Illustration of two possible pump-probe geometries to simultaneously detect angular distributions in a surface Velocity Map Imaging spectrometer. The sheet-dot configuration (a) has advantages in terms of probe laser fluence due to the focusing, while the dot-sheet configuration (b) is more intuitive but requires refined Velocity Map Imaging conditions.
Schematic of our Velocity Map Imaging spectrometer showing the placement of the ion optics within the vacuum apparatus, and the propagation direction of the REMPI laser which can be translated vertically.
Close-up cut-out of the first few electrodes showing the vertically offset holes in the repeller plate intersected by the REMPI laser. Distance of the molecular beam valve and skimmer to the ion optics not to scale.
Ion trajectories simulated using SIMION 8.0 demonstrating the resolution obtained when (b and c) three electrodes, and (d) 12 electrodes are used. The origin of the ions is ±7.5 mm offset from the center axis of the spectrometer. The close-ups show a resolution better than 2% for the more elaborate setup compared to 8% for either 3-plate setup.
Ion images (top-left quadrants, not averaged by mirroring) of NO+ after dissociation of NO2 at ∼226 nm and 1 + 1 REMPI ionization of NO fragments. The potential on the first plate is 1000 V, and the laser polarization is indicated by ɛ . Final graph is the raw velocity distribution showing the linear relationship between image radius and velocity.
(a) Displacement of the center-of-inversion (COI) of the seven images shown in Fig. 5 expressed as a perceived velocity shift (compared with the molecular beam aligned along the center axis) as a function of molecular beam offset. (b) Peak kinetic energy of the NO fragment of the most intense channel after NO2 photodissociation at 226 nm. Also shown on the vertical axis is the kinetic energy distribution of this peak for comparison. (c) Resolution of the most intense NO production channel after 226 nm photolysis of NO2 as a function of molecular beam offset.
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