Production and Detection of Selectively Laser Ionized Species
Resonant multiphoton ionization is a general method either for analyzing the chemical composition of samples (trace analysis) or for inducing modification of isotopic natural abundance (isotope enrichment). Selectivity is achieved by a multiple-step process in which different laser beams are tuned to resonant transitions of the element of interest. Since the steps involved are, in general electric-dipole transitions, pulsed lasers are required to saturate resonant transitions and, thus, to achieve a high degree of ionization. The ions produced are accelerated by an external electric field, and then fed into a time-of-flight (TOF) mass spectrometer. This simple scheme can be applied to a wide range of molecular or atomic samples1, providing that the resonant transitions belong to a range of the electromagnetic spectrum in which laser sources are available. In the case of samples whose first resonant step is in the VUV region, e.g. noble gases, excitation by electron impact into metastable states may be combined with selective laser ionization2.
KeywordsExternal Field Resonant Transition Internal Field Collective Effect Needle Injector
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