Lasers and Spectroscopy The laser is acquiring an increasingly important role in analytical chemistry, and it is important for the practicing analyst to have an appreciation of both its capabilities and its problems. In this REPORT, we review the fundamentals of the interaction of radiation with chemical systems that form the basis of all analytical spectroscopy techniques, and some of the major spectroscopic techniques which have benefited from the unique characteristics of the laser. To build a strong intuitive picture of both the laser and its applications, let's examine the idea of coherence— both spatial and temporal coherence. We first set up a series of observation points throughout a sample that will measure the amplitude and direction of the electric field associated with an oscillatory electromagnetic wave propagating through the sample. If the wave is spatially coherent, there will be a relationship between the amplitudes and phases at all of the observation points at any instant in time. For example, the observation points along the wavefront of a plane wave would all record the same amplitude and phase (see Figure 1). If the wave is temporally coherent, there will be a relationship between the amplitude and phase at one particular observation point at different times. Light from a light bulb is incoherent because there is no relationship in phase and amplitude between light emitted from different parts of the filament. The molecules in a sample serve as observation points. An electric field acting on a molecule will distort the electron clouds and create a polarization. If the field is oscillatory and has a frequency that matches one of the natural frequencies of the molecule (either rotational, vibrational, or electronic), large oscillations can be induced in the molecule. Everyone is familiar with the analogy of resonance
in a simple harmonic oscillator, be it a weight on a spring or a pendulum on a string. One can deliver energy to the simple harmonic oscillator by pushing it in phase at its resonance frequency. The simple harmonic oscillator absorbs energy from the outside world (i.e., you) and is set into oscillation. Similarly, an oscillating simple harmonic oscillator can deliver energy to the outside world if it pushes you at its resonance frequency. The difference between the oscillator absorbing and emitting energy lies completely in the phases between the oscillator and the outside world. Molecules act in exactly the same way. If the majority of molecules are in their ground states (analogous to a stationary oscillator), they can absorb energy from an electromagnetic wave. If the majority are in an excited state (analogous to an active oscillator), they can emit energy to the electromagnetic wave. This event is called stimulated emission. Molecules, being quantum mechanical systems, have a third option open to them. They may spontaneously relax to a lower energy state and generate an electromagnetic wave. This event is called spontaneous emission. If the molecule is being distorted by a field whose frequency is far from resonance, the spontaneous emission is called Raman scattering. The oscillatory distortion is called a virtual level because it does not correspond to an eigenstate of the molecule. If the frequency of the driving field is close to a resonance of the molecule, the molecular distortions can become quite large. If the spontaneous emission occurs from a molecule which has maintained its proper phase relationship with the driving field, the spontaneous emission is called resonance Raman scattering. A molecule cannot always maintain its phase relationship, however, because it undergoes collisions with all of its neighboring molecules
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and becomes dephased. Spontaneous emission that results after the molecule has become dephased relative to the driving field is called fluorescence. One of the startling characteristics of laser sources is that the electric field associated with the emission can be as large as the fields binding electrons to molecules. As this condition is approached, the polarization that is induced in samples cannot increase linearly with the electric field associated with the laser light. Generally one must use a lens to focus the laser output into a sample in order to achieve such fields. If the incoming light has an electric field component that is oscillating sinusoidally at a frequency O>L, the induced polarization in the sample must also be oscillating at ÎOL; however, the sinusoidal functionality will be distorted because the polarization is unable to follow the driving field. The distortions become increasingly pronounced as the electric fields approach the binding energy of the electrons in the sample, and eventually, the sample will experience dielectric breakdown. The distortions in an oscillating signal can be described in terms of the harmonics of O>L. For example, the distorted wave shown in Figure 2a is composed of the wave at ω (Figure 2b) and the waves at 2co and 0 which represent the distortion (Figure 2c). The oscillating polarization must also generate an electromagnetic wave since it is associated with charge accel erations. The polarization shown in figure 2a will produce waves at ω, 2ω, and a dc field. The wave at ω corre sponds simply to the propagation of light through a sample, and it gives rise to the index of refraction. The wave at 2ω corresponds to second har monic generation. Light is produced at twice the input frequency with an effi ciency which depends upon the non linear susceptibility of the material and the intensity of the input electric 0003-2700/80/0351-988A$01.00/0 © 1980 American Chemical Society
Report J . C . Wright a n d M . J . Wirth D e p a r t m e n t of Chemistry University of W i s c o n s i n Madison, W i s . 5 3 7 0 6
Wavefront
Figure 1. The induced polarization at each atom for an ordered array of atoms at any given instant of time as an electromagnetic wave propagates through the material Lines of constant phase would define the direction of propagation
field. T h e dc field corresponds to opti cal rectification. One will need t h e other harmonics in addition to those a t 0 a n d 2co in order to describe t h e distortion of a real polarization. T h e same reasoning is used when t h e r e are two electromagnetic waves with frequencies COL a n d cos entering the sample where COL > cos- New frequencies will be created a t t h e s u m and difference frequencies, (COL + cos) and (COL — cos). T h i s process is called three-wave mixing. T w o waves result in t h e creation of a third one. Other frequencies will be present for this case as well. T h e wave a t (COL + s) can combine with COL t o create new frequencies a t (2COL + cos) a n d cos or it
can combine with cos t o create new frequencies a t (2cos + COL) a n d COL. Similarly by combining t h e wave a t (COL — cos) with COL or cos, new frequen cies a t (2coL — co s ), &S, (2cos — coL), a n d COL are created. T h e s e processes are examples of four-wave mixing. Addi tional frequency components can be present if one considers other combi
nations for four-wave mixing a n d t h e higher order mixing. T h e r e is nothing magical in these processes; they sim ply represent t h e distortions t h a t occur in t h e induced polarization be cause of t h e strong electric fields t h a t can be created with a laser. T h e oscillatory polarization induced in a sample is generally small if t h e electromagnetic waves are n o t in reso nance with any of t h e n a t u r a l frequen cies of t h e system. An i m p o r t a n t case arises when t h e difference frequency (COL — cos) between two lasers illumi nating t h e sample matches a vibra tional frequency coy. Sample mole cules now feel a component of pulling and pushing a t a frequency they want to vibrate a t a n d vibrational motion can be induced. T h e resulting large polarization causes more efficient gen eration of t h e waves a t (2COL — cos), ws, (2cos — COL), a n d COL. N o t e t h a t for this situation, t h e waves a t (2COL — cos) a n d (2cos — COL) correspond t o frequencies of (COL + ωγ) a n d (cos ~~ ων), t h e antiStokes a n d Stokes frequencies. N o t e
also t h a t t h e oscillating polarization is linked intimately with t h e driving fields a t COL a n d a>s, a n d therefore is coherent with t h e i n p u t lasers. T h e processes t h a t generate their frequen cies are called coherent anti-Stokes R a m a n scattering (or CARS) a n d co h e r e n t Stokes R a m a n scattering (or C S R S — p r o n o u n c e d "Scissors" (1). It is also possible t o have higher order R a m a n scattering. For example, since fields exist within t h e sample a t (2COL — cos) a n d OIL a n d since
[(2L — cos) — w j matches t h e vibra tional frequency, ωγ, a new field will be created a t (3COL — 2cos) or (COL + 2ωγ). In fact, a series of frequencies will be created which are spaced even ly by t h e vibrational frequency from b o t h OIL a n d cosIf t h e laser intensity is high enough, a single laser a t O>L can create t h e sec ond frequency a s = (COL — coy) from s p o n t a n e o u s R a m a n scattering. One t h e n still obtains a series of new frequencies a t (COL ± ncoy) where η is an integer, even though only a single
A N A L Y T I C A L CHEMISTRY, V O L . 5 2 , NO. 9, AUGUST 1980 · 9 8 9 A
Figure 2. Distortion in an oscillating signal (a) As an electromagnetic wave travels through a medium, the induced polarization can be distorted as shown. The distortion in this example is simplified and consists of the two components in (b) and (c). (b) The fundamental frequency makes up a fraction of the distorted polarization, (c) One also has components at twice the fequency (frequency doubling) and at dc (optical rectification)
laser is used. This phenomenon is called stimulated Raman scattering (2). There is a high threshold intensity for this process primarily because of the need for generating appreciable amounts of spontaneous Raman scattering initially. These general spectroscopic principles find application in both the construction of lasers and other coherent sources, and the analytical techniques that use those sources. Applications
Absorption. Lasers can be used in many of the traditional absorption measurements where conventional light sources are not used. To displace a conventional light source from these applications, the laser would have to offer some substantial advantages. There is little the laser can offer for UV/visible spectrophotometry. Perhaps a high peak power will allow a measurable amount of light to get through an optically dense sample so high absorbances can be measured. One might also take advantage of the highly collimated beam of a laser to measure absorption over a very long path length. Both of these situations are specialized. Lasers have been investigated as
possible replacements for hollow cathode lamps in atomic absorption spectroscopy. They have several potential advantages: One laser system could replace many hollow cathode lamps because of its tunability; the large number of photons from a laser results in a very favorable shot noise component of the absorption measurement; and the laser output bandwidths can be made narrower than the linewidths of atomic transitions. Many improvements would need to be made, such as obtaining the proper wavelength quickly and reliably, reducing the temporal fluctuations in intensity, improving the tunability to include the important wavelength regions, and simplifying the operation of the lasers. Photoacoustic spectroscopy is an alternative means of detecting absorption. (3,4). In this method a modulated light source generates acoustical waves in a sample because the sample generates heat at the modulation frequency. Since lasers can have large powers, large amounts of heat can be generated from light absorption in the sample. The laser has become an important source in photoacoustic spectroscopy when the sample is a gas (4). The narrow bandwidths from lasers can supply a large amount of energy to
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the sharp absorption lines of gas phase molecules. The continuum from conventional lamps is not suited for gas phase work because there is so little power within the narrow range of wavelengths where the molecule absorbs. Photoacoustic spectroscopy is compatible with many types of infrared and visible lasers, although increased sensitivity is gained from high power pulsed lasers such as the CO2 laser and the flashlamp pumped dye laser. Photoacoustic detection has also been used with the sample placed inside of the laser cavity. This accesses the high intracavity peak powers, thus increasing the sensitivity. This approach is compatible with CW lasers, for which the intracavity powers are significantly higher than the output powers. Gas phase infrared spectroscopy is an area where the laser has been able to make a substantial contribution. Again the difficulty is that gas phase molecules have sharp vibrational transitions in the infrared. The low intensity of light sources and the poor efficiencies of detectors in the infrared have hampered traditional gas phase measurements. Tunable diode lasers have relatively large powers concentrated within a narrow bandwidth and they make good sources for infrared spectroscopy (5). This is of particular interest in using laser-based infrared systems for detection and quantitation of particular gases in such areas as air pollution or explosives detection. Intracavity absorption is one of the first analytical procedures that took advantage of the unique characteristics of lasers (6,7). A typical system for intracavity absorption is shown in Figure 3. A sample is placed within a highly reflective, low loss resonator cavity where light can make many passes back and forth. With such an arrangement, even a small amount of gain from the laser medium can lead to laser oscillation since the gain is cumulative for successive passes. Similarly, small amounts of loss within the cavity can lead to a marked decrease or a total quenching of the laser power. Since an absorbing sample provides a loss mechanism when it is in the cavity, the laser power becomes very sensitive to sample absorption. Environmental and instrumental factors that also cause changes in laser power can be serious interferences. The successful implementation of intracavity absorption thus requires a very stable and reproducible laser so that changes in laser power between sample and blank are related only to the sample absorption. This has been recently accomplished using null techniques to balance the sample absorption (8). Intracavity absorption experiments are compatible with the stable
Mirror
Mirror
Gain Medium
Sample
Figure 3. Intracavity absorption schematic The sample is placed inside the laser cavity to induce a concentration-dependent loss
CW lasers, with the argon ion laser being used most often. The use of tun able CW lasers and pulsed dye lasers is presently being developed. The phenomenon of thermal lensing has been applied to the problem of measuring small absorptions (9). This technique utilizes the high power and beam quality of the laser in order to achieve greater sensitivity in the ab sorption measurement. If a highly di rectional laser beam is passed through an absorbing sample, the resulting heating of the sample will cause a change in its index of refraction. Since the heating is greatest in the beam center, the index of refraction will vary across the beam profile and will act as a lens to defocus the beam. The beam emerging from the sample is generally broader than it would be if there were no sample absorption. De tection can be achieved by measuring either the change in beam profile or the beam power at a particular point in the profile. The magnitude of the effect depends linearly upon the laser power. Thermal lensing methods re quire a well defined beam profile; thus the CW lasers such as He:Ne, He:Cd, argon ion and CW dye lasers are appli cable for the technique. Emission Spectroscopy. Fluores cence spectroscopy has been a particu larly fertile area for laser applications. Since the number of photons emitted by a sample increases linearly as the number of photons put into the sam ple, the best signal levels are obtained with high excitation powers. This in crease in signal level is limited by sat uration effects which occur when the number of sample molecules in the ex cited state becomes comparable to the number in the ground state. General ly, however, the detectivity for fluo rescence methods is not limited by the signal strength but rather by the back ground in the blank that arises from Raman scattering or low level impuri ties in the solvent or from instrumen tal problems (JO). Thus, the laser power does not become the principal
concern in choosing an excitation source. In fact, if the sample is a broadband absorber as most organic solutions are, a conventional blackbody source can be as effective as a laser in depositing photons into a sam ple. Instead, other properties of lasers can make them attractive as excitation sources for conventional fluorescence spectroscopy. A particularly good il lustration is the use of lasers as the ex citation source for a fluorescence HPLC detector (11). Lasers can be conveniently focused into the small volumes required for good HPLC de tection. Since they are highly mono chromatic, the monochromators re quired for a broadband excitation source are not needed. In addition, the wavelengths of Raman scattering from the solvent are sharply defined and the monochromator used to observe fluorescence can be tuned to avoid these lines. A major advantage of lasers for fluo rescence excitation is realized when the sample absorption occurs as sharp spectral lines. Although lasers and conventional light sources have com parable powers, the laser concentrates that power in a very narrow spectral region. A broadband source has very little power within the line-width of a narrow absorption line. Atomic fluo rescence spectroscopy is a traditional analytical method that is well suited for laser excitation because of the nar row atomic absorption lines. Excellent results have been obtained with graphite furnace atomizers where con centrations of 0.5 pg/mL of Tl were detectable (12). Molecules can also exhibit sharp lines—even quite complex molecules. Molecular absorption and fluores cence spectra from solutions are gen erally broad and featureless because the molecules can experience a contin uum of different conformations and environments that shift their electron ic levels. In particular frozen solvent systems, however, large molecules can fit uniquely into the lattice structure
of the frozen solvent and become fixed in a particular conformation and envi ronment. These solvent systems are called Shpol'skii systems after the Russian scientist who discovered the effect (13). Polyaromatic hydrocar bons (PAH compounds) dissolved in frozen alkane matrices are the best ex ample of such systems. The absorp tion and fluorescence features are nar row and highly characteristic of the particular PAH compound. An exam ple of such a spectrum is shown in Fig ure 4 (14). Since the absorption lines of different PAH compounds differ in wavelength, a laser can be tuned to the absorption of one particular com pound and excite its fluorescence without exciting other compounds which might also be present in the sample. Shpol'skii systems are diffi cult to use in an analytical procedure, however, because of aggregation and irreproducibility associated with the freezing. Matrix isolation procedures have recently been developed which avoid these problems by forming the solvent lattice under controlled condi tions (15). A related technique is laser-induced fluorescence line narrowing (16). Again the sample molecules are frozen in a solvent, usually one which forms a glass. Conventional absorption and fluorescence spectra of the glass are broad because the molecules are not uniquely fixed in the matrix. If a nar row-band laser is tuned to the low en ergy side of the absorption profile and if the molecules of interest have strong ν = 0 to ν = 0 transitions, the only molecules which will be excited will be the ones that have their electronic en ergy level resonant with the laser. Molecules that have different elec tronic energies because they have dif ferent conformations and environ ments will not be excited. Thus, the excitation process itself is selecting a specific conformation and environ ment from the continuum of possible arrangements, and the resulting fluo rescence spectrum exhibits the nar row-line features that were seen in Shpol'skii systems. If the molecules of interest are placed in the gas phase, they can achieve their equilibrium conforma tion, and their spectra are again sharplined. A great deal of success has been achieved using molecular beam techniques where sample molecules are cooled in the expansion that forms the molecular beam (17). The re sulting depopulation of rotational and vibrational states of the molecules sharpens and simplifies the spectra considerably. Each of these techniques for pro ducing sharp-lined molecular spectra is well-suited for laser applications. The selectivity that results from the sharp line features is particularly im-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 · 991 A
Figure 4. Fluorescence s p e c t r u m of coronene in a frozen η-heptane matrix ( 14) The sharp lines arise because of the unique conformation of the coronene in the matrix
portant for fluorescence analysis where one is limited by interference from other fluorescence substances or by background fluorescence from the solvent. The methods that rely upon solvent matrices for line narrowing will not work for a majority of fluo rescent materials. They have been ap plied only to particular classes of com pounds, such as the PAH compounds. The lanthanide and actinide ele ments are spectroscopically unique. Their optical transitions result be cause of transitions between electronic levels of the unfilled 4f" or 5f" shells. These shells are screened from outside influences by outer s and ρ orbitals and consequently their transitions are inherently sharp. Nevertheless, when these ions are placed in a condensed phase, the crystal fields they encoun ter split the electronic levels into a se ries of crystal field levels that are unique to the immediate environment. The frequencies and intensities of the transitions between the crystal field levels can be used as a fingerprint for that environment. If there are multi ple environments that are encoun tered, a narrow-band laser can be tuned to a specific absorption line characteristic of a particular environ ment, and the resulting fluorescence spectrum can come only from the site excited. By systematically tuning to different absorption lines, the spec trum of a material can be dissected into the component sites which make it up. This method has been called SEPIL—selective excitation of probe ion luminescence (18,19). The use of lanthanides as probe ions has been demonstrated in a number of different situations. Lanthanide and actinide analysis can be performed with great specificity and low detection limits by coprecipitating the lanthanides in CaF2 and selectively exciting fluores cence from the precipitate. Other inor ganic ions can be analyzed if they can be associated with lanthanides in a matrix. The analyte ions change the crystal fields at the site of the lan
thanide and result in new lines in the spectra, which can be selectively excit ed. The sharpness of the lines permits high selectivity for the analysis while the sensitivity is determined by the associated lanthanide. Lanthanide ions have been used as probes of the solid-state chemistry of point defects. One can also use the spectra of lan thanides in solution to identify or monitor complexes. Since lanthanides can replace the Ca 2+ in many proteins or enzymes, the spectra can reflect the local environments at the active site in biological materials. The natural abundance of lanthanides can be used to fingerprint, identify, or monitor the microscopic environments in natural materials. It is expected that the de tailed microscopic information that lanthanides provide coupled with the specificity and selectivity that laser excitation provides will find many practical applications in the area of chemical measurement. Time discrimination is a valuable method in fluorescence measurements for discriminating between the fluo rescence of interest and other interfer ing sources of light (20). Fluorescence originates from a state which has a characteristic lifetime. Competing flu orescences will come from states which have different lifetimes, and one can take advantage of the differ ences to distinguish the fluorescence of interest from the competition. Such procedures are particularly effective if there is a marked difference in the lifetimes. Lanthanide lifetimes for ex ample are typically between tens of microseconds to tens of milliseconds while competing fluorescence occurs in the nanosecond regime (18). In this case, it is easy to use a pulsed laser and to delay the detection electronics until the fast component has disap peared. Time discrimination also al lows one to reject Raman scattering or scattered excitation light since both of these sources are coincident with the laser (20). These interfering sources of light become important when one is
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performing low level analyses by mo lecular fluorescence. Experimentally, however, it is more difficult to imple ment time discrimination because the characteristic lifetimes are on the ns scale. It becomes important therefore to use lasers that have ps pulsewidths and techniques that permit ps detec tion resolution. Most lasers have sufficiently narrow bandwidths for high resolution mea surements of fluorescent materials and, since fluorescence is usually background limited, most have suffi cient power to allow good sensitivity. Bandwidth and power are not primary factors in the choice of a laser for fluorimetry. Lasers are more likely to be selected for wavelength and temporal characteristics. The N2 laser provides output at 337 nm which excites a vari ety of analytically important systems. In order to scan excitation spectra, tunability is required, and a number of pulsed dye lasers are commonly used. CW dye lasers do not provide as wide a tunability range as the pulsed dye lasers. Tunable UV radiation is one of the more desirable characteris tics for fluorimetry, and pulsed dye la sers can be frequency doubled or Raman shifted to produce tunable UV. The temporal characteristics of the laser are important in experiments where the lifetime is to be determined or when time discrimination is to be used. Pulsed lasers driven by flashlamps can measure lifetimes in the 100 ns range. N2 lasers and N2 pumped dye lasers allow measurement of the lifetimes of many fluorescent mole cules in solution, which are typically in the ns range. The synchronously pumped dye laser outputs pulses in the ps range; thus it is capable of mea suring the fluorescence lifetime of most species of interest. In addition, the synchronously pumped dye laser provides the unique capability of tem porally discriminating between Raman scattering and fluorescence. Photoionization. A milestone was reached in chemical measurement when it was demonstrated that laser techniques could achieve single atom detection (21). The key to achieving such sensitivity was to detect the ion ization that resulted after a two pho ton excitation, first to an excited atomic electronic state and then to the ionization continuum. The photon ex citation energies were chosen so that it was not possible to ionize the atom with a single photon. Proportional counter techniques permit one to de tect individual ionization events and, by requiring that the ionization event be coincident with the laser, the ma jority of possible interfering events from other ionization sources can be rejected. The requirement of a reso nant intermediate state enhances the selectivity of the method.
Photoionization has been of interest to analytical chemists for many years, particularly in relationship with mass spectroscopy. The multiple photon ionization techniques now promise an explosion in the capabilities and information for the analytical chemist. The same ideas that provided selectivity and sensitivity to the single atom detection method are applicable to molecular detection (22). One chooses intermediate states that are characteristic of the molecule. Optical-optical double resonance, for example, could excite a gas phase molecule to an excited electronic state from which it could be ionized. Infrared-optical double resonance could excite the molecule to a vibrational state from which it could be ionized. Intense infrared multiple photon absorption can excite a molecule up a vibrational ladder until it ionizes or fragments. The efficiencies of photoionization induced by multiple photon absorption depend nonlinearly on the excitation intensities within the absorption linewidths. One would like high intensities with narrow linewidths from the laser sources in the wavelength interval of interest. Almost every type of laser has been used for these types of experiments. In fact, incoherent blackbody sources have been used for the second excitation to the ionization continuum where narrow linewidths are no longer required for selectivity. A related technique, called optogalvanic spectroscopy, has recently been developed and has found widespread application (23). If the atoms within a plasma or a flame are electronically excited, they become easier to ionize. The change in ionization efficiency causes changes in the impedance of the medium which can be sensed as a change in the voltage drop across the plasma or as a change in current between two electrodes that measure the impedance of the flame. This phenomenon has been used as a method for trace analysis or for calibrating the wavelengths of a tunable laser. It can be used effectively with any of the tunable sources. Nonlinear Excitation. As mentioned previously, several laser frequencies can mix in a sample to generate polarizations at unique frequencies. In two-photon absorption, two input frequencies, COL and cos, sum to excite levels at (COL + ^s). Oftentimes, one laser is used for twophoton absorption at 2COL for reasons of economy and simplicity. There are several chemical applications of twophoton spectroscopy (24, 25). The excited state symmetry of a molecule determines the selection rules for the different polarizations the two excitation beams can have relative to each other. If there is an intermediary state which can resonantly enhance the ab-
sorption, selectivity can be gained over species without the intermediary state. Finally, two-photon spectroscopy can be done in samples which absorb at the energy of the excited state when the energy of each photon is less than any single-photon absorptions. Two-photon absorption spectroscopy requires high peak power lasers for efficient excitation and is often combined with fluorescence detection in order to increase the sensitivity. Pulsed dye lasers and synchronously pumped dye lasers have been used. Scattering. Historically, the laser was first applied in an analytical context to Raman spectroscopy, and Raman spectroscopy continues to be the most far-reaching application of the laser in analytical chemistry. The availability of convenient multiwavelength lasers has permitted resonance Raman spectroscopy to acquire increased importance (26). In resonance Raman spectroscopy, one is exciting near an excited electronic state. The electric field of the laser can drive the large distortions in the molecule that are characteristic of the excited state and consequently the Raman scattering can be greatly enhanced. Simultaneously though, one can excite fluorescence which can interfere with the Raman measurement. Again, time discrimination can be used to distinguish between the fluorescence and the Raman scattering except now one wants to detect the light that is coincident with the laser. The discrimination is improved if the fluorescence lifetime is long compared with the laser pulsewidth. The argon ion laser is one of the most widely used lasers for Raman spectroscopy and serves as the source in commercial instruments. The argon ion laser outputs stable, high intensity, visible radiation to allow reliable scanning of Raman spectra. Spatial resolution has been incorporated into Raman spectroscopy with the commercialization of the MOLE, or laser Raman molecular microprobe (27). This technique combines a microscope and a laser Raman spectrometer to map the spatial distribution of a desired component in the sample. A vidicon allows operation of the device in two modes, one in which the twodimensional map of the individual component is generated; and the other in which the Raman spectrum is generated for a 1 /mi point in the sample. Applications of this method include geological studies, industrial quality control and biological research. There is a whole series of coherent Raman spectroscopies that are currently being researched (1, 2, 27). We saw earlier that two lasers having frequencies COL and cos will generate a polarization which oscillates at all of the combinations of frequencies COL
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and cos, including one at (2COL — cos).
The oscillating polarization in turn creates a new electromagnetic wave which is the basis for CARS. The new beam at (2COL — cos) caused by Raman scattering is a visible, coherent beam of light that is as directional as a laser beam and is spatially separated from the exciting beams. The intensity depends quadratically upon the intensity at COL and linearly upon the intensity at cos- Thus, high power lasers are generally preferred. Despite the high Raman intensities that characterize CARS, it has not proven to be a replacement for conventional Raman spectroscopy (2). There are always high background levels from the solvent which obscure the CARS signal at low sample concentrations. As discussed in the introduction, the solvent's polarization is still driven by the excitation beams, even in the absence of vibrational resonances, and will produce an output beam regardless of the frequency. CARS measurements are complex, and the analyst must be skilled in their execution. The excitation beams must be directed through the sample at a slight angle to each other in order to achieve phase matching. The angle must be changed if the excitation wavelengths are scanned over several hundred wave numbers. The beams must also overlap spatially and temporally, and this overlap must be maintained over the time period of the experiment and as the laser is tuned. CARS has proven to be useful in a number of situations (1, 2). It is a good method to use in probing hostile environments such as plasmas or flames, and it is particularly effective in situations where the samples fluoresce such as in resonance Raman spectroscopy. Since the beam is coherent and directional, it can be isolated and spatially filtered without appreciable loss, but the incoherent background fluorescence is almost totally rejected. Raman loss and Raman gain spectroscopy are a relatively recent addition to the coherent Raman spectroscopies that are analytically interesting (27). The difference frequency between the two lasers at COL and cos will drive a vibration at coy when they are resonant. If one monitors the intensity of either beam (COL or cos) as the difference frequency is changed, one will see a change whenever a vibrational resonance is encountered in the sample. If there are no vibrational resonances, no energy can be dissipated and no changes are observed in the laser intensities. At resonance, COL increases in intensity and cos decreases in intensity. Note that in this form of coherent Raman spectroscopy, the sample blank does not contribute a background. Instead one measures changes in light intensity, a very familiar prob-
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lem to an analytical chemist. T h e magnitude of the changes d e p e n d s lin early upon the peak intensities of both lasers. T h e o p t i m u m laser would therefore have very high and extreme ly stable peak intensities. T h e most i m p o r t a n t laser charac teristic for nonlinear excitation is the peak power. T h e lasers primarily used for nonlinear techniques such as twophoton absorption and CARS are t h e pulsed dye lasers and t h e synchro nously p u m p e d dye lasers. W h e n two lasers are required for beams a t OJL a n d a>s, it is best if the dye lasers are excited by a single laser like a Nd: YAG, excimer, nitrogen, or argon laser so the beams are temporally coinci dent. R a m a n loss/gain spectroscopy can be performed with high peak power pulsed dye lasers which give large losses or gains b u t have large pulse-to-pulse intensity changes t h a t limit the losses or gains t h a t can be seen. CW dye lasers give smaller losses or gains b u t the intensity stability is quite good, and small changes can be observed more readily. T h e laser is one of t h e newest tools in the analytical chemist's arsenal of i n s t r u m e n t s , and it is clear t h a t we have become excited a b o u t t h e poten tial applications t h a t it has in our field. It is also clear t h a t m a n y chal lenges remain to be m e t before its po tential can be realized. Despite the m a n y applications t h a t have been dis cussed in this article, t h e r e are m a n y more which we unfortunately could not include. In t h e following INSTRU MENTATION article, we want n e x t to examine t h e properties of p r e s e n t co h e r e n t sources in order to appreciate when and how they should be used in analytical applications. References (1) W. M. Toiles, J. W. Nibler, J. R. McDonald, and A. B. Harvey, Appl. Spectrosc, 31, 253 (1977).
John Wright is currently professor of chemistry at the University of Wis consin-Madison. He received his Ph.D. from The Johns Hopkins Uni versity in 1970. His research interests include site-selective laser spectros copy, double resonance methods, ul tratrace organic and inorganic analy sis, nonradiative relaxation and ener gy transfer, and solid state defect chemistry.
Send for Tech Note # 1 To get your free copy promptly, contact Rheodyne, Inc.. RO. Box 996, Cotati, California 94928. Phone (707)664-9050
Mary Wirth received her Ph.D. from Purdue University in 1978 and is cur rently an assistant professor of chem istry at the University of WisconsinMadison. Her research interests in clude picosecond spectroscopy, twophoton polarization measurements and nonlinear Raman spectroscopy.
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