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Show ) before they convert to the triplet state. Two-color REMPI can be used to effectively ionize the triplet state after internal conversion, but the triplet molecules are usually highly vibrationally exci ted and require a much shorter wavelength to effect photo ionization. It is possible that such a second color laser would also be in resonance with higher singlet electronic states (S2 , etc .) that could produce a large number of background ions via one-color 1+1 REMPI. There is another technique that can be used for molecules that exhibit rapid intersystem crossing. In this scheme jet-cooled molecules are excited near the nozzle orifice, undergo intersystem crossing to the long-lived triplet state, and finally strike a sensitized phosphor screen. The electronic excitation is transferred from the triplet molecules to the phosphor and a photomultiplier is then used to monitor the phosphorescence from the screen. This jet-cooled sensitized phosphorescence technique has been recently used successfully on naphthalene, } CN, and 2 CN.14 The key question concerning this approach is its sensitivity since it involves an energy transfer step of unknown efficiency. Nonetheless it is a general technique that may hold promise for a wide variety of molecules and should be investigated more fully. Finally it is important to make the point that any of these laser-based spectroscopic techniques (LIF, REMPI, etc.) require a fairly detailed knowledge of the absorption spectrum and photophysics of the molecule of interesL To a large degree this information is at present totally unavailable for the species of most importance, i.e. PCB's, dioxins, etc. Therefore a large amount of basic research must be done before we can answer the question of whether the techniques discussed here can be used effectively for such molecules. In particular there is a clear need for accurate spectroscopic studies in a jet-cooled environment. Such laboratory studies are difficult for the more toxic species for a variety of reasons (not the least of which is safety). For example, the vapor pressure of 2,3,7,8 TCDD at room temperature (7xlO-10 torr16) is far too small to allow detection under jet-cooled conditions for room temperature seeding. Even at the highest temperatures attainable with pulsed valves (-300 OC) it is unlikely that 2,3,7,8 TCDD will have a high enough vapor pressure. Therefore more elaborate techniques will be required for such involatile samples. One possibility that has been successfully implemented for biological species is pulsed laser desorption of the sample from a target that is placed in the throat of the supersonic nozzle expansion.} 7 Desorbed molecules are entrained in the carrier gas flow at relatively high instantaneous densities prior to free-jet expansion. This technique has the added advantage of minimizing the amount of sample required since the pulse of desorbed molecules is utilized more efficiently by a pulsed laser detection scheme. The last criterion set forth in Sec. I is that our detection technique be at least quasi-real-time. Either of our techniques meet this criterion, however the extent to which "quasi" is used depends on how they are implemented. For example it takes -30 min. to acquire the REMPI spectrum shown in Fig. 6 and -2 min. to acquire each reflectron TOF mass spectrum shown in Fig. 3. This much information may be necessary if one wants to detect several different species in the sample (for example the DCB isomers). However it takes only -30 s to scan ,an individual line in the REMPI spectrum, and this would be the amount of time needed to detect just one species. Alternately single species detection could be arranged 15 |