Supersonic Jets, Rotational Cooling, and Analytical Chemistry

Instrumentation

John M. Hayes Gerald J. Small Ames Laboratory-USDOE and Department of Chemistry Iowa State University Ames, Iowa 50011

Supersonic Jets, Rotational Cooling, and Analytical Chemistry Supersonic beams produce a vapor stream in which thermal broadening is dramatically reduced

Given a complex sample, such as shale oil or a biological specimen, which with slight exaggeration can be viewed as containing infinitesimal amounts of an infinite number of species, is it too much to ask for a direct quantitative analysis that affords selectivity and sensitivity? We think not and hope that this article will allow the reader to share our optimism. To date the most widely used direct analysis technique for such samples has been capillary column/gas chromatography/mass spectroscopy (CC/ GC/MS). Its success, of course, is due to the marriage of GC and MS, which serve, respectively, as fractionator and selective detector. A major problem, however, is that "dirty" samples cause rapid degradation of the capillary column. Thus, an extraction or prefractionation step frequently is used to clean up the sample prior to analysis. In addition, for isomeric species, CC/ GC/MS must achieve chromatographic resolution to effect an analysis. Clearly, it is highly desirable to explore the utility of detectors that afford greater selectivity. In principle, electronic absorption or emission spectroscopy is the basis for one such detector. This is because electronic-vibrational (vibronic) transition energies and relative intensities are extremely sensitive to subtle changes in molecular structure. For example, moving a methyl substituent from one skeletal position on a polycyclic aromatic hydrocarbon (PAH) to other symmetry-inequvialent positions produces vibronic energy shifts on the order of tens of cm - 1 . Other substituents produce even larger 0003-2700/83/0351 -565A$01.50/0 © 1983 American Chemical Society

Figure 1 . D i a g r a m m a t i c r e p r e s e n t a t i o n of a s u p e r s o n i c e x p a n s i o n f r o m a r e s e r v o i r at initial t e m p e r a t u r e T0 and p r e s s u r e PQ into a l o w - p r e s s u r e r e g i o n , Pt Green region indicates shock wave around the jet. The location of the onset of free flow, XF, and of the Mach disc XM is indicated

shifts; the amino group, for example, affords shifts of hundreds of cm - 1 . To be successful, however, optical spectroscopic methods must overcome severe degradation of spectral resolution caused, for example, by thermal broadening, collisions, and spectral congestion. Degradation in emission generally parallels that in absorption. In recent years several low-tempera-

ture solid-state laser-induced fluorescence techniques have been demonstrated to have the sensitivity and selectivity to be useful for direct analysis of PAHs in real complex mixtures (1,2). The use of low temperatures, 4-20 K, eliminates spectral congestion in absorption due to hot band transitions and minimizes thermal broadening of individual vibronic transitions.

ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983 · 565 A

In addition, narrow band excitation with a laser can essentially eliminate site-inhomogeneous line broadening in fluorescence (the so-called fluorescence line-narrowing phenomenon). Although such techniques are exceptionally sensitive and selective, they are limited to molecules with fluorescence quantum yields â 10~ 3 and by solid-state broadening effects that no amount of sorcery can eliminate. Unlike GC/MS they do not have an analogous prefractionation step. In one sense this is an advantage. However, for very complex mixtures in which, for example, complexation causes fluorescence quenching or broadening, sample cleanup is required. To couple an integral gas chromatographic step with selective detection of absorption or fluorescence would require that thermal broadening be eliminated in the vapor phase. For large molecules with boiling points > 100 °C, maintaining the sample as vapor and minimizing thermal broadening seem to be mutually exclusive. However, in the past few years, it has been demonstrated that supersonic molecular beams can be used to produce a vapor stream in which thermal broadening is dramatically reduced compared to that in static gas samples. Spectroscopic applications of this technique have grown rapidly. Reports of analytical applications have only recently appeared (3-6), although we are confident that they also will grow as the power of the technique becomes more widely known. In this paper we will attempt to explain the physics of cooling in supersonic expansions and to demonstrate the analytical potential of the technique by describing our work on analytical applications of supersonic expansions. Broadening in Vapor-Phase Spectroscopy Before proceeding to an explanation of the cooling observed in supersonic expansions, a brief summary of the extent and causes of broadening in the vapor phase will indicate under what conditions a supersonic expansion will eliminate broadening. Causes of broadening have been divided into two categories: intrinsic and trivial (i.e., extrinsic) (7). The distinction between the two is that given appropriate experimental conditions, trivial broadening can be eliminated, while intrinsic broadening cannot. Thus, a supersonic expansion will not be useful in studying species whose absorption spectra are intrinsically broad. Fortunately, the examples of significant intrinsic broadening for the lowest excited singlet state of molecules are rare. For purposes of this discussion, the only important intrinsic broadening mechanism is intramo-

lecular vibrational energy redistribution (IVER). This is the broadening that occurs when a molecule is excited to an energy above the zero-point vibrational level of the first excited state, at which the density of vibrational levels is high. Although only a single vibronic band may be optically active in this energy region, coupling between the vibrations results in the energy being redistributed among many vibrations. Spectroscopically, this manifests itself as band broadening. Since IVER only occurs at energies well above the zero-point level of the lowest excited state it usually will not prevent the observation of narrow absorptions, but will only limit the energy range over which such observations may be made. Thus, IVER will be a problem only for those molecules in which large geometry changes cause the low energy transitions to be Franck-Condon forbidden. Trivial broadening is perhaps an unfortunate term: In principle, trivial broadening can be eliminated; in practice, this often has not been a trivial matter, requiring wizardry of the first order for solution. This is especially true if one requires high sensitivity, i.e., a high density for absorbers. The two major trivial broadening mechanisms, rotational broadening and vibrational broadening, are temperature dependent. Thus, as the temperature of a sample is raised to increase its vapor pressure, and thereby the number density of vapor-phase molecules, rotational and vibrational broadening blossom. Rotational broadening arises because large molecules have large moments of inertia and hence minuscule energy spacings between rotational levels. Thus, at ambient temperatures many rotational levels are populated, and these levels usually cannot be resolved. Consequently, each vibronic absorption band exhibits a rotational contour rather than discrete rotational structure, and as the temperature is increased the width of the contour increases. By itself, however, rotational broadening would lead to bandwidths on the order of tens of cm - 1 , sufficiently sharp that selective absorption could be accomplished in many cases. The major cause of diffuseness in vapor absorption spectra is vibrational sequence congestion. This is a manifestation of the fact that at the temperatures required to obtain an appreciable vapor pressure, there is significant population of several low-lying vibrational levels of the ground state. For every principal or "cold" absorption band of the molecule, there will be associated bands for transitions from populated vibrational levels that terminate at corresponding levels in the excited state. The energies of

566 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983

these hot band transitions will differ from those of the principal transition by the difference in ground- and excited-state vibrational frequencies. These frequency differences are on the order of the rotational bandwidths, so that severe spectral congestion occurs. The combination of rotational and vibrational broadening in favorable cases produces an absorption spectrum consisting of an unresolved broad background on which the fundamental absorption bands and the strongest sequence bands are superimposed. When such a spectrum can be obtained, it is only within a very restricted range of experimental conditions. In most cases, the spectra are far worse, exhibiting only broad diffuse features. For aromatic hydrocarbons, based on the known vibrational frequencies and computed densities of vibrational states, it has been predicted that with attainable experimental conditions, vibrational structure would be resolvable only in molecules with three or fewer rings (7). Cooling in Supersonic Expansions Let us see how the thermal broadening effects that make high-resolution spectroscopy of vapors of large molecules so difficult are essentially eliminated in a supersonic expansion. To understand the cooling that occurs in a supersonic molecular beam, and also to clarify the sometimes confusing nomenclature that is used, let us consider a molecular beam source (Figure 1). The source consists of a reservoir containing a gas at high pressure, P„, and temperature, T 0 , an orifice in the reservoir, and a low-pressure region downstream of the orifice. If the orifice diameter, D, is much smaller than the mean free path, λ 0 , of the gas in the reservoir, then the flow from the ' reservoir into the low-pressure region is effusive. Alternatively, the orifice diameter may be much larger than λ 0 , in which case the orifice acts as a su­ personic nozzle or jet. In the effusive case, because the ori­ fice dimensions are less than the mean free path of the gas molecules, mole­ cules pass from the reservoir into the low-pressure region without colliding with other gas molecules. The absence of collisions in and downstream of the orifice means that the velocity distri­ bution of molecules in the low-pres­ sure region will be a Maxwellian dis­ tribution characteristic of the reser­ voir temperature, T0. In addition, the population of internal (rotational, vi­ brational) energy states will also be the same as in the reservoir. In the case of supersonic flow, where D » λ 0 , there will be many col­ lisions in and downstream of the ori­ fice. In particular, those collisions that

impart a large velocity component in the axial direction will be most effec­ tive in driving molecules from the res­ ervoir. Thus there is a conversion of random molecular motion into di­ rected mass flow along the beam axis. This directed mass flow has three consequences: First, the peak of the velocity distribution of molecules along the axis is shifted to higher ve­ locities and the distribution is nar­ rowed. The narrowing of the distribu­ tion means that the characteristic temperature for the distribution has decreased. The increase in flow veloci­ ty, u, together with the decrease in the local speed of sound, a, as the gas ex­ pansion proceeds, means that the Mach number, M = u/a, increases. In an ideal nozzle, M = 1 at the most constricted point within the nozzle, and M > 1 downstream, hence the terms supersonic jet, supersonic nozzle, and supersonic expansion. A second consequence of the directed mass flow is that the density of mole­ cules along the axis is much higher than for an effusive source. This in­ crease in beam intensity was a pri­ mary motivation for the development of nozzle beam sources. A third characteristic of supersonic expansions, and perhaps the most im­ portant for spectroscopic applications, is that the initial part of the expansion is isentropic, i.e., there is equilibration between the translational and the in­ ternal degrees of freedom. Thus in the isentropic core of the jet, the tempera­ ture characterizing rotational and vi­ brational energy distributions will de­ crease. As the collision frequency de­ creases, the vibrational temperatures and then the rotational temperatures begin to lag behind the translational temperature and at some point freeze. Eventually, the collision frequency be­ comes negligible and the beam enters the free molecular flow regime. In Fig­ ure 1, the point XF marks the start of the free flow regime for typical expan­ sion conditions. Beyond this point the beam constitutes an ideal spectroscop­ ic medium: isolated molecules, rotationally and vibrationally cold, free of any matrix or collisional perturba­ tions. Furthermore, the beam is pro­ tected from interactions with hot background molecules in the chamber by a shock wave that forms around the expansion. The shape of the shock wave is shown in the diagram. The leading edge of the shock wave is called the Mach disc and is at the point Xjif in the figure. The final temperatures attained in the expansion depend upon the prop­ erties of the gas being expanded. The lowest translational temperatures are obtained for monatomic gases that lack internal degrees of freedom. A monatomic gas can be seeded with a

Figure 2. Block diagram of Instrumentation used to obtain laser-induced photoexci­ tation and fluorescence spectra from a seeded supersonic beam

polyatomic species and, in the isentro­ pic region, the polyatomic will also be in equilibrium with the monatomic. The final translational temperature obtained in a seeded expansion differs only slightly from that obtained in an expansion of the pure monatomic. The rotational temperature of the large molecule will then be on the order of 1-10 K. These remarkably low tem­ peratures for vapor-phase species have resulted in the technique being called rotationally cooled spectroscopy. Vi­ brational cooling is by no means as extensive, being on the order of 30-100 K. However, with the larger energy spacings of vibrational levels, the excited vibrational states are al­ most completely depopulated. Even when some population remains in the lowest vibrational level, the narrowing of the rotational contour results in the hot band absorption being a well-re­ solved, weak spectral feature. Thus, vibrational sequence congestion, the most troublesome thermal-broadening mechanism, is virtually eliminated in a supersonic expansion. A common misconception regarding supersonic expansions is that super­ sonic cooling is still difficult in prac­ tice, requiring very expensive and large vacuum systems. Although large systems with several stages of differ­ ential pumping are required with effu­ sive sources and have been used with supersonic sources, they are not neces­ sary for the latter. For a free jet ex­ pansion—a supersonic jet in which there are no downstream beam colli­ mators—a chamber with a cross-sec­ tional dimension of a few inches, pumped by a single high-throughput diffusion pump, is adequate. Indeed, supersonic cooling can be achieved

using a mechanical pump with high pumping speed. Spectroscopy of Supersonic Expansions

Having prepared a seeded superson­ ic beam, what can be done with it, and what are the analytical applications? Although the density of molecules of interest in the beam is much higher than in an effusive beam, the beam di­ mensions are small, so the spectro­ scopic interaction region will be small. For a beam seeded with a typical aro­ matic hydrocarbon (e ~500), under typical expansion conditions, the number density of absorbers in the beam will be ~ 5 Χ 10 11 molecules c m - 3 . Accounting for the angular di­ vergence of the beam, at a distance from the nozzle of 1.0 cm, the optical pathlength will be ~0.5 cm. Under these conditions an absorbance < 10~4 would be obtained. Thus, in general, direct absorption measurements on seeded beams are not feasible, al­ though absorption measurements are possible on pure beams (8) or with slit nozzles (6). By far the most common type of spectroscopy performed on seeded su­ personic beams is laser-excited fluo­ rescence. A block diagram of the equipment used to generate such spec­ tra is shown in Figure 2. A frequencydoubled, Nd:YAG laser-pumped dye laser is used as a source of tunable ul­ traviolet radiation. This beam is fo­ cused onto the molecular beam axis through a quartz window and a set of light baffles, which minimize stray light reaching the detector. The laser beam crosses the molecular beam 2-10 mm downstream of the nozzle or­ ifice. Fluorescence from the beam is

ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983 · 569 A

detected at right angles to both the molecular and the laser beams. The fluorescence is dispersed with a highthroughput medium-resolution monochromator and detected by a photo­ tube. The phototube output is ampli­ fied, measured by a gated integrator, and divided by a reference signal pro­ portional to the laser intensity to pro­ duce a normalized output. Using this arrangement, either photoexcitation spectra or fluorescence spectra can be obtained. An example of a photoexcitation spectrum is shown in Figure 3. In this mode of operation, the dye laser wave­ length is scanned while the detection monochromator is fixed at a wave­ length at which the molecules of inter­ est fluoresce. In the example shown, the sample is a mixture of naphtha­ lene, α-methylnaphthalene, and /?-methylnaphthalene. From left to right in the figure, the labeled bands are the Si origin of /3-methylnaphthalene, the Si origin of a-methylnaph­ thalene, the Si origin of naphtha­ lene, a 422-cm - 1 vibration of a-meth­ ylnaphthalene, and a 438-cm - 1 vibra­ tion of naphthalene. If the fluores­ cence quantum yield of each of the vibronic levels were equal, the photoex­ citation spectrum would be equivalent to an absorption spectrum. In general this will not be the case. Nevertheless, the peak positions are invariant, and the photoexcitation spectrum serves as a fingerprint of the molecule in the expansion. It is clear from the figure that even with the moderate resolu­ tion (bandwidths are laser-limited, ~ 2 cm - 1 ) of this spectrum, the bands are well resolved and allow for easy identification of the three components of the sample. Thus, a qualitative analysis of a complex mixture can be obtained easily from the photoexcita­ tion spectrum. Bear in mind that in obtaining such a spectrum, photoexcitation peaks will be observed only for those molecules that emit in a spectral region within the bandpass of the detection mono­ chromator. In the spectrum shown, the detection wavelength and band­ pass were chosen so that all three mol­ ecules could be detected. If the band­ pass were narrowed and a detection wavelength specific to one of the three molecules chosen, the photoexcitation spectrum of that single species would be obtained. When peaks due to each of the components of a mixture are re­ solvable there is no advantage to be gained by this procedure. However, if a component of the mixture absorbed in the same spectral region as other molecules of interest, but was intrinsi­ cally broadened (due to absorption into very high vibrational levels or into a higher excited state), then by a judicious choice of detection wave-

Figure 3. Laser-induced photoexcitation spectrum of a mixture of naphthalene (N), α-methylnaphthalene (αΜΝ), and /3-methylnaphthalene (/?MN) seeded in a super­ sonic expansion of He

λ θ = 310.6 nm

ke = 315.4 nm

ι • ι 80

• ι • ι 160

. ι • ι 240

• ι • Ι ι Ι ι Ι ι 320 400

Time (s)

Figure 4. Analysis of a crude oil sample for naphthalene and methylnaphthalenes by rotationally cooled fluorescence The traces a-c show chromatograms using selective excitation wavelengths for /3-methylnaphthalene, α-methylnaphthalene, and naphthalene, respectively. Trace d shows a chromatogram using a nonselec­ tive excitation wavelength

length, the broad peaks could be elim­ inated from the spectrum. The same apparatus used for ob­ taining photoexcitation spectra can be used for obtaining fluorescence spec­ tra. In this case, the laser wavelength would be chosen to coincide with one of the bands in the photoexcitation spectrum and the monochromator would be scanned. In analytical appli­ cations, the fluorescence spectrum would be of use in determining the species responsible for the observed bands. For example, if one has ob­ tained the photoexcitation spectrum

570 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983

of a supposedly pure substance, then the fluorescence spectrum from each band in photoexcitation should be analyzable in terms of the vibrations of that molecule. Bands that are not readily analyzable probably indicate impurities. For many nonfluorescent species, spectra with the same resolution can be obtained via multiphoton ioniza­ tion. This technique requires a more intense laser source to achieve ioniza­ tion, but since in principle every ion is detectable whereas every photon is not, it can be as sensitive as fluorés -

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cence detection. If the ion detector is also a mass detector, then mass-selec­ tive detector would be analogous to wavelength-selective fluorescence de­ tection.

Quantitative Jet Spectroscopy As described in the previous sec­ tion, the dramatic narrowing of vibronic bands in a supersonic expan­ sion makes qualitative analysis quite straightforward. A quantitative analy­ sis is more difficult. The intensities of bands in the photoexcitation spectrum are proportional to the partial vapor pressure in the reservoir of the mole­ cule causing the band. Therefore, ob­ taining quantitative data from band intensities requires knowing the par­ tial pressure of each species in the res­ ervoir. If the sample to be analyzed is a complex multicomponent mixture, these data would not be available. As an alternative to knowing all the partial pressures, one can set the res­ ervoir temperature high enough that the analyte is totally vaporized and flows through the laser excitation re­ gion quickly. Then by fixing the exci­ tation wavelength to an absorption of the species of interest and monitoring the fluorescence as a function of time, the area of a peak in the fluorescence vs. time graph will be proportional to the amount of absorbing material that passed through the excitation region. We have recently described how the reservoir of the supersonic nozzle can be converted to a simple GC to achieve quantitation in this manner (4). In that work the reservoir was packed with GC packing material and heated to ~100 °C. A heated injection port was attached to the nozzle inlet. Solutions of analytes were then inject­ ed into the GC column. Figure 4 shows fluorescence vs. time graphs for a sam­ ple of dilute crude oil at several exci­ tation wavelengths. The wavelengths of traces a-c correspond to strong ab­ sorption bands of /8-methylnaphthalene, «-methylnaphthalene, and naphthalene. The peaks in each trace are due to the respective molecules, and the peak areas are proportional to the concentration of each species in the sample. The top trace, d, uses a nonselective wavelength and gives an indication of the complexity of the sample (only the low-melting com­ pounds absorbing at 266 nm and emit­ ting at 345 nm show up in this trace). The limit of detection demon­ strated in Reference 4 was ~ 1 0 - 8 g. That work used a continuous-flow nozzle and a pulsed excitation laser. Thus, the duty factor was quite low, 10 - 7 . Use of a pulsed nozzle synchro­ nized to the laser firing would result in an increase of duty factor and, hence, of limit of detection, by a factor of 103.

572 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983

Roger Adams: Scientist and Statesman by D. Stanley Tarbell and Ann Tracy Tarbell

Examines the life of Roger Adams — a man unparalleled in his con­ tribution to the development of organic chemistry. A comprehensive biographical sketch of Roger Adams, whose work was important in the development of American chemistry and chemical education. Adams's early years, edu­ cation, and career achievements in academia, industry, research, and government are described. His con­ tributions to Illinois chemistry in par­ ticular and the education of chemists are expounded. CONTENTS Introduction · Early Years and College · Germany and Harvard, 1912-16· Illinois, 1916-26 · Academic Progress · Service and Research to 1942 · Government Service, 1940-48 · Illinois and Research, 1943-67 · Broader Horizons · Career Achievements of Roger Adams's Ph.D.s. 1918-58 · Career Achievements of Roger Adams's Postdoctorates, 1936-59 240 pages (1981) Clothbound US & Canada $13.95 Export $16.95 LC 81 -17625 ISBN 0-8412-0598-1 240 pages (1981) Paper US & Canada $9.95 Export $11.95 LC 81 -17625 ISBN 0-8412-0711 -9 Order from: American Chemical Society Distribution Office — 1 6 1155 Sixteenth St., N.W. Washington, D.C. 20036 or CALL TOLL FREE 800-424-6747 and use your credit card.

The Analytical Approach Hie Analytical Approach

Jeanette G. Grasselli, Editor Brings together 52 papers from The Analytical Approach column in ANALYTICAL CHEMISTRY. Provides unique approaches to analytical science and focuses on real-world problems. Discusses topical and interesting subjects such as the analysis of the JFK assassination bullets, Mt. St. Helens ash, flavor changes in food, and failure mechanisms in spacecraft parts. Written in a "popular" style yet is highly informative. Will serve as a teaching aid in higher education or as a guide in corporate training programs on analytical capabilities.

Increasing the optical pathlength through the use of a slit nozzle (6) could also provide another order of magnitude increase in sensitivity. Conclusion The rapid growth in spectroscopic applications of supersonic nozzles can be demonstrated by considering that between 1974 and 1976 fewer than a half-dozen papers about this tech­ nique appeared, while in the past two years a noncomprehensive count indi­ cated that well over 100 papers were published. A wide variety of uses are reported in these latter papers. Mole­ cules as large as tetraphenylporphyrins have been studied (9), picosecond decays have been observed (9,10), and dimeric structures have been elu­ cidated (11). Further applications and descriptions of the technique are con­ tained in several review articles (12, 13). Although this list of applications is by no means exhaustive, the variety of information available does explain the growing popularity of the tech­ nique. Analytical applications have only begun to appear (3-6). Such a lag be­ tween fundamental studies and ana­ lytical applications is not unusual, and we are confident that as the simplici­ ty, high selectivity, and sensitivity of

this technique become more widely appreciated, the growth in analytical applications will mirror the rapid growth of the fundamental studies. References (1) Brown, J. C; Duncanson, J. Α.; Small, G. J. Anal. Chem. 1980,52,1711. (2) Yang, Y.; D'Silva, A. P.; Fassel, V. A. Anal. Chem. 1981,53,894. (3) Warren, J. Α.; Hayes, J. M.; Small, G. J. Anal. Chem. 1982,54, 138. (4) Hayes, J. M.; Small, G. J. Anal. Chem. 1982,54,1202. (5) Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982,54,660. (6) Amirav, Α.; Even, U.; Jortner, J. Anal. Chem. 1982,54,1666. (7) Byrne, J. P.; Ross, I. G. Aust. J. Chem. 1971,24,1107. (8) Vaida, V.; McClelland, G. M. Chem. Phys. Lett. 1980,77,436. (9) Even, U.; Jortner, J. J. Chem. Phys. 1982 77 4'}91 (10) Felke'r, P. M.; Lambert, W. R.; Zewail, A. H. Chem. Phys. Lett. 1982,89, 309. (11) Levy, D. H. J. Chem. Phys. 1980, 73, 5380. (12) Levy, D. H.; Wharton, L.; Smalley, R. E. In "Chemical and Biochemical Ap­ plications of Lasers"; Academic Press: New York, 1977; Vol. 2, Chapter 1. (13) Levy, D. H. Ann. Rev. Phys. Chem. 1980,37, 197. Ames Laboratory is operated for the U.S. Depart­ ment of Energy by Iowa State University under Contract No. W-7405-Eng-82. This research was supported by the Office of Health and Environ­ mental Research, Office of Energy Research.

CONTENTS

Sections include: • Production Processes • Products • Environmental • Toxicity • Forensic • Miscellaneous 240 pages. (1983) Cloth bound LC 82-22618 ISBN 0-8412-0753-4 US & Canada $29.95 Export $35.95 A paperbound student edition is available in bulk quantity. For price .ind ordering information, call toll tree 800-424-6747. Order from: American Chemical Society Distribution Office — 50 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your credit card.

Gerald J. Small (left) is a professor of chemistry at Iowa State University and a senior chemist at the Ames Laboratory-USDOE. After obtaining a BSc degree in chemistry and math­ ematics from the University of Brit­ ish Columbia, he undertook graduate studies in chemical physics at the University of Pennsylvania, where he received his PhD in 1967. Following a two-year appointment as a research fellow at the Australian National University, he joined the faculty of Iowa State University in 1969. In ad­ dition to lasers in chemical analysis, his research interests include the photophysics and photochemistry of

574 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983

organic molecules and their solids, nonlinear optical phenomena, and electronic energy transfer in the con­ densed phase. John M. Hayes is an associate chem­ ist at the Ames Laboratory-USDOE. His undergraduate studies in chemis­ try were pursued at Providence Col­ lege and the State College at Boston. He received his PhD in physical chemistry from Boston University in 1974. Analytical applications of laser spectroscopy and spectroscopic stud­ ies of structure and dynamics in gases and solids are among his research interests.