Time-of-flight photoelectron momentum microscopy with 80–500 MHz photon sources: electron-optical pulse picker or bandpass pre-filter

Abstract

The small time gaps of synchrotron radiation in conventional multi-bunch mode (100–500 MHz) or laser-based sources with high pulse rate (∼80 MHz) are prohibitive for time-of-flight (ToF) based photoelectron spectroscopy. Detectors with time resolution in the 100 ps range yield only 20–100 resolved time slices within the small time gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron-optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows electron-optical `pulse-picking' with any desired repetition period. Aberration-free momentum distributions have been recorded at reduced pulse periods of 5 MHz (at MAX II) and 1.25 MHz (at BESSY II). The approach is compared with two alternative solutions: a bandpass pre-filter (here a hemispherical analyzer) or a parasitic four-bunch island-orbit pulse train, coexisting with the multi-bunch pattern on the main orbit. Chopping in the time domain or bandpass pre-selection in the energy domain can both enable efficient ToF spectroscopy and photoelectron momentum microscopy at 100–500 MHz synchrotrons, highly repetitive lasers or cavity-enhanced high-harmonic sources. The high photon flux of a UV-laser (80 MHz, <1 meV bandwidth) facilitates momentum microscopy with an energy resolution of 4.2 meV and an analyzed region-of-interest (ROI) down to <800 nm. In this novel approach to `sub-µm-ARPES' the ROI is defined by a small field aperture in an intermediate Gaussian image, regardless of the size of the photon spot.

The SXP instrument at the European XFEL

“…The energy resolution of the ToF microscope is defined by the length and potential of the drift tube and the temporal resolution of the detector. The SXP k -microscope can achieve an energy resolution ∆E between 70 meV and 9 meV [16] when working with drift energies between 60 eV and 10 eV, exceeding the energy resolution provided by the SASE 3 monochromator. Given the 150 ps time resolution of the DLD, the time gap of 222 ns (4.5 MHz mode) allows for resolving 1,500 time slices, thus enabling high-resolution work.…”

Spin- and time-resolved photoelectron spectroscopy and diffraction studies using time-of-flight momentum microscopes

“…6 in Ref. 35). By switching lenses to the real-space imaging mode (PEEM mode), this aperture and its position on the sample surface can be viewed on the detector.…”

“…III A) allow exploiting the small-area mode of the ToF-MM. Regions of interest down to the sub-micrometer range 35,[88][89][90] can be defined by a small field aperture in the first Gaussian image [Fig. 1(a)].…”

“…34 More recently, a hybrid instrument of single hemisphere plus ToF section has been developed. 32,35 The functioning scheme of a MM exploits a basic principle of optics, namely the fact that the backfocal plane of an objective lens hosts a reciprocal image (in mathematical language, also called Fourier image). In charged-particle optics this reciprocal image is nothing else than the lateral momentum distribution, briefly termed k-image.…”

Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS$_2$

“…The sample is an exfoliated monolayer of WS 2 stacked on an exfoliated buffer layer of hexagonal boron nitride on a silicon substrate. We use the spatial imaging capabilities of the momentum microscope [72] to isolate the photoelectron signal from the ∼10×10 µm 2 monolayer region of interest of the sample. The valence band structure of the sample for a cut along the K-Γ-K axis of WS 2 is shown in Fig.…”