A velocity map imaging apparatus for gas phase studies at GAPH
The design and evaluation of a velocity map imaging spectrometer specifically optimised for experimentsat the GAs Phase pHotoemission beam line has been perfomed. (P. O'Keeffee et al. Rev. Sci. Instrum. 2011)
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Since its initial introduction in 1997 the velocity map imaging (VMI) technique has become one of the most extensively used methods for the investigation of photoionization and photodissociation dynamics of atoms and molecules. Indeed, the number of publications based on the use of this technique has been constantly increasing since that date. VMI is based on the projection of 3D distributions of charged particles onto a position sensitive detector (PSD) by inhomogeneous electrostatic fields which have the property of mapping particles of the same initial velocity to the same point on the detector, independent of the point within the interaction zone at which the particle was born. The original 3D distribution can then be recovered from this 2D projection by a mathematical inversion procedure, provided the system has cylindrical symmetry around the axis parallel to the face of the detector. This technique has been applied to the measurement of the kinetic energies and angular distributions of photofragments produced by the photodissociation of molecules as well as photoelectrons emitted, following photoionization of atoms, molecules, and clusters. Furthermore, the method has been coupled to numerous sources such as intense femtosecond lasers, 3 high harmonic generation sources, 4 synchrotron light sources, 5–9 free electron lasers, 10,11 and nanosecond lasers in combination with resonant multiphoton ionization and even electron impact sources. 1A velocity map imaging/ion time-of-flight spectrometer designed specifically for pump–probe experiments combining synchrotron and laser radiations has been developed for experiments at the GAs Phase pHotoemission low-energy branch line of the beamline, which is equipped with a tunable Ti:Sapphire oscillator (Tsunami, SpectraPhysics). The in-house built delay line detector can be used in two modes: the high spatial resolution mode and the coincidence mode. In the high spatial resolution mode a kinetic energy resolution of 6% has been achieved. The coincidence mode can be used to improve signal-to-noise ratio for the pump–probe experiments either by using a gate to count electrons only when the laser is present or by recording coincidences with the ion formed in the ionization process. |
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A photoelectron velocity map imaging spectrometer for experiments combining synchrotron and laser radiations |
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