F. Penent

Threshold photoelectrons coincidence spectroscopy of N22+ and CO2+ ions

Abstract Threshold photoelectrons coincidence (TPEsCO) spectra of the nitrogen and carbon monoxide dications have been recorded with improved resolution (≈ 40 meV) allowing us to clearly resolve the lowert three vibrational progressions in each species. For the nitrogen dication we have identified, for the first time, the 3 Σ g − state predicted by theory and determined its constants. In the case of the CO dication, we have determined that the ν = 0 level of the 1 1 Σ + state is almost degenerate with X 3 Π, ν = 1 and is thus one vibrational quantum lower than previously believed.

Double photoionization of below the double ionization potential

Three separate experiments demonstrate that an indirect process of double ionization can populate part of the potential energy surface of the ion at energies below the lowest bound vibrational level. The process seems to involve autoionization at a curve crossing of a singly ionized (or possibly neutral) state with a repulsive part of the adiabatic ground state doubly charged ion surface.

Investigation of the CS22+ dication using threshold photoelectrons coincidence spectroscopy

Abstract A threshold photoelectrons coincidence (TPEsCO) spectrum of CS2 has been recorded in the 26.8–29.8 eV range with a resolution of 40 meV using a synchrotron radiation source. The X 3 Σ − g , a 1 Δ g , b 1 Σ + g and c 1 Σ − u states of CS22+ have been observed in the spectrum and their double ionisation potentials determined. Vibrational structure of these states was also observed for the first time and comparison with recent theoretical results allowed its tentative assignment to vibrational progressions associated with the symmetric stretching mode. The experimental spectrum also revealed important contributions from indirect photoionisation processes in the formation of CS22+ from the neutral CS2 molecule.

Very low energy electron-H(2p) interaction studied in H−-rare-gas collisions by translation electron spectroscopy

Detached electron energy spectral have been observed in the forward direction for collisions of H- ions with helium, neon and argon gas targets at 4 keV. Specifically, the channel corresponding to H(2p) excitation has been examined by recording the electron energy distributions in coincidence with Lyman- alpha photons. Near the 0 observation angle, electrons with very low energy in the projectile frame (1<Ep<100 meV) are observed that are kinematically shifted in the laboratory and which yield important information on the H(n=2)-e- interaction near threshold. The spectra reveal, in addition to the well known 1Po shape resonance of H-, a cusp typical of convoy electrons which is attributed to the long range dipole interaction between H(n=2) and an electron. In addition, weak structure near 4 meV above the H(2p) threshold could constitute further evidence in support of the existence of dipole oscillations.

Post-collision interaction effects in near-threshold Ar -shell photoionization

Near-threshold photoionization of argon in the () inner-shell region has been studied by means of threshold photoelectron - photoion coincidence spectroscopy. The observed line profiles are clearly influenced by the post-collision interaction effect and this influence is observed to be stronger for double than for single-Auger decay. The spectra have been interpreted using a classical theory which takes into account the possibility that a single photoelectron can interact with several Auger electrons.

Positive ion production in halogen negative ion collisions

Presents the results of electron spectroscopy and total cross section measurements of one- and two-electron loss processes in collisions of halogen negative ions with various gases for collision energies below 5 keV. The electron spectroscopy study illustrates that the production of singly excited autoionising states of the halogen atoms can be important for energies as low as 100 eV and is responsible for positive ion production, as has been also recently shown for O- collisions by Penent et al. These conclusions concerning positive ion production should also be applicable to collisions involving the parent atoms, being a characteristic of these systems rather than that of the negative ion.

Excitation and decay of H-**(n = 3) Feshbach resonances in H-/rare gas collisions

Excitation of H−** resonances in H− /rare gas collisions for energies from 2 to 7 keV has been studied. The resonances are observed using translational electron spectroscopy. In addition to well known Feshbach resonances lying below H (n = 2) and decaying to H(1s), we have identified Feshbach resonances lying below H (n = 3) decaying to H (n = 2) and H(1s). The resonance profiles exhibit strong interference patterns, depending on the target gas. Resonances below H (n = 4) have also been observed. Excitation of shape resonances lying above the H (n = 3) threshold is also suspected.

Zero-kinetic-energy electrons at threshold in core ionization. The shape of the yield curve

The question of the shape of the curve that describes the yield of zero-kinetic-energy electrons near the threshold for ionization of an inner shell is considered. The theoretical situation appears to be ambiguous as to whether or not the theoretical curves should have an additional Lorentzian broadening. Comparison with experimental data for argon ionization shows a significantly better fit to the data with curves that have been broadened than with those that have not.

Translation electron spectroscopy study of the 1P0 shape resonance of H- excited in H- rare-gas collisions (0.5-6 keV)

The study of electron spectra in H- -rare-gas collisions (0.5-6 keV) at low observation angles ((5) reveals structure due to decay of the H- * (2s2p 1 P0 ) shape resonance to H (n=2). Projectile kinematics allow the transformed cross section in the H- frame to be obtained with a resolution that can be as high as 1 meV which then yields the resonance parameters. At very low angles ((1) a cusp is observed due to zero-energy electrons in the projectile frame, implying a nonzero cross section at threshold. This behaviour originates from a long range potential interaction which can be the dipole interaction between the excited hydrogen atom and the detached electron. Furthermore the authors have observed a target effect on the angular distribution of these near-zero energy electrons which could be related to the target polarizability. Very weak structure in the electron spectra corresponding to energies below 4 meV could be the first observation of the predicted dipole oscillations.

Role of cascade double Auger decay in the PCI distortion of the Ar 2s photoelectron line

We have investigated both experimentally and theoretically the post collision interaction effect on Argon 2s photoelectrons in the case of cascade double Auger decay. New features of this effect are revealed.