SCIENCE/TECHNOLOGY
Cavityringdownoffers sensitivity, simplicity in absorption spectroscopy Elizabeth K. Wilson C&EN West Coast Bureau n principle, absorption spectroscopy is an ideal chemical probe. But in practice, it's a major headache. It sounds simple enough: shine light through a sample, and measure how much of the light has been absorbed by the sample. Not only does this nondestructive method identify chemical species by their characteristic absorptions at different wavelengths, it also provides a measure of their concentrations. Unfortunately, light sources aren't stable. Even the most quiescent lasers available suffer from intensity fluctuations that overpower the faint spectroscopic ripples produced by many absorbing samples. Only the most intense absorption peaks rise above the sea of noise. Of course, physical chemists have plenty of other spectroscopic tools at their disposal, including Fourier-transform absorption spectroscopy, laser-induced fluorescence, and Raman scattering, to name just a few. All are powerful techniques that work very well for plenty of applications. But in their heart of hearts, many spectroscopists yearn for a simple absorbance method to quickly and elegantly produce detailed molecular spectra, probe kinetics of species that don't fluoresce, or perform trace analysis. An ingenious new technique, which some chemists say is on the verge of exploding in popularity, may fit the bill. Cavity ringdown laser absorption spectroscopy (CRLAS), as it is known, is several orders of magnitude more sensitive than traditional absorption spectroscopy and, in some cases, rivals some of the most sensitive spectroscopic methods available. If s nondestructive, relatively inexpensive, and easy to set up. The secret to CRLAS's sensitivity is that it measures absorbance indirectly through the relationship of absorbance to the time of photon decay in a cavity—a quantity that is not affected by stability of the initial light source. In a typical CRLAS experiment, a laser pulse fires into a cavity formed by two reflecting mirrors. Because the mirror reflectance isn't perfect, the photons
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FEBRUARY 19,1996 C&EN
1238 (1984)] developed precursors to cavity ringdown. Their work led to breakthroughs in the late 1980s by physical chemists Anthony O'Keefe and David A. Deacon of Deacon Research, Palo Alto, Calif., who developed a commercial cavity ringdown apparatus to measure mirror reflectances as part of the Strategic Defense Initiative. In their investigations, however, they found that oxygen in air was interfering with their measurements, absorbing some of the light energy, and changing the ringdown time. "That's when we realized it was a complement to traditional absorption spectroscopy techniques," says O'Keefe, now with Los Gatos Research in Mountain View, Calif. Turning their reflectance assessor into a spectroscopic tool, they were able to detect the spectral lines
of light gradually dribble out of the cavity. The intensity of the leaked light decreases exponentially, and the time it takes to decay to the inverse natural log (1/e) of the initial intensity is known as the "ringdown" time. If the cavity holds a vacuum, the ringdown time is a measure of the reflective properties of the cavity mirrors. But a sample such as a gas inside the cavity can absorb some of the injected light, affecting the ringdown time. It's then a simple matter to obtain the absorbance, which is inversely proportional to the ringdown time. What makes CRLAS possible is the extremely high reflectance of modern mirror coatings. ReCavityringdowntechnique measures flectances of up to 99.99% ensure that absorbance as function of decay time the photons stay Mirrors trapped in the cavity for microseconds, bouncing back and Pulsed forth thousands of laser times. Detector Cavity As a tunable laser is scanned over a In cavity ringdown laser absorption spectroscopy, pulsed multitude of frelaser light is injected into an optical cavity formed by two quencies, each pulse mirrors with reflectance greater than 99%. The trapped records a ringdown light gradually escapes from the cavity, its intensity decreasing exponentially. The time required for the signal to time that represents decay to the inverse natural log (1/e) of the initial output an absorbance meapulse is the "ringdown" time. A sample placed inside the surement. Eventucavity absorbs light, changing the ringdown time which is ally, a spectrum is inversely proportional to the absorbance of the sample. produced that, regardless of the stability of the light source, highlights even weak transitions of a doubly forbidden transition in moand can detect fractional absorption in lecular oxygen [Rev. Sci. lustrum., 59, parts per million, typically orders of 2544 (1988)]. magnitude greater than traditional abMost of the CRLAS experiments use sorbance spectroscopy. pulsed lasers, which, ironically, are wildEven as far back as the 1960s, re- ly unstable compared with continuoussearchers at the Army Signal Research wave lasers. Before cavity ringdown, us& Development Lab in Fort Mon- ing pulsed lasers for absorbance specmouth, N.J., and others were looking at troscopy "would have been laughable," decay of millimeter waves in reflecting says Alec M. Wodtke, chemistry profescavities. sor at the University of California, Santa But it was during the early 1980s that Barbara. But since intensity fluctuations scientists such as John M. Herbelin of do not affect CRLAS, pulseddye lasers Aerospace Corp., Los Angeles [Appl. are the ideal source for CRLAS. However, several camps are currently Opt, 19,144 (1980)], as well as Dana Z. Anderson and colleagues at California wrangling over the theoretical capabiliInstitute of Technology [Appl. Opt., 23, ties of CRLAS, hotly debating issues of
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Spectra show sensitivity of cavity ringdown method Absorbance (x 1CT4)
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Absorption cross section, sqcm (x10~18)
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University of California, Berkeley, researchers Saykally (front), (and from left) Bjorn Steiner, Joshua Paul, Ryan McLaughlin, and their coworkers have used the nngdown method to study metal clusters and metal silicides.
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cavity mode interferences and the use of continuous-wave versus pulsed lasers. Most chemists involved in CRLAS re search, including Princeton University physical chemistry professor Kevin K. Lehmann and Stanford chemistry pro fessor Richard N. Zare, have devoted much attention to the fundamental ex planations of CRLAS. Experiments are currently being done that may resolve those questions, says University of Cal ifornia, Berkeley, chemistry professor Richard J. Saykally. But that hasn't stopped research groups around the world from forging ahead with exciting experiments that demon strate the potential of CRLAS for many different types of chemical problems. Saykall/s group at UC Berkeley, in cluding former graduate student James J. Scherer, now a postdoctoral researcher with physical chemist David J. Rakestraw at Sandia National Laboratories7 facility in Livermore, Calif., utilized CRLAS for molecular beam spectroscopy to pro duce the first absorption spectra of some metal clusters, including a copper trimer. Scherer, O'Keefe, Saykally, and col leagues have published a host of pa pers on the absorption bands of metal silicides, species important in the semiconductor and microelectronics industry. They started with a copper silicide, searching for absorption bands through out the spectrum. "I scanned for months from red to blue/' Scherer says. "I knew there had to be optical transitions, but
5
there were no ab initio calculations" to base the search on. I I I I 0J The group then did the same thing 214 215 216 217 218 with gold, silver, and platinum sili Wavelength, nm cides. Scherer says all the metal sili Cavity ringdown spectrum of methyl cides had transitions in the ultraviolet radical (top) compared with a spec region, perhaps due to excitation of the trum obtained by using multiple silicon atoms. pass laser absorption spectroscopy Thanks to developments in mirror (bottom) reveals similar features. technology, Saykally's group, Scherer However, the density of the methyl and colleagues at Sandia, and O'Keefe radical column used to measure the have also extended the range of CRLAS cavity ringdown spectrum was more to the infrared region, studying absorp than one order of magnitude smaller. tion bands of methane [Chem. Phys. Lett., Source: Chem. Phys. Lett. 234, 269 (1995) 245, 273 (1995)] CRLAS can now be used in the 1- to 10-μιη range, where much crucial in formation from vibrational structure company Bruker Instruments, Meijer can be observed. 'The entire mid-IR is has also developed a hybrid technique open now/ 7 O'Keefe says. "As a device that combines CRLAS with standard that gives broad coverage, it's probably Fourier-transform absorption spectros the most sensitive available in the IR." copy. Instead of using a monochro Gerard Meijer, physics professor at matic laser, a broad bandwidth light the University of Nijmegen, the Nether source can be injected into the cavity, lands, together with Wodtke, was the in effect eliminating the need for scan first to extend the capabilities of CRLAS ning frequencies one by one. The ab to the UV region—detecting hydroxyl sorbance information from each fre radicals generated in flames, weak tran quency can then be extracted with Fou sitions of carbon monoxide, and atomic rier transforms. This method, Meijer mercury vapor at one part per trillion says, should be ideal for exploration in [Chem. Phys. Lett, 217, 111 (1994); /. Mol.the spectroscopically valuable infrared Spec., 165, 303 (1994); Rev. Sci. lustrum., regions. 66,2821 (1995)]. Meijer and colleagues at Building on earlier cavity reflectance Stanford Research Institute, Menlo Park, work, such as that done by Herbelin, Calif., have also recently found previ Meijer and colleagues Richard Engeln ously undetectable vibrational levels in and Gert von Helden have developed a molecular oxygen. method of CRLAS that uses continu In collaboration with the German ous-wave light sources, as opposed to FEBRUARY 19,1996 C&EN
35
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FEBRUARY 19,1996 C&EN
O'Keefe: method covers broad applications
pulsed lasers. Using intensity-modulat ed beams, the technique measures ab sorption through the relative phase changes between the entering and exit ing beams, and is even more sensitive than pulsed laser CRLAS, Meijer says. CRLAS can also be useful for study ing the kinetics of species that show weak absorption, or ones that don't fluo resce and therefore can't be studied by methods like laser-induced fluorescence. Ming-Chang Lin and colleagues at Emo ry University, Atlanta, are pioneering this line of research, with examinations of the kinetics of reactions involving phenyl radicals and other species [/. Am. Chem. Soc, 115,4371 (1993)]. The behavior of the phenyl radical, which plays an important role in hy drocarbon combustion, has been partic ularly intractable. "At the time we started the investigation of the kinetics of the phenyl radical, nobody could measure the absolute rate constant in the gas phase, because of weak absorp tion/' Lin says. Lin and colleagues first created the radical by photodissociation, then mea sured absorbance with CRLAS. By varying the time between dissociation and the CRLAS measurement from 0 to 10 milliseconds, they were able to de termine rate constants for several reac tions. They have also applied the tech nique to reactions with the phenoxy radical, and are studying other nonfluorescing radical species. Lehmann and former graduate student Daniele Romanini have used CRLAS to study the high overtone spectrum of hy drogen cyanide, which has very weak transitions in the visible range, as well as
collisional processes by observing line shapes as a function of pressure. Many researchers are also exploring CRLAS's potential as a detector of abso lute quantities. At Sandia, Scherer and Rakestraw are measuring absorption spectra of various radicals—including the important combustion intermediate hydroxyl radical, generated in flames— in the IR region of 1.5 to 4.2 jim. "We're able to probe with a level of sensitivity never achieved before/' Scherer says. "The spectral sensitivity level is about 20 ppm—that's 100 times better than Fourier-transform spectroscopy in flames, and about 10 times better than diode laser techniques." Zare's group also is interested in ap plying CRLAS to flame and plasma techniques where traditional methods have been found difficult to use [Chem. Phys. LOt., 234,269 (1995)]. Zare sees real power in CRLAS's ability to measure trace quantities. Some applications that "might seem mundane," he says, like measuring trace quantities of water va por, could revolutionize the food indus try, for example. Keeping track of water amounts "makes all the difference in the world for when you bake your dough. For companies that make pastries or crackers, this really matters," he says. Water vapor is also a concern at the National Institute of Standards & Tech nology in Gaithersburg, Md., where J. Patrick Looney, Roger D. van Zee, and colleagues believe CRLAS will be able to help them reduce current standards of about 10 ppm of water and air in process gas streams to possibly below 1 ppb and establish measurement stan dards for water in a vacuum. Minute traces of water affect the growth of thin films on silicon wafers in semiconduc tor processing, requiring stringent moisture restrictions. "The semicon ductor industry is extremely worried about this," Looney says. Lehmann says there exist mass spec trometry techniques with sensitivities of about a few parts per billion, but the ma chines cost several hundred thousand dollars. "If we come up with a technique, we could use semiconductor diode lasers which only cost a couple of thousand dollars, and are very specific," he adds. At Mississippi State University, post doctoral researcher Christopher B. Winstead believes cavity ringdown could be an ideal environmental monitor, particu larly in hazardous waste disposal sites, for measurement of toxic metals. "Cavi-
which would actual ly go into hazardous waste treatment fa cilities and monitor gas for toxic materi als," he says. The power and simplicity of CRLAS also make it ideal for a physical chemistry lab because even undergraduates can handle it, says chem istry professor Fred J. Grieman, at Pomo na College, Pomona, Calif. "We're going to start with the same Tao Yu (left) and Lin study kinetics of phenyl radical reactions thing [aKeefe] did— with cavity Hngdown apparatus at Emory University, Atlanta, just using air between the two mirrors," ty ringdown promises really great sensi Grieman says. "One of the exciting things tivity," Winstead says. His group is ex about CRLAS is that you don't need an ploring the development of an appara expensive, powerful laser, you don't need tus they hope will become a useful real expensive apparatus, and you can do industrial tool. "We're trying to put some spectroscopy on systems that are • something together for the real world, easily obtained."
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