Complete characterization of photodetachment
processes in the xuv regime using FLASH
H. B. Pedersen, L. S. Harbo, A. Becker1, S. Dziarzhytski2, C. Domesle1, N. Guerassimova2,
and A. Wolf1
Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
1Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany
2HASYLAB at DESY, Hamburg, Germany
Photodetachment processes of negative ions provide a detailed probe into fundamental properties of
atomic and molecular systems [1] in particular regarding dynamics dominated by electron
correlation. At synchrotron radiation facilities, photodetachment has indeed been studied with
beautiful experimental setups [2-3] where beams of ions and photons are merged co-linearly and
where resulting heavy charged fragments are observed: benchmarking results of wavelength
resolved absolute cross sections for especially double electron photodetachment (A- + e-  A+ +
2e-) have thus been obtained for a number of cases involving both inner-shell and valence-shell
electron detachment. However, these experimental systems are inherently limited to observe the
charged atomic fragments, whereas neither neutral fragments originating for instance from single
electron detachment (A- + e-  A0 + e-) nor emerging photoelectrons are accessible. For example,
a general problem in the understanding for photodetachment processes in the xuv regime is the
significance of single detachment compared to double (and multiple) detachment processes.
Moreover, an experimental analysis of the emitted photoelectrons would evidently ease the
identification and interpretation of particular electron detachment mechanisms.
With an exploratory study of the photodetachment of the oxygen anion at 41.7 nm (29.8 eV)
performed at FLASH [4], we have demonstrated that these two experimental limitations can be
overcome by using an experiment where ions and photons are brought to interact in a crossed
beams geometry. The experimental station called [5-6] TIFF (Trapped Ion Fragmentation with a
FEL) is set up at the Plane Grating monochromator (PG2) beam line [7] of FLASH. Figure 1
displays schematically the TIFF station in the region around the ion-photon crossing zone. By
using the electron detectors (eDET 1-2) and one of the fragment detectors (DET 2) all emitted
photofragments from the detachment reactions of O- anions, i.e. fragments O0, O+, and e-, can be
detected in coincidence. The heavy neutral (O0) and charged (O+) fragments are separated in time
and position by applying a retarding field in the region of the electrostatic mirror.
Figure 1: Schematic drawing of a part of the TIFF experiment [5-6] around the ion-photon
crossing region. All emerging particles from photodetachment processes can be detected
ion coincidence.
Figure 2: Detection of reaction products after photodetachment of O- at 41.7 nm. (a) Heavy
fragments detected on DET2. (b) Electrons detected on eDET 1 in coincidence with O0 on DET
2. Dashed lines show distributions obtained from Monte Carlo simulations.
Figure 2(a) displays the observed distributions of Time of-Flight (TOF) for particles impacting on
DET 2 as referenced to the arrival time of the FLASH photon pulse and corrected for background.
The two types of heavy fragments, O0 and O+, resulting from the detachment process are readily
identified as indicated. From the absolute counts in the peak corresponding to O0 and using the
values for ion current, photon intensity, detector efficiency, and beam overlap for the present
experiment, an absolute cross section for single detachment of σ1e = (2.1 ± 0.6) × 10−19 cm2 is
derived at 41.7 nm. From the relative number of counts in the two peaks, the ratio of single and
double detachment is found to be σ1e/σ2e = 4.12 ± 0.17. Our work [4] is in fact the first direct
determination of a ratio of single and double detachment in the xuv regime and explicitly shows the
dominance of the single detachment process that has so far not been observable in experiments
using synchrotron radiation.
The detection of ejected photoelectrons poses a more complicated experimental challenge since the
electrons arising from photodetachment must be identified within a total signal dominated by a
background of photoelectrons stemming from ionization of the residual gas in the vacuum chamber.
Photoelectron detection can, however, be accomplished by requiring coincidence between detected
events on DET2 and events on the electron detectors. Figure 2(b) shows the resulting distribution of
the electron TOFs toward eDET 1, requiring coincidence between electrons and neutral O0 particles
detected on DET 2 and subtracting the background from random coincidences. By comparison to
Monte Carlo simulations, the distribution can be interpreted to originate from detachment processes
leading the neutral oxygen in either the 3P ground state or the 1D first excited state, excluding the 1S
final level
Summarizing, our exploratory study of the oxygen anion at 41.7 nm demonstrates how all channels
of photodetachment processes can be analyzed simultaneously in a crossed beams setup, taking
particular advantage of intense free-electron laser sources in the xuv regime. The present results
complement research on anion photodetachment performed with synchrotron radiation since all
emerging fragments including neutrals and photoelectrons can be detected simultaneously, allowing
a complete characterization of such processes.
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