Photoelectron photoion coincidence spectroscopy (PEPICO) 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 (VUV) 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 those of ions, which allows electron detection to 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 the use of 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.
History
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 were used in later setups, in which fixed, mostly zero kinetic energy electrons were detected, and the mass spectra were 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. Secondly, threshold electrons are stationary and can be detected with higher collection efficiencies, thereby increasing signal levels. Threshold electron detection was first based on line-of-sight, i.e. a small positive field was 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.
Overview
The primary application of TPEPICO is the production of internal energy selected ions to study their unimolecular dissociation dynamics as a function of internal energy. The electrons are extracted by a continuous electric field and are velocity map imaged depending on their initial kinetic energy. Ions are accelerated in the opposite direction and their mass is determined by time-of-flight mass spectrometry. The data analysis yields dissociation thresholds, which can be used to derive new thermochemistry for the sample. The electron imager side can also be used to record photoionization cross sections, photoelectron energy and angular distributions. With the help of circularly polarized light, photoelectron circular dichroism (PECD) can be studied. A thorough understanding of PECD effects could help explain the homochirality of life. Flash pyrolysis can also be used to produce free radicals or intermediates, which are then characterized to complement e.g. combustion studies. In such cases, the photoion mass analysis is used to confirm the identity of the radical produced. Photoelectron photoion coincidence spectroscopy can be used to shed light on reaction mechanisms, and can also be generalized to study double ionization in (photoelectron) photoion photoion coincidence ((PE)PIPICO), fluorescence using photoelectron photon coincidence (PEFCO), or photoelectron photoelectron coincidence (PEPECO). Times-of-flight of photoelectrons and photoions can be combined in a form of a map, which visualizes the dynamics of the dissociative ionization process. Ion–electron velocity vector correlation functions can be obtained in double imaging setups, in which the ion detector also delivers position information.
Energy selection
The relatively low intensity of the ionizing VUV radiation guarantees one-photon processes, in other words only one, fixed energy photon will be responsible for photoionization. The energy balance of photoionization comprises the internal energy and the adiabatic ionization energy of the neutral as well as the photon energy, the kinetic energy of the photoelectron and of the photoion. Because only threshold electrons are considered and the conservation of momentum holds, the last two terms vanish, and the internal energy of the photoion is known:
E int ion = E int neutral + h ν − I E ad {\displaystyle E_{\text{int}}^{\text{ion}}=E_{\text{int}}^{\text{neutral}}+h\nu -IE_{\text{ad}}}
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