Laser-Induced Fluorescence in Spectroscopy, Dynamics, and Diagnostics David R. Crosley Molecular Physics Laboratory. SRI International. Menlo Park. CA 94025
In the technique of laser-induced fluorescence, or LIF, a laser is tuned so that its frequency matches that of an ahsorption line of some atom or molecule of interest. The ahsorption of the laser photons by this species produces an electronicallv excited state which then radiates. The flnorescent emission i i decected using a filter or a monochromator followed hv a ~hotomt~ltiirlier. Because a l~arricularahsorptiun line is selected, the excitkd state bas definite and identifiable vibrational. rotational, and fine structure quantum numbers. This cleanstate has significant advantages for spectroscopic and collision studies, in contrast to the congestion often found in ordinary emission spectra from, for example, a discharge. Since the lower state responsible for the absorption is also definite, considerable selectivity is provided by LIF when used as a diagnostic tool. In addition, its high degree of sensitivitv. " ., the soatial and temooral resolution available, and its non-intrusive nature are important attrihutes for this nurnose. LIF methods not possible . . Finallv." .soecial . in non-laser spectroscopy, such as two photon excitation, yield new information and make oossihle new diagnostic probes. These features of LIF are;llustrated in th& paperming as examoles a variety of experiments conducted in the author's lahor&ies. LIF as a whole has had a tremendous impact on the study of the electronic spectra of small molecules,' and it should be noted that the experiments discussed here form but a tiny portion of the many ways LIF has been used to further our knowledge of molecular structure and hehavior. Nonetheless, it is hoped that the highly personalselection presented will serve to describe some of the important aspects of this exciting and rapidly progressing technique. ~~
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LIF Experiments
Given a laser, the experimental config~rationemployed for most LIF studies is quite simple. The laser beam is directed into a samole. which is contained in some suitable cell if necessary. ~ h flborescence k emitted a t a right angle to the beam direction is focussed through a filter into a photoelectric detector. The filter may he a t a particular wavelength (such as a glass color filter or interference filter) or scannable (i.e., a monochromator). - - - - ~ ~ - ~ A single frequency laser (such as from a rare gas ion laser) mav be used if its freouencv hannens to coincide with that of some absorption ~ i n e , b uciear&a t tunable (dye) laser is more versatile. It permits the performance of experiments on different molecules, or on a sequence of excited levels in one species so as to compare their hehavior. The most rapid growth in the number of LIF studies has coincided with the availability of commercial tunahle dye lasers. Continuous duty 1;tsers have ndwmtnyrs d much narrower linewidth and morc stahle otltput amplitudes, wherwi pulsed lasers have higher peak pow& and thus higher instantaneous signal levels, and with them is possible a variety of non-linear processes including frequency doubling and shifting methods. All of the experiments described in this paper involve pulsed lasers having repetition rates of typically 10 Hz, pulse lengths of 10 m~a-r or 1 usec. and in all but one afre~uencyshift from the dye laser fundamental. When operatingwitha pulsed laser, it is eenerallv advantaeeous to use a gated detection system. Also termed -a boxcarvintegrator when used to average over a ~
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Journal of Chemical Education
Figure 1. Schematicdiagram of an excitation scan &wing the potential curves (electronicstates)and some levels involved. The detection system measures fluorescence(downward arrow)from any excited level; this will occur each time the laser is tuned to match the frequency of some absorption line (upward arrows).As the laser is tuned through the series of absorption lines, an excitation Scan results. number of laser pulses, this device basically turns the detection electronics on only for a short period during or immediately after the laser pulse. Because all of the LIF signal occurs in this brief time span, gated detection greatly enhances the ratio of signal to that background which is continuously nresent. such as ohotomultiolier dark current or ordinary flame emission. The two chief methods of snectroscooic data acouisition in an LIF experiment are excitation and fluorescence scans. In an excitation scan, depicted schematically in Figure 1, the detector and filter are chosen so that fluorescence from any excited level can he detected. The laser frequency is then scanned through the absorption region of the molecule; each time it matches that of an absorption line, fluorescenceresults. Thus an excitation scan mimiis the ahsorption spectrum of the molecule. The main difference is that instead of looking a t a small dip in a large transmitted signal, LIF forms a positive signal on a null background. I t is thus much more sensitive: total absorptions of or less can produce readily measured signals. In addition, higher selectivity can he possible than in an absorption measurement. Suppose two (or more) species are present, hoth absorbing a t the same wavelength hut fluorescing a t different wavelengths. The choice
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(12) Crosiey,D. R..Smith, G.P.,and Bischel,W. K..Lo bepubliahed. (131 Bischel, W. K.. Perry, B. E.,and Cms1ey.D. R.,Chrm. Phya L~tt..SI,85 (19311.
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Figure 17. Excitation scans through each line structure component in 0 transition. The figurer are denoted by the 3P, ground state level in each case: note that me gain varies. The upper state splitting is not resolved by the laser: the theoretically expected positions and relative intensities due to each Is indicated by the stick diagrams, which are labdled by the upper state value of J. (The lower two scans have only two conwibuting upper state components each due to selection rules.) [Reprinted h o m Chemical Physics Lenerr]
Figure 18 Lifetme scans f a me 3p 3Pstate 01 oxygen. The laser pLmps e m l e d state b d shuts offaller -8nsec. leaving lneexcoted s m t e t o f l ~ o r e r c e at its characteristic decay rate. The top scan is in the absence of added gas, and b e middleand b c m m scam are wilh 2 and 5 tan of NZBdded, r e s p ~ c t i i l yF. r m these data are obtained the radiative lifetime of the state and cross section for its collisional quenching by N,.
Volume 59
Number 6 June 1982
455
