Selective Resonance Ionization

After the third laser pulse a small field is applied to extract ions resulting from excitation to the Spnl autoionizing states. If the rf is off or not tuned to a Sr 5s(n + l)d-Ssnf resonance we see very little ion signal. However, a sharp increase in the ionization signal is observed as the rf frequency is swept through the 5s( + 2)d-Ssnf resonance. 

It is quite apparent that this technique is very restricted in its applicability since we must have a laser to drive the transition to an autoionizing state. Thus while it is potentially interesting for alkaline earth atoms it is useless for alkali atoms. In addition, it is most useful when there is a /-dependent difference in the wavelength of the transitions to the autoionizing states, and experience to date indicates that this is not a common occurrence. 

From the preceding discussion of approaches to radiofrequency spectroscopy it is apparent that a variety of means exist to observe transition between any two Rydberg states. Since there is no fundamental limit to the frequency range over which the method is applicable it is the most general approach to measuring small intervals in Rydberg state energies and has found wide application in the measurement of A/ intervals, fine and hyperfine levels, and polarizabilities. 

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Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

Weber, Th., von Bargen, A., Riedle, E., and Neusser, H. J. (1990), Rotationally Resolved Ultraviolet Spectrum of the benzen-Ar Complex by Mass-Selected Resonance-Enhanced Two-Photon Ionization, J. Chem. Phys. 92,90. 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

It is reported by Fassett. et al that the resonance ionization process has an inherently high elemental selectivity. Mass spcctromciric dcieeiion provides an increased selectivity that is a practical necessity lor analytical problems in which nonspecific background iomzatinn must he characterized and differentiated. Resonance ionization is ideally suited to mass spcclnnn-dry- ionizaiion is well defined in both time and space, and only a small excess of translational kinetic energy is added to the atom by the process. 

A good example of the use of the electric resonance technique is the measurement of the Na nd fine structure intervals and tensor polarizabilities.38 These transitions were observed using selective field ionization, although they appear to be unlikely prospects for field ionization detection because of the small separations of the levels, 20 MHz. The nd3/2 states were selectively excited from the 3p1/2 state in a small static electric field and the = 0 transitions to the nd5/2 states induced by a... 

Resonance ionization spectroscopy (RIS) with pulsed tunable lasers offers new possibilities for constructing pulsed, highly selective ion sources with high efficiencies. Fig. 4 shows the principle of RIS and the planned set up. The efficiency of the ion source is determined mainly by the ratio of the repetition rate... 

Recently, new 2D-methods for the analysis of complex mixtures have been developed for time-of-flight mass spectrometry (22), which could also be utilized in external ionization FTMS. Specifically, the combination of IR-laser desorption of nonvolatile neutrals, followed by adiabatic cooling to 2°K in a supersonic jet, and subsequent compound-selective Resonance-Enhanced Multiphoton Ionization (REMPI) could increase the role of FTMS in the analysis of biological mixtures. The coupling of supersonic jets to the external ion source would also be of interest in ion- and neutral cluster experiments. 

Whereas in LIMS only one laser with defined wavelength (e.g., Nd YAG - 1064 nm) is used for direct vaporization and ionization of solid samples in laser plasma, in resonance ionization mass spectrometry (RIMS) " one or more lasers are tuned precisely to the wavelength required for the excited states and ionization of evaporated atoms in order to get a highly selective ionization of the analyte. The basic principles of resonant ionization were first described by Hurst and coworkers at Oak Ridge National Laboratory as well as by Letokhov et in Russia. The technology... 

Thermal ionization mass spectrometry has been used extensively in the geological nuclear and analytical sciences for stable isotope measurements A new technique resonance ionization mass spectrometry offers a comprehensive approach to sensitive and selective elemental and isotopic analysis Recent developments in thermal and resonance ionization mass spectrometry are reviewed) and specific applications of the technology to zinc and calcium metabolism studies and to trace element analysis of foodstuffs are summarized ... 

Two basic and unique advantages are implicit in the resonance ionization process ionization efficiency and wavelength selectivity Absorption cross-sections for excitation to the first energy... 

Vibrational spectra of aniline/cyclohexane and aniline/benzene clusters investigated by infrared depletion spectroscopy with selective resonance—enhanced multiphoton ionization with time-of-flight mass spectrometry (REMPI-TOF mass spectrometry) show186 an... 

The experiments are done by exciting Na atoms in an atomic beam to the 17s state, by the route 3s —> Sp —> 17s, using two 5 ns dye laser pulses. The atoms are allowed to collide for 3 ps, and then the final states are analyzed by selective field ionization. In particular, atoms in the 17p state are detected as the tuning electric field is slowly swept over many shots of the laser. In Fig. 2 we show the resonances observed in this way. There are four resonances corresponding to the two possible rni values of the up states, mi = 0 and 1. It is convenient to label the four resonances by the m values of the upper and lower p states, e.g., (0,0) has m = 0 in both. 

Cao, L. Muhlberger, F. Adam, T. et al. Resonance-enhanced multiphoton ionization and VUV-single photon ionization as soft and selective laser ionization methods for on-line time-of-flight mass spectrometry investigation of the pyrolysis of typical organic contaminants in the... 

We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

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