Velocity map photoelectron-photoion coincidence imaging on a single detector

LaserLaB Amsterdam, VU University Amsterdam, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
The Review of scientific instruments (Impact Factor: 1.61). 09/2012; 83(9):093103. DOI: 10.1063/1.4749843
Source: PubMed


Here we report on a new simplified setup for velocity map photoelectron-photoion coincidence imaging using only a single particle detector. We show that both photoelectrons and photoions can be extracted toward the same micro-channel-plate delay line detector by fast switching of the high voltages on the ion optics. This single detector setup retains essentially all the features of a standard two-detector coincidence imaging setup, viz., the high spatial resolution for electron and ion imaging, while only slightly decreasing the ion time-of-flight mass resolution. The new setup paves the way to a significant cost reduction in building a coincidence imaging setup for experiments aiming to obtain the complete correlated three-dimensional momentum distribution of electrons and ions.

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    • "In this article, we make use of velocity map imaging (VMI) of photoelectrons in coincidence with the corresponding cations [17] [18] in order to determine the amount of ionization to the ground state versus excited states, and to distinguish between direct and indirect ionization to excited states. Here we use the term 'indirect' to refer to the process whereby the final excited state of the cation is not produced directly from the neutral, but rather via the ground ionic state. "
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    ABSTRACT: We use velocity map imaging of photoelectrons in coincidence with molecular cations to determine which ionic states are populated via strong field ionization, and whether the ionization to excited ionic states proceeds indirectly via the ground ionic state or directly from the neutral. We carry out measurements on a series of molecules that have different energy gaps between the ground ionic state and dissociative excited states. We measure both direct and indirect ionization to excited states of the molecular cations, and find that the energy gap between non-dissociative and dissociative states plays an important role in determining the amount of excited state ionization. Direct ionization to dissociative states is generally comparable to ionization to the ground state for gap energies less than the photon energy, but is suppressed for gap energies larger than the photon energy.
    Journal of Physics B Atomic Molecular and Optical Physics 10/2014; 47(20):204023. DOI:10.1088/0953-4075/47/20/204023 · 1.98 Impact Factor
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    ABSTRACT: We have established through simulations and experiments the area over which Velocity Map Imaging (VMI) conditions prevail. We designed a VMI setup in which we can vary the ionization position perpendicular to the center axis of the time-of-flight spectrometer. We show that weak extraction conditions are far superior over standard three-plate setups if the aim is to increase the ionization volume without distorting VMI conditions. This is important for a number of crossed molecular beam experiments that already utilize weak extraction conditions, but to a greater extent for surface studies where fragments are desorbed or scattered off a surface in all directions. Our results on the dissociation of NO2 at 226 nm show that ionization of the fragments can occur up to ±5.5 mm away from the center axis of the time-of-flight spectrometer without affecting resolution or arrival position.
    The Review of scientific instruments 04/2013; 84(4):044101. DOI:10.1063/1.4798646 · 1.61 Impact Factor
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    ABSTRACT: We present a versatile double imaging particle coincidence spectrometer operating in fully continuous mode, named DELICIOUS III, which combines a velocity map imaging device and a modified Wiley-McLaren time of flight momentum imaging analyzer for photoelectrons and photoions, respectively. The spectrometer is installed in a permanent endstation on the DESIRS vacuum ultraviolet (VUV) beamline at the French National Synchrotron Radiation Facility SOLEIL, and is dedicated to gas phase VUV spectroscopy, photoionization, and molecular dynamics studies. DELICIOUS III is capable of recording mass-selected threshold photoelectron photoion coincidence spectra with a sub-meV resolution, and the addition of a magnifying lens inside the electron drift tube provides a sizeable improvement of the electron threshold∕ion mass resolution compromise. In fast electron mode the ultimate kinetic energy resolution has been measured at ΔE∕E = 4%. The ion spectrometer offers a mass resolution-full separation of adjacent masses-of 250 amu for moderate extraction fields and the addition of an electrostatic lens in the second acceleration region allows measuring the full 3D velocity vector for a given mass with an ultimate energy resolution of ΔE∕E = 15%, without sacrificing the mass resolution. Hence, photoelectron images are correlated both to the mass and to the ion kinetic energy and recoil direction, to access the electron spectroscopy of size-selected species, to study the photodissociation processes of state-selected cations in detail, or to measure in certain cases photoelectron angular distributions in the ion recoil frame. The performances of DELICIOUS III are explored through several examples including the photoionization of N2, NO, and CF3.
    The Review of scientific instruments 05/2013; 84(5):053112. DOI:10.1063/1.4807751 · 1.61 Impact Factor
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