. Photoionization (PI) of this Br-like ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. Resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurements are benchmarked against state-of-the-art theoretical cross-section calculations from the Dirac-Coulomb R-matrix method.

Photoionization of ions of the nitrogen isoelectronic sequence: experiment and theory for F2+ and Ne3+

We used a pulsed Townsend technique to measure the effective ionization coefficients, electron drift velocities and positive ion drift velocities in methane-argon mixtures over the combined density-normalized electric field intensity, E/N, range from 0.05 to 700 × 10-17 V cm2. The mixture studied contained 0.5, 3, 25, 50 and 75% CH4, including pure methane and pure argon. We found a well defined dependence of the effective ionization coefficient on the amount of CH4 in the mixture at low E/N, while at the higher E/N end, all the curves merged into a single one. The electron drift velocities in the mixture show a pronounced negative differential conductivity region, the maxima and minima of which depend on the mixture ratio.