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A velocity map imaging detector with an integrated gas injection system
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10.1063/1.3085799
/content/aip/journal/rsi/80/3/10.1063/1.3085799
http://aip.metastore.ingenta.com/content/aip/journal/rsi/80/3/10.1063/1.3085799
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Figures

Image of FIG. 1.
FIG. 1.

The velocity map imaging spectrometer introduced in this paper. Ions/electrons are formed at the crossing point of a laser beam and the symmetry axis of the spectrometer and are accelerated toward a screen assembly by applying appropriate voltages to repeller and extractor electrodes. In a conventional velocity map imaging spectrometer, the repeller electrode is a flat plate, and the gas typically enters the detector through a small hole drilled in the repeller electrode (i.e., along the symmetry axis of the spectrometer) or from the side (i.e., along an axis that lies in a plane perpendicular to the detector axis, and containing the laser beam). In our new design, the repeller electrode incorporates a capillary tube from which the gas escapes in close proximity to the laser focus. The central part of the repeller electrode is flat (i.e., parallel to the detection plane). From there the repeller electrode assumes a conical shape with a 154° cone angle to accommodate focusing of the laser in front of the capillary.

Image of FIG. 2.
FIG. 2.

(a) Simulated image determined by performing a Monte Carlo simulation where particles with kinetic energies of 20, 40, , 100 eV were projected onto a 2D detector plane by means of an open electron/ion optics design as introduced by Eppink and Parker (Ref. 3). (b) Slice through the 3D velocity distribution that was obtained from (a) by means of an inverse Abel transform. (c) Kinetic energy distribution derived from (b) allowing an assessment of the energy resolution of the spectrometer.

Image of FIG. 3.
FIG. 3.

Calculated energy resolution for a conventional velocity map imaging spectrometer as function of the kinetic energy of the fragments and the voltage applied to the extractor electrode. The geometry shown in Fig. 1 is assumed, with 10 kV applied to a flat repeller plate and with a 26 cm separation between the repeller and the detection plane. The ejection of the fragments is assumed to be isotropic. The fragment source volume is assumed to be the intersection of a molecular beam with a Gaussian radial profile (2 mm FWHM) and a Gaussian laser beam that is focused to a spot size (FWHM) of .

Image of FIG. 4.
FIG. 4.

(a) Simulated image determined by performing a Monte Carlo simulation where particles with kinetic energies of 20, 40, , 100 eV were projected onto a 2D detector plane by means of the new velocity map imaging design that incorporates a capillary in a conical repeller. (b) Slice through the 3D velocity distribution that was obtained from (a) by means of an inverse Abel transform. (c) Kinetic energy distribution derived from (b) allowing an assessment of the energy resolution of the spectrometer.

Image of FIG. 5.
FIG. 5.

Calculated energy resolution for the new velocity map imaging spectrometer where the gas injection is integrated in the repeller electrode as a function of the kinetic energy of the fragments and the voltage applied to the extractor electrode. The geometry shown in Fig. 1 is assumed, with 10 kV applied to the conical repeller electrode and with a 26 cm separation between the repeller electrode and the detection plane. The ejection of the fragments is assumed to be isotropic. The diameter of the capillary tube that is integrated in the repeller electrode is . The fragment source volume is assumed to be the intersection of an effusive flow with a Gaussian radial density distribution a 1 sr opening angle and a Gaussian laser beam that is focused to a spot size (FWHM) of . The distance from the repeller electrode to the laser axis is assumed to be 1 mm.

Image of FIG. 6.
FIG. 6.

Dependence of the calculated kinetic energy resolution of the velocity map imaging spectrometer as a function of (a) the distance from the repeller to the interaction region and (b) the width of the flat region of the repeller. In the former case the width of the flat region is held a 1 mm, while in the latter case the distance from the repeller to the interaction region is held at 1 mm. The kinetic energy resolution shows a clear dependence on the distance from the repeller to the interaction but does not show any dependence on the width of the flat region

Image of FIG. 7.
FIG. 7.

(a) Experimental photoelectron image obtained for six- and seven-photon ionization of Xe atoms using the second harmonic (532 nm) of a Nd:YAG laser. For each -photon ionization process, two rings are observed that correspond to the formation of the ion in its ground state or in the spin-orbit excited state. (b) Slice through the 3D velocity distribution obtained by applying the iterative inversion procedure in Ref. 15. (c) Photoelectron kinetic energy distribution derived from integration of the 3D velocity distribution within a narrow cone (5°) along the polarization axis. Two contributions arising from seven-photon ionization are measured with a kinetic energy resolution of . When the photoelectron kinetic energy distribution is evaluated by full angular integration of the 3D velocity distribution, the resolution degrades to 5% due to imperfections in the roundness of the measured images, which are likely due to imperfections in the magnetic shielding of the spectrometer.

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/content/aip/journal/rsi/80/3/10.1063/1.3085799
2009-03-18
2014-04-21
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A velocity map imaging detector with an integrated gas injection system
http://aip.metastore.ingenta.com/content/aip/journal/rsi/80/3/10.1063/1.3085799
10.1063/1.3085799
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