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| '''Photoelectron photoion coincidence spectroscopy''', '''PEPICO''' for short, is a combination of photoionization [[mass spectrometry]] and [[Photoemission spectroscopy|photoelectron spectroscopy]].<ref>{{cite book |last1=Baer | first1=Tomas |last2=Booze | first2=Jon |last3=Weitzel | first3=Karl-Michael | editor-last=Ng |editor-first=Cheuk-Yiu |title=Vacuum Ultraviolet Photoionization and Photodissociation of Molecules and Clusters |publisher=World Scientific Pub Co Inc |date=1991-02 |pages=259–296 |chapter=Photoelectron Photoion Coincidence Studies of Ion Dissociation Dynamics |isbn=981-02-0430-2}}</ref> Gas phase sample, i.e. isolated molecules are ionized by incident vacuum [[ultraviolet]] (VUV) radiation. In the ensuing [[Photoelectrochemical_processes#Photoionization|photoionization]], a [[Ion|cation]] and a photo[[electron]] 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 [[Photofragment-ion_imaging#Velocity_Map_Imaging|velocity map imaged]] onto a [[Microchannel_plate_detector#Delay_line_detector|position sensitive detector]]. 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 for the use of continuous light sources, e.g. a [[Gas-discharge lamp|discharge lamp]] or a [[synchrotron]] light source. No more than a few 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.
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| == History ==
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| [[File:IPEPICO by Jonelle Harvey.jpg|thumb|A painting featuring the PEPICO endstation at the [[Swiss Light Source]].]]
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| Brehm and von Puttkammer published the first PEPICO study on methane in 1967.<ref>{{cite journal |last1=Brehm |first1=B. |last2= von Puttkammer |first2= E. |title=Koinzidensmessung von Photoionen und Photoelektronen bei Methan |journal=Zeitschrift Für Naturforschung. Teil A, Astrophysik, Physik, Physikalische Chemie |volume=22 |year=1967 |issue=1 |page=8}}</ref> In the early works, a fixed energy light source was used, and the electron detection was carried out using retarding grids or [[media:ARPESgeneral.png|hemispherical analyzers]]: the mass spectra were recorded as a function of electron energy. Tunable vacuum ultraviolet light sources have been used in later setups,<ref>{{cite journal |last1=Stockbauer |first1=R. |title=Threshold electron-photoion coincidence mass spectrometric study of CH<sub>4</sub>, CD<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, and C<sub>2</sub>D<sub>6</sub> |journal=Journal of Chemical Physics |volume=58 |year=1973 |issue=9 |pages=3800–3815|bibcode = 1973JChPh..58.3800S |doi = 10.1063/1.1679733 }}</ref><ref>{{cite journal |last1=Werner |first1=AS. |last2=Baer | first2=T. |title=Absolute unimolecular decay rates of energy selected C<sub>4</sub>H<sub>6</sub><sup>+</sup> metastable ions |journal=Journal of Chemical Physics |volume=62 |year=1975 |issue=7 |pages=2900–2910|bibcode = 1975JChPh..62.2900W |doi = 10.1063/1.430828 }}</ref> 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 principle|Franck–Condon]] factors in the photoelectron spectrum, but threshold electrons can still be emitted via other ionization mechanisms.<ref>{{cite doi|10.1063/1.445141}}</ref> Second, threshold electrons are stationary and can be detected with higher collection efficiencies, thereby increasing signal levels.
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| 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.<ref>{{cite doi|10.1016/0009-2614(69)80174-0}}</ref> The inherent compromise between resolution and collection efficiency was resolved by applying velocity map imaging<ref>{{cite doi|10.1063/1.1148310}}</ref> conditions.<ref>{{cite doi|10.1063/1.1593788}}</ref> Most recent setups offer meV or better (0.1 kJ mol<sup>–1</sup>) resolution both in terms of photon energy and electron kinetic energy.<ref>{{cite doi|10.1063/1.3079331}}</ref><ref>{{cite doi|10.1063/1.3082016}}</ref>
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| The 5–20 eV (500–2000 kJ mol<sup>–1</sup>, [[Wavelength|''λ'']] = 250–60 nm) energy range is of prime interest in [[Valence_electron|valence]] photoionization. Widely tunable light sources are few and far between in this energy range. The only laboratory based one is the H<sub>2</sub> discharge lamp, which delivers quasi-continuous radiation up to 14 eV.<ref>{{cite doi|10.1364/AO.10.001904}}</ref> The few high resolution [[laser]] setups for this energy range are not easily tunable over several eV. Currently, VUV [[beamline]]s 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]].<ref>{{cite doi|10.1063/1.1150009}}</ref>
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| == Overview ==
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| [[File:Scheme - Photoelectron photoion coincidence apparatus.png|thumb|left|Velocity map imaging photoelectron photoion coincidence apparatus. Electrons with different kinetic energies are shown as well as ions with a room temperature kinetic energy distribution.]]
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| The primary application of TPEPICO is the production of internal [[Photoelectron photoion coincidence spectroscopy#Energy selection|energy selected ions]] to study their [[Unimolecular ion decomposition|unimolecular dissociation dynamics]] as a function of internal energy. The electrons are extracted by a continuous electric field and are e.g. 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 [[Photoelectron photoion coincidence spectroscopy#Thermochemical applications|thermochemistry]] for the sample.<ref>{{cite doi|10.1039/b502051d}}</ref>
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| 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.<ref>{{cite doi|10.1039/b714095a}}</ref> A thorough understanding of PECD effects could help explain the homochirality of life.<ref>{{cite doi|10.1063/1.2336432}}</ref> Flash pyrolysis can also be used to produce free radicals or intermediates, which are then characterized to complement e.g. combustion studies.<ref>{{cite doi|10.1016/j.ijms.2006.09.023}}</ref><ref>{{cite doi|10.1002/cphc.201000892}}</ref> In such cases, the photoion mass analysis is used to confirm the identity of the radical produced. | |
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| Photoelectron photoion coincidence spectroscopy can be used to shed light on reaction mechanisms,<ref>{{cite doi|10.1255/ejms.943}}</ref> and can also be generalized to study double ionization in (photoelectron) photoion photoion coincidence ((PE)PIPICO),<ref>{{cite doi|10.1080/00268978700101421}}</ref> fluorescence using photoelectron photon coincidence (PEFCO),<ref>{{cite doi|10.1016/0301-0104(80)80106-6}}</ref> or photoelectron photoelectron coincidence (PEPECO).<ref>{{cite doi|10.1016/j.chemphys.2003.08.001}}</ref> Ion–electron velocity vector correlation functions can be obtained in double imaging setups, in which the ion detector also delivers position information.<ref>{{cite doi|10.1063/1.1458063}}</ref>
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| == Energy selection ==
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| [[File:PEC Scheme.tif|thumb|Potential energy diagram for dissociative photoionization. When only zero kinetic energy electrons are detected, the photon energy above the adiabatic ionization energy is converted into the internal energy of the photoion AB<sup>+</sup>]]
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| 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 [[Ionization energy|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 [[Momentum#Conservation_of_linear_momentum|conservation of momentum]] holds, the last two terms vanish, and the internal energy of the photoion is known:
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| <math>E_{\text{int}}^{\text{ion}} = E_{\text{int}}^{\text{neutral}} + h \nu - IE_{\text{ad}}</math> | |
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| Scanning the photon energy corresponds to shifting the internal energy distribution of the parent ion. The parent ion sits in a potential energy well, in which the lowest energy exit channel often corresponds to the breaking of the weakest [[chemical bond]], resulting in the formation of a fragment or daughter ion. A mass spectrum is recorded at every photon energy, and the fractional ion abundances are plotted to obtain the breakdown diagram. At low energies no parent ion is energetic enough to dissociate, and the parent ion corresponds to 100% of the ion signal. As the photon energy is increased, a certain fraction of the parent ions (in fact according to the [[cumulative distribution function]] of the neutral internal energy distribution) still has too little energy to dissociate, but some do. The parent ion fractional abundances decrease, and the daughter ion signal increases. At the dissociative photoionization threshold, ''E''<sub>0</sub>, all parent ions, even the ones with initially 0 internal energy, can dissociate, and the daughter ion abundance reaches 100% in the breakdown diagram.
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| If the potential energy well of the parent ion is shallow and the complete initial thermal energy distribution is broader than the depth of the well, the breakdown diagram can also be used to determine adiabatic ionization energies.<ref>{{cite doi|10.1021/jp208018r}}</ref>
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| == Data analysis ==
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| The data analysis becomes more demanding if there are competing parallel dissociation channels or if the dissociation at threshold is too slow to be observed on the time scale (several μs) of the experiment. In the first case, the slower dissociation channel will appear only at higher energies, an effect called competitive shift, whereas in the second, the resulting kinetic shift means that the fragmentation will only be observed at some excess energy, i.e. only when it is fast enough to take place on the experimental time scale. When several dissociation steps follow sequentially, the second step typically occurs at high excess energies: the system has much more internal energy than needed for breaking the weakest bond in the parent ion. Some of this excess energy is retained as internal energy of the fragment ion, some may be converted into the internal energy of the leaving neutral fragment (invisible to mass spectrometry) and the rest is released as kinetic energy, in that the fragments fly apart at some non-zero velocity.
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| More often than not, dissociative photoionization processes can be described within a [[Statistical mechanics|statistical]] framework, similarly to the approach used in [[collision-induced dissociation]] experiments. If the [[ergodic hypothesis]] holds, the system will explore each region of the [[phase space]] with a probability according to its volume. A [[transition state]] (TS) can then be defined in the phase space, which connects the dissociating ion with the dissociation products, and the dissociation rates for the slow or competing dissociations can be expressed in terms of the TS phase space volume vs. the total phase space volume. The total phase space volume is calculated in a [[microcanonical ensemble]] using the known energy and the density of states of the dissociating ion. There are several approaches how to define the transition state, the most widely used being [[RRKM theory]]. The unimolecular dissociation [[Reaction rate|rate]] curve as a function of energy, ''k''(''E''), vanishes below the dissociative photoionization energy, ''E''<sub>0</sub>.<ref>{{cite book |last1= Baer |first1= Tomas |last2= Hase |first2= William L. |title= Unimolecular Reaction Dynamics: Theory and Experiments |publisher= Oxford University Press |year= 1996 |isbn= 0-19-507494-7}}</ref>
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| Statistical theory can also be used in the microcanonical formalism to describe the excess energy partitioning in sequential dissociation steps, as proposed by Klots<ref>{{cite doi|10.1063/1.1679153}}</ref> for a canonical ensemble. Such a statistical approach was used for more than a hundred systems to determine accurate dissociative photoionization onsets, and derive thermochemical information from them.<ref>{{cite doi|10.1002/jms.1813}}</ref>
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| == [[Thermochemistry|Thermochemical]] applications ==
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| Dissociative photoionization processes can be generalized as:
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| AB + ''hν'' <math>\rightarrow</math> A<sup>+</sup> + B + e<sup>–</sup>
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| If the [[Standard_enthalpy_of_formation|enthalpies of formation]] of two of the three species are known, the third can be calculated with the help of the dissociative photoionization energy, ''E''<sub>0</sub>, using [[Hess's law]]. This approach was used, for instance, to determine the enthalpy of formation of the [[Methyl_group#Methyl_cation|methyl ion]], CH<sub>3</sub><sup>+</sup>,<ref>{{cite doi|10.1063/1.480169}}</ref> which in turn was used to obtain the enthalpy of formation of [[iodomethane]], CH<sub>3</sub>I as 15.23 kJ mol<sup>–1</sup>, with an uncertainty of only 0.3 kJ mol<sup>–1</sup>.<ref>{{cite doi|10.1039/b915400k}}</ref>
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| If different sample molecules produce shared fragment ions, a complete thermochemical chain can be constructed, as was shown for some methyl trihalides,<ref>{{cite doi|10.1021/jp8056459}}</ref> where the uncertainty in e.g. the CHCl<sub>2</sub>Br, ([[Halomethane|Halon-1021]]) heat of formation was reduced from 20 to 2 kJ mol<sup>–1</sup>. Furthermore, dissociative photoionization energies can be combined with [[Computational chemistry|calculated]] [[isodesmic reaction]] energies to build thermochemical networks. Such an approach was used to revise primary alkylamine enthalpies of formation.<ref>{{cite doi|10.1021/jp064739s}}</ref>
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| == References ==
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| {{Reflist}}
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| == External links ==
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| * [http://www.psi.ch/sls/vuv/endstations PEPICO endstation at the Swiss Light Source]
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| * [http://www.synchrotron-soleil.fr/Soleil/ToutesActualites/2009/DELICIOUS2 DELICIOUS2: a PEPICO experiment at SOLEIL, France]
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| * [http://www1.pacific.edu/~bsztaray/research/pepico/ PEPICO page at the University of the Pacific]
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| * [http://physchem.ox.ac.uk/~jhde/techniq.html PEPICO page at Oxford University]
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| <!--- Categories --->
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| [[Category:Physical_chemistry]]
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| [[Category:Chemical_kinetics]]
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| [[Category:Thermochemistry]]
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