We have established through simulations and experiments the volume in 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 NO 2 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. a) sven.koehler@manchester.ac.uk 2

Dissociation dynamics of chloroform near the threshold investigated using the velocity-map imaging technique show that the three lowest states 3Q1, 1Q1, and 3Q0 are involved in the dynamics.

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