REMPI in a focusing rf-quadrupole: a new source for mass-, energy-, and state-selected ions

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Abstract
Using resonance enhanced multiphoton ionization (REMPI), ions are created in a small volume centred on the axis of an rf-quadrupole. The pulsed beam of neutral molecular precursors is injected coaxially. This combination leads to high collection efficiency without the need of strong electrostatic extraction fields. The quadrupole is operated in the adiabatic limit. Therefore, the influence of the oscillatory rf-field on the ion trajectories can be described by an effective harmonic potential, which has focusing properties. Using these energy and mass dependent focusing properties, a primary H2 + beam of well-defined kinetic energy (< 5 meV (fwhm) at a mean of 50 meV) is prepared. In a typical case of multiphoton ionization of polyatomic molecules using C2H2, suppression of fragment ions by more than a factor of 10 is achieved, without losing the narrow kinetic energy spread of the parent ion beam.