Multiphoton excitation and mass-selective ion detection for neutral and ion spectroscopy

The combination of resonant laser excitation, supersonic beams, and mass-selective detectors is one of the outstanding techniques for neutral and ion spectroscopy. It offers a large variety of configurations and therefore may encourage many new experiments. Resonant laser excitation facilitates multiphoton ionization spectroscopy of neutrals and dissociation spectroscopy of molecular ions and also forms a very special ion source. Furthermore, secondary excitation with further lasers is possible. Concerning the mass-selective detectors, the combination of time-of-flight analyzers with pulsed lasers turned out to be very favorable. This is due to a high sensitivity, a low signal-to-noise ratio, and the experimental possibilities available when working with small ion clouds. In this article, important features of multiphoton ionization and fragmentation for laser spectroscopy are discussed as well as some modern arrangements of time-of-flight analyzers. The application of mass-selective analyzers to a special field of neutral spectroscopy, namely, to isotope selective spectroscopy, is presented. As examples of mass-selective ion spectroscopy, resonance-enhanced multiphoton dissociation spectra of some molecular cations are shown. Finally, the application of mass-selective laser spectroscopy to several problems, i.e., Rydberg states, isomers, large molecules, or clusters, is discussed.