page_title: Photoelectron photoion coincidence spectroscopy
section_title: History
hierarchical_section_title: Photoelectron photoion coincidence spectroscopy / History
caption_attribution_description: English: Artistic representation of the iPEPICO apparatus at the VUV beamline of the Swiss Light Source.
context_page_description: Photoelectron photoion coincidence spectroscopy is a combination of photoionization mass spectrometry and photoelectron spectroscopy. It is largely based on the photoelectric effect. Free molecules from a gas-phase sample are ionized by incident vacuum ultraviolet radiation. In the ensuing photoionization, a cation and a photoelectron are formed for each sample molecule. The mass of the photoion is determined by time-of-flight mass spectrometry, whereas, in current setups, photoelectrons are typically detected by velocity map imaging. Electron times-of-flight are three orders of magnitude smaller than ion ones, which means that the electron detection can be used as a time stamp for the ionization event, starting the clock for the ion time-of-flight analysis. In contrast with pulsed experiments, such as REMPI, in which the light pulse must act as the time stamp, this allows to use continuous light sources, e.g. a discharge lamp or a synchrotron light source. No more than several ion–electron pairs are present simultaneously in the instrument, and the electron–ion pairs belonging to a single photoionization event can be identified and detected in delayed coincidence.
context_section_description: Brehm and von Puttkammer published the first PEPICO study on methane in 1967. In the early works, a fixed energy light source was used, and the electron detection was carried out using retarding grids or hemispherical analyzers: the mass spectra were recorded as a function of electron energy. Tunable vacuum ultraviolet light sources have been used in later setups, in which fixed, mostly zero kinetic energy electrons are detected, and the mass spectra are recorded as a function of photon energy. Detecting zero kinetic energy or threshold electrons in threshold photoelectron photoion coincidence spectroscopy, TPEPICO, has two major advantages. Firstly, no kinetic energy electrons are produced in energy ranges with poor Franck–Condon factors in the photoelectron spectrum, but threshold electrons can still be emitted via other ionization mechanisms. Second, threshold electrons are stationary and can be detected with higher collection efficiencies, thereby increasing signal levels.
  Threshold electron detection was first based line-of-sight, i.e. a small positive field is applied towards the electron detector, and kinetic energy electrons with perpendicular velocities are stopped by small apertures. The inherent compromise between resolution and collection efficiency was resolved by applying velocity map imaging conditions. Most recent setups offer meV or better (0.1 kJ mol⁻¹) resolution both in terms of photon energy and electron kinetic energy.
  The 5–20 eV (500–2000 kJ mol⁻¹, λ = 250–60 nm) energy range is of prime interest in valence photoionization. Widely tunable light sources are few and far between in this energy range. The only laboratory based one is the H₂ discharge lamp, which delivers quasi-continuous radiation up to 14 eV. The few high resolution laser setups for this energy range are not easily tunable over several eV. Currently, VUV beamlines at third generation synchrotron light sources are the brightest and most tunable photon sources for valence ionization. The first high energy resolution PEPICO experiment at a synchrotron was the pulsed-field ionization setup at the Chemical Dynamics Beamline of the Advanced Light Source.
caption: A painting featuring the PEPICO endstation at the Swiss Light Source.
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