Lasers in the Undergraduate Curriculum I. Features and Philosophy Jack K. Steehler Roanoke College, Salem, VA 24153 Teachers of chemistry have long realized that the most effective teaching methods are those that create excitement and interest in students. However, there is often a discrepancy hetween topics that are viewed as being simple and reliable enough for classroom use and those topics that excite students. The use of lasers in science is a ease in paint. Students are fascinated by the idea of lasers. They read or see stories about laser surgery, laser welding, fiber optic communication, or laser-based weaponry an a daily basis. Every visit to a supermarket brings exposure to modern applications of lasers for bar code scanning. In scientific research, as many as unr-third of chemists and phyairirte use lasers nmtincly. Set the ~cienttlicimoortance of lasers and the high level of student interest in lasers are rareiy combined in undergraduate education. The reason is the perception among teachers of science that lasers are too complex or unreliable to he used in the instructional setting. Beyond the diffraction experiments of introductory physics courses, lasers are rarely seen by students. While this perception of complexity may once have been true, it is true no longer. Most laser systems are turnkey systems. You turn a key, and they work. Period. Similarly, the perception that Lasers in undergraduate coursework are limited to optics experiments in physics is also incorrect. Many fascinating student experimente and lecture demonstrations using lasers can be done, with excellent reception hy students. Cost, too, is perceived as a major limitation. Yet many of the experiments of interest need only the most inexpensive of lasers, the red helium neon laser, which costs only several hundred dollars. And the next level of capability, the nitrogen laser, lists a t less than $4000, well within the instrumentation budgets of many colleges and universities, especially when efforts are made to utilize the instrument across the curriculum, in three or four different courses. The henefits of incorporating lasers in lectures and lab experiments are real. Student interest in such experiences is quite high. Most groups ofstudents (collegefreshmen or even as young as fifth grade) who can barely define the term chemistry, can explain details of how lasers work and give five or six maior exam~lesof different laser auplieations. Lecture demonstrations using lasers are followed by lengthy student initiat-
eddismsions. Duringlsser lab experiments the rapt attention of the students is remarkable. Students are willing to undertake quite difficult experiments and experimental analyses in such eases. And, they ask questions! Compared with run of the mill experiments with traditional low-level equipment or instrumentation, the contrasts in student attitudes, efforts and overall understanding ere striking. This article and the companion article to be puhlished neat month seek to provide the guidance necessary far using lasers effectively in undergraduate instruction in chemistry. This first article will provide a review of the features of lasers that make them useful to chemists, as well as a hit of philosophy about how the excitement of lasers should be used in the chemistrycurriculum. The second article will provide specific exoeriments (with references). annlicable to different courser, and some discuskirm of the use of lasers in undergraduate rrsearch projects.
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Features of Lasers Why should chemists he interested in lasers? What features of laser light makes it better than light from a light bulb? What kinds of different lasers are available and what are the key differences? These and other questions come to mind whenever the subject of lasers in chemistry is discussed. A basic understanding of the answers to these questions is necessary before considering specific examples. A variety of references are available for these general topics, including past Journal of Chemical Educa-
tion articles (I, 2) and various books. With one exception, the available resources are rather narrowly focussed, omitting the large picture of lasers related to chemistry. The exception is a book by D. L. Andrews entitledLasers in Chemistry (3). The relevance, and indeed the centrality, of laser methodologies to chemistry are presented most clearly in this book. Solid references on molecular spectroscopy include books by Hollas ( 4 ) and Steinfeld (5).For anunderstandable low-level explanation of lasers themselves, the book by OShea (6) and two articles by Wright and Wirth (7,8) are recommended. Higher level reviews of areas of laser spectroscopy are readily found, such as J. Chem. Edue. articles on picosecond spectroscopy (9, 101, two photon spectroscopy ( 1 0 , photoionization (12, 131, infrared lasers (14), lasers in chemical kinetics (151, and laser induced fluorescence (16, 17), many of which are included in the "Lasers from the Ground Up" J. Chem. Educ. offprint (I).Analytical chemists will find the book Analytical Applications of Lasers edited by Piepmeier (18) to be a major resource. Among the attractive features of lasers are high intensity, high monochromaticity, high directionality, high time resolution, and coherence. While not all lasers offer each of these features, each feature is readily available and each can be powerfully applied to chemistry. Typically pulsed lasers offer the highest intensities and time resolution, while essentially all lasers offer monochromaticity. The table lists a variety of (Continued on page A38)
Commonly Available Lasers
~ o a e s cost t system IS mmea ass low end commercially ava~lamesystem. ~ a w e coat r kits or nomemads laserscan be COnStlYCIed. Gosh of the highest peak power shown are signiticantiy higher.
In/trument~tlon
of enormous use to chemisrs. Nanonecond ( 1 0 - ' 4 , picosecond (10:- s ) and even fem.. - .- -. ... - toaerond (10 "s, pulsesarecommonly used to monitor very fast chemical processes (9, 10). Such processes include vibrational relaxation, protein conformational changes, available lasers. For more detailed informaphotosynthetic reactions, and even molecutian, consult the annual "Buyer's Guide" issues of Laser Foew World (19) or Lasers lar collisions. And let us not forget that the and Ootronics (20). most central caneeot in ebemistrv. . .~Students should be eiv.. the en e bariic understanding of these features chemical reaction, is hased on molecular colwhenever lasers are used in the curriculum. lisions and the enerm redirtributionr which High light intenaitrer (megawatt powers reau11 from such rollrsionr. Since no method other than laser spectroscopy can offer subfor nanosecond pulsed lasers, compared to hundred of watts for "light bulb" sources) nanosecond resolution, this "less puhliciyield enhanced signal levels for processes zed" feature of lasers may well be the moat dependent on peak power. Such applicaimportant chemically. Students rarely have any grasp of such short timeframes, a probtions include luminescence methods such as fluorescence and phosphorescence (see Fig lem which involvement with nanosecond pulsed lasers can remedy. Simple demonI), surface ablation sampling for mass specstration of the time required far a light pulse trometry, photoionization, and nontradito travel the Length of a room and back can tional spectroseopies such as second barpnnvide this timeframe understanding. monrc generation ior the study of surfaces Such erpcriments typrcelly measure the and surface advorbatcs r2l1.'These m e t h d s are the most denritive availnhle, with hoth speed of light to withrn :rot using a photodifluorescence and photoionization having ode detect& and an oseillosope & determine the roundtrip time. I t is important to been demonstrated to detect single molerelate these ideas to chemical timeframes cules in favorablecases (22). From astudent such as the 10-'2-s vibrational period, the point of view the concept of "zapping" 10-13-s energy transfer event in photosynsomething with a laser is m w t enticing! thesis, the 10-14-s collision interaction time While demonstration of such potential does in liquids, and the nanosecond timescale of hare its place, a discussion of the question protein internal motions. Students uniform"What good does rr do a chemirt to "rap" somethina?" is certainly approprinte. Adly report a greatly increased understanding of rapid chemical events following such laser vanced students are well able & come up with applications such as surface sampling, demonstrationa. photoionization, and enhanced emission of Generalizing from the features discussed light from such discussions. above, two main links between lasers and chemistry can be stated. The first is the use High monochromaticity translates into of laser spectroscopy of many varieties to excellent spectral specificity. In analytical probe chemical systems. The second is the chemistry this spectral resolution translates use of lasers to induce chemical reactions. into multicomponent mixture analysis. In This latter use of lasers is less widely apprephysical chemistry the resolution provides ciated than spectroscopic applications, hut detailed molecular energy level structures is equally important. Excitation af highly (particularly in the gas phase, where collispecific molecular excited states leads to sesional broadening is small), allowing ever lectivity among a large number of possible more detailed investigations of molecular reaction pathways. Specific examples instructure and reaction dynamics. In organic clude: (1) the synthesis of vitamin Dg(23), photochemistry, the spectral purity of laser whcre a low-yield thermal step in the reaciight can reduce side reacttons common to tion sequence ran be replaced by a L'V laser photoreaetiona with broadband light induced step with YO': yield, (2) the d i s ~ o sources !"lieht bulb" sources). The anal\zieiation of 2-&opanol(24), where the ratioof cal advantage of selectivity is readily two possible products (acetone end prograsped by undergraduates, but the more pene) can be varied from 6:l to 1:50 by variasubtle uses in high-resolution rotationallvition of the angle of the infrared laser with brational spectroscopy and photochemistry respect to the catalytic CuO surface used, require extensive discussion if in depth unand (3) synthesis of silicon nitride ceramics derstanding is desired. (25), where laser induced reactions produce The time resolution of pulsed laaers is also ~~
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Figure 1. Typical layout for a laser-induced fluarescenceexperiment
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Journal of Chemical Education
essentially 100%yield, with highly uniform particle diameters, compared to thermal methods where lower yields and more disperse particle sizes occur. These differences translate into higher strength and better catalytic performance for the laser produced ceramic powders. Lasers a n d Chemistry Lasers, like computers, fall into an interesting category of scientific tools. That category is one in which the tool itself threatens to become the primary focus, rather than the science the tool makes possible. Just as there are computer hackers among the country's scientists, there are laser "jocks" whose primary interest is in the biggest and most powerful laser, or a t least in playing with Lasers as their mast important activity. As educators, we must strenuously resist such tendencies in ourselves. The chemistry must come first! Lasers should be viewed as tools that make communication of ideas and concepts easier, or more effective than other tools. Yes, we must allow students to enjoy the lasers, to use the fun and fascination of nifty instrumentation to provide necessary motivation for learning science. But the tool does not replace the science! It is instructive to examine two similar cases wine brizht lieht sources. for which cation of lasers is in fluorescence spectroscopy, we will use fluorescence as the basis of both cases (see Fig. 2). One experiment consists of determining low-level concentrations of an organic fluorophor in solution. The second case is the determination of low levels of flumercent lanthanide bans m solution. In neither case ia time r~aolutionof inrerest, since only straighlfurwnrd quantitation of a single component sample is desired. For the first case, a light bulb excitation source is likely to he a better choice than a laser Light source. In solution, molecular spectra are quite broad, so that the complete absorption profile is spread over several hundred nanometers of wavelength. The laser source, though it is brighter than a traditional arc lamp source, interacts with only a tiny integrated absorbance cross section, due to its monochromaticity (bandwidths of 0.001 nm are common). The continuous spectrum of the arc lamp encompasses the full absorbance band of the compound. Thus the total number of photons absorbed when the are lamp is used (and thus the number of fluorescent photons emitted) can in fact exceed that found when the higher intensity laser source is used. In the second case, the lanthanide ions in solution possess very sharp absorption and emission spectra because the f electrons involved in absorption are shielded from environmental effects by electrons lying further from the nucleus. In this case, the many photons of the are lamp which are spread over wide ranges of wavelength cannot be utilized, while the monochromaticity of the laser is well matched to the narrow band of colors ahsorhed by the lanthanide ion. In this second case, laser excitation sources provide major advantages over traditional sources, as long as the necessary wavelength for absorption is available, such as from a tunable dye laser. The examples demonstrate a need to think, and to understand (Continued on page A40) Volume 67
Number 2
February 1990
A39
6. 0'Shsa.D. C.: Csllen, W.R.:Rhodea. W. T. Infrodu~. tionloLasersondfheirApplicoliom:Addison-Wes-
ley: Reading. 1977. 7. Wlighf. J. C.; Wirth. M. J. A n d Chem. 1980. 52, l"17A
8. Wright,J.C.:Wirth.M.J.Anol.Chem.1580.52,988A. 9. Wane, Y.: Eisenthal. K. B. J. Chem. Edui. 1982, 59, 482. 10. Teay,w.: Hoehatrsswr, R M. J.Ch*m.Educ. 19s2,59,
underlying processes before choosing a laser as a light source. A further illustration of this uoint is seen when examining the most common apectnrscopir method, abaurption sprcrrosropy. The baris of traditional absorption s p ~ c troscopy is the measurement of the intensity ratio of incident light and light transmitted through a sample. Quantitative measurements (Beer's law) are based on this ratio. Thus the intensity of the light source is immaterial, as long as sufficient light is present to minimize the importance of detector noise. In most cases laser sources offer no sensitivity advantage over "light bulb" sources. Specialized absorption spectroscopy applications such as high resolution gas phase spectroscopy or ultratrace spectrwcopy using photothermal absorption methods (26) (see Fig. 3) may require laser sources, hut not most routine cases. Routine absorption spectroscopy requires Light sources which are easily tuned over wide wavelength regions. No convenient widely and eontinously tunable lasers exist. Dye lasers offer UV-visible tunability, but only aver short ranges, since tedious changes of dye solutions are required for wider tunability. Students should come to an understanding of when the w e of lasers is appropriate and when it is not. As another example, it is possible to quantitate suspended solids in fluids by laser light scattering. In many cases, simple filtration, drying, and weighing can provide equivalent information, with orders of magnitude less expense and complexity. In other cases where particle size distributions are of key importance (in the paint industry for example) laser light scattering may provide information unavailable with other common tools. T h e bottom line is of course that the mere availahility of an analytical tool does not make it the method of choice in all eases. This lesson is applicable t o almost all scientific'methodologies, where the use of judgement in choosing the correct method can mean the difference hetween success and failure. ~~
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Conclusions Lasers are a very strong motivating force for students of chemistry. They are fun to use! The widespreadusage of lasers in chemical research adds to the reasons for using lasers in the undergraduate curriculum. This article has focussed on reviewing the main features offered to the chemist by lasers and on emphasizing the proper place of any advanced tool in the chemist's toolbox. T article in this will~ de- h e second ~ ~ ~ seauence scribe specific experiments using lasers ap~~~
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11. Friedrich, D. M. J. Chem. Edue. 1982.59,472. 12. Travis,J. C. J. Chrm. Educ 1982.59.909. 13. Smalloy,R.E.J. Chem.Edur 1382.59.934. 14. John. P. J. Chem. Edue. 1582.59.135. 15. Kovslenko. L. J.; Lpane, S. R. J. Chm.Edue. 1988.65, 681. 16. Cmsley,D. R. J Chsm. Edue. 1982.59.446 17. Lytle, F. E. J. Cham. Educ. 1982.59.914, 18. Piepmeier, E. H.A n o h r i d Appi~cationsof Losers: Wiley: New York. 1986. 19. Laser Fmua World. PennWell Publishing Co.. 1421S. Sheridan,Tulsa.OK 74112. 20. Laosra & Ootronier. P.O. Box 1952., Dover.,NJ 07801. 21. ~iehmo"d,'G.L.; Ghantalsb,H. M.:Robinso", J. M.; Shannon,V. L.J. OD,. Sor. Amer. B. 1987.4.228. 22. Hirshehfeld, T. ~ p p Opf. i 1916.15. 2965. 23. Malatesta,B.: Wil1is.C.; Hacketl, P. A. J, Am. Cham. Soc. 198l,103.6781. 24. Fsrnefh, W. D.; Zimmeman, P. G.Hogenkamp, D. J.; Kennedy,S.D. J A m . Chem.Soe. 1983,105,1126. 25. Andrew%D. L. Losers in Chrmkfry; Springer:Berlin, 1986;~. 162. 26. Sell, J. A. Phofofharmolinwstigotionso/Solids ond Fluids; Academic: Baston. 1989. ,.. ,,,. ...,, , , ,,'., , ~
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Figlne 2. Camperison of spectral profiles of laser output, molecular absorption specha, and lanthanide ion absorption spectra.
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propriate LO organic chemistry, physical chemistry, hiwhemistry, a n d analytical chemistry.
Literature Cited 1. Lnaers /,om the Ground Up; J. Chem. Educ.: Eaaton PA. 1982. 2. Findam. E. W.. Ondrias. M. R. J. Chem Educ. 1986, 63,479. 3. Andrew, D. L. L a w s in Chmlatry: Springer Bedin, 1986. 4. Hollas. J. M. Modern Spectroscopy; Wiley: Chiehestrr, 1987. 5. Steinie1d.J.I. MoleculesondR~di~fion,2ndad.:MIT Cambridge. 1985.
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Journal of Chemical Education
Figure 3. Concepml sketchof a photothermal beamMttlection experiment. Absorption ofthe intense pump laser light by Wsampleresults in mereleaseof heat. The resulting change in refractiveindex, shown by Uw low power HeNe probe laser. The deflection is a very renshive curved lines, results in a defledion of measurement of amount of light absorbed by lha sample.
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