LASERS FOR PLASMA DIAGNOSTICS, TIME-RESOLVED MEASUREMENT AND MOLECULAR FLUOROMETRY IN ANALYTICAL SCIENCE

In chemical measurement and characterization, lasers are playing a uni que role in improving sensitivity, enhancing spatial and spectral reso lution, and enabling time resolution on the fastest time scales that a re chemically significant. Furthermore, lasers have permitted entirely new classes of measurements to be undertaken that would not be possib le without the high radiant power, directionality, and coherence of a laser beam. In this paper, a number of these capabilities are illustra ted with examples from the authors' laboratory. Prominent among these examples is the use of a high-power pulsed laser for producing scatter ing and fluorescence from species in an inductively coupled plasma (IC P). With the appropriate laser and photometric equipment, such measure ments enable the determination of time-resolved and spatially resolved values for electron concentration, electron energy distribution, gas- kinetic temperature, and the concentrations of important sample and in trinsic species that the plasma contains. Another example shows how ei ther a continuous wave (CW) or repetitively pulsed laser can be couple d with relatively simple electronic instrumentation to permit measurem ents to be obtained on a sub-nanosecond time scale. Interestingly, a r ecent development might obviate the need for a sophisticated laser in such schemes. Lastly, a relatively simple experimental configuration c an be used to determine as few as 10(6) molecules in a real sample. In this arrangement, a single aliquot of the sample is dispensed in a vo lume as small as 6 nL. This aliquot, in droplet from, then constitutes the sample cell itself. As the droplet falls through the focused lase r beam, its contents can be determined with extraordinarily high sensi tivity. Methods to improve even this detection capability will be outl ined.