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ed~tedby FRANKA SETTLE JR V~ Ler r ~ngton n~M VA~24450 ~lav~~st~lufe
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Lasers in the Undergraduate Curriculum II. Coursework Experiments and Research Projects Jack K. Steehler Roanoke, College, Salem, VA 24153 In Part I of this series of ankles on lasers in the undergraduate chemistry curriuulum, the features of lasers were hriefly reviewrd and some guiding principles were given for the proper use of laser experimentation ( I ) . In this second part, the focus will he on specific experiments suitahle for different topical areas in chemistry. In each case a brief description of the experiment will be given, the applicable laser features will he pointed out, the required laser instrumentation will be specified, and references will he given. Since this article is designed to provide direct guidance on incorporating Lasers into lab courses, most references will he to -tid e s describing student experiments rather than references to the original research level publications. More general references can be found in the part I article. Within a given section, experiments are listed in order of increasing complexity. High levels of student interest in laser experimentation lead naturally to their use in long-term undergraduate research projects. Students pursuing such projects are enthusiastic and committed, which certainly enhances the quantity of results they obtain and the knowledge gained. This article will briefly discuss the centraland peripheral benefits of undergraduate laser research and will list some criteria and cautions for the choice of appropriate research topics. I t is not intended that all or even most of the suggested experiments and demonstrations he adopted a t a given institution. As with any tool, overuse of the laser must be avoided. Three or four laser experiments spread throughout a curriculum are certainly sufficient. As with moat major instrumentation, student laser experiments are best suited to lower enrollment upper level courses and to those lab courses where several different experiments are used simultaneously by different groups of students. The list of experiments below illustrates the range of types and levels of experiments that can be performed, hut is by no means exhaustive.
cost lasers are well matched to the experiments shown here; while higher power (more expensive) lasers are typically used in more intensive research environments. As mentioned in part I, HeNe lasers cost several hundred dollars, a low-power nitrogen laser costs about $4000, and a small dye laser costs $2500. Homemade versions of the nitrogen or dye lasers can he constructed from literature references at some savings but are recommended for experienced technicians only. The cost savings is often negated by the loss of reliability in a homemade system. Laser spectroscopy typically has a reputation for fancy optical setups, expensive hardware, and expensive electronic detection systems. These research-level require-
ments are not necessary for instructional systems or for undergraduate research. Homemade optical tables of a variety of materials are quite suitable for most experiments. Simple post holders for lenses and sample mounts are also sufficient. Commercial optical mounts are very expensive, and homemade alternatives are easily found. If commercial mounts are the only option, a recommended source is the micro and mini series of components of the Newport Corporation, P. 0.BOX8020,18235 Mt. Baldy Circle, Fountain Valley, CA 92728. Most experiments involve simple light detectors (photodiodes or inexpensive photomultiplier tubes). Such detectors cost under $100, (Continued on page A66)
Instrumentallon Requlred The experiments below (with one exception) require one of three possible laser sources. These sources are a helium neon laser, a nitrogen laser, or a dye laser (usually pumped by a nitrogen laser). Mbre sophisticated laser sources such a s Nd:YAG lasers or excimer lasers can readily he substituted for the nitrogen laser if availahle. The lower Volume 67
Number 3
March 1990
A65
although the photomultiplier tube does re. quire a high wltage power supply ($350). Photodiode detectors yield electrical arwslv that for continuous iasers can be directly measured with lab multimeters. The pulsed signals found when nitrogen or nitrogen pumped dye lasers are used require timegated detection electronics (see Fig. 1). These data systems average the signal only when it is present, outputting that average until the next laser pulse occurs. Such pulsed data averaging is most commonly done with a gated integrator. Commercial systems are expensive ($4500, Stanford Research Systems, 460 California Avenue, Palo Alto, CA 94306), but lower cost options exist. Evans Electronics (P. 0. Box 5055, Berkeley, CA 94705) offer a gated integrator circuit board (completely assembled) for $178. Simple TTL electronic timing circuits for the gate and a power supply are the only necessary additions, keeping the total cost below $225. Access to an oscilloscope and optional interfacing to a computer should also be considered. A final optical equipment item is a monochromator. Only those experiments that require wavelength scanning require a monochromator. Fixed (or nonselective) wavelength detection of fluorescence, for example, requires only a simple optical glass filter to keep the excitation w~arele!l~th from r e a c h i ~ ; the ~ detector. Such filters cost approrimamly S50.00 each ~Corion Coro... 73 Jeftrev Aw.. Hollist
