Time-of-Flight Photoelectron Momentum Microscopy at 100-500 MHz Synchrotron Sources: Electron-Optical Chopping or Bandwidth Pre-Selection

TL;DR

This paper introduces two techniques for efficient time-of-flight electron spectroscopy at high-repetition-rate synchrotron sources, enabling high-resolution momentum microscopy despite small time gaps.

Contribution

It presents a fast electron-optical beam blanking method and compares it with energy pre-selection, advancing ToF spectroscopy at 100-500MHz sources.

Findings

Aberration-free momentum distributions at 5MHz and 1.25MHz pulse periods.

Effective ToF spectroscopy with few-meV resolution at high repetition rates.

Benchmarking of chopping versus dispersive element approaches.

Abstract

The small time gap of synchrotron radiation in conventional multi-bunch mode (100-500MHz) is prohibitive for time-of-flight (ToF) based electron spectroscopy. Even the new generation of delay-line detectors with improved time resolution (<100ps) yields only 20-100 resolved time slices within a 2-10ns 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 chopping the photon-pulse train to any desired repetition period. Aberration-free momentum distributions have been recorded at chopped pulse periods of 5MHz (at MAX II) and 1.25MHz (at BESSY II). The approach is benchmarked against the alternative way of implementing a dispersive element, e.g. a hemispherical analyzer, in the electron optics. Both approaches, chopping in the time domain as well as bandpass pre-selection in the energy domain, can enable efficient ToF spectroscopy, spectroscopic real-space imaging and momentum microscopy with few-meV resolution using 100-500MHz Synchrotron radiation, highly-repetitive lasers or cavity-enhanced high-harmonic sources. For comparison, we show results recorded at BESSY II with a parasitic 4-bunch island-orbit pulse train, coexisting with the 500MHz filling pattern on the main orbit.