Instrumental configuration for direct measurement of optical absorption of ion cyclotron resonance mass-selected trapped ions

Abstract

Identification and structural analysis of gas-phase ions is presently based on methods similar to those used in condensed-phase chemistry 75 years ago: namely, breaking the ion apart and weighing the fragments, and/or using chemical reactions to identify groups or reactive centers. For example, dissociation of mass-selected parent ions by (e.g.) collision-induced dissociation [CID], photodissociation, electron impact dissociation, surface-induced dissociation, etc., yields a product ion mass spectrum from which parent ion structure and bonding are indirectly inferred.Optical spectroscopy, on the other hand, can reveal directly the structure of the absorbing species. Directly measured optical absorption spectra of ions have yielded structures of a few species, such as H3+. Most such experiments have been carried out in a discharge tube although a few mass selected ion spectra have been obtained in a fast ion beam. Here, we propose to conduct optical absorption experiments on mass-selected ions in an ICR ion trap; such experiments require that both optical absorption sensitivity and the maximum number of trapped ions be improved by an order of magnitude. To increase absorption sensitivity, we have chosen a newly developed cavity ringdown method which has previously been demonstrated for visible spectra of neutrals. By use of quadrupolar excitation and collisional cooling to axialize and mass-select ions in a multi-chamber trap, we hope to trap as many as 109 ions with an effective optical path length of 10 000 m, making it possible to detect ions of 10−16 cm2 absorption cross-section.