A velocity map imaging mass spectrometer for photofragments of fast ion beams.

We present the details of a fast ion velocity map imaging mass spectrometer that is capable of imaging the photofragments of trap-cooled (≥7 K) ions produced in a versatile ion source. The new instrument has been used to study the predissociation of NO produced by electric discharge and the direct dissociation of Al formed by laser ablation. The instrument's resolution is currently limited by the diameter of the collimating iris to a value of Δv/v = 7.6%. Photofragment images of NO show that when the predissociative state is changed from Σ(200) to Σ(300) the dominant product channel shifts from a spin-forbidden ground state, N (S) + NO(v = 5), to a spin-allowed pathway, N(D) + NO. The first photofragment images of Al confirm the existence of a directly dissociative parallel transition (Σ ← Σ) that yields products with a large amount of kinetic energy. D of ground state Al (Σ) measured from these images is 138 ± 5 kJ/mol, which is consistent with the published literature.