P. Lablanquie

Efficient production of metastable fragments around the 1s ionization threshold in N2

Metastable formation has been investigated in the inner-shell region of N2. Enhanced N* formation has been observed around the 1s threshold, and is shown to result from dissociation of high N2+* Rydberg states. These N2+* states are populated by spectator Auger decay from core-excited states, as well as by the recapture of slow photoelectrons into the Rydberg orbitals. The present measurement demonstrates that the metastable observation is a new and sensitive tool to study the decay dynamics of core-excited states and the photoelectron recapture due to post-collision interaction.

Dynamics of double photoionization near the Ar 2p threshold investigated by threshold electron?Auger electron coincidence spectroscopy

Threshold electron–Auger electron coincidence spectroscopy measurements were carried out near the Ar 2p thresholds. Such a method allows us, for the first time, to observe the threshold electron yields associated with the selected final states of the Ar2+ ion: 3p4(1D2), 3p4(1S0), 3p4(3P) resulting from the 2p hole Auger decay. All the spectra reveal strong PCI distortion. Comparison with calculations carried out in the framework of a quantum-mechanical PCI model allows us to clarify the mechanisms and the dynamics of threshold electron production. In the 3p4(1D2) and 3p4(1S0) channels, contribution to the threshold electron yield comes essentially from the PCI retardation of slow photoelectrons. In the 3p4(3P) final state channel, an additional process of valence multiplet decay of the 3p4(1D2)6d state plays a role at and below the L2, L3 thresholds.

Multicoincidence measurements of double photoionization in helium

A new toroidal analyser has been used to study the double photoionization of helium, for energies down to 4 eV above threshold. The two electrons are detected with equal energies, and coincidences between them for various angles of emission can be obtained simultaneously. The plane of detection contains the photon beam, in contrast with recent experiments performed in the plane perpendicular to the photon beam. The angular correlation function is extracted directly from the present measurements, which are shown to be insensitive to the Stokes parameters of the photon beam. The width of that function is not observed to vary significantly from 4 eV to 17.6 eV above threshold.

The formation of fluorescent and metastable fragments by photoexcitation of some diatomic molecules in the vacuum ultraviolet region

The formation of fluorescent and metastable fragments from four diatomic molecules, i.e. O2, N2, NO and CO, has been investigated in the vacuum ultraviolet region. The neutral particles are detected by using a microchannel plate stack, where retarding electric potentials prevent charged particles from reaching the detector. Every diatomic molecule investigated here shows the formation of fluorescent and metastable fragments in particular photon energy regions. Three Rydberg states of O2 converging to O2+(a 4Πu) undergo both neutral dissociation processes forming fluorescent [O+(4S)]3s 3S1 and metastable [O+(4S)]3s 5S2; the yield curve for each fragment is determined. Direct and cascade formation of the fluorescent [N+(3P)]3s 4P fragment from N2 are separated, and it is found that the dissociation of the 4s σ Rydberg state converging to N2+(C 2Σu+) preferentially produces [N+(3P)]3p fragments, but not or weakly the [N+(3P)]3s 4P fragment. High -n Rydberg states converging to NO+(c 3Π) and CO+(D 2Π) undergo neutral dissociation described by the core ion model, resulting in large peaks for neutral particle formation.

Angular distribution of the fluorescence of helium doubly photo-excited states converging on the He+(N = 2) ionization threshold

The asymmetry parameter β for fluorescence photons emitted during the radiative decay of the sp2,n+(1Po), sp2,n−(1Po), 2pnd(1Po) and 2pnd(3Do) series of helium doubly excited states, formed by the absorption of linearly polarized synchrotron radiation, has been measured. The results are compared with recent theoretical calculations (Žitnik et al 2002 Phys. Rev. A 65 032520). At high n the β parameter is shown to increase and oscillate due to the excitation of triplet states, optically accessible from the helium ground state due to spin–orbit effects.

Competition between autoionization and dissociation in the [O2+(B 2Σg−)]nl and [O2+(c 4Σu−)]nl Rydberg states investigated by photon-induced dissociation to neutral fragments

Formation of fluorescing and metastable fragments from photon-induced dissociation of O2 has been investigated in the 20.2–21.2 eV and 22.5–25.0 eV photon energy regions under high resolution. The neutral particles are detected using a microchannel plate stack, where electric potentials prevent charged particles from reaching the detector. Yield curves of excited neutral particles exhibit peaks at Rydberg states converging to O2+(B 2Σg−) and O2+(c 4Σu−). Furthermore, these yields are strongly enhanced at high-n, enabling the measurement of their energy location and width. A simple model for the decay of the Rydberg states converging to O2+(c 4Σu−) based on the competition between autoionization and neutral dissociation is used to describe qualitatively the observed spectrum. The results indicate that the lifetime of the O2+(c 4Σu−)v = 0 level is two orders of magnitude longer than that of the O2+(c 4Σu−)v = 1 level.

Theoretical and experimental study of inner-valence-shell satellites above the double-photoionization threshold of argon

The Ar photoelectron lines located above the double-ionization threshold have been investigated both theoretically and experimentally. Their Auger decay, sorted in a coincidence experiment, is found to occur predominantly to the Ar2+ (1 D) final state. Calculations demonstrate that while some photolines can be labelled as 3s3p5 (3 P)nl , none of them can be described as 3s3p5 (1 P)nl because such a configuration is always weaker than doubly excited ones in the configuration-interaction (CI) description of the state. This is evidence of `second-order satellites, which we define as satellites dominated by a doubly excited configuration. The intensity of the photolines in the 45-58 eV binding energy region is qualitatively demonstrated to reflect correlation in the Ar(1 S) ground state, and is related to the singly and doubly excited configurations in the CI development of the latter.

Symmetry-favoured double photoionization of argon

Experiments where the energies and angles of the two emitted electrons are resolved have been performed for double photoionization of argon in the valence shells. So far such experiments were always restricted to the final states of doubly charged rare gases and to helium. Here we report on first measurements for the state of and we show that the symmetry of the residual ion leads to a situation which is completely new compared with all previous studies, since the two symmetries which are allowed for the pair of outgoing electrons ( and ) both have a `favoured character. As a consequence the observed angular patterns exhibit a sharp maximum when the two electrons fly in opposite directions with equal energies, whereas a node exists in the unfavoured cases such as helium. Dynamical effects which appear when the photon energy is varied are also discussed.

Detailed observations of photo-accessible triplet doubly excited states in helium

High resolution synchrotron radiation has been used to make extensive measurements of the doubly excited states converging on the He+(N = 2) ionization threshold of helium. The excitation of two series of triplet doubly excited states, 2pnd(3Do) and sp2,n−(3Po), have recently been observed by Penent et al (2001 Phys. Rev. Lett. 86 2758). In this paper the excitation of a third weakly excited series of triplet doubly excited states, 2pnd(3Po), which was suggested by the previous measurements, is confirmed. The metastable atom yields, resulting from the radiative decay of these three observable series of triplet doubly excited series, are reported and compared to the recent theoretical calculations of Žitnik et al (2002 Phys. Rev. A 65 032520).

Site-specific Auger electron spectra of ethyl trifluoroacelate molecules studied by magnetic bottle electron spectrometer

We performed multielectron coincidence measurements for inner-shell photoionizations of ethyl trifluoroacelate molecules (C4H5F3O2) using a magnetic bottle electron spectrometer. From a two dimensional coincidence map between a photoelectron and Auger electron for C 1s ionizations, we extracted site-specific Auger electron spectra for each carbon site and corresponding binding energy of doubly charged states.