ION MOLECULE REACTIONS, MASS-SPECTROMETRY AND OPTICAL SPECTROSCOPY INA LINEAR ION-TRAP/

A linear-geometry, radio-frequency, quadrupole ion trap has been devel oped to generate, purify, accumulate and study atomic and molecular io ns in the gas phase. By employing a trap-based system, both reactant a nd product ions can be stored for significant time periods, which can both enhance the efficiency of gas-phase reaction processes and create an environment to observe collision products after vibrational and ro tational excitations have had time to relax. Relaxation occurs via vis cous cooling with a dilute buffer gas or via laser cooling. Furthermor e, the setup is particularly useful for performing optical spectroscop y on the trapped ions. Atomic and molecular ovens are used to generate thermal beams of neutral species, which are then ionized by electron bombardment. The ions can be trapped, or they can be collided with neu tral molecules (e.g. C-60) under well defined experimental conditions. The collision energies are variable over a range from nearly 0 to 200 eV, This feature makes possible studies of complex formation, charge transfer and collision-induced fragmentation as a function of kinetic energy, A wide range of masses of up to 4000 u can be stored and manip ulated with this apparatus. Two mass spectrometric techniques for the analysis of trapped ionic species are presented. In one method, parame tric excitation of the secular motion is used to generate mass spectra with resolutions as high as 1 part in 800 with a simple experimental setup. The second method is capable of quickly generating mass spectra over the entire range of trapped masses, but has only moderate resolu tion. These spectra are generated by linearly sweeping the rf-trapping voltage to zero and detecting ions as their trap depth falls below a threshold value. In the trapping volume, which is 10 cm in length and 200 mu m in diameter, 10(6) ions can be loaded and stored, correspondi ng to an ion density above 10(8) cm(-3), Such densities facilitate spe ctroscopy of the stored ions. Both laser-induced fluorescence and phot odissociation measurements have been carried out with a cw laser syste m providing near-infrared, visible, and ultraviolet beams. Absolute, t otal cross-sections and branching ratios of the photodissociation of M gC60+ have been measured. (C) 1998 Elsevier Science B.V.