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Laser Applications in Spectroscopy
Chapter 20 Laser Applications in Spectroscopy
The availability of laser sources has substantially changed the field of spectroscopy. Perhaps the most obvious form of laser-based spectroscopic measurement is simply to use a tunable laser, direct the beam through the sample to be measured, tune the laser wavelength, and measure the absorption spectrum of the sample. Thus, because of their high radiance and spectral purity, lasers have substantially advanced the capabilities of conventional spectroscopic techniques, like absorption spectroscopy. But lasers have been employed in many more innovative approaches to spectroscopic investigation. Many new classes of spectroscopic methods and equipment have been developed to take advantage of the unusual properties of laser light. The laser is far more than simply a bright new source of tunable light. Rather, it offers new and highly versatile capabilities for novel measurements of the structure and dynamics of molecules. The availability of the laser has led to a qualitative revolution in spectroscopic capabilities and techniques. Figure 20-1 illustrates the impact that lasers have had on the resolution of spectroscopic measurements. The top portion of the figure shows the absorption spectrum of sulfur hexafluoride at a wavelength near 10 ~m obtained with a grating spectrograph. This represents capabilities available in the late 1960s with a resolution around 0.7 wavenumbers. (The wavenumber, expressed in reciprocal centimeters, is a measure of the photon energy associated with a particular wavelength.) The middle portion of the figure represents an expanded portion of the spectrum, obtained with a tunable diode laser. This represents capabilities available in the early 1970s, with a resolution around 0.001 wavenumbers. The bottom portion shows a further expanded portion obtained using saturation spectroscopy, to be discussed later. This represents capabilities available in the late 1970s, with a resolution around 10 -6 wavenumbers. The figure illustrates the impact that lasers have had on spectroscopic capabilities. In this chapter, we first discuss the types of lasers that are commonly used for spectroscopic studies, then we describe a few of the different types of laser spectroscopy that have been developed, and finally we describe some of the applications of laser spectroscopy. 491
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20. Laser Applications in Spectroscopy
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Wave Number (cm -1) Figure 20-1 Absorption spectrum of sulfur hexafluoride near 10/zm. Top: Obtained with grating spectrograph. Middle: Obtained with tunable laser diode. Bottom: Obtained using saturation spectroscopy. (Based on information from L. J. Radziemski, Los Alamos National Laboratory.)
A.
Lasers for Spectroscopic Applications
Lasers used in spectroscopy include many of the tunable lasers described in Chapter 4 and summarized in Table 4-2. The tunable dye laser has probably been the most commonly employed laser for spectroscopic applications. It has been used routinely in spectroscopic studies for many years. It is now being replaced in some applications by tunable solid state lasers, like Ti:sapphire, especially in the visible and near infrared regions. Tunable lead salt lasers have been widely used for studies in the far infrared, and III-V compound diode lasers are beginning to be used in the near infrared. The tuning range of any specific device, though, is limited.
B. Types of Laser Spectroscopy
493
Other lasers that do not offer continuous tuning have also been employed in spectroscopic applications. The CO 2 laser, which offers line tuning over the range 9-11/.Lm, has been employed in far infrared applications. Even lasers that strictly are not tunable, or at least are tunable to only a few discrete lines, have been used, especially for some of the more exotic types of spectroscopy, like Raman spectroscopy. The argon laser has frequently been used, both at its various fundamental wavelengths and as a frequency-doubled device with output in the ultraviolet. The Nd:YAG laser, as a frequency-doubled, -tripled, and -quadrupled device, offers several useful wavelengths, especially in the ultraviolet. One limiting factor has been the availability of good laser sources in the ultraviolet, but advances in laser instrumentation, especially the use of nonlinear optics to generate ultraviolet sources, are removing this limitation.
B.
Types of Laser Spectroscopy
A wide variety of configurations and methods for laser spectroscopy have been developed. Table 20-1 summarizes some of the many types of laser spectroscopy. The table lists the types, briefly describes their principles, and compares the materials for which they are useful, their applications, and their advantages and disadvantages. Many articles in the technical literature describe these spectroscopic methods and their application. It is not possible to describe all the methods in detail in a reasonable space. We will consider a few selected types of laser spectroscopy as examples. 1.
A B S O R P T I O N SPECTROSCOPY
The first and perhaps most obvious type of laser spectroscopy is absorption spectroscopy, which involves tuning the laser. An example of the spectrum of sulfur hexafluoride in the l0 p~m region, as obtained by tuning a semiconductor diode laser, has already been shown in the middle portion of Figure 20-1. Because of the narrow linewidth and high radiance of the laser, spectra with very high resolution may be obtained. The laser beam is transmitted through the sample the spectrum of which is desired. The intensity of the transmitted beam is monitored by a photodetector, and the wavelength of the laser light is tuned through the region of interest. There is no need for gratings, prisms, or any of the other dispersive elements used in conventional spectrometers. The combination of narrow spectral linewidth, high radiance, and tunability available with lasers has led to great improvement in resolution for absorption spectroscopy. The resolution of a tunable laser system is far better than that of the best conventional dispersive spectrometers. The status and availability of tunable laser sources suitable for applications in spectroscopy has been discussed in Chapter 4. Many types of tunable lasers are under development; these advances should continue to improve laser sources for spectroscopic applications.
20. Laser Applications in Spectroscopy
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Table 20-1
Type
Comparison of Laser-Based Spectroscopic Methods
Principle
Samples
Uses
AdvantagesDisadvantages
Laser absorption spectroscopy
Resonant absorption of light
Mainly gases & liquids
Quantitative analysis, remote sensing
Speciesspecific, broad applicability
Wavelength & intensity stabilization needed
Laser-induced fluorescence
Emission of light from level populated by resonant absorption
Gases & liquids
Analytical spectrochemistry, species concentrations in flames
Selectivity, reduction of interference
Low laser output in UV, where many transitions are
Photoionization spectroscopy
Measurement of change in ionization equilibrium
Gases
Ultrasensitive detection
Very low detection limits
Costly & complicated
Photoacoustic spectroscopy
Absorbed light transformed to acoustic energy
Gases, liquids, solids
Analytic spectrochemistry
Simple & sensitive
Sample must be in contact with detector
Doppler-free spectroscopy
One beam saturates absorption, 2nd beam probes it
Gases
Very high resolution spectroscopy
Highest spectral resolution
Needs extra components
Laser-induced breakdown spectroscopy
Laser creates hot plasma, which emits light
Particles, surfaces, gases, liquids
Real-time spectrochemistry
Minimal sample preparation
Potentially difficult calibration
Raman spectroscopy
Excitation of molecular vibrational frequencies
Molecular gases, liquids, solids
Trace species identification
Species-specific, detects many species at same time
Background may cause high detection threshold
Coherent anti-Stokes Raman spectroscopy (CARS)
Two lasers coherently excite vibrational frequency
Molecular gases, liquids, solids
Trace species identification, composition profiling
Much higher signal than conventional Raman
Costly & complex
UV resonance Raman spectroscopy
Raman scattering from electronic state
Molecular gases, liquids, solids
Analytical chemistry, structure & dynamics of excited states
Highly versatile
Availability of suitable UV lasers
Differential absorption lidar
Absorption spectroscopy using distributed backscatter
Gases, aerosols, atmospheric particulates
Remote sensing
Allows monitoring of remote samples from one spot
May be low signal levels
B. Types of Laser Spectroscopy
495
Laser absorption spectroscopy has been carried out in the visible, near infrared, and near ultraviolet regions of the spectrum for many years using tunable dye lasers. More recently, Ti:sapphire lasers, including frequency-multiplied devices and covering most of the spectral range available to dye lasers, have been replacing dye lasers for many spectroscopic studies. Ti:sapphire and other tunable solid state lasers offer a number of advantages compared with liquid dye systems, and these lasers seem likely to become the leading source for spectroscopy in the visible, near infrared, and near ultraviolet regions. Advances in tunable III-V compound semiconductor lasers have led to their use as spectroscopic sources in the near infrared (in the 1-2/.~m region). In the longer wavelength infrared, lead salt semiconductor lasers have been used in spectroscopic applications for many years. These lasers are cryogenic and expensive. Further advances in laser technology are needed to provide sources for absorption spectroscopy in this region. Many applications of laser absorption spectroscopy are available. They include as examples: 9 Detection of trace components in automobile exhaust using far infrared tunable semiconductor diode lasers. 9 Determination of the structure of atoms, molecules, and ions through measurement of hyperfine splittings, Zeeman splittings, and Stark splittings. 9 Determination of relative populations of rotational and vibrational levels of molecules. 9 Real-time measurements of the concentration and flow patterns of combustion
products. 9 Remote sensing of atmospheric pollutants. This application will be described in more detail in a later section. 2.
R A M A N SPECTROSCOPY
The Raman effect involves scattering of light by molecules of gases, liquids, or solids. The Raman effect consists of the appearance of extra spectral lines near the wavelength of the incident light. The Raman lines in the scattered light are weaker than the light at the original wavelength. The Raman-shifted lines occur both at longer and shorter wavelengths than the original light; the lines at the shorter wavelengths are usually very weak. The Raman spectrum is characteristic of the scattering molecule. The Raman lines occur at frequencies v +_ v~, where v is the original frequency and v~ are the frequencies corresponding to quanta of molecular vibrations or rotations. If the scattered frequency is lower than the original frequency (longer wavelength), the incident light has excited a molecular vibration or rotation and the optical photon has decreased energy. This situation is called Stokes scattering. If the frequency of the scattered light is higher than the incident light (shorter wavelength), the light has gained energy from the vibrational or rotational quanta. This is called anti-Stokes
496
20. Laser Applications in Spectroscopy
scattering. Because the values of z,k are characteristic of individual molecules, investigation of the Raman spectrum provides a sensitive analytical tool. Figure 20-2 shows a schematic example of a Raman scattering spectrum. A number of Stokes lines are present at frequencies lower than the original frequency, corresponding to absorption of one, two, three, and so forth, quanta of vibrational or rotational energy from the incident light. The spacings of the lines are equal and yield an identifying characteristic of the scattering molecule. A smaller number of anti-Stokes lines, with lower intensity, are present at frequencies higher than the original frequency. These lines correspond to transfer of one, two, and so forth, quanta of vibrational or rotational energy from the molecule to the optical field. Raman spectroscopy has long been used for qualitative analysis and for identification of characteristic localized units of structure within molecules. The availability of laser sources has provided an important new device for use in Raman spectroscopy. The narrow linewidth and high radiance of laser sources make it much easier to identify the scattered light and to determine the amount of the wavelength shift. These same properties permit higher resolution of Raman spectra than was possible with conventional sources. The variety of available laser wavelengths makes it possible to carry out Raman spectroscopy while avoiding interfering absorption bands of the molecule being studied. With a tunable laser, the excitation frequency can be tuned to a resonant frequency of the molecule to produce a larger Raman signal. Although a tunable laser is not necessary for Raman spectroscopy,
Original Frequency
Anti-Stokes Lines
Stokes Lines
Wavelength Frequency Figure 20-2 Schematic illustration of a Raman scattering spectrum. The separation between the different lines is equal to the frequency of a molecular vibration.
B. Types of Laser Spectroscopy
497
the use of a tunable source can greatly enhance the Raman signal. In practice, many of the studies involving Raman spectroscopy have been carried out with argon lasers. Laser Raman spectroscopy has been used in a wide variety of applications, such as for identifying drugs, for detecting trace quantities of drugs in blood or urine samples, for detecting the metabolic by-products of drugs, for determining the presence of impurities in products like medicines, for studying polymers in solution, and for characterization of the kinetics of combustors and coal gasifiers. Raman spectroscopy has also been used for remote detection of pollutants in the atmosphere. This application will be described in a later section. The Raman spectroscopic technique is applicable to very small samples, because Raman spectroscopy requires a sample only of sufficient size to fill the focused beam of an argon laser, a volume about 10/xm in diameter. Thus, Raman spectroscopy can be useful for small samples of fibers, coatings, or finishes. Because Raman spectroscopy is a scattering technique, samples do not have to be transparent. Thus, materials such as pills, chemicals, drugs, small particles, and coatings can be sampled as received. These capabilities make the chemical laboratory an important market for laser Raman spectroscopic instrumentation. One important variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS). In CARS, one combines two laser beams with angular frequencies o)1 and o)2, within the sample to be measured. Then, one tunes o)2 so that the frequency difference o)l - o)2 i s equal to the frequency of a Raman mode for the molecule of interest. Raman scattering then occurs through a nonlinear interaction at frequency 2o)1 - o)2. The characteristics of the Raman signal yield a signature dependent on the molecular species, the temperature, and the pressure. Because of these capabilities, the CARS technique has been used for measurement of flow parameters. It is useful in environments with high-velocity gas flow and in situations where the gas is hot and luminous. The technique can provide both high temporal and spatial resolution.
3.
DOPPLER-FREE SPECTROSCOPY
A limitation to resolution in absorption spectroscopy is the Doppler effect, the broadening of the spectral lines in gases and vapors because of the motion of the atoms or molecules. The spectral linewidths of absorption lines in gases should be very narrow. But the Doppler effect, caused by the random thermal motion of the molecules, can broaden the observed linewidth considerably. The Doppler linewidth, which is proportional to the square root of the temperature, can even be larger than the separation between neighboring narrow spectral lines. Thus, Doppler broadening can hide much of the fine structure contained in atomic or molecular spectra. The use of lasers in ordinary absorption spectroscopy does not help this limitation, because even though the laser linewidth is very narrow, the observed broadening arises from the motion of molecules in the sample. The broadening can be
498
20. Laser Applications in Spectroscopy
eliminated by use of techniques termed Doppler-free spectroscopy. The resulting spectra reveal the natural width of the narrow spectral lines, not the width resulting from motion of the molecules. One common Doppler-free technique is called saturation spectroscopy. In this approach, the laser beam is split into two beams, one strong and one weak. The two beams are introduced into a gas cell, traveling in opposite directions. The monochromatic laser beam, tuned to a specific wavelength, interacts only with a small fraction of the molecules. It interacts only with those molecules that happen to be moving with a velocity such that their absorption is at the same wavelength as the light. The stronger beam can saturate the absorption, that is, it can interact with all the atoms or molecules in the narrow portion of the Doppler linewidth corresponding to the laser wavelength. Thus, the absorbing atoms or molecules are depleted, and the absorption is weakened in this portion of the absorption line. The stronger beam thus bleaches a path for the weaker probing beam. The strong beam is chopped with a modulator. If the two beams are interacting with the same molecules, then the probe beam will be modulated also. A detector viewing the probe beam sees the modulation. The modulated signal is detected only when the laser is tuned to the center of the absorption line, so that the light is absorbed by molecules having zero component of velocity in the direction of the laser beam. For those molecules, neither beam is Doppler shifted and the two beams are in resonance with the same molecules. In this way, the effects of Doppler broadening are eliminated. Figure 20-1, in the bottom portion, has already shown how the resolution of a spectroscopic measurement may be increased by use of saturation spectroscopy. As another example, Figure 20-3 shows the hyperfine structure of a line of molecular iodine-127. The measurements were made with an argon laser that could be tuned over a very small range by a piezoelectric drive on one mirror. The Doppler width is shown for comparison. In conventional absorption spectroscopy, none of the detail on a scale narrower than the Doppler width would have been observable. The figure shows dramatically the ability of saturation spectroscopy to overcome the broadening introduced by the Doppler effect. Another variation of Doppler-free spectroscopy is polarization spectroscopy, which developed later than saturation spectroscopy. Polarization spectroscopy yields greater sensitivity than saturation spectroscopy in the sense that it is possible to work with fewer molecules. The apparatus for polarization spectroscopy is similar to that for saturation spectroscopy, but the sample is between crossed polarizers, so that the background light is reduced to near zero. Because the probability for absorbing polarized light depends on the orientation of the absorbing molecule, the strong beam preferentially depletes the population of molecules with a specific orientation. Then, in its interactions with the oriented molecules, the weaker probe beam acquires a component of polarization that passes through the second polarizer. A detector then sees an increase of light intensity against a zero background. As with saturation spectroscopy, this method requires that both
C. Applications of Laser Spectroscopy
499
Doppler Width '
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beams interact with the same molecules near the center of the Doppler-broadened line.
C.
Applications of Laser Spectroscopy
The unique properties of laser light, particularly its monochromaticity and high radiance, have made possible many advances in spectroscopic applications. Lasers provide greatly increased resolution in conventional spectroscopic measurements and also allow many innovative new spectroscopic techniques. Laser spectroscopy has become a very broad and varied subject, with many, diverse applications. It is not possible to describe all the many applications in chemistry, physics, medicine, life sciences, forensic science, and so forth that are made possible by lasers. We will simply present some examples.
500
1.
20. Laser Applications in Spectroscopy
CHEMICAL APPLICATIONS
Laser spectroscopy provides chemists with an extremely versatile tool for analytical spectrochemistry, for probing the dynamic behavior of chemical reactions, and for determining molecular structure and energy levels. One significant application made possible by lasers is the technique of picosecond and subpicosecond spectroscopy, which is used to study ultrafast kinetics of chemical reactions. A number of tunable lasers, such as liquid dye lasers, can emit extremely short pulses with durations in the picosecond, or even femtosecond (10 -15 sec), regimes. The availability of such lasers permits many new applications in the study of transient chemical phenomena. Examples include determination of the lifetimes of excited states and measurements of relaxation processes in atoms and molecules with extremely high temporal resolution. Chemical kinetic processes can be monitored in a fashion that was not previously possible. Processes taking place on a picosecond time scale include vibrational and orientational relaxation of molecules in liquids, radiationless transitions of electronically excited large molecules, solvation of photoejected free electrons, and isomerization of photoexcited states of complex molecules like visual pigments, We present one example that will illustrate some of the techniques and serve to demonstrate the capabilities of picosecond spectroscopy. The experiment [1] involved the formation and decay of a biophysical compound called prelumirhodopsin, which is considered to be the initial step in photoinduced chemical changes that begin the chain of physiological processes constituting vision. Vision begins when rhodopsin, a photosensitive pigment in the retina, is bleached by light. When rhodopsin in the eye is exposed to light, prelumirhodopsin is formed faster than can be followed by conventional methods. The experiment involved photoexcitation of rhodopsin by a picosecond pulse of light at a wavelength of 530 nm. The absorption band of prelumirhodopsin at 560 nm was monitored by a train of short pulses at that wavelength. The original laser was a mode-locked Nd:glass laser, from which one pulse of 6 psec duration was extracted. This pulse was amplified and frequency-doubled in a potassium dihydrogen phosphate crystal to yield light at 530 nm wavelength. Part of that light was used to excite the rhodopsin. Another part was split off and Raman scattered to yield pulses of 560 nm light. This light was reflected from an echelon, a stepped optical element that introduced different optical delays into different spatial parts of the wavefront. Thus, the 560 nm light was transformed into a series of short interrogating pulses to monitor the desired absorption band. These pulses were focused onto the same area of the sample as the 530 nm exciting light. After emerging from the sample, the 560 nm light was imaged onto a camera. Light from each optical segment of the echelon was imaged on a different area of the film. Because the light from each element had a unique incremental optical path, the temporal pulse separation (about 20 psec) was recorded as a spatial separation. The results of the measurement showed that the rise time of the prelumirhodopsin was less than 6 psec after the start of the 530 nm exciting pulse. The data indicated that production of prelumirhodopsin is the primary photochemical event.
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Probing time-resolved chemical processes by means of picosecond spectroscopy has been the subject of extensive research, which has led to significant new knowledge about mechanisms of chemical reactions, energy distributions, and relaxation processes in molecules and structures of the excited states of biological systems. 2.
REMOTE SENSING AND ENVIRONMENTAL MONITORING
There is a need for improved, accurate methods for remote measurement of the concentration of pollutants in the atmosphere. Many different types of pollutants are present in the atmosphere. Some of the major pollutants of interest include oxides of nitrogen, carbon monoxide, sulfur dioxide, and ozone. In addition, there are various types of particulate materials, such as ash, dust, and soot. There are also requirements to monitor sources of pollution, in order to comply with governmental regulations. Moreover, there are needs for mapping, on a global scale, constituents such as carbon dioxide and water vapor. Many problems are associated with measurement of concentrations of pollutants in the atmosphere. Conventional measurements are difficult. For example, if one desires to measure gases from a smokestack, there are problems involving the inhospitable environment inside the smokestack, problems with accessibility, and problems involving perturbing the quantities that are to be measured. Traditional methods have involved collecting a sample of gas from the smokestack and making a chemical or spectroscopic analysis. Such methods are conducted remotely, and results are not immediately available. Stratification of gases and particulate materials also could lead to error. Sensors embedded in the smokestack have used the electrochemical properties of the gases as a sensing mechanism. These sensors require contact with the gas sample and, for the most part, have slower response than desired. Such devices have had problems with maintenance because of the hot, corrosive gases that contact them. Although there have been numerous approaches to monitoring smokestack emissions, no completely satisfactory solutions have yet been developed. The problem becomes even more severe when one considers monitoring the concentrations of specific gases remotely over broad areas. Approaches based on optical technology appear attractive for areal mapping. Fourier transform infrared spectroscopy (FTIR) based on nonlaser sources has been employed for this application. There are commercial FTIR monitoring instruments available. Laser technology offers attractive features for remote monitoring of atmospheric quantities. It is possible to perform measurements at great distances. The measurements are completely noncontact and involve no sample collection nor chemical processing. There is no disturbance of the quantities to be measured. Generally, the results of the measurement can be available immediately. Tunable semiconductor laser diodes with very narrow spectral linewidth offer many attractive opportunities for remote noncontact spectroscopic sensing of gaseous species. The use of laser spectroscopy for detection of pollutant gases has been studied for a number of years. The emphasis in early work has been on relatively long wavelength infrared spectroscopy, using lasers operating in the 5-10/xm
502
20. Laser Applications in Spectroscopy
wavelength range. The lasers have been tunable lead salt diode lasers, such as lead selenide sulfide. Such lasers are expensive, operate at cryogenic temperatures, and require the use of expensive cryogenic detectors. Operation in the long wavelength infrared does offer the advantage that the molecular absorption lines are very strong for gases of interest. A number of approaches for laser-based remote sensing have been studied over several decades. These include 9 Optical radar (lidar) 9 Raman backscattering 9 Resonance fluorescence 9 Absorption methods, particularly differential absorption laser backscatter (DIAL) We shall discuss each of these approaches briefly.
1.
Optical Radar
Optical radar, also called lidar, which stands for light detection and ranging, operates similarly to microwave radar. In its basic form, it employs a pulsed laser, the beam of which is directed to the atmospheric sample that is to be probed. Energy backscattered by the atmosphere is sensed by a photodetector. The resulting signal is processed as a function of time from the transmission of the pulse, just as in a radar ranging system. Generally, lidar systems have relied on Mie scattering, due to particulate material and aerosols, to provide the backscattered light. Thus, lidar is most useful for determining concentrations of particulate material, and it gives no information about gas concentrations. Lidar has most often been used for measurements in which concentrations of particulates or aerosols are desired. It can determine concentration as a function of distance from the measurement position. Figure 20-4 presents what is perhaps a typical example of a lidar measurement. The results are derived from laser backscattering from the emission of an 800-foothigh smokestack. The figure shows the range-corrected signal in decibels relative to that from the ambient background aerosols. The contours represent relative particulate concentration in a vertical cross section of the emission from the smokestack. The example shows clearly how lidar can provide information about particulate concentration that would otherwise be difficult to obtain. The measurement may easily be performed remotely. Lidar can also measure quantities like stratification, flow, and changing profiles of turbid layers in the atmosphere.
2.
Raman Backscattering
The use of the Raman effect, which involves scattering of light by molecules of gases with a shift in the wavelength of the scattered light, has been discussed previously in terms of its spectroscopic applications. It also has application to remote sensing, particularly for identification of gaseous species. Because the values of the
C. Applications of Laser Spectroscopy
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Figure 20-4 Vertical cross section through a smoke plume from a smokestack, as obtained by lidar measurements. The contours represent relative particulate concentrations. (From W. B. Johnson, J. Air Pollution Contr. Assoc. 19, 176 (1969).)
wavelength shift are characteristic of individual molecules, examination of the spectrum of the light backscattered from a remote sample of gas can provide information about the gases present in the sample. Figure 20-5 presents an example of the spectral composition of the backscattered return from an oil-smoke plume emitted by a smokestack at a distance around 30 m. The figure shows the strength of the return signal over a range of wavelengths. A number of pollutants, such as SO 2 and H2S, are identified. Also, the major constituents of the atmosphere, oxygen and nitrogen, appear as relatively strong signals. Raman-based remote sensing requires the use of visible or near ultraviolet lasers. Use of the Raman effect with a pulsed laser also offers range discrimination, so that relative pollutant concentration as a function of distance can be obtained. But because the cross section for Raman scattering is small, the laser used must have high values of peak power. This requirement leads to problems with eye safety. Also, Raman methods are mainly useful for measuring relatively high concentrations and at relatively short range. Although there have been many demonstrations of Ramanbased remote sensing over a long period, the use of Raman sensing has not been adopted for routine monitoring use.
3.
Resonance Fluorescence
In resonance fluorescence, the beam is directed to a gaseous sample that contains molecules that fluoresce when excited with the proper wavelength. An example is
504
20. Laser Applications in Spectroscopy
N2
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Figure 20-5 Spectral distribution of the Raman-shifted signal from a variety of molecular gases in a remote laser Raman measurement. (From H. Inaba and T. Kobayasi, OptoElectron. 4, 101 (1972).)
NO2, which fluoresces when excited by blue light, such as 488 nm light from an argon laser. The fluorescence is collected by a telescope and delivered to a detector. An example showing results of laser detection of NO 2, using an argon laser on samples collected in Los Angeles on a smoggy day, is presented in Figure 20-6. The concentration of NO 2 varies as a function of the time of day, with peaks near the times of peak automobile traffic. The rapidity of the changes shows the desirability of real-time monitoring with fast response. In addition to use for atmospheric monitoring, fluorescence techniques have been used for measurements underground, to determine the presence of pollutants in groundwater and in sites where toxic wastes may be present. Test holes may be drilled, or a sensing element may be driven into the ground in a cone penetrometer. Such devices can be rammed tens of feet into the ground in suitable soils. A fiber optic cable is used to deliver laser light to the underground region being tested. Fluo-
C. Applications of Laser Spectroscopy
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Figure 20-6 Nitrogen dioxide concentration measured as a function of local time during a smoggy period in the Los Angeles basin. The data were obtained using a resonance fluorescence technique. (From J. A. Gelbwachs et al., Opto-Electron. 4, 155 (1972).)
rescence emitted by materials in the ground is collected and delivered back through the fiber optic cable to a detector at the surface. The presence of specific chemicals is revealed by the spectrum of the fluorescence. In one example, an excimer laser was used with a Raman shifter to produce light at 248, 276, and 317 nm. The light was delivered through optical fibers into boreholes near a leaking gas pump. The fluorescence from hydrocarbons in the soil was returned to the surface by the fiber and detected with a monochromator and a photomultiplier. The presence of diesel fuel in the soil was detected, and the concentrations could be readily mapped. Figure 20-7 shows experimental results for diesel fuel contamination in a vertical layer of soil near the pump. The results demonstrate the usefulness of laser techniques for in situ monitoring of subsurface contaminants. The system could distinguish contaminants such as fuels, benzene, toluene, xylene, and polycyclic aromatic hydrocarbons. The status of this type of measurement is still developmental, but a number of demonstrations have been performed. The technique offers the advantage of realtime in situ determination of the presence of underground pollutants, without the need for chemical processing.
506
20. Laser Applications in Spectroscopy
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4.
Absorption Methods
Absorption methods provide a sensitive class of methods of detection for specific gases. The usual method of absorption spectroscopy involves transmitting the beam through the sample. One requires a two-ended system, with a detector on the opposite side of the sample from the laser source. This may be inconvenient for remote monitoring. One possibility for relieving this difficulty is the use of a remote topographic reflector. One may use reflection from some feature present in the countryside, such as a large building or a hill, to direct light back to the position of the source. This can lead to a single-ended system in which the transmitter and receiver are located at the same position. This does, however, require that a suitable scattering feature be available. Another variation of absorption spectroscopy for remote gas monitoring involves differential optical absorption spectroscopy (DOAS). DOAS relies on determining
C. Applications of Laser Spectroscopy
507
the differences between local maxima and minima in the absorption spectrum of the gas being probed. DOAS seems to have been used mostly in the ultraviolet region of the spectrum. It requires a source that can cover a relatively large spectral range. DOAS has been demonstrated using laser sources, but it probably has been used more with broadband sources, like high-pressure xenon lamps. With any of the foregoing absorption techniques for remote sensing, one obtains measurements of concentration integrated over the length of the absorbing path. The use of differential absorption laser (DIAL) backscattering techniques removes this difficulty and provides range resolution in a single-ended system. It also preserves the high sensitivity of absorption measurements and the specificity for particular gases. DIAL spectroscopy uses ambient atmospheric particulates and molecules as a distributed reflector to provide a return signal. The laser is tuned alternately on and off a characteristic molecular absorption peak to provide sensitivity for a specific gas. The gas concentration is determined from the relative strength of the return signal for on-peak and off-peak operation. The essential feature of the DIAL technique is that it uses Rayleigh and Mie scattering in the atmosphere to provide reflection of light back to the position of the source, so that no physical reflector is needed. The tuning of the source wavelength on and off the peak of the absorption line provides high sensitivity and makes the method self-calibrating. If a short laser pulse is used, monitoring the return as a function of time after the pulse provides range resolution. DIAL spectroscopy appears to be the leading candidate for laser-based remote monitoring of the environment. There have been many demonstrations of its use for remote sensing of a wide variety of materials. Some commercial DIAL systems are becoming available. Figure 20-8 illustrates mapping of the distribution of SO 2 in a horizontal plane over the area of a zinc works, as determined using DIAL spectroscopy. The laser was located at the coordinate origin, off the right-hand side of the diagram. The measurements were performed using a Nd:YAG pumped dye laser as the source. The laser was tuned between the wavelengths of 296.17 and 297.35 nm to provide on-peak and off-peak conditions. The plume of SO 2, traveling with the wind, is mapped with a claimed uncertainty around 2 percent. As another example, NASA has used an airborne ultraviolet DIAL system to monitor ozone concentrations [2]. The system uses two dye lasers pumped by Nd:YAG lasers, to generate 301 nm light for the on-peak probe and 311 nm light for the off-peak probe. The system has been flown on many missions to map the concentrations of ozone in the troposphere and stratosphere around the world, particularly the depletion of ozone in the arctic regions. Mobile DIAL spectroscopy systems have been developed, suitable for remote monitoring of pollution at changing sites. DIAL spectroscopy has become an established technique for atmospheric monitoring. It is used for applications like industrial site inspection, monitoring of waste cleanup, enforcement of air quality regulations, and environmental research.
508
20. Laser Applications in Spectroscopy
150
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Figure 20-8 Distributionof S O 2 in a horizontal plane over the area of a zinc works, as determined with DIAL spectroscopy. (From U.-B. Goers, Laser remote sensing of sulfur dioxide and ozone with the mobile differential absorption lidar ARGOS, Opt. Eng. 34, 3097 (1995).) 5.
Summary
There have been many demonstrations of the application of lasers to remote monitoring of atmospheric parameters. Many different methods have been employed. Table 20-2 compares the features of the different methods we have described. Some commercial systems for laser monitoring of the environment have become available. Models that are being offered make use of a variety of laser types, including Nd:YAG (both 1064 and 532 nm devices), excimer, tunable dye, tunable semiconductor, and CO 2 lasers. Systems have been designed for a number of specific applications, including measurement of smokestack opacity, monitoring of ozone concentration, leak detection, and cloud mapping. Although commercial laser-based environmental monitoring equipment is still in a relatively early stage, it appears to be developing steadily. Remote sensing of atmospheric parameters involves directing a laser beam into the atmosphere. Issues of laser eye safety become important. The requirements for eye safety may limit the laser power that can be used in a probing beam. The relatively low values for maximum permissible eye exposure in the visible and ultraviolet may restrict the use of high-power pulsed lasers in these spectral regions. The optimum wavelength from the point of eye safety would be in the infrared, at wavelengths greater than 1.5 ~m. The widespread application of laser technology for remote atmospheric monitoring will have to be compatible with public safety. Although many workers have been active in this field for a number of years and have shown the capability for obtaining remote measurements of air pollutants and
Selected Additional References Table 20-2
Comparison of Laser-Based Remote Monitoring Methods
Method Optical radar Raman scattering Resonance fluorescence Absorption Transmission Topographic
DOAS DIAL
509
Identification of gases
Range resolution
Ease
Sensitivity
No Yes Yes
Yes Yes Yes
Single-ended Single-ended Single-ended
Moderate Low Moderate
Yes Yes
No No
High High
Yes Yes
No Yes
Double-ended Single-ended, but needs reflecting feature Double-ended Single-ended
High High
other atmospheric constituents, this technology is still largely in development. It has been useful as a research tool. Global scale measurements by NASA have provided useful information about atmospheric composition. Concentrations of specific gases around specific industrial sites have been derived. The amount of work being performed in this area indicates that laser-based remote sensing of atmospheric quantities will continue to grow.
References [1] G.E. Busch et al., Proc. Nat. Acad. Sci. 69, 2802 (1972). [2] E.V. Browell, Proc. IEEE 77, 419 (1989).
Selected Additional References R. M. Measures, ed., Laser Remote Chemical Analysis, Wiley, New York, 1988. E. R. Menzel, Laser Spectroscopy: Techniques and Applications, Marcel Dekker, New York, 1994. Optical Sensing for Environmental Monitoring, Proceedings of an International Specialty Conference, Atlanta, GA, October 11-14, 1993, Air and Waste Management Assn., Pittsburgh, PA, 1994. L. J. Radziemski, R. W. Solarz, and J. A. Paisner, eds., Laser Spectroscopy and Its Applications, Marcel Dekker, New York, 1986.
The availability of laser sources has substantially changed the field of spectroscopy. Perhaps the most obvious form of laser-based spectroscopic measurement is simply to use a tunable laser, direct the beam through the sample to be measured, tune the laser wavelength, and measure the absorption spectrum of the sample. Thus, because of their high radiance and spectral purity, lasers have substantially advanced the capabilities of conventional spectroscopic techniques, like absorption spectroscopy. But lasers have been employed in many more innovative approaches to spectroscopic investigation. Many new classes of spectroscopic methods and equipment have been developed to take advantage of the unusual properties of laser light. The laser is far more than simply a bright new source of tunable light. Rather, it offers new and highly versatile capabilities for novel measurements of the structure and dynamics of molecules. The availability of the laser has led to a qualitative revolution in spectroscopic capabilities and techniques. Figure 20-1 illustrates the impact that lasers have had on the resolution of spectroscopic measurements. The top portion of the figure shows the absorption spectrum of sulfur hexafluoride at a wavelength near 10 ~m obtained with a grating spectrograph. This represents capabilities available in the late 1960s with a resolution around 0.7 wavenumbers. (The wavenumber, expressed in reciprocal centimeters, is a measure of the photon energy associated with a particular wavelength.) The middle portion of the figure represents an expanded portion of the spectrum, obtained with a tunable diode laser. This represents capabilities available in the early 1970s, with a resolution around 0.001 wavenumbers. The bottom portion shows a further expanded portion obtained using saturation spectroscopy, to be discussed later. This represents capabilities available in the late 1970s, with a resolution around 10 -6 wavenumbers. The figure illustrates the impact that lasers have had on spectroscopic capabilities. In this chapter, we first discuss the types of lasers that are commonly used for spectroscopic studies, then we describe a few of the different types of laser spectroscopy that have been developed, and finally we describe some of the applications of laser spectroscopy. 491
492
20. Laser Applications in Spectroscopy
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A.
Lasers for Spectroscopic Applications
Lasers used in spectroscopy include many of the tunable lasers described in Chapter 4 and summarized in Table 4-2. The tunable dye laser has probably been the most commonly employed laser for spectroscopic applications. It has been used routinely in spectroscopic studies for many years. It is now being replaced in some applications by tunable solid state lasers, like Ti:sapphire, especially in the visible and near infrared regions. Tunable lead salt lasers have been widely used for studies in the far infrared, and III-V compound diode lasers are beginning to be used in the near infrared. The tuning range of any specific device, though, is limited.
B. Types of Laser Spectroscopy
493
Other lasers that do not offer continuous tuning have also been employed in spectroscopic applications. The CO 2 laser, which offers line tuning over the range 9-11/.Lm, has been employed in far infrared applications. Even lasers that strictly are not tunable, or at least are tunable to only a few discrete lines, have been used, especially for some of the more exotic types of spectroscopy, like Raman spectroscopy. The argon laser has frequently been used, both at its various fundamental wavelengths and as a frequency-doubled device with output in the ultraviolet. The Nd:YAG laser, as a frequency-doubled, -tripled, and -quadrupled device, offers several useful wavelengths, especially in the ultraviolet. One limiting factor has been the availability of good laser sources in the ultraviolet, but advances in laser instrumentation, especially the use of nonlinear optics to generate ultraviolet sources, are removing this limitation.
B.
Types of Laser Spectroscopy
A wide variety of configurations and methods for laser spectroscopy have been developed. Table 20-1 summarizes some of the many types of laser spectroscopy. The table lists the types, briefly describes their principles, and compares the materials for which they are useful, their applications, and their advantages and disadvantages. Many articles in the technical literature describe these spectroscopic methods and their application. It is not possible to describe all the methods in detail in a reasonable space. We will consider a few selected types of laser spectroscopy as examples. 1.
A B S O R P T I O N SPECTROSCOPY
The first and perhaps most obvious type of laser spectroscopy is absorption spectroscopy, which involves tuning the laser. An example of the spectrum of sulfur hexafluoride in the l0 p~m region, as obtained by tuning a semiconductor diode laser, has already been shown in the middle portion of Figure 20-1. Because of the narrow linewidth and high radiance of the laser, spectra with very high resolution may be obtained. The laser beam is transmitted through the sample the spectrum of which is desired. The intensity of the transmitted beam is monitored by a photodetector, and the wavelength of the laser light is tuned through the region of interest. There is no need for gratings, prisms, or any of the other dispersive elements used in conventional spectrometers. The combination of narrow spectral linewidth, high radiance, and tunability available with lasers has led to great improvement in resolution for absorption spectroscopy. The resolution of a tunable laser system is far better than that of the best conventional dispersive spectrometers. The status and availability of tunable laser sources suitable for applications in spectroscopy has been discussed in Chapter 4. Many types of tunable lasers are under development; these advances should continue to improve laser sources for spectroscopic applications.
20. Laser Applications in Spectroscopy
494
Table 20-1
Type
Comparison of Laser-Based Spectroscopic Methods
Principle
Samples
Uses
AdvantagesDisadvantages
Laser absorption spectroscopy
Resonant absorption of light
Mainly gases & liquids
Quantitative analysis, remote sensing
Speciesspecific, broad applicability
Wavelength & intensity stabilization needed
Laser-induced fluorescence
Emission of light from level populated by resonant absorption
Gases & liquids
Analytical spectrochemistry, species concentrations in flames
Selectivity, reduction of interference
Low laser output in UV, where many transitions are
Photoionization spectroscopy
Measurement of change in ionization equilibrium
Gases
Ultrasensitive detection
Very low detection limits
Costly & complicated
Photoacoustic spectroscopy
Absorbed light transformed to acoustic energy
Gases, liquids, solids
Analytic spectrochemistry
Simple & sensitive
Sample must be in contact with detector
Doppler-free spectroscopy
One beam saturates absorption, 2nd beam probes it
Gases
Very high resolution spectroscopy
Highest spectral resolution
Needs extra components
Laser-induced breakdown spectroscopy
Laser creates hot plasma, which emits light
Particles, surfaces, gases, liquids
Real-time spectrochemistry
Minimal sample preparation
Potentially difficult calibration
Raman spectroscopy
Excitation of molecular vibrational frequencies
Molecular gases, liquids, solids
Trace species identification
Species-specific, detects many species at same time
Background may cause high detection threshold
Coherent anti-Stokes Raman spectroscopy (CARS)
Two lasers coherently excite vibrational frequency
Molecular gases, liquids, solids
Trace species identification, composition profiling
Much higher signal than conventional Raman
Costly & complex
UV resonance Raman spectroscopy
Raman scattering from electronic state
Molecular gases, liquids, solids
Analytical chemistry, structure & dynamics of excited states
Highly versatile
Availability of suitable UV lasers
Differential absorption lidar
Absorption spectroscopy using distributed backscatter
Gases, aerosols, atmospheric particulates
Remote sensing
Allows monitoring of remote samples from one spot
May be low signal levels
B. Types of Laser Spectroscopy
495
Laser absorption spectroscopy has been carried out in the visible, near infrared, and near ultraviolet regions of the spectrum for many years using tunable dye lasers. More recently, Ti:sapphire lasers, including frequency-multiplied devices and covering most of the spectral range available to dye lasers, have been replacing dye lasers for many spectroscopic studies. Ti:sapphire and other tunable solid state lasers offer a number of advantages compared with liquid dye systems, and these lasers seem likely to become the leading source for spectroscopy in the visible, near infrared, and near ultraviolet regions. Advances in tunable III-V compound semiconductor lasers have led to their use as spectroscopic sources in the near infrared (in the 1-2/.~m region). In the longer wavelength infrared, lead salt semiconductor lasers have been used in spectroscopic applications for many years. These lasers are cryogenic and expensive. Further advances in laser technology are needed to provide sources for absorption spectroscopy in this region. Many applications of laser absorption spectroscopy are available. They include as examples: 9 Detection of trace components in automobile exhaust using far infrared tunable semiconductor diode lasers. 9 Determination of the structure of atoms, molecules, and ions through measurement of hyperfine splittings, Zeeman splittings, and Stark splittings. 9 Determination of relative populations of rotational and vibrational levels of molecules. 9 Real-time measurements of the concentration and flow patterns of combustion
products. 9 Remote sensing of atmospheric pollutants. This application will be described in more detail in a later section. 2.
R A M A N SPECTROSCOPY
The Raman effect involves scattering of light by molecules of gases, liquids, or solids. The Raman effect consists of the appearance of extra spectral lines near the wavelength of the incident light. The Raman lines in the scattered light are weaker than the light at the original wavelength. The Raman-shifted lines occur both at longer and shorter wavelengths than the original light; the lines at the shorter wavelengths are usually very weak. The Raman spectrum is characteristic of the scattering molecule. The Raman lines occur at frequencies v +_ v~, where v is the original frequency and v~ are the frequencies corresponding to quanta of molecular vibrations or rotations. If the scattered frequency is lower than the original frequency (longer wavelength), the incident light has excited a molecular vibration or rotation and the optical photon has decreased energy. This situation is called Stokes scattering. If the frequency of the scattered light is higher than the incident light (shorter wavelength), the light has gained energy from the vibrational or rotational quanta. This is called anti-Stokes
496
20. Laser Applications in Spectroscopy
scattering. Because the values of z,k are characteristic of individual molecules, investigation of the Raman spectrum provides a sensitive analytical tool. Figure 20-2 shows a schematic example of a Raman scattering spectrum. A number of Stokes lines are present at frequencies lower than the original frequency, corresponding to absorption of one, two, three, and so forth, quanta of vibrational or rotational energy from the incident light. The spacings of the lines are equal and yield an identifying characteristic of the scattering molecule. A smaller number of anti-Stokes lines, with lower intensity, are present at frequencies higher than the original frequency. These lines correspond to transfer of one, two, and so forth, quanta of vibrational or rotational energy from the molecule to the optical field. Raman spectroscopy has long been used for qualitative analysis and for identification of characteristic localized units of structure within molecules. The availability of laser sources has provided an important new device for use in Raman spectroscopy. The narrow linewidth and high radiance of laser sources make it much easier to identify the scattered light and to determine the amount of the wavelength shift. These same properties permit higher resolution of Raman spectra than was possible with conventional sources. The variety of available laser wavelengths makes it possible to carry out Raman spectroscopy while avoiding interfering absorption bands of the molecule being studied. With a tunable laser, the excitation frequency can be tuned to a resonant frequency of the molecule to produce a larger Raman signal. Although a tunable laser is not necessary for Raman spectroscopy,
Original Frequency
Anti-Stokes Lines
Stokes Lines
Wavelength Frequency Figure 20-2 Schematic illustration of a Raman scattering spectrum. The separation between the different lines is equal to the frequency of a molecular vibration.
B. Types of Laser Spectroscopy
497
the use of a tunable source can greatly enhance the Raman signal. In practice, many of the studies involving Raman spectroscopy have been carried out with argon lasers. Laser Raman spectroscopy has been used in a wide variety of applications, such as for identifying drugs, for detecting trace quantities of drugs in blood or urine samples, for detecting the metabolic by-products of drugs, for determining the presence of impurities in products like medicines, for studying polymers in solution, and for characterization of the kinetics of combustors and coal gasifiers. Raman spectroscopy has also been used for remote detection of pollutants in the atmosphere. This application will be described in a later section. The Raman spectroscopic technique is applicable to very small samples, because Raman spectroscopy requires a sample only of sufficient size to fill the focused beam of an argon laser, a volume about 10/xm in diameter. Thus, Raman spectroscopy can be useful for small samples of fibers, coatings, or finishes. Because Raman spectroscopy is a scattering technique, samples do not have to be transparent. Thus, materials such as pills, chemicals, drugs, small particles, and coatings can be sampled as received. These capabilities make the chemical laboratory an important market for laser Raman spectroscopic instrumentation. One important variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS). In CARS, one combines two laser beams with angular frequencies o)1 and o)2, within the sample to be measured. Then, one tunes o)2 so that the frequency difference o)l - o)2 i s equal to the frequency of a Raman mode for the molecule of interest. Raman scattering then occurs through a nonlinear interaction at frequency 2o)1 - o)2. The characteristics of the Raman signal yield a signature dependent on the molecular species, the temperature, and the pressure. Because of these capabilities, the CARS technique has been used for measurement of flow parameters. It is useful in environments with high-velocity gas flow and in situations where the gas is hot and luminous. The technique can provide both high temporal and spatial resolution.
3.
DOPPLER-FREE SPECTROSCOPY
A limitation to resolution in absorption spectroscopy is the Doppler effect, the broadening of the spectral lines in gases and vapors because of the motion of the atoms or molecules. The spectral linewidths of absorption lines in gases should be very narrow. But the Doppler effect, caused by the random thermal motion of the molecules, can broaden the observed linewidth considerably. The Doppler linewidth, which is proportional to the square root of the temperature, can even be larger than the separation between neighboring narrow spectral lines. Thus, Doppler broadening can hide much of the fine structure contained in atomic or molecular spectra. The use of lasers in ordinary absorption spectroscopy does not help this limitation, because even though the laser linewidth is very narrow, the observed broadening arises from the motion of molecules in the sample. The broadening can be
498
20. Laser Applications in Spectroscopy
eliminated by use of techniques termed Doppler-free spectroscopy. The resulting spectra reveal the natural width of the narrow spectral lines, not the width resulting from motion of the molecules. One common Doppler-free technique is called saturation spectroscopy. In this approach, the laser beam is split into two beams, one strong and one weak. The two beams are introduced into a gas cell, traveling in opposite directions. The monochromatic laser beam, tuned to a specific wavelength, interacts only with a small fraction of the molecules. It interacts only with those molecules that happen to be moving with a velocity such that their absorption is at the same wavelength as the light. The stronger beam can saturate the absorption, that is, it can interact with all the atoms or molecules in the narrow portion of the Doppler linewidth corresponding to the laser wavelength. Thus, the absorbing atoms or molecules are depleted, and the absorption is weakened in this portion of the absorption line. The stronger beam thus bleaches a path for the weaker probing beam. The strong beam is chopped with a modulator. If the two beams are interacting with the same molecules, then the probe beam will be modulated also. A detector viewing the probe beam sees the modulation. The modulated signal is detected only when the laser is tuned to the center of the absorption line, so that the light is absorbed by molecules having zero component of velocity in the direction of the laser beam. For those molecules, neither beam is Doppler shifted and the two beams are in resonance with the same molecules. In this way, the effects of Doppler broadening are eliminated. Figure 20-1, in the bottom portion, has already shown how the resolution of a spectroscopic measurement may be increased by use of saturation spectroscopy. As another example, Figure 20-3 shows the hyperfine structure of a line of molecular iodine-127. The measurements were made with an argon laser that could be tuned over a very small range by a piezoelectric drive on one mirror. The Doppler width is shown for comparison. In conventional absorption spectroscopy, none of the detail on a scale narrower than the Doppler width would have been observable. The figure shows dramatically the ability of saturation spectroscopy to overcome the broadening introduced by the Doppler effect. Another variation of Doppler-free spectroscopy is polarization spectroscopy, which developed later than saturation spectroscopy. Polarization spectroscopy yields greater sensitivity than saturation spectroscopy in the sense that it is possible to work with fewer molecules. The apparatus for polarization spectroscopy is similar to that for saturation spectroscopy, but the sample is between crossed polarizers, so that the background light is reduced to near zero. Because the probability for absorbing polarized light depends on the orientation of the absorbing molecule, the strong beam preferentially depletes the population of molecules with a specific orientation. Then, in its interactions with the oriented molecules, the weaker probe beam acquires a component of polarization that passes through the second polarizer. A detector then sees an increase of light intensity against a zero background. As with saturation spectroscopy, this method requires that both
C. Applications of Laser Spectroscopy
499
Doppler Width '
1
I
.... t
I
~1
I
I
_.!_
0
100 200 300 400 500
I
-700 --600 -500 -400 -300-200-100
I
I
1
Av (MHz) Figure 20-3 Hyperfine structure of an absorption line of 12712,probed by saturation spectroscopy. The frequency shift from line center is denoted A v. (From A. L. Schawlow, in Fundamental and Applied Laser Physics (M. S. Feld, A. Javan, and N. A. Kurnit, eds.), Wiley, New York, 1973.)
beams interact with the same molecules near the center of the Doppler-broadened line.
C.
Applications of Laser Spectroscopy
The unique properties of laser light, particularly its monochromaticity and high radiance, have made possible many advances in spectroscopic applications. Lasers provide greatly increased resolution in conventional spectroscopic measurements and also allow many innovative new spectroscopic techniques. Laser spectroscopy has become a very broad and varied subject, with many, diverse applications. It is not possible to describe all the many applications in chemistry, physics, medicine, life sciences, forensic science, and so forth that are made possible by lasers. We will simply present some examples.
500
1.
20. Laser Applications in Spectroscopy
CHEMICAL APPLICATIONS
Laser spectroscopy provides chemists with an extremely versatile tool for analytical spectrochemistry, for probing the dynamic behavior of chemical reactions, and for determining molecular structure and energy levels. One significant application made possible by lasers is the technique of picosecond and subpicosecond spectroscopy, which is used to study ultrafast kinetics of chemical reactions. A number of tunable lasers, such as liquid dye lasers, can emit extremely short pulses with durations in the picosecond, or even femtosecond (10 -15 sec), regimes. The availability of such lasers permits many new applications in the study of transient chemical phenomena. Examples include determination of the lifetimes of excited states and measurements of relaxation processes in atoms and molecules with extremely high temporal resolution. Chemical kinetic processes can be monitored in a fashion that was not previously possible. Processes taking place on a picosecond time scale include vibrational and orientational relaxation of molecules in liquids, radiationless transitions of electronically excited large molecules, solvation of photoejected free electrons, and isomerization of photoexcited states of complex molecules like visual pigments, We present one example that will illustrate some of the techniques and serve to demonstrate the capabilities of picosecond spectroscopy. The experiment [1] involved the formation and decay of a biophysical compound called prelumirhodopsin, which is considered to be the initial step in photoinduced chemical changes that begin the chain of physiological processes constituting vision. Vision begins when rhodopsin, a photosensitive pigment in the retina, is bleached by light. When rhodopsin in the eye is exposed to light, prelumirhodopsin is formed faster than can be followed by conventional methods. The experiment involved photoexcitation of rhodopsin by a picosecond pulse of light at a wavelength of 530 nm. The absorption band of prelumirhodopsin at 560 nm was monitored by a train of short pulses at that wavelength. The original laser was a mode-locked Nd:glass laser, from which one pulse of 6 psec duration was extracted. This pulse was amplified and frequency-doubled in a potassium dihydrogen phosphate crystal to yield light at 530 nm wavelength. Part of that light was used to excite the rhodopsin. Another part was split off and Raman scattered to yield pulses of 560 nm light. This light was reflected from an echelon, a stepped optical element that introduced different optical delays into different spatial parts of the wavefront. Thus, the 560 nm light was transformed into a series of short interrogating pulses to monitor the desired absorption band. These pulses were focused onto the same area of the sample as the 530 nm exciting light. After emerging from the sample, the 560 nm light was imaged onto a camera. Light from each optical segment of the echelon was imaged on a different area of the film. Because the light from each element had a unique incremental optical path, the temporal pulse separation (about 20 psec) was recorded as a spatial separation. The results of the measurement showed that the rise time of the prelumirhodopsin was less than 6 psec after the start of the 530 nm exciting pulse. The data indicated that production of prelumirhodopsin is the primary photochemical event.
C. Applications of Laser Spectroscopy
501
Probing time-resolved chemical processes by means of picosecond spectroscopy has been the subject of extensive research, which has led to significant new knowledge about mechanisms of chemical reactions, energy distributions, and relaxation processes in molecules and structures of the excited states of biological systems. 2.
REMOTE SENSING AND ENVIRONMENTAL MONITORING
There is a need for improved, accurate methods for remote measurement of the concentration of pollutants in the atmosphere. Many different types of pollutants are present in the atmosphere. Some of the major pollutants of interest include oxides of nitrogen, carbon monoxide, sulfur dioxide, and ozone. In addition, there are various types of particulate materials, such as ash, dust, and soot. There are also requirements to monitor sources of pollution, in order to comply with governmental regulations. Moreover, there are needs for mapping, on a global scale, constituents such as carbon dioxide and water vapor. Many problems are associated with measurement of concentrations of pollutants in the atmosphere. Conventional measurements are difficult. For example, if one desires to measure gases from a smokestack, there are problems involving the inhospitable environment inside the smokestack, problems with accessibility, and problems involving perturbing the quantities that are to be measured. Traditional methods have involved collecting a sample of gas from the smokestack and making a chemical or spectroscopic analysis. Such methods are conducted remotely, and results are not immediately available. Stratification of gases and particulate materials also could lead to error. Sensors embedded in the smokestack have used the electrochemical properties of the gases as a sensing mechanism. These sensors require contact with the gas sample and, for the most part, have slower response than desired. Such devices have had problems with maintenance because of the hot, corrosive gases that contact them. Although there have been numerous approaches to monitoring smokestack emissions, no completely satisfactory solutions have yet been developed. The problem becomes even more severe when one considers monitoring the concentrations of specific gases remotely over broad areas. Approaches based on optical technology appear attractive for areal mapping. Fourier transform infrared spectroscopy (FTIR) based on nonlaser sources has been employed for this application. There are commercial FTIR monitoring instruments available. Laser technology offers attractive features for remote monitoring of atmospheric quantities. It is possible to perform measurements at great distances. The measurements are completely noncontact and involve no sample collection nor chemical processing. There is no disturbance of the quantities to be measured. Generally, the results of the measurement can be available immediately. Tunable semiconductor laser diodes with very narrow spectral linewidth offer many attractive opportunities for remote noncontact spectroscopic sensing of gaseous species. The use of laser spectroscopy for detection of pollutant gases has been studied for a number of years. The emphasis in early work has been on relatively long wavelength infrared spectroscopy, using lasers operating in the 5-10/xm
502
20. Laser Applications in Spectroscopy
wavelength range. The lasers have been tunable lead salt diode lasers, such as lead selenide sulfide. Such lasers are expensive, operate at cryogenic temperatures, and require the use of expensive cryogenic detectors. Operation in the long wavelength infrared does offer the advantage that the molecular absorption lines are very strong for gases of interest. A number of approaches for laser-based remote sensing have been studied over several decades. These include 9 Optical radar (lidar) 9 Raman backscattering 9 Resonance fluorescence 9 Absorption methods, particularly differential absorption laser backscatter (DIAL) We shall discuss each of these approaches briefly.
1.
Optical Radar
Optical radar, also called lidar, which stands for light detection and ranging, operates similarly to microwave radar. In its basic form, it employs a pulsed laser, the beam of which is directed to the atmospheric sample that is to be probed. Energy backscattered by the atmosphere is sensed by a photodetector. The resulting signal is processed as a function of time from the transmission of the pulse, just as in a radar ranging system. Generally, lidar systems have relied on Mie scattering, due to particulate material and aerosols, to provide the backscattered light. Thus, lidar is most useful for determining concentrations of particulate material, and it gives no information about gas concentrations. Lidar has most often been used for measurements in which concentrations of particulates or aerosols are desired. It can determine concentration as a function of distance from the measurement position. Figure 20-4 presents what is perhaps a typical example of a lidar measurement. The results are derived from laser backscattering from the emission of an 800-foothigh smokestack. The figure shows the range-corrected signal in decibels relative to that from the ambient background aerosols. The contours represent relative particulate concentration in a vertical cross section of the emission from the smokestack. The example shows clearly how lidar can provide information about particulate concentration that would otherwise be difficult to obtain. The measurement may easily be performed remotely. Lidar can also measure quantities like stratification, flow, and changing profiles of turbid layers in the atmosphere.
2.
Raman Backscattering
The use of the Raman effect, which involves scattering of light by molecules of gases with a shift in the wavelength of the scattered light, has been discussed previously in terms of its spectroscopic applications. It also has application to remote sensing, particularly for identification of gaseous species. Because the values of the
C. Applications of Laser Spectroscopy
~1000 f
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Figure 20-4 Vertical cross section through a smoke plume from a smokestack, as obtained by lidar measurements. The contours represent relative particulate concentrations. (From W. B. Johnson, J. Air Pollution Contr. Assoc. 19, 176 (1969).)
wavelength shift are characteristic of individual molecules, examination of the spectrum of the light backscattered from a remote sample of gas can provide information about the gases present in the sample. Figure 20-5 presents an example of the spectral composition of the backscattered return from an oil-smoke plume emitted by a smokestack at a distance around 30 m. The figure shows the strength of the return signal over a range of wavelengths. A number of pollutants, such as SO 2 and H2S, are identified. Also, the major constituents of the atmosphere, oxygen and nitrogen, appear as relatively strong signals. Raman-based remote sensing requires the use of visible or near ultraviolet lasers. Use of the Raman effect with a pulsed laser also offers range discrimination, so that relative pollutant concentration as a function of distance can be obtained. But because the cross section for Raman scattering is small, the laser used must have high values of peak power. This requirement leads to problems with eye safety. Also, Raman methods are mainly useful for measuring relatively high concentrations and at relatively short range. Although there have been many demonstrations of Ramanbased remote sensing over a long period, the use of Raman sensing has not been adopted for routine monitoring use.
3.
Resonance Fluorescence
In resonance fluorescence, the beam is directed to a gaseous sample that contains molecules that fluoresce when excited with the proper wavelength. An example is
504
20. Laser Applications in Spectroscopy
N2
C2H4
C2H4
l
I
350
....
I
360
,
i
370 Wavelength (nm)
I
380
Figure 20-5 Spectral distribution of the Raman-shifted signal from a variety of molecular gases in a remote laser Raman measurement. (From H. Inaba and T. Kobayasi, OptoElectron. 4, 101 (1972).)
NO2, which fluoresces when excited by blue light, such as 488 nm light from an argon laser. The fluorescence is collected by a telescope and delivered to a detector. An example showing results of laser detection of NO 2, using an argon laser on samples collected in Los Angeles on a smoggy day, is presented in Figure 20-6. The concentration of NO 2 varies as a function of the time of day, with peaks near the times of peak automobile traffic. The rapidity of the changes shows the desirability of real-time monitoring with fast response. In addition to use for atmospheric monitoring, fluorescence techniques have been used for measurements underground, to determine the presence of pollutants in groundwater and in sites where toxic wastes may be present. Test holes may be drilled, or a sensing element may be driven into the ground in a cone penetrometer. Such devices can be rammed tens of feet into the ground in suitable soils. A fiber optic cable is used to deliver laser light to the underground region being tested. Fluo-
C. Applications of Laser Spectroscopy
505
0.10
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0
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6
8
10
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2
4
6
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Pacific S t a n d a r d T i m e
Figure 20-6 Nitrogen dioxide concentration measured as a function of local time during a smoggy period in the Los Angeles basin. The data were obtained using a resonance fluorescence technique. (From J. A. Gelbwachs et al., Opto-Electron. 4, 155 (1972).)
rescence emitted by materials in the ground is collected and delivered back through the fiber optic cable to a detector at the surface. The presence of specific chemicals is revealed by the spectrum of the fluorescence. In one example, an excimer laser was used with a Raman shifter to produce light at 248, 276, and 317 nm. The light was delivered through optical fibers into boreholes near a leaking gas pump. The fluorescence from hydrocarbons in the soil was returned to the surface by the fiber and detected with a monochromator and a photomultiplier. The presence of diesel fuel in the soil was detected, and the concentrations could be readily mapped. Figure 20-7 shows experimental results for diesel fuel contamination in a vertical layer of soil near the pump. The results demonstrate the usefulness of laser techniques for in situ monitoring of subsurface contaminants. The system could distinguish contaminants such as fuels, benzene, toluene, xylene, and polycyclic aromatic hydrocarbons. The status of this type of measurement is still developmental, but a number of demonstrations have been performed. The technique offers the advantage of realtime in situ determination of the presence of underground pollutants, without the need for chemical processing.
506
20. Laser Applications in Spectroscopy
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2 Figure 20-7 Profile of diesel fuel contamination in a vertical soil layer near a leaking gas pump. (From J. Bublitz et al., Appl. Opt. 34, 3223 (1995).)
4.
Absorption Methods
Absorption methods provide a sensitive class of methods of detection for specific gases. The usual method of absorption spectroscopy involves transmitting the beam through the sample. One requires a two-ended system, with a detector on the opposite side of the sample from the laser source. This may be inconvenient for remote monitoring. One possibility for relieving this difficulty is the use of a remote topographic reflector. One may use reflection from some feature present in the countryside, such as a large building or a hill, to direct light back to the position of the source. This can lead to a single-ended system in which the transmitter and receiver are located at the same position. This does, however, require that a suitable scattering feature be available. Another variation of absorption spectroscopy for remote gas monitoring involves differential optical absorption spectroscopy (DOAS). DOAS relies on determining
C. Applications of Laser Spectroscopy
507
the differences between local maxima and minima in the absorption spectrum of the gas being probed. DOAS seems to have been used mostly in the ultraviolet region of the spectrum. It requires a source that can cover a relatively large spectral range. DOAS has been demonstrated using laser sources, but it probably has been used more with broadband sources, like high-pressure xenon lamps. With any of the foregoing absorption techniques for remote sensing, one obtains measurements of concentration integrated over the length of the absorbing path. The use of differential absorption laser (DIAL) backscattering techniques removes this difficulty and provides range resolution in a single-ended system. It also preserves the high sensitivity of absorption measurements and the specificity for particular gases. DIAL spectroscopy uses ambient atmospheric particulates and molecules as a distributed reflector to provide a return signal. The laser is tuned alternately on and off a characteristic molecular absorption peak to provide sensitivity for a specific gas. The gas concentration is determined from the relative strength of the return signal for on-peak and off-peak operation. The essential feature of the DIAL technique is that it uses Rayleigh and Mie scattering in the atmosphere to provide reflection of light back to the position of the source, so that no physical reflector is needed. The tuning of the source wavelength on and off the peak of the absorption line provides high sensitivity and makes the method self-calibrating. If a short laser pulse is used, monitoring the return as a function of time after the pulse provides range resolution. DIAL spectroscopy appears to be the leading candidate for laser-based remote monitoring of the environment. There have been many demonstrations of its use for remote sensing of a wide variety of materials. Some commercial DIAL systems are becoming available. Figure 20-8 illustrates mapping of the distribution of SO 2 in a horizontal plane over the area of a zinc works, as determined using DIAL spectroscopy. The laser was located at the coordinate origin, off the right-hand side of the diagram. The measurements were performed using a Nd:YAG pumped dye laser as the source. The laser was tuned between the wavelengths of 296.17 and 297.35 nm to provide on-peak and off-peak conditions. The plume of SO 2, traveling with the wind, is mapped with a claimed uncertainty around 2 percent. As another example, NASA has used an airborne ultraviolet DIAL system to monitor ozone concentrations [2]. The system uses two dye lasers pumped by Nd:YAG lasers, to generate 301 nm light for the on-peak probe and 311 nm light for the off-peak probe. The system has been flown on many missions to map the concentrations of ozone in the troposphere and stratosphere around the world, particularly the depletion of ozone in the arctic regions. Mobile DIAL spectroscopy systems have been developed, suitable for remote monitoring of pollution at changing sites. DIAL spectroscopy has become an established technique for atmospheric monitoring. It is used for applications like industrial site inspection, monitoring of waste cleanup, enforcement of air quality regulations, and environmental research.
508
20. Laser Applications in Spectroscopy
150
4000
direction
9
lai, ai,a[,l,,[,i
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1000
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m
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1000 i~g/m 3 -50 -600
I
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-500
i
I
-400
I
I
-300
i
i
-200
Distance in m
Figure 20-8 Distributionof S O 2 in a horizontal plane over the area of a zinc works, as determined with DIAL spectroscopy. (From U.-B. Goers, Laser remote sensing of sulfur dioxide and ozone with the mobile differential absorption lidar ARGOS, Opt. Eng. 34, 3097 (1995).) 5.
Summary
There have been many demonstrations of the application of lasers to remote monitoring of atmospheric parameters. Many different methods have been employed. Table 20-2 compares the features of the different methods we have described. Some commercial systems for laser monitoring of the environment have become available. Models that are being offered make use of a variety of laser types, including Nd:YAG (both 1064 and 532 nm devices), excimer, tunable dye, tunable semiconductor, and CO 2 lasers. Systems have been designed for a number of specific applications, including measurement of smokestack opacity, monitoring of ozone concentration, leak detection, and cloud mapping. Although commercial laser-based environmental monitoring equipment is still in a relatively early stage, it appears to be developing steadily. Remote sensing of atmospheric parameters involves directing a laser beam into the atmosphere. Issues of laser eye safety become important. The requirements for eye safety may limit the laser power that can be used in a probing beam. The relatively low values for maximum permissible eye exposure in the visible and ultraviolet may restrict the use of high-power pulsed lasers in these spectral regions. The optimum wavelength from the point of eye safety would be in the infrared, at wavelengths greater than 1.5 ~m. The widespread application of laser technology for remote atmospheric monitoring will have to be compatible with public safety. Although many workers have been active in this field for a number of years and have shown the capability for obtaining remote measurements of air pollutants and
Selected Additional References Table 20-2
Comparison of Laser-Based Remote Monitoring Methods
Method Optical radar Raman scattering Resonance fluorescence Absorption Transmission Topographic
DOAS DIAL
509
Identification of gases
Range resolution
Ease
Sensitivity
No Yes Yes
Yes Yes Yes
Single-ended Single-ended Single-ended
Moderate Low Moderate
Yes Yes
No No
High High
Yes Yes
No Yes
Double-ended Single-ended, but needs reflecting feature Double-ended Single-ended
High High
other atmospheric constituents, this technology is still largely in development. It has been useful as a research tool. Global scale measurements by NASA have provided useful information about atmospheric composition. Concentrations of specific gases around specific industrial sites have been derived. The amount of work being performed in this area indicates that laser-based remote sensing of atmospheric quantities will continue to grow.
References [1] G.E. Busch et al., Proc. Nat. Acad. Sci. 69, 2802 (1972). [2] E.V. Browell, Proc. IEEE 77, 419 (1989).
Selected Additional References R. M. Measures, ed., Laser Remote Chemical Analysis, Wiley, New York, 1988. E. R. Menzel, Laser Spectroscopy: Techniques and Applications, Marcel Dekker, New York, 1994. Optical Sensing for Environmental Monitoring, Proceedings of an International Specialty Conference, Atlanta, GA, October 11-14, 1993, Air and Waste Management Assn., Pittsburgh, PA, 1994. L. J. Radziemski, R. W. Solarz, and J. A. Paisner, eds., Laser Spectroscopy and Its Applications, Marcel Dekker, New York, 1986.