Time-of-Flight PEEM

A. Oelsner (now Surface Concept GmbH), M. Cinchetti, A. Gloskowski, G. Schönhense

Cooperation in the early stage of the development:
H. Spiecker (LaVision Biotec GmbH)
In time-of-flight (ToF) PEEM [1] the advantages of ToF energy dispersion are combined with high lateral resolution. The electrons that are emitted from the sample surface with different energies are dispersed in a drift tube at low beam energy thus retaining a linear column of the imaging optics, see Fig.1. Two methods of fast image detection have been explored: an ultrafast gated intensified CCD camera (≥300 ps gate time) [2] and later a 3D (x,y,t)-resolving delayline detector (time resolution 140-240 ps) [3]. The latter device has a lateral resolution of 20 -100 μm (depending on the layout of the anode) in the image plane, being equivalent to up to more than 1000 pixels along the image diagonal. An energy resolution of < 100 meV can easily be achieved in spectroscopic imaging, with potential of improvement to the 10-20 meV region. The operation scheme of a ToF energy filter in comparison with a dispersive energy filter is shown in Fig. 1. The potential of TOF photoemission spectromicroscopy in a PEEM was demonstrated for pulsed synchrotron radiation (in the soft X-ray range) and femtosecond lasers (in the visible and infrared spectral range). For a review, see [4].


Figure 1: Comparison of time-of-flight (a) with dispersive electron spectroscopy (b) (from [4]).


If the ToF drift tube is operated at low pass energies (typically 50–100 eV), the energy distribution of the electrons is spatially dispersed, leading to different arrival times like in conventional ToF electron spectroscopy. As the drift tube is part of the microscope column, the image is retained and can be recorded by a time-resolving detector. At the end of the drift tube we find a spatiotemporal dispersion of the energy distribution of the electron beam along the electron optical axis as illustrated in Fig.1. The time-resolving detector allows observing the energy distribution along z, depicted in (a), in a manner similar to a conventional dispersive analyzer e.g., of hemispherical type, as depicted in (b). It is near at hand that the electron optical properties of the ToF arrangement bear principal advantages compared with a common dispersive arrangement because a linear electron optical axis z is retained. The dispersion along x, perpendicular to the electron momentum direction z in image (b) requires elaborate techniques to avoid additional aberrations in imaging dispersive analyzers.

Figure 2 shows the schematic layout of a ToF PEEM and calculated time/energy resolution as functions of the drift energy. At typical drift energies between 10 and 100 eV and the present time resolution of the DLD (150 ps), the energy resolution lies between 10 and 100 meV. As the retardation is done behind the PEEM lenses, the image size is already large, so that the ToF section does not induce significant additional aberrations. Typical examples are shown in Fig. 3 and in the next section.

A crucial element of a ToF PEEM is the delay line detector (DLD), a position (x,y) and time (t) sensitive microchannel plate area detector with special delayline anode. It allows imaging of single counting events with high acquisition rates of up to 107 counts per second with a very high dynamic range of 106. Unlike for other pico-second imagers, delay line detectors collect all incoming particle hits continuously without any gate window duty cycles. Thus (besides the device dead time limits) all hits are acquired even when they represent random time positions within the excitation cycle time period. For single-shot experiments special multi-segment devices allow registration of a high number of events in the same time window.

[1] G. Schönhense, A. Oelsner, O. Schmidt, G. H. Fecher, V. Mergel, O. Jagutzki, H. Schmidt-Böcking; Time-Of-Flight Photoemission Electron Microscopy - A New Way To Chemical Surface Analysis; Surf. Sci. 480 (2001) 180-187
[2] H. Spiecker, O. Schmidt, Ch. Ziethen, D. Menke, U. Kleineberg, R. G. Ahuja, M. Merkel,
U. Heinzmann, G. Schönhense; Time-Of-Flight Photoelectron Emission Microscopy TOF-PEEM-First Results; Nucl. Instr. and Meth. A406 (1998) 499-506
[3] A. Oelsner, O. Schmidt, M. Schicketanz, M.J. Klais, G. Schönhense, V. Mergel, O. Jagutzki, H. Schmidt-Böcking; Microspectroscopy and  Imaging Using a Delayline-Detector in Time-of-Flight Photoemission Microscopy; Rev. Sci. Instrum. 72 (2001) 3968-3974
[4] G. Schönhense, H. J. Elmers, S. A. Nepijko, C. M. Schneider; Time-Resolved Photoemission Electron Microscopy; In: Advances in Imaging and Electron Physics(Ed. P. Hawkes) 142 (2006) 159-323



Figure 2: Operation principle of the time-of-flight PEEM (top) and relation between time resolution and resulting energy resolution as function of the drift energy (for L=40mm).  The dots mark typical working points corresponding to the time resolution of the delayline detector.


Figure 3: Time-of-flight energy discrimination and contrast (from homepage of Surface Concept GmbH).





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