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Direct IR excitation in a fast ion beam: application to NO- photodetachment cross sections

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Background: Optical access to a travelling ion packet is required in many ion beamexperiments that study ion-photon interactions.Methods: An approach is described for carrying out direct infrared excitation of a fastion beam that uses an optical-quality reflective beam blocker to illuminate a counterpropagating pulsed ion beam in a collinear configuration. This arrangement providesoptical access along the axis of ion beam propagation by placing a mirror in the beampath at a 25 degree angle. The ion packet is bumped over the mirror, which is alsoused to block fast neutral particles produced during ion beam acceleration that alsopropagate along the beam path.Results: The efficiency of this setup is demonstrated in a photodetachment experimenton NO− anions, where a photoinduced depletion of up to 90% of the beam is achievedin a single laser shot. To demonstrate the application of this configuration, the relativephotodetachment cross section for NO− has been measured in the range of 2800 –7200 cm-1. The measured relative cross section shows a set of sharp peaks that areidentified as vibrational autodetachment resonances.Conclusion: The new setup paves the way for future experiments where parentanionic species are vibrationally excited via direct infrared excitation first andundergo photodetachment/photodissociation in a subsequent step.

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