Introduction

Development and application of the spectroscopic technique of optical Resonance Ionization (RI) with powerful tunable lasers, in combination with Mass Spectrometry (MS) and single ion detection, opens a wide range of research fields.

RIMS covers fundamental atomic and quantum-optical spectroscopy, the production of exotic species at on-line production plants in large-scale research centers like CERN as well as elemental ultra-trace analysis. In all cases highest selectivity in respect to element and isotope is required. These specifications are subject to further research and development, both concerning suitable laser systems as well as mass spectrometers, ion coolers, bunchers and traps.

Resonant optical laser excitation of atoms up to final ionization in general provides highest elemental (=isobaric) selectivity due to the uniqueness of atomic resonances. Narrow bandwidth continuous wave lasers which induce sequential multi-step excitation processes in addition generate highest optical isotope selectivity by employing small isotope shifts in each individual atomic transition. Subsequent ion optical treatment of the generated photo-ions further increases isotopic selectivity in a mass selection process. High overall efficiency, sensitivity and lowest detection limits are ensured by efficient and low background detection of individual ions. Specifications realized in the Mainz LARISSA (LAser Resonance Ionization Spectroscopy for Selective Applications) experiments by far exceed conventional mass spectrometry. They complement and compete against the results of the well known but very extensive technique of Accelerator Mass Spectrometry yet using table top equipment.

The interest in ultra-trace determination of rare isotopes of natural occurrence is multifold. Figure 1 shows a compilation of long-lived and stable isotopes with relative abundances below 1E-9 of their neighboring isotope together with their class of origin. Even though investigation on these isotopes would be of highest interest for a wide variety of applications, they are in most cases not accessible by conventional techniques. Many of them represent sensitive probes for geochemical and environmental studies in the atmosphere, the hydrosphere, the lithosphere, and the technosphere. A wealth of information is encoded in their production, transport, and decay mechanisms, which govern their rarity on earth. Further applications on fundamental aspects concern cosmochemistry and astrophysics with analysis on extraterrestrial material, as well as nuclear and particle physics. Using artificially enriched samples, numerous bio-medical studies and investigations in material sciences can be performed. For all these investigations, highest selectivity, far greater than 1E+9, is required and efficiency in the range of 1E-3 or higher is preferred.

Figure 1: isotopic ratios of long living and stable isotopes on earth
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Mandatory basis for the evolution and refinement of the RIMS technique is the development and upgrade of laser sources as well as mass spectrometers. Additionally, detailed spectroscopic studies on excitation and ionization path ways in the elements under study are needed. The simultaneous irradiation with several laser light fields and the presence of electric fields from the mass spectrometer ion optics lead to effects, which can only be understood and analysed using quantum optical approaches.