Photoacoustic Spectroscopy

Chapter 13

Photoacoustic Spectroscopy: Applications in Security and Biology Surya N. Thakur Benares Hindu University, Varanasi, India

Chapter Outline 1. Introduction 1.1 The Photophone 1.2 The Spectrophone 2. Radiative and Nonradiative Transitions in Molecules 2.1 Production of Photoacoustic Signal in Gases 2.2 Production of Photoacoustic Signal in Solids 3. Experimental Methods in Photoacoustic Spectroscopy 3.1 Photoacoustic Cells for Static Gas Samples 3.2 Photoacoustic Cell for Flowing Gaseous Samples 3.3 Photoacoustic Cells for Solid Samples 3.3.1 Helmholtz Resonant Photoacoustic Cell for Solid Samples 3.4 Piezoelectric Transducers for Photoacoustic Detection 3.5 Quartz Tuning Fork for Photoacoustic Detection 4. Photoacoustic Detection of Harmful Chemicals 4.1 Photoacoustic Detection of Gaseous Molecules 4.1.1 Photoacoustic Spectroscopy of Methanol and Ethanol 4.1.2 Photoacoustic Spectroscopy of Ethylene

283 284 284 285 286 288 289 290 291 293 294 294 295 297 298 298 298

4.1.3 Photoacoustic Spectroscopy of Aerosols 4.1.4 Photocoustic Spectroscopy of Ozone 4.1.5 Photoacoustic Spectroscopy of Gases Emanating From Human Body 4.1.6 Photoacoustic Spectroscopy of Improvised Explosive Device Precursors in Vapor Phase 4.2 Photoacoustic Detection of Condensed Matter 4.2.1 Photoacoustic Spectroscopy of Dangerous Drugs 4.2.2 Photoacoustic Spectroscopy of Explosives 4.2.3 Photoacoustic Spectroscopy of Contaminated Water 5. Hyperspectral Imaging and Photoacoustic Spectroscopy 5.1 Photoacoustic Imaging 5.2 Photoacoustic Tomography 5.3 Combined Photoacoustic TomographyeOptical Coherence Tomography Imaging 6. Conclusion Acknowledgments References

300 301 302 303 303 304 304 307 308 310 311 314 314 314 314

1. INTRODUCTION The Laser Revolution in Spectroscopy, which started in early 1960s, has greatly enriched the science of atoms and molecules both by adding novel spectroscopic techniques and reviving some old ones. During a conference on “Fundamental and Applied Laser Physics” in Iran in 1971, the eminent Raman spectroscopist B.P. Stoicheff had predicted a great upsurge of laser spectroscopy techniques in the infrared (IR) and visible regions in view of the revolution that had been witnessed in Raman spectroscopy [1]. The same year Kreuzer and Patel used tunable IR laser in photoacoustic spectroscopy (PAS) for detecting atmospheric pollutants [2]. However, unlike Raman spectroscopy where molecules inelastically scatter photons to reveal their spectral features recorded by optical detectors, in PAS the radiation absorbed by molecules is converted into thermal motion of the gas by intermolecular collisions, which produce a pressure rise to be detected by a sensitive microphone. This technique, based on the photoacoustic (PA) effect, first discovered in solids by Graham Bell in 1880 had remained dormant for almost 100 years [3].

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1.1 The Photophone PAS involves measurements of optical absorption by materials, in condensed as well as vapor phase, by its subsequent conversion into heat and sound due to nonradiative transitions in atomic and molecular species. The origin of this highly sensitive technique is historically related to the 19th-century experiments of Graham Bell in sending sound waves over a beam of light, which he called a “photophone.” He used the newly discovered selenium cell in the receiver in view of selenium’s property to react to modulated intensity of sunlight incident on it, as the resistance of selenium crystal depends on the incident light. His assistant, Sumner Tainter, used a flexible mirror, at the speaking end of the photophone, that would bend and vibrate from the sound waves to alter the light, creating a fluctuating beam of light capable of being received from greater distances. The selenium receiver would then act like an optical version of the electric coil in a telephone receiver, converting the modulated light back into sound waves. The photophone was found to carry the sound waves up to 200 m using a parabolic mirror. Thus Bell succeeded in wireless audio communication about two decades before the first radio transmission. Bell was very proud of his invention and proclaimed, “In the importance of the principles involved, I regard the photophone as the greatest invention I have ever made, greater than the telephone.” Writing to his father Bell said: “I have heard articulate speech by sunlight! I have heard a ray of the sun laugh, cough and sing! I have been able to hear a shadow and I have even perceived by ear the passage of a cloud across the sun’s disc.” Bell was a very curious person and he observed that sound waves were also produced directly from a solid sample when exposed to a periodically modulated beam of sunlight. He used a hearing tube whose other end was tightly attached to the open end of a transparent glass test tube with the solid material placed at its closed end. When a beam of sunlight focused on the sample was rapidly interrupted with a rotating slotted wheel at an audible frequency, he noticed that the intensity of sound in the hearing tube was dependent on the type of material. He used cotton wool of different colors in addition to solids as the sample and found that the loudness of sound increased for fibrous samples as compared to the compact ones. The loudest sound was heard when the sample was lampblack (carbon black), and Bell was correctly led to the conclusion that the PA effect was caused by the absorbed light energy, which subsequently heats the sample. It was, however, rather incorrectly concluded that fibrous samples with air pockets trapped inside them led to larger volume expansion compared to compact samples and thus led to louder sound generation. A comparative diagram of Bell’s photophone and its modern version is shown in Fig. 13.1.

1.2 The Spectrophone Bell visited Europe in 1880 to share his experimental findings with the eminent physicists of that era, where John Tyndall performed the PA experiment with gases in presence of Bell in England on November 29, 1880. He used a Siemen’s lamp as the source of radiation, and a glass lens was used to concentrate the rays which afterward passed through two more lenses. The first lens rendered the rays parallel, while the second caused them to converge to a point about 20 cm from this lens. A circular sheet of zinc, with radial slits cut through it, was mounted to rotate rapidly across

FIGURE 13.1 (A) Bell’s photophone receiver with hearing tube fitted on the open end of the air-filled test tube containing light-absorbing sample. (B) Modern version of the photophone where a sensitive microphone converts acoustic waves into electrical signal.

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FIGURE 13.2 Alexander Graham Bell and his spectrophone for listening to the intensity variation of sound generated by optical absorption by the sample at different wavelengths.

the light beam near the focus. The passage of light through the rotating slits provided the desired intermittence of the focal spot inside a glass flask containing the gas or vapor to be studied. A tube of India rubber from the mouth of the flask was connected to the thinner end of a conical tube of wood or ivory to form the listening device (see Fig. 13.1A). This experimental setup demonstrated generation of sound in a number of gases whose tone (frequency) was dependent on the rate of rotation of the circular zinc sheet. Although the PA effect was confirmed by Tyndall, he was of the view that it was caused mainly by the radiant heat [4]. Bell could have been happy and satisfied with his discovery of the PA effect; however, he was driven by rare intellectual curiosity to learn and it led him to invent the spectrophone [5]. He wanted to find out the wavelengths in the solar spectrum that were more efficient for the radiant heat, which according to Tyndall caused the PA effect. For this purpose he converted a prism spectroscope into a spectrophone. The eyepiece of the telescope was replaced by a hearing tube where a thin wire mesh coated with lampblack was fitted in the position of the cross wires and connected to the hearing tube (see Fig. 13.2). The sunlight was focused on the entrance slit of the collimator through a mechanical chopper for its periodic intermittence, and observations were made by fixing the position of the telescope in different spectral regions of the solar spectrum. It was found that the loudness of sound was proportional to the spectral intensity. Because lampblack completely absorbs radiation of every wavelength in the visible region, this observation confirmed the fact that the PA effect was due to optical absorption. To investigate the spectral absorption of different substances, similar coatings were put in the focal plane and their audibility was detected by the hearing tube. Bell made the following comment about the importance of his spectrophone, “I recognize the fact that spectrophone must ever remain a mere adjunct to the spectroscope: but I anticipate that it has a wide and independent field of usefulness in the investigation of absorption spectra in the ultra-red.” The process of sound generation can be described in terms of three simple steps: (1) the absorbed energy of pulsed or modulated optical radiation is converted into heat; (2) the temperature at the site of optical absorption rises when radiation is absorbed and falls when the radiation stops; and (3) the expansion and contraction following these temperature changes lead to periodic pressure variation and generate sound. However, the temperature and pressure changes involved in the process are extremely smalldtypically a micro- to a millidegrees and nano- to microbars. This was the reason that the field of PAS remained dormant till the advent of tunable laser sources and sensitive audio detectors. In recent times this technique has come into great prominence for homeland and defense applications in rapid detection of hazardous materials as well as in the field of medicine for analysis of biological matter.

2. RADIATIVE AND NONRADIATIVE TRANSITIONS IN MOLECULES The heat generation on optical absorption is caused by internal motions in molecules and those of the matrix in which atoms are imbedded in condensed matter. These internal motions convert part or whole of the absorbed optical energy into heat. According to the quantum mechanical description, the excited states of atoms and molecules, reached by the optical absorption, have two channels of relaxation. The radiative decay leads to optical emission by the excited species, whereas the nonradiative decay causes heat generation. A molecule optically excited to a vibronic or rovibrational state, loses a part of its excitation energy as heat by nonradiative transition. The PA spectrum is similar to the absorption spectrum, but its intensity at the exciting wavelength is proportional to the product of the absorption cross section and the probability of nonradiative decay of the excited state. We will illustrate the mechanism of nonradiative transitions in molecules undergoing optical absorption with the help of Fig. 13.3. The energy level diagram of a typical organic molecule is shown in the Born-Oppenheimer approximation. The

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FIGURE 13.3 Radiative and nonradiative transitions in an organic molecule, where S and T stand for singlet and triplet electronic states, respectively, with the subscript representing the degree of excitation. Bold horizontal lines correspond to the electronic energy and lighter ones to vibrational levels associated with them. Radiative processes A, F, and P stand for absorption, fluorescence, and phosphorescence, respectively. The nonradiative processes IC and ISC refer to internal conversion and intersystem crossing, respectively.

total internal energy of the molecule is the sum of its electronic, vibrational, and rotational energies (and can be written as E ¼ Ee þ Ev þ Er) by neglecting interactions among the three types of motions. The rotational energy levels are not shown in Fig. 13.3 to avoid the complexity due to congestion resulting from such levels associated with each of the vibronic energy levels. A typical organic molecule has even number of electrons, and the ground electronic state is a singlet state (S0). When one of the electrons from the outermost filled orbital is excited to the ith vacant higher energy orbital, its spin may remain unchanged or reverse its orientation resulting in a singlet (Si) or a triplet (Ti) electronic state, respectively. For a molecule containing N atoms, there are 3N6 or 3N5 different modes of vibrational motion depending on whether the molecular geometry is nonlinear or linear, respectively. In Fig. 13.3 we show vibronic levels associated with only one of the normal modes, and these levels associated with different electronic states are shown in the harmonic approximation. Only the vibronic transitions are illustrated in Fig. 13.3 but it is to be remembered that a vast number of rotational lines result from each vibronic transition due to transitions among the rovibronic levels. In large molecules such as benzene, nonradiative decay from optically excited states occurs even in the vapor phase. Michael Kasha pioneered the investigations on nonradiative transitions in molecules, and on the basis of his extensive work, it was found that the emitting state of any particular multiplicity is the lowest excited state of that multiplicity. This is known as the famous Kasha’s rule, which determines the energetics of electronic transitions in molecules [6]. The nonradiative transfer of molecules from an excited state to a lower state of same multiplicity is known as internal conversion and that between a singlet and a triplet state is called intersystem crossing (ISC). Lewis and Kasha formulated a fundamental law concerning fluorescence and phosphorescence through ISC [7]. They were the first to conclude that phosphorescent state in all molecules, simple or complex, organic or inorganic, is the triplet state.

2.1 Production of Photoacoustic Signal in Gases Let us consider two quantum states E0 and E1 of a molecule connected with radiative (rij) and nonradiative (collisional) transitions (cij) between them. The radiative transition rate rij is the sum of the Einstein coefficients for stimulated (Bij) and spontaneous (Aij) emission of the form rij ¼ rn Bij þ Aij

(13.1)

where Bij ¼ Bji and rn is the spectral energy density at the frequency of the transition between E0 and E1. For the two-level model of the molecule, we have B01 ¼ B10 and A01 ¼ 0 because spontaneous emission from a state of lower energy to that of higher energy does not occur. We also assume that the probability of a collisional excitation from E0 to E1 is very small, approximately zero, so that c01 ¼ 0.

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FIGURE 13.4 Two-level molecular model showing rate of radiative (rij) and nonradiative (cij) transitions between the energy states E0 and E1.

To determine the rate of molecular density change in our two-level system of Fig. 13.4, we assume the population densities of absorbing molecules in the ground and excited states to be n0 and n1, respectively. The rate of change of upper-state population depends on the number of molecules entering and leaving the excited state: dn1 =dt ¼ ðr01 þ c01 Þn0  ðr10 þ c10 Þn1 ¼ rn B01 n0  ðrn B01 þ A10 þ c10 Þn1 ¼ rn B01 ðn0  n1 Þ  ðA10 þ c10 Þn1

(13.2)

We define the radiative and nonradiative lifetimes to be sr ¼ 1=A10 and sc ¼ 1=c10 , respectively, and the total lifetime to be sð1=s ¼ 1=sr þ 1=sc Þ, to get the following expression for the rate of change of excited state population: dn1 =dt ¼ rn B01 ðn0  n1 Þ  ð1=sr þ 1=sc Þn1 ¼ rn B01 ðn0  n1 Þ  n1 =s

(13.3)

In a similar manner we find the following expression for the rate of change of ground-state population: dn0 =dt ¼ rn B01 ðn1  n0 Þ þ n1 =s

(13.4)

dn1 =dt  dn0 =dt ¼ 2rn B01 ðn1  n0 Þ  2n1 =s

(13.5)

Hence,

In steady state (for PA experiment), the incident light intensity I may be assumed to vary slowly so that we consider the upper- and lower-state population density change an adiabatic interchange, and we can set the left-hand side of Eq. (13.5) to zero. Because the total molecular density for the two-level system is N ¼ n1 þ n0 , Eq. (13.5) takes the following form: 0 ¼ 2rn B01 ðN  n0  n0 Þ  2ðN  n0 Þ=s
 0 ¼ 4rn B01 n0  2N rn B01 þ s1 þ 2s1 n0

 n0 ¼ N rn B01 þ s1 2rn B01 þ s1 Similarly, n1 ¼ Nrn B01


 2rn B01 þ s1

(13.6)

(13.7)

The spectral radiant energy density is directly proportional to the intensity I of the light source, and we define the constant B ¼ rn B01 =I, so that Eq. (13.7) takes the following form: 
 n1 ¼ NBI 2BI þ s1 (13.8) Mechanical chopping of the light source at a frequency u can be expressed as the periodic turning “on” and “off” of the intensity by I ¼ I0 cosut for simplicity, and we get the following expression for the excited-state population density:  
(13.9) n1 ¼ NBI0 cosut 2BI0 cosut þ s1 If the gas is very weakly absorbing, we assume that most molecules are in state E0 or n0 >> n1. Hence, BI << 1/s so that Eq. (13.9) takes the following form: n1 ¼ sNBI0 cosut

(13.10)

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To quantify the coupling between absorption of light and generation of sound waves, we have to consider the total internal energy density U of our two-level gas model: U ¼ n1 E1 þ K

(13.11)

ðdU=dtÞ ¼ ðdn1 =dtÞE1 þ ðdK=dtÞ

(13.12)

where K is the kinetic energy. The rate of change of energy is given by

This change of energy is equal to the difference between the absorbed and radiated optical energy, so that ðdU=dtÞ ¼ ðr01 n0  r10 n1 ÞE1

(13.13)

Since A01 ¼ 0 and c01 ¼ 0, Eqs. (13.12) and (13.13) lead to the following relation in the light of Eqs. (13.1) and (13.2): ðdK=dtÞ ¼ c10 n1 E1

(13.14)

Thermodynamic evaluation of the change in the kinetic energy at constant volume gives dK ¼ ðvK=vTÞV dT þ ðvK=vVÞT dV (13.15) ¼ ðvK=vTÞV dT Since the specific heat capacity of a gas Cv ¼ ðvK=vTÞV , by integrating Eq. (13.15) we get K ¼ Cv T þ fðVÞ

(13.16)

where f(V) is a constant of integration. The pressure of the ideal gas P ¼ NkT, where N is the number density of molecules, k is the Boltzmann constant, and T is the temperature. Thus substituting for T from Eq. (13.16) in the expression for P, we get P ¼ Nk½K  fðVÞ=Cv

(13.17)

Because f(V) is a constant and pressure wave (sound) is given by vP=vt, we get the following relation from Eq. (13.17) in the light of Eq. (13.14): vP=vt ¼ ðNk=Cv ÞvK=vt (13.18) ¼ ðNk=Cv Þc10 n1 E1 Substituting for n1 from Eq. (13.10) and using the relation c10 ¼ 1/sc, Eq. (13.18) reduces to 
vP=vt ¼ kN2 E1 CV ðs=sc ÞBI0 cosut

(13.19)

Integrating Eq. (13.19) we get the following expression for the PA signal, which is detected by the sensitive microphone: 
PðtÞ ¼ kN2 E1 CV ðs=sc ÞBI0 sinut (13.20)

2.2 Production of Photoacoustic Signal in Solids Let us consider the cylindrical cell shown in Fig. 13.1B where the light-absorbing solid sample is surrounded by optically transparent gas (air) on the front side and surrounded by a backing material, which is a poor conductor of heat. The process of PA signal generation by the solid material is schematically illustrated in Fig. 13.5. Light of a particular wavelength, partially (in transparent material) or totally (in opaque material) absorbed by condensed matter, is converted into heat by nonradiative transitions. The acoustic signal produced in a gas-microphone PA cell is due to periodic heat flow from solid sample to the surrounding gas. The most widely used model by Rosencwaig and Gersho [8] for PA signal generation depends on the optical as well as thermal properties of the solid sample. It may be described in terms of four distinct stages consisting of light absorption (Fig. 13.5A), diffusion of heat produced by nonradiative transitions (Fig. 13.5B), transfer of heat to the solidegas interface (Fig. 13.5C), and the production of sound due to density waves in the gas (Fig. 13.5D). A thin layer of surrounding gas (about 1 mm for air at chopping rate of

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FIGURE 13.5 (A) Penetrations of photons in the solid sample before being absorbed, (B) production of heat and its diffusion into the solid from the sites of absorption, (C) heat transfer to the solidegas interface, and (D) production of density waves in the surrounding gas.

100 Hz) adjacent to the surface of the solid responds to the periodic heat flow from the solid. This layer may be imagined as a vibratory gas piston creating acoustic signal detected by the microphone (see Fig. 13.1B). The intensity of light of wavelength l transmitted through the thickness “x” of a solid material of absorption coefficient b is given by IðlÞ ¼ I0 ebx

(13.21)

where I0 is the incident intensity for wavelength l, and if u is the modulation frequency of the incident radiation, the temporal variation of I0 is given by I0 ðtÞ ¼ ð1=2ÞI0 ð1  cosutÞ

(13.22)

The heat diffusion equation has to be solved for the solid sample, the backing material and the gas but here we will consider that for the sample only, as given below:
2  2 v T vx ¼ ð1=aÞvT=vt  ðbsnr I0 =2kÞ expðbxÞð1  cosutÞ (13.23) where T is the temperature, “k” is the thermal conductivity of the solid, and a is its thermal diffusivity defined as a ¼ ðk=9cp Þ 9 is density of the solid and cp is its specific heat capacity. The thermal diffusion length m of the solid sample is defined as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi m ¼ 1=a ¼ ð2a=uÞ and a ¼ ðu=2aÞ

(13.24)

(13.25)

where “a” is called the thermal diffusion coefficient of the solid sample. According to Rosencwaig and Gersho [8] the periodic temperature variation, in the surrounding gas, due to heat transfer from the solid sample is given by Tðx; tÞ ¼ expðag xÞ ½q1 cosðut þ ag xÞ  q2 sinðut þ ag xÞ

(13.26)

where ag represents the thermal diffusion coefficient of the gas, and the complex amplitude of periodic temperature variation is given by q ¼ q1 þ iq2

(13.27)

The temperature variation in the gas dies out within a thickness of 2pmg from the surface of the solid sample where mg is thermal diffusion length of the gas. This forms the gas piston, creating the PA signal, as illustrated in Fig. 13.5D. It is to be noted that the optical absorption length in the solid sample is mb ¼ 1/b and its thermal diffusion length is m.

3. EXPERIMENTAL METHODS IN PHOTOACOUSTIC SPECTROSCOPY In principle, the whole range of electromagnetic wavelengths from microwaves up to X-rays can be used for investigating all kinds of materialsdgases and vapors, solids, liquids, gel, powder, nanomaterials, etc. Xenon arc lamps and lasers have been widely used in recording PA spectra, and the emergence of a variety of IR lasers during the past one decade has led to many spectroscopic applications in environmental science, medicine, homeland security, and biology, as well as optical absorption of nonconventional and highly scattering samples including nanoparticles. In laboratory-based investigations, PA cell fitted with a sensitive microphone or piezoelectric transducer forms the heart of the detection system. In field experiments and standoff detection, novel miniature devices including quartz crystal tuning forks have been used with great success. Because the optical absorption is first converted into heat, it is also possible to detect the density changes caused by temperature fluctuations in the sample such as the detection of thermal lens formation, using a laser probe. In this chapter, we will, however, discuss only those methods of PAS, which are based on the detection of acoustic vibrations.

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3.1 Photoacoustic Cells for Static Gas Samples The PA cell, for gaseous samples, is usually in the form of a cylindrical tube fitted with glass or quartz windows at its front and back ends. The acoustic signals generated by periodically chopped light can be significantly enhanced if the gas cell is made to be acoustically resonant at the chopping frequencies. Fig. 13.6A exhibits a pyrex tube 2.5 cm in diameter and 62.5 cm in length, fitted with quartz windows at Brewster’s angle, which can resonate at 335 and 669 Hz [9]. The ports for locating the microphone are at the positions of the three possible antinodes of the stationary acoustic waves formed in the cell (Fig. 13.6A). Two of these ports, not in use during the measurements, are sealed by means of O-rings and flat teflon discs. Acoustic isolation of the PA cell was achieved by mounting it in a wooden box filled with sand. PA measurements were carried out on iodine vapor at room temperature in presence of air at atmospheric pressure in the cell. Iodine vapor is known to have absorption in the yellow-green region of the visible spectrum, so parametric PA measurements were carried out using 514.5 nm argon laser radiation with a power output of 20 mW. Chopping frequency of the incident laser beam was varied between 27 and 1000 Hz, and the resulting PA signals are shown in Fig. 13.6BeD. The PA signal is found to be maximum at the middle port for the chopping frequency of 335 Hz and it is maximum at 669 Hz for the front and the rear ports. The measured ratio of PA signals at the front antinode and node in the middle is 7.0, and that for the rear antinode and the node is 6.0 in good agreement with a theoretically predicted value of 6.3 [10]. These measurements demonstrated the fact that acoustically resonant PA cells are very effective in measurements on vapors with extremely small absorption coefficients.

10

(B) PAS SIGNAL IN RELATIVE UNITS

(A) MICROPHONE LOCATION FRONT MIDDLE REAR

0

STANDING

W/WES

RESONANCE AT 335 Hz RESONANCE AT 669 Hz

5

1

20

10

1

20

103

(D)

10

PAS SIGNAL IN RELATIVE UNITS

PAS SIGNAL IN RELATIVE UNITS

(C)

40 60 80 102 CHOPPER FREQUENCY(Hz)

103 40 6080 102 CHOPPER FREQUENCY(Hz)

1

20

40 60 80 102 CHOPPER FREQUENCY(Hz)

103

FIGURE 13.6 (A) Longitudinally resonant photoacoustic (PA) cell with three ports for microphone located from front window at l/2 for 335 Hz and at l/4 and 3l/4 for 669 Hz chopper frequencies. (B) Resonant PA signal observed for 335 Hz. (C) and (D) Resonant signals observed for 669 Hz.

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FIGURE 13.7 Microphone detection of laser-induced photoacoustic signals from gaseous samples using an acoustically resonant cylindrical photoacoustic cell.

A typical experimental setup for gas-phase PA spectroscopy is schematically shown in Fig. 13.7. The length of the PA cell is acoustically resonant for a particular frequency of the mechanical chopper corresponding to stationary wave formation with a single antinode in the center of the cylindrical cell. The microphone is located near the cell close to the middle of the PA cell to maximize the acoustic signal. The PA cell is fitted with two side arms containing stopcocks for evacuation by a pump and for entry of the gaseous sample, respectively. The continuous-wave CO2 laser beam was attenuated to a few milliwatts before meeting the chopper, and its power through the fully evacuated cell was measured at each wavelength using a power meter at the exit end of the PA cell. The evacuated cell was then filled with the sample vapor corresponding to its room-temperature vapor pressure, and the stopcocks leading to the pump and sample reservoir were shut tight to acoustically isolate the PA cell. The PA signal from the microphone was preamplified before its processing and the output of the lock-in amplifier was manually recorded at each wavelength of the rotational lineetunable CO2 laser. The spectra were plotted after normalizing PA signal at each wavelength using the corresponding power meter readings. Characteristic vibrational bands of CH3OH and C2H5OH vapors were located in the 9.6 mm region only and those of C2HCl3 were found only in 10.6 mm region [11]. These data were found useful in estimating the relative concentrations of CH3OH and C2H5OH in a mixture of the two organic vapors. The results of a similar experiment for the PAS of iodine vapor are shown in Fig. 13.8. In this case the source of exciting radiation was an Nd-YAG-pumped tunable dye laser, and the PA signals were processed with a boxcar integrator. Dye laser pulses were of 7 ns duration, 0.05 nm bandwidth, and 2 mJ/pulse energy at a pulse repetition rate of 10 Hz. The PA cell was evacuated and filled with I2 vapor from a side arm containing solid iodine obtained from double distillation. The lowest limit of air pressure reached by our vacuum pump was 15 Torr, and the spectrum recorded under this condition is shown in the upper part of Fig. 13.8. Air was introduced into the PA cell from a side arm connected to the pump to fill it up to atmospheric pressure for recording the spectrum shown in the lower half of Fig. 13.8. It is seen from Fig. 13.8B that the relative intensity of vibronic bands in the low-pressure spectrum (15 Torr) monotonically increases in going up to the dissociation limit, where I2 splits into a ground-state (2P3/2) iodine atom and an excited-state (2P1/2) iodine atom, whereas that in the atmospheric pressure spectrum monotonically decreases after reaching a maximum value around 19,225 cm1(see Fig. 13.8A). This reduction of intensity, in the PA spectrum at atmospheric air pressure, has been explained in terms of energy transfer from excited (2P1/2) iodine atoms to O2 molecules following the dissociation of I2 [12].

3.2 Photoacoustic Cell for Flowing Gaseous Samples In PA spectroscopy applications relating to pollution monitoring, environmental studies, and aerosol detection, it is required to record the spectra when the air is passing through the PA cell. A PA cell for this kind of measurements has been described by Arnott et al. [13] and by Lewis [14], as schematically shown in Fig. 13.9. The stationary pattern of the acoustic wave has been indicated in Fig. 13.9, with the horizontal section of U-shaped cell of acoustic length l/2 and each of the two vertical sections, l/4 in length. Low-pressure nodes of the stationary wave exist at the two bent corners of the PA cell, whereas high-pressure antinodes are formed at the center of the horizontal section

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3.5

Dissociation limit (2P +2P ) 3/2 1/2

(B) 15 mm pressure

2.65

0 20297

(24,1)

(24,0)

(26,1)

(34,0)

0 5.3

(44,0)

PHOTOACOUSTIC INTENSITY (in arbitrary units)

Pre-dissociation

(A) Atm. pressure

19215

18133

VIBRONIC TRANSITION ENERGY (in cm-1) FIGURE 13.8 Photoacoustic spectra of I2 vapor in the presence of air at atmospheric pressure (A) and at 15 Torr pressure (B) in the wavelength region 492e552 nm (20,300e18,100 cm1). The little vertical downward arrow indicates the wavelength of exciting radiation that dissociates I2 into two iodine atoms (20,043 cm1).

FIGURE 13.9 The U-shaped resonant photoacoustic (PA) cell has a total length of one acoustic wavelength (l). The length of the horizontal section is one half of the acoustic wavelength (l/2) with an antinode at the center and two nodes at the corners. The two vertical ends of the cell are at the two antinodes of the stationary acoustic wave. The entrance and exit of laser beam as well as those of the air sample are located in the regions of low-pressure sections (nodes) of the stationary acoustic wave causing minimal disturbance to the PA signal.

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and at both ends of the vertical sections. The laser and sample inlet and outlet, in the PA resonant cell, reside at the nodes of the stationary acoustic wave to prevent pressure fluctuations in PA signals at the antinodes. The microphone detects the PA signal, whereas the piezoelectric disc is used to detect the resonance frequency of the PA cell and the resonator quality factor to calibrate the system. The center of the horizontal section, where heat is generated, and the two ends where microphone and piezoelectric signals are measured contain high pressure points of the standing wave, which are 180 degree out of phase. The flow rate of air through the PA cell is regulated by a special orifice on the outlet end before the sampling pump (see Fig. 13.9). This special orifice is a narrow hole that causes a pressure drop of at least 50% directly on the outlet side compared to pressure on the side of the PA cell. Flow rate through the PA cell is critical and remains constant, whereas the airflow velocity through the orifice reaches the speed of sound. Thus the special orifice blocks the noise from the pump entering into the PA cell and cluttering the PA signal. Because the air traveling through the special orifice is moving at the speed of sound, any sound from the pump traveling toward the PA cell, also at the same speed, meets a barrier in air moving in the opposite direction and goes no further. The PA spectrometer is the most important instrument to directly measure light absorption by airborne particulate matter (aerosols) over the entire range of sunlight entering the atmosphere. In an experimental setup using PA cell of Fig. 13.9, the particles suspended in air (aerosols) must have enough time to absorb radiation from the laser beam and then transfer the heat to the surrounding air before the next laser pulse arrives, to build stationary pressure wave in the PA cell. This PA cell can be used in conjunction with a single laser, or it can be used with two lasers of different wavelengths impacting the same gaseous sample. In the dual-wavelength excitation mode, the first laser beam is modulated at the resonance frequency “f” and the second laser beam is modulated at a slightly different frequency “f þ df.” It has been found that the PA signal from the second laser still gets adequate resonance enhancement for small values of “df,” a typical value being 5 Hz [14]. In a dual-wavelength mode of operation, Lewis et al. [15] have used two compact diode lasers operating at 405 and 870 nm, for assessment of spectral variations in aerosol optical properties from near-ultraviolet (UV) to near-IR wavelengths. These authors have measured light absorption and scattering simultaneously, at 405 and 870 nm, by smoke from the combustion of a variety of biomass fuel. These measurements strongly indicate that organic material is present in wood smoke. Thus, spectral variation in optical properties provides insight into the differentiation of aerosols from mobile or industrial sources against those from biomass burning.

3.3 Photoacoustic Cells for Solid Samples PA cells for recording spectra of solid samples, using gas-microphone detection system, may be acoustically resonant or acoustically nonresonant. A typical single beam nonresonant PA cell shown in Fig. 13.10 has been constructed from aluminum blocks for effective shielding from extraneous sound [9]. The cavities, for locating sample cuvette and the microphone with its preamplifier, are drilled into the main block. The aluminum plate, covering the top of the sample and microphone chambers, has double quartz windows in front of the sample for entry of the chopped light beam. The thickness of air film connecting the sample and the microphone is about 1 mm and its total volume is about 1 cc. The sample cuvettes are made from stainless steel with identical exterior to fit into the aluminum cavity but with different depths for accommodating different thicknesses of samples. Carbon black is used as the standard sample for recording the power spectrum of the excitation source to normalize the PA signals of samples under investigation. Cross sectional view

3

1 4cm 2

2

1cm

1 FIGURE 13.10 Design of a nonresonant phtoacoustic cell: (1) main body, (2) microphone chamber, and (3) sample cuvette.

294 Molecular and Laser Spectroscopy

3.3.1 Helmholtz Resonant Photoacoustic Cell for Solid Samples The Helmholtz resonant PA cell shown in Fig. 13.11 is similar to the nonresonant cell described in Section 3.3 with the major difference that the sample and microphone chambers, in this case, are connected by a 1-cm-long cylindrical channel whose diameter can be varied between 0.75 and 1.20 mm [9]. The volume of air above the sample (V1) is about 0.35 cc, whereas that above the microphone (V2) can be varied by means of an airtight opaque piston located in the top plate above the microphone chamber (see Fig. 13.11). The total volume of air in the two chambers is kept on the order of 1 cc for optimum sensitivity of the PA cell. The resonant frequency “f” of the PA cell is given by [16]: f ¼ ðcd=4Þ½ðV1 þ V2 Þ=pV1 V2 ð1 þ 0:85dÞ

1=2

(13.28)

where “c” is the velocity of sound in air and “d” is the diameter of the cylindrical channel connecting the two chambers. Carbon black was used as the sample to record the frequency response of the PA cell for three different diameters of the connecting channel as shown in Fig. 13.11. The observed and calculated resonance frequencies are given in Table 13.1, and the large differences (almost a factor of 2) between the two indicate a need for refinement of the theoretical model [16].

3.4 Piezoelectric Transducers for Photoacoustic Detection Piezoelectric transducers have been widely used in PAS of liquid and solid samples. The acoustic voltage developed in the piezoelectric transducer is proportional to the optical energy of the excitation pulse at a particular wavelength and depends on the absorption coefficient of the sample as well as on the acoustic wave propagation in the medium. In the present case [17], Nd-YAG laserepumped dye laser was used as the source of excitation, and pulsed laseregenerated PA signals from

TOP VIEW 5.0

2.0 PAS Signal (a.u.)

BACK VIEW

1

d = 0.75 mm d = 1.00 mm d = 1.20 mm

1 2

1.0 0.5

3 0.2 0.1 20

50

100

200

500

1000

2000

Chopping frequency (f)

FIGURE 13.11 Design of the Helmholtz resonant photoacoustic cell (on the left) showing the microphone chamber in the main body (3) and the protruding cylindrical piston for changing volume of air above the microphone (see top view of the covering plate (1)). A narrow channel connects the sample chamber with the microphone chamber (see back view of covering plate (1)). Double quartz windows above the sample chamber are also shown (2). The observed variation of resonant frequency with change in the diameter of the tube connecting the sample and microphone chambers is shown on the right half of the figure.

TABLE 13.1 Observed and calculated resonant frequencies for the Helmholtz Photoacoustic cell Diameter of Channel (mm)

Observed Frequency (Hz)

Calculated Frequency (Hz)

0.75

350

700

1.00

450

920

1.20

600

1000

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295

POWER METER FeNe HOLLOW CATHODE LAMP BEAM SPLITTER 2

Nd:YAG LASER

MIRROR 1

DYE LASER

BEAM SPLITTER 1

Polished stainless steel diaphragm Thin layer of grease PZT cylinder Lead disc Copper cylinder

GLASS PLATES SAMPLE

LENS

Teflon cylinder Stainless steel spring

MIRROR 2

Brass disc Teflon disc Stainless steel casing

PZT TRANSDUCER

PRE-AMPLIFIER

BOXCAR

RECORDER Connector

REFERENCE

Signal

FIGURE 13.12 The homemade piezoelectric transducer, used in photoacoustic (PA) spectral measurements of solid samples, is shown on the right-hand portion. For good acoustic coupling, a thin layer of grease separates the PZT (lead zirconate titanate) disc from the polished stainless steel diaphragm of the outer casing. The experimental setup for recording PA spectra is shown on the left side of the diagram.

powder samples were recorded using the experimental arrangement shown in Fig. 13.12. The construction details of the PZT transducer used for recording the PA spectra are shown on the right of the experimental arrangement in Fig. 13.12. A small fraction of the dye laser intensity is sent to the power meter and a FeeNe hollow cathode lamp for calibration of spectral wavelengths using the optogalvanic spectrum. The major part of the dye laser intensity is incident on the powder sample sandwiched between two glass plates. The power spectral profile of the dye laser was recorded using carbon black powder as the sample, for which the separation between the position of the piezoelectric transducer and that of the sample was adjusted to maximize the PA signal. A thin circular layer of the powder sample was sandwiched between two plane glass plates, and the PZT transducer was tightly held with its front surface in firm contact with the glass plates. The linear separation between the sample and the PZT transducer was adjusted corresponding to that for the maximum PA signal while using the carbon black powder to record the power spectrum of the tunable dye laser. The light-absorbing species in the powder sample are Ho3þ in the microcrystals of Ho2O3. The acoustic pressure wave generated in the glass plate, following the pulsed optical absorption of the sample, propagates to the PZT transducer and gets converted into transient electric voltage as a result of the piezoelectric effect. These transient signals are then processed by the boxcar averager and the spectrum recorded by tuning over the dye laser emission wavelengths. A comparison between the raw PA spectrum of Fig. 13.13A and the normalized spectrum at the bottom right of Fig. 13.13 clearly demonstrates that the optical poweredependent influence on the PA signal is removed by the process of normalization.

3.5 Quartz Tuning Fork for Photoacoustic Detection The basic idea of using quartz tuning fork (QTF), for detecting PA signals, is to accumulate the acoustic energy not in a gas-filled cell but in a sharply resonant acoustic transducer [18e20]. Crystal quartz is an easy material for such a transducer because of its low loss piezoelectric property and QTFs can be designed to resonate at any frequency in the 4e200 kHz range and beyond. The resonance frequencies are defined by the properties of the piezoelectric material and by the geometry of the tuning fork. The widely used QTFs, intended for use in electronic clocks as frequency standard, resonate at 32,768 Hz in vacuum. The detection of PA signal by QTF is also based on the piezoelectric effect. In the PA spectroscopy of gaseous samples, the interaction between the modulated laser beam and a trace gas generates acoustic waves that mechanically bend the QTF prongs. Hence the electrode pairs of the QTF will be electrically charged due to the quartz piezoelectricity (see Fig. 13.14), and this will give rise to the PA signal. We may approximate the QTF as a system of two weakly coupled beams each of which is a cantilever with a fixed end. The tuning fork has two vibrational modes each with a different natural frequency. One is symmetric in which the two

PA Signal intensity (arb.units)

296 Molecular and Laser Spectroscopy

4 3 2 1 (c)

PA Signal intensity (arb.units)

0 4 3 2 1 11.40

6717.0428

4

6402.246

6

6163.5959

OG Signal intensity (arb.units)

8

2 0

–2 6156.196

(a) 6351.364

6546.530 WAVELENGTH (Å)

6741.696

PA Signal intensity (arb.units)

(b) 0

8.07

4.73

1.40 6280 6354.3

6428.7

6503.0 6577.3

6651.7

6726

WAVELENGTH (Å)

FIGURE 13.13 Photoacoustic (PA) spectroscopy of Ho3þ in Ho2O3 powder: (A) Optogalvanic spectrum of neon for wavelength calibration. (B) Spectral power profile of tunable dye laser. (C) Unnormalized spectrum of Ho3þ. The normalized spectrum is on the bottom right.

FIGURE 13.14 A typical quartz tuning fork (QTF) with each prong of length “L,” thickness “T” and width “Y.” The two electrodes can be connected at the bottom of QTF.

prongs move in opposite direction with respect to the central plane and the other is antisymmetric with one prong approaching the central plane while the other going away from it. The QTF is so designed that only the symmetric mode induces an electric signal via the piezoelectric effect. The standard QTF has a resonant frequency of about 32 kHz, with a gap between the prongs of about 300 mm, with prongs that are 3.2 mm long, 0.33 mm wide, and 0.40 mm thick (see Fig. 13.14). These QTFs have a quality factor “Q” of 100,000 when enclosed in vacuum and Q of 10,000 in normal atmospheric pressure. The width of QTF resonance at normal pressure is about 4 Hz so that only frequency components in this narrow acoustic band can produce efficient excitation of the QTF vibration. An experimental arrangement for recording PA spectra of gaseous samples is shown in Fig. 13.15. When the periodically modulated laser beam is focused at the center of the gap between the two prongs, the optical energy absorbed by the gas generates a weak acoustic pressure wave. The pressure wave makes the two prongs of QTF to move apart two times during each acoustic cycle, created by the laser modulation at half the QTF resonant frequency (f). In this case, the laser light is modulated at “f/2” and PA signal is demodulated by lock-in amplifier at “f,” the demodulated signal is referred to as the 2f-PA signal. In this case, the QTF detects sound oscillations at the second harmonic of the modulation frequency caused by the double intersection of the absorption line by the laser line during a modulation period. The PA spectrum is recorded by varying the wavelength of the tunable laser.

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FIGURE 13.15 Experimental setup for gas-phase photoacoustic spectroscopy with quartz tuning fork (QTF) detector. The excitation diode laser source is current modulated at half the QTF resonant frequency (f).

FIGURE 13.16 Schematic experimental arrangement for solid-phase photoacoustic spectroscopy with quartz tuning fork (QTF) detector, with sample adsorbed on one of the prongs of QTF. The function generator controls the pulse repetition frequency of the quantum cascade laser to produce resonance with the QTF symmetric vibration.

The commercial availability of quantum cascade laser (QCL) provides a compact narrow linewidth, mid-IR source that combines single-frequency operation with powers up to tens of milliwatt [21]. The large wavelength coverage coupled with their room-temperature operation makes it possible to monitor trace amounts of numerous molecular species. In the experimental setup of Fig. 13.16, adsorbed molecular sample, on the surface of one of the prongs of QTF, is illuminated by the laser pulse frequency equal to the mechanical resonant frequency of QTF. This arrangement generates acoustic waves at the airesurface interface, giving rise to oscillating localized pressure that sets QTF into resonance. The amplitude of this vibration, and the resulting piezoelectric voltage, is proportional to the amount of heat produced by optical absorption on the surface. The resulting PA signal is demodulated by the lock-in amplifier at the laser light modulation frequency “f” and spectrum recorded by varying the wavelength of the tunable laser. The experimental setup of Fig. 13.17 is used for remote detection of adsorbed chemicals on surfaces located away from the laser, the QTF, and the electronic recording system. The incident light from tunable QCL, scattered from the target, is focused on one of the prongs of the QTF. When the wavelength of the laser is tuned across the absorption wavelength of the adsorbed molecule, there is a sudden decrease in the intensity of scattered radiation on the QTF prong, which leads to a change in the electrical voltage that produces the PA signal.

4. PHOTOACOUSTIC DETECTION OF HARMFUL CHEMICALS PAS has been widely used in chemical sensing applications in homeland security, industrial process control, environmental science, and medical diagnostics. It is useful in rapid detection of hazardous materials (biological and energetic), illicit drugs, and nerve agents, without much sample preparation. For example, there is need for evaluation of anesthetic gaseous components in hospitals. Although hospital staff are exposed to much lower anesthetic concentrations than the patients, this exposure extends over many years. Under inadequate hygiene conditions, people working in hospitals or factories often complain of headaches and fatigue due to traces of harmful gases in the environment. Illicit drug trafficking and terrorist attacks pose many challenges for detection of dangerous chemicals that threaten life and property. In the following sections, we will present examples of point detection as well as standoff detection of chemical compounds using PAS.

298 Molecular and Laser Spectroscopy

FIGURE 13.17 Standoff photoacoustic spectroscopy of solid samples with quartz tuning fork (QTF) detector. The scattered quantum cascade laser (QCL) light from the sample adsorbed on a remote surface is focused on one of QTF prongs and the spectrum recorded by varying the incident wavelength. With permission from, C.W. Van Neste, L.R. Senesac, T. Thundat, Applied Physics Letters 92 (2008) 234102.

4.1 Photoacoustic Detection of Gaseous Molecules PAS with IR laser sources is an extremely effective tool for the detection and quantification of molecular gases and vapors. Very high sensitivities are achieved using fundamental vibrational bands of molecules in the IR range from 3 to 24 mm. There is an atmospheric spectral window centered between the bending fundamental vibration of water molecule around 6.2 mm and the water’s OH stretching vibrations below 3.1 mm. The atmospheric window, at wavelengths above water bending vibration, extends from 6.2 to 12.5 mm. Absorption spectra of several molecules, of great interest in controlling atmospheric processes, fall in the mid-IR region whose trace detection is of great importance. Because the molecular bands of vibrational fundamentals have the largest absorption coefficients, they are the most suitable for high sensitivity trace gas detection of small and large molecules, provided appropriate tunable sources are available. For laboratory experiments the size of the laser sources is not important, but for real-world applications these should be compact, in addition to operation at room temperature for long periods of time. The real-world applications include environmental monitoring, biomedical diagnostics, and industrial process control. In addition to the versatile CO2 laser and CO laser, most of the tunable lasers operating in the mid-IR include lead salt diode lasers, optical parametric oscillators, and coherent sources based on difference frequency generation. In recent times, QCLs have become commercially available and they are increasingly being used in trace gas monitoring. QCLs can be fabricated to operate at any of the very wide range of mid-IR wavelengths. In the following sections we describe the PA applications of mid-IR laser systems for measurements on gaseous samples.

4.1.1 Photoacoustic Spectroscopy of Methanol and Ethanol The use of ethanol as a motor fuel has as long a history, as the car itself. Bioethanol is a renewable green fuel and can be produced from corn stalks, rice straw, and sugarcane, and it can be used as a 10% blend with gasoline without need for any engine modification. Thus, both the consumption of crude oil and the environmental pollution are reduced. Methanol and ethanol act as oxygenates when added to gasoline, enhancing its octane value. The oxygen present in the gasoline makes the total amount of oxygen (from air and from oxygenates) more, relative to carbon and hydrogen and greatly reduces the emission of harmful CO. Free tropospheric concentrations of methanol and ethanol range from 400 to 700 ppt and play significant role in atmospheric processes [23]. PA spectroscopy with CO2 laser has great advantage due to its high power and the spectral coverage from 9 to 11 mm where more than 200 molecular gases and vapors, of environmental concern for atmospheric, medical, and industrial spheres, exhibit strong absorption bands. We have used the experimental setup of Fig. 13.7 to record the PA spectra of mixed vapors of ethanol (C2H5OH) and methanol (CH3OH) as shown in Fig. 13.18. The relative change of PA intensity according to the proportion of the two components can be seen from the characteristic bands of ethanol located at 1050 and 1045 cm1 and those of methanol at 1040 and 1035 cm1.

4.1.2 Photoacoustic Spectroscopy of Ethylene Mexico City, located in a valley at a height of 2 km above the sea level and surrounded by mountains, suffers from high degree of air pollution. This is due to large amount of volatile organic compounds, nitric oxide, and hydrocarbons emitted

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(A) 80 70 1/4 : 3/4

60 50 40 30 20 10

(B) 0 70 1/2 : 1/2 PA signal (in a. u.)

60 50 40 30 20 10

(C) 0 70 3/4 : 1/4

60 50 40 30 20 10 0 9.25

9.75

10.25

10.75

Wavelength (in µm) FIGURE 13.18 Photoacoustic (PA) spectra of mixtures of ethanol and methanol vapors. The fractional ratios shown refer to methanol: ethanol in (A) ¼:¾, (B) ½:½, and (C) ¾:¼.

to the troposphere from local industries and from a very large human population. Because of its geographical location, the mountains act as wind barriers, obstructing the airflow that, otherwise, could scatter the pollutants. As a consequence, it suffers from severe smog and ozone pollution. Ethylene (C2H4) is a well-known emission fingerprint from vehicle exhausts, and its reaction with nitric oxides under solar UV radiation produces ozone. Altuzar et al. have used PA spectroscopy to analyze ethylene concentration in the Mexico City atmosphere [24].

300 Molecular and Laser Spectroscopy

Altuzar et al. [24] have used a CO2 laser with the PA cell located inside the laser cavity, between the laser waveguide and the outcoupling mirror, to profit from the intracavity laser power. A mechanical chopper modulated the intensity of the laser beam at a frequency that matches with the acoustic resonant frequency of the PA cell. The rotational line laser emissions, overlapping with the ethylene absorption band, corresponding to 10P14 at 949.48 cm1 and 10P12 at 951.19 cm1 were used for monitoring the ethylene concentration in the sample. The relative absorption of ethylene at 10P14 and 10P12 are 30.4 and 4.31, respectively. The 10P14 line of CO2 laser is in exact resonance with Q branch of the n7 band of C2H4. Air samples from different locations in Mexico City were collected daily in two batches: from 6 to 9 a.m. in the morning and from 12 to 15 p.m. in the afternoon. The raw samples, collected in stainless steel vessels, went through the process of removing their CO2 and water vapor content in the laboratory before being transferred into the PA cell. The results of ethylene concentration analysis of the atmospheric air exhibit distinct temporal and spatial variations. Ethylene concentrations are high in the morning and decrease in the afternoon. This has been interpreted in terms of higher amount of ethylene emission from vehicles when the morning traffic is heavy. The decrease of ethylene concentration in the afternoon samples has two possible causes. Firstly, there is expansion in the thickness of atmospheric air, due to heating by sunlight, making it change from an average 200 m in the morning, to about 2000 m in the afternoon, leading to dilution of ethylene and other primary pollutants in the afternoon samples. Secondly, ethylene reacts with nitrogen oxides in the presence of solar UV radiation to produce ozone and other oxidants, causing its concentration to decrease in the afternoon samples. The spatial variations in ethylene concentration exhibited average values of 40.3 ppbV in highly polluted industrial locations, 37 ppbV in commercial areas, and 18.7 ppbV in residential areas. Most of the samples were also examined by gas chromatography, and results were found to be consistent with those obtained by PA technique. It is to be emphasized that PA technique has the advantage of performing online measurements with excellent time resolution.

4.1.3 Photoacoustic Spectroscopy of Aerosols Atmospheric aerosols are suspended particulate matter. In addition to reducing visibility and posing health hazard when inhaled, the role of these particles in modifying the atmospheric energy and climate change has been much investigated. The atmospheric modifications brought about by aerosols result from their properties to absorb and scatter radiation. It is the proportion of its light scattering to its light absorption capacity that determines whether a particular type of aerosol would give rise to cooling or warming effect on the atmosphere. Nitrate and sulfate aerosols, resulting from anthropogenic sources, predominantly scatter sunlight and thereby produce cooling. Aerosols produced by incomplete combustion of carbonaceous fuels warm the atmosphere because of predominant absorption of sunlight. The fraction of incident optical energy that is scattered by a surface is called albedo. A high albedo means the object scatters majority of the radiation that hits it and absorbs the rest, whereas a low albedo means the object scatters only a small amount of the incoming radiation and absorbs the most of it (see Fig. 13.19). Typical examples of albedos in visible light for fresh snow correspond to 0.9 and for charcoal, which is one of the darkest substances, it is 0.04. Arnott and coworkers [25] have used a single 532 nm laser beam configuration of Fig. 13.20 with NO2 gas to develop a calibration method for PA measurement of optical absorption by atmospheric aerosols. The inlet and outlet for NO2 are located close to the node of the acoustic stationary wave formed in the PA cell, whereas the microphone is located near the antinode (see Fig. 13.7). This configuration has minimal perturbation on the PA signal detected by the microphone during the airflow. Laser power, during the operation, is measured by the photodiode for normalization of the PA signal from the microphone.

FIGURE 13.19 Atmospheric carbonaceous aerosols cause smoke of low albedo (A), whereas nitrate and sulfate aerosols cause smoke with high albedo (B).

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Air Inlet

Microphone

301

Photodetector

Light (532 nm)

Chopper to pump FIGURE 13.20 Schematic representation of the photoacoustic cell for calibration measurements of absorption by NO2 gas.

Lewis et al. [15] have measured aerosol light absorption and scattering simultaneously, at 405 and 870 nm, by smoke from the combustion of a variety of biomass fuel. Single scattering albedo values at 405 nm ranging from 0.37 to 0.95 were measured for different fuel types, and the spectral dependence of absorption was quantified using the Angstrom exponent of absorption. For biomass smoke, Angstrom exponents as high as 3.5 were found in association with smoke having single scattering albedo near unity. Angstrom exponent of absorption is the negative slope of the absorption coefficient in a loglog plot. An absorption Angstrom exponent near unity is commonly observed for motor vehicle emissionegenerated black carbon aerosol. For data from dual-wavelength measurements at 532 and 870 nm, the absorption Angstrom exponents as large as 2.5 were found for smoke with single scattering albedos near unity. These results show that light-absorbing organic material is present in wood smoke and aerosols arising from vehicles can be differentiated from those due to biomass burning.

4.1.4 Photocoustic Spectroscopy of Ozone Ozone (O3), in our atmosphere, plays a very important role in shielding the Earth’s surface from harmful solar UV radiations, which are known to cause skin cancer. Almost 90% of atmospheric ozone is found in the stratosphere. The presence of ozone in the troposphere is, however, not good for life and human activities. It is the major ingredient in smog and poses risk to human life because it attacks cells and breaks down tissue. It also decreases the ability to breathe and increases the risk of respiratory diseases. Ozone also causes damage to agriculture and forests due to its toxic activities in plants. Ozone is a strong oxidant, and in combination with water vapor, in the atmosphere, it produces OH radical and enhances the rate at which many natural and anthropogenic compounds are eliminated from the atmosphere. It has been found that, in urban areas, people are periodically exposed to larger concentrations of ozone for which a health-based air quality standard of 120 ppbV has been determined. A simple PA system based on QCL source, to detect ozone at ambient pressure, has been proposed by da Silva et al. [26] as schematically shown in Fig. 13.21. The wavelength-tunable 9.5-mm pulsed QCL was modulated by an external TTL signal at 3.8 kHz to excite the first longitudinal mode of the resonant PA cell, and the PA signal from the microphone was measured by a digital lock-in amplifier. Gaseous ozone samples were produced by a commercial ozone generator

FIGURE 13.21 Experimental arrangement for quantum cascade laser (QCL)ebased quantitative photoacoustic (PA) detection of ozone in ambient air. Flow of N2 and that of “synthetic air,” into the ozone generator, are controlled by needle valve (NV) and the flow controller (FC), respectively. The inlet and outlet, of the ozone-mixed air, are located close to the regions of nodes in the resonant PA cell with microphone located at the antinode.

302 Molecular and Laser Spectroscopy

based on electric discharge. Synthetic air (20.5% O2 and 79.5% N2) and pure nitrogen were mixed to generate mixtures with different oxygen concentrations, which were continuously fed through the ozone generator. The amount of ozone in ambient air was monitored with a commercial UV photometric ozone analyzer, which has a precision of about 2 ppbV in detecting ozone. The ozone-mixed air was pumped through the PA cell, and the pressures, at the inlet of the PA cell and at the inlet of ozone analyzer, were kept at ambient value while the amount of ozone was varied by changing the mixed sample. The peak at 1050.9 cm1 of the n3 vibrational band of O3 was used for PA intensity measurements of different mixed samples to determine their O3 concentrations. The lowest value of measured O3 concentration in this PA experiment was 102 ppbV with a signal-to-noise ratio (SNR) of unity.

4.1.5 Photoacoustic Spectroscopy of Gases Emanating From Human Body The most common body odor is the unpleasant smell emanating from human body, when the bacteria that live on the skin break down sweat into acids. It is not the smell of bacteria growing on the body, but it results from certain acids produced in the process of protein broken down by the bacteria. A person’s smell escapes not just from their skin but their breath, blood, and urine, and subtle differences reveal just how healthy the person is. The smells pouring from various parts of the body are unique to an individual, made up of specific chemical compounds that vary depending on age, diet, metabolism, and health. If one eats a meal with a lot of garlic, it will emanate from the breath for 48 h. Some diseases result in a characteristic odor emanating from different sources on the body of a sick individual. The knowledge of medical science has established that uncontrolled diabetes produces a sweet fruity odor, advanced liver diseases cause a fishy reek, failing kidneys give rise to urinelike smell, and lung abscess produces a putrid stench. Ammonia (NH3) plays a role in both normal and abnormal human physiology. It is biosynthesized through normal amino acid metabolism and is toxic in high concentrations. The liver converts ammonia to urea through a series of reactions known as the “urea cycle.” Liver dysfunction, such as in cirrhosis, may lead to elevated amounts of ammonia in the blood. Several investigators have developed near-IR diode laser (1.53 mm)ebased PA cells, fitted with microphone as well as QTF detector, for trace detection of NH3 in the ppmV and ppbV range [27e29]. These sensors involve harmonic vibrational band for trace detection of ammonia. There is great potential for mid-IR QCL-based PA sensors to make portable ammonia sensors. These sensors would need ultrasmall gas volumes, and concentration limits can be lowered by using fundamental absorption bands of NH3 molecule. Kreuzer and Patel [2] were the first to use CO laserebased PA spectroscopy, about 45 years ago, to detect NO concentrations of 0.01 ppmV. Detection of nitric oxide (NO) at trace levels is of major importance for medical, biological, and environmental applications. The presence of endogenous NO in exhaled air was observed for the first time in 1991 [30] and since then exhaled NO was found to be a sensitive marker for asthmatic airway inflammation [31]. The diagnosis of lowerairway inflammation is associated with a number of lung diseases and illnesses. Elia et al. [32] have developed a QCL-based resonant PA cell with microphone detector for concentration measurements of NO. Commercially available distributed feedback QCL was operated in the pulsed mode at a wavelength around 5.3 mm, and the laser beam intensity was modulated by a mechanical chopper at the first longitudinal resonance frequency of the PA cell. The PA signal for different NO concentrations was measured by tuning the laser emission over the doublet P(1.5) absorption lines located at 1871.051e1871.066 cm1. A minimum NO concentration limit of 500 ppbV was achieved by this PA sensor. Spagnolo et al. [33] have fabricated a QTF detectorebased PA cell for use with an externally controlled QCL with a tuning range 1763e1949 cm1. Using the NO R(6.5) absorption doublet at 1900.075 cm1, they achieved a minimum NO concentration limit of 15 ppbV. Harren et al. [34] have demonstrated the emission of ethylene (C2H4) from human breath and skin under exposure to UV radiation. CO2 laser was used as the source of PA excitation, and a small amount of air, from the exhaled air, was cleaned for its content of CO2, water, and other spectroscopically interfering gases, before introducing it into the PA cell. To examine ethylene emission from skin, a specially designed PA cell was placed on the skin. The 10P14 emission line of CO2 laser, which is in resonance with Q branch of the n7 vibrational band of C2H4, was used for monitoring ethylene emission. This PA sensor has a lower detection limit of 6 ppbV ethylene in nitrogen. The emissions of ethylene from the human breath and the skin are shown in Fig. 13.22. UV radiation leads to the formation of reactive species in the skin that can damage the lipids in the cell membranes to produce small hydrocarbon molecules such as ethylene, ethane, and pentane [35]. PA measurements are carried out on test persons while resting under a solar bench. When the UV radiation is switched on, a steady increase in ethylene emission, in exhaled air (Fig.13.22A), is observed after 2 min, which continues to reach a maximum limit of more than 3 ppbV. When the solarium is switched off, ethylene emission starts decreasing and two emission decays are found to occur. The fast decay is caused by washout of ethylene from the blood and the slow decay results from ethylene stored in the body tissue. In the case of ethylene emission from skin, a steady and constant production of ethylene was observed, immediately after the start of UV radiation (see Fig. 13.22B).

Photoacoustic Spectroscopy: Applications in Security and Biology Chapter | 13

(A)

(B) 6 3 5 C2H4 (ppb)

UV exposure C2H4 (ppb)

303

2

UV shielding

UV on

4 3 UV off

1 2 0.0

0.2

0.4

0.6

Time (h–1)

1.0

1.2

1.4

1.6

Time (h–1)

FIGURE 13.22 Ethylene emission from human body after exposure to ultraviolet (UV) radiation: (A) variation of ethylene production in exhaled air and (B) variation of ethylene production from skin. The thick black bar represents the duration of UV exposure. With permission from F.J.M. Harren, G. Cotti, J. Oomens, S. te Lintel Hekkert, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, John Wiley, 2000 2203e2226.

4.1.6 Photoacoustic Spectroscopy of Improvised Explosive Device Precursors in Vapor Phase It has been known for more than a decade that powerful explosives can be made from precursor chemicals found in common consumer goods. This poses a great threat for a nation’s efforts to ensure a homeland that is secure against terrorism because these chemicals are readily available commercially. Homemade explosives have been used in several high-profile incidents in what have come to be known as improvised explosive devices (IEDs). The detection of hidden IEDs, in public places and among the crowd, is of utmost importance for the safety of people and property. Viola et al. [36] have developed a PA spectroscopyebased gas sensor for the point detection and identification of IED precursors. The sensor consists of a broadly tunable external cavity QCL and a specially designed PA cell with a miniaturized internal volume and fitted with a QTF detector. This sensor has been successfully tested in the laboratory with several volatile IED precursor compounds including acetone, nitric acid, nitromethane, and hydrogen peroxide in liquid state and, hexamine and DNT in solid form at normal ambient conditions. The PA cell can be heated up to 150 C for detection of low volatile compounds. The external cavity tuning of the EX-QCL provides a laser linewidth of 1 MHz with peak power in excess of 400 mW in the wavelength range between 7.1 and 8.5 mm. This wide spectral range covers the IR fingerprints of IED precursors (see Fig. 13.23) but avoids the absorption bands of water below 7 mm, which could mask the precursors. The laser was operated in pulse mode at the pulse repetition frequency corresponding to the resonance frequency of the QTF to achieve the highest sensitivity. Because of the change in the PA cell temperature, the resonance frequency of the QTF drifts from 32,768 Hz but there is provision for automatic calibration, so that PA signals always correspond to the changed resonance frequency. A custom-designed hardware machine interface allows computer-controlled operation of the sensor and the acquisition of data. The PA detection is carried out with the precursor vapor in ambient air sampled directly inside the PA cell by means of a micropump. The lowest limit of detection for acetone, nitric acid, nitromethane, and hydrogen peroxide, at their peak absorption wavelengths of 8.271, 7.54, 7.254, and 7.992 mm, have been found to be 110, 486, 257, and 593 ppbV, respectively [36]. One of the advantages of this PA gas-detection system is the fact that the laser controller and all fluids necessary for direct sampling of air have been assembled inside a small box measuring 45 cm  35 cm  25 cm.

4.2 Photoacoustic Detection of Condensed Matter Harshbarger and Robin [38] and Rosencwaig [39] were one of the first investigators to use PA spectroscopy for the study of condensed matter. The microphone detector system for recording PA spectra has been described in Section 3.3 and that with a piezoelectric transducer in Section 3.4. There is an acoustic impedance mismatch between the condensed phase sample and the coupling gas used in microphone detection. This mismatch allows only a small fraction, of the acoustic signal generated in the condensed phase sample, to be transmitted to the microphone. This feature of the microphone detector, however, makes it possible to successfully record the PA spectra of relatively opaque solid materials. A piezoelectric transducer, attached to a solid sample or immersed in a liquid, does not suffer from the problem of impedance mismatch, making it very useful for studies on transparent materials [40]. Because of its ability to carry out studies of opaque or highly diffusive materials, PA spectroscopy has been used in the investigation on condensed materials of great relevance to environment, biology, and security. In the following sections we present some examples of the wide-ranging applications of this technique with microphone as well as piezoelectric detection systems. There is no sample preparation necessary in this technique, it is suitable for opaque material, and measurement is nondestructive.

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FIGURE 13.23 Mid-infrared spectral bands of some commonly available chemicals that can be used for making improvised explosive devices. Adapted from R. Viola, N. Liberatore, D. Luciani, S. Mengali, QEPAS for detection of IEDs and precursors, Advances in Optical Technologies 2016 (2016) Article ID:5757361; Pacific Northwest National Lab (PNNL) database of IR absorption spectra. https://secure2.pnl.gov/nsd/nsd.nsf/Welcome.

4.2.1 Photoacoustic Spectroscopy of Dangerous Drugs Morphine is the prototype narcotic drug and it is the standard against which all other opioids are tested. It is believed to be the first isolation of an active ingredient from plant in 1803 by Friedrich Serturner, who named the substance morphium after the Greek god of dreams. In 1897, Felix Hoffmann, working at Bayer pharmaceutical company in Germany, was instructed by his supervisor to acetylate morphine with the objective of producing codeine, a constituent of poppy. Codeine is pharmacologically similar to morphine but less potent and less addictive. Instead, the experiment produced an acetylated form of morphine almost two times more potent than morphine itself (see Fig. 13.24). The new drug’s name “heroin” was coined by head of Bayer’s research department based on the German “heroisch,” which means “heroic, strong” from the ancient Greek word “heros.” Animal and human studies and clinical experience back up the contention that morphine is one of the most euphoric of drugs on earth. Both morphine and heroin are used for pain medication (heroin in severe physical trauma and other terminal illnesses), but both are addictive and identified as illegal drugs. The experimental setup, for recording the PA spectra of heroin and morphine with CO2 laser, was identical to that shown in Fig. 13.7 with the difference that the PA cell used in this case was similar to that shown in Fig. 13.10. The oval cavity in the aluminum block of Fig. 13.10 was covered with an airtight flat ZnSe window, leaving about 1-mm-thin layer of air between the sample surface and the window. The PA spectra (Fig. 13.25) of powder samples were recorded manually from the lock-in amplifier and normalized using the power meter reading of the corresponding laser line. Microgram quantities of powder samples were used for recording the spectra, which exhibit the characteristic vibrational bands of the two molecules [41]. At present, rapid detection of different molecules in small concentrations is a common requirement in applications in waste monitoring, medical use, drug detection, and human health. These small concentrations of compounds can be detected in different sample matrices such as skin, fingernails, saliva, and blood using PA spectroscopy. A QCL-based PA setup, with extremely sensitive optical cantilever microphone detector, has been developed for recording PA spectra of solid samples with capability for drug detection in human hair [42].

4.2.2 Photoacoustic Spectroscopy of Explosives The solid circles in the upper half of Fig. 13.26 represent the normalized PA signal, for the pure explosive powder samples of 1,3,5-trinitro perhydro-1,3,5-triazine (RDX) and 2,4,6-trinitrotoluene (TNT), at each of the rotational lines of the CO2 laser described in Section 3.3, which is tunable in 9.6 and 10.6 mm regions. The PA spectra of explosives at lower

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FIGURE 13.24 Typical powder forms of heroin (A) and chemical structure of heroin (diacetylmorphine) (B) and of morphine (C).

FIGURE 13.25 Rotational line-tunable CO2 lasereinduced photoacoustic (PA) spectra of heroin (left) and morphine (right).

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FIGURE 13.26 CO2 lasereinduced photoacoustic (PA) spectra of RDX (1,3,5-trinitro perhydro-1,3,5-triazine) and TNT (2,4,6-trinitrotoluene) powders. The persistent vibrational bands of the two molecules can be seen in the highly diluted samples in lower half of the figure.

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concentrations are shown in the bottom half of Fig. 13.26. The samples were diluted by uniformly mixing 10 mg of each explosive with 8 g of SiO2, which does not have any absorption in this region. Thus, the concentrations of two explosives are of the order of a few ppm, and four persistent bands of RDX and five of TNT appear with appreciable intensity in the PA spectra [43]. The methods of explosive detection can be subdivided into those that detect vapors or particles emitted from the materials, those that probe solid samples, and those that detect dissolved or suspended solids in solution. A wide variety of methods have been developed for environmental analysis of hazardous chemicals including explosives and their degradation products. There is, however, an urgent need for standoff detection of concealed explosives, in view of the increasing terrorist activities. TNT is one of the oldest used military explosives, whereas RDX is more recent and still in very wide use today. Plastic-bonded explosives (PBXs) use both RDX and PETN (pentaerythritoltetranitrate). HMX (octogen) also chemically related to RDX is a very powerful explosive for military warheads. There have been many experiments using different optical detection techniques to investigate such chemicals [44e46]. Van Neste et al. [22] have recorded the standoff PA spectra of TNT, RDX, PETN, and TBP (tributyl phosphate) using the experimental setup shown in Fig. 13.17. In these experiments, clean stainless steel was used as the target surface and solutions of explosive samples, uniformly deposited on the surface, were allowed to dry till the solvent was completely evaporated. This was done to make sure that solvent does not contribute to the photoabsorption on the target surface. A 100-mW-power QCL source was tunable in the wavelength range from 9.25798 mm (9257.98 nm) to 9.80407 mm (9804.07 nm) in increments of 0.01 nm. The laser light scattered from the target surface was focused onto one of the prongs of the QTF using a spherical mirror. The QTF detector with a resonant frequency of 32,768 Hz was that used in Citizen electronic clocks. A function generator was used to control the pulse repetition frequency of the QCL to produce QTF resonance, and a lock-in amplifier was employed to record the amplitude and phase data from the QTF signal. When the wavelength of light emitted by the QCL is changed, the target will either scatter or absorb the light depending on the absence or presence of spectral absorption of the sample, respectively. Increased absorption by the sample will decrease the intensity of reflected light falling on the QTF. Thus, with wavelength of laser light corresponding to that of an absorption peak in the sample, the vibrational amplitude of the QTF decreases and the plot of PA signal against wavelength resembles the conventional IR absorption spectrum of the sample (see Fig. 13.27B). The PA spectral measurements were performed, by varying the target distance from 0.5 to 20 m, to get the best results for different samples. In the case of RDX sample shown in Fig. 13.27, the target was separated by 20 m from the detection system. It is to be noted that the spectral resolution of the standoff PA spectrum is much higher than that of the conventional IR spectrum shown in Fig. 13.27B. The PA signal at 9.64 mm corresponds to one of the persistent vibrational bands shown in Fig. 13.26, obtained in the laboratory with CO2 laser, at the reduced concentration of RDX sample. In an upgraded version of standoff detection of explosives, Van Neste and coworkers [47] have extended the region of PA spectrum by using two tunable QCLs emitting at different wavelengths and coupled to two QTFs with different resonant frequencies. The first QCL had a tuning range from 7.38 to 8.0 mm and the other from 9.25 to 9.80 mm, and the

FIGURE 13.27 (b) Mid-infrared absorption spectrum of RDX (1,3,5-trinitro perhydro-1,3,5-triazine) and the 9.6 mm absorption band recorded in the standoff photoacoustic experiment, at higher resolution, with tunable quantum cascade laser source and quartz tuning fork detector. With permission from C.W. Van Neste, L.R. Senesac, T. Thundat, Applied Physics Letters 92 (2008) 234102.

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emitted wavelength from each could be changed in steps of 0.01 nm. Identical QTFs exposed to air generally exhibit different resonant frequencies, and in the present experiment a pair was chosen with a resonance frequency difference of 143 Hz. The light from both the QCLs was simultaneously illuminating the explosive-coated target surface, and the reflected light was focused by a common spherical mirror on the two QTFs (separated from each other by a gap of 0.5 mm). The baseline spectrum of the target surface is taken before it is coated with the sample residue. Each QCL is tuned simultaneously from the starting wavelength of its range to the ending wavelength. The outputs of the two QTFs are processed by two separate lock-in amplifiers, and the whole process is controlled by a computer. The process of recording each set of spectra takes about 1 min, and the normalized PA spectra using the baseline data are plotted on the computer. This experiment demonstrates that a wide spectral range can be covered in standoff PA detection of chemical species, using multiple QCLs each set at a different pulsing frequency matching with the corresponding QTF in the array of detectors [47].

4.2.3 Photoacoustic Spectroscopy of Contaminated Water According to the environmental campaign organization WWF, “pollution from toxic elements threatens life on this planet.” As the world’s population continues to grow, people are putting ever-increasing pressure on the Earth’s water resources so that quality of water is reduced. Poor water quality means water pollution. In addition to many other pollutant species, much emphasis has been put to monitor water pollution by actinides and by pesticides. Actinides are a group of 14 radioactive elements following element actinium in the periodic table, whereas the pesticides adversely affect human health and some of the polycyclic aromatic hydrocarbons (PAHs) possess properties such as carcinogenicity and mutagenicity. Actinides are known to migrate, and at Los Alamos, plutonium and americium were detected in monitoring wells over a kilometer from a liquid waste outfall [48]. Understanding of chemical behavior of actinides, in natural aquatic systems, is necessary for safety of nuclear waste disposal. The solubility of actinides, in neutral solutions, is very low (<106 mol/L) so that conventional absorption spectroscopy is not possible. The technique of PA detection was first used in mid-1980s to obtain information on actinide ions in natural aquatic systems [49,50]. A typical experimental arrangement shown in Fig. 13.28, for trace detection of pollutants including actinides, consists of a tunable dye laser pumped by an excimer laser or Nd-YAG laser. The dye laser light is focused into a 600-mm-core multimode fiber for remote detection of polluted water samples. Polluted water samples are kept in a quartz cuvette acoustically coupled to a piezoelectric transducer. A 10 microscope objective is used to collimate the light from the fiber into the quartz cuvette. A photodiode assembly monitors the light exiting from the cuvette to correct for pulse-to-pulse energy fluctuation, to ratio the measured PA spectra to the dye laser power profile. The photodiode signal is recorded using a second boxcar gate, and the PA spectra are recorded by scanning the dye laser wavelength with computer after processing in the boxcar. The experimental assembly consisting of the quartz cuvette, the microscope objective, and the photodiode is mounted on a small platform that can be taken to remote locations for analyzing the polluted solutions. The elements uranium, neptunium, and plutonium show a wide variety of oxidation states, and because these elements are radioactive, special laboratories are necessary to handle them. In the tetravalent oxidation state, the solubility is very low and hydrolysis is the most important reaction. The small portable unit, consisting of the cuvette, the microscope

FIGURE 13.28 Schematic photoacoustic spectroscopy system for trace detection of chemical species in polluted water. PD, photodiode; PZT, lead zirconate titanate.

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objective and photodiode, can be easily positioned within the glove box used in special laboratories for handling radioactive samples. The actinide chemistry involving redox reactions and colloid generation, which are important for understanding migration behavior of these elements, have been studied by PA spectroscopy [51,52]. Klenze et al. [53] have used PA spectroscopy with excimer laserepumped dye laser system to understand some important aspects for the safety analysis of nuclear waste disposal and have determined ultralow concentrations of Am3þ and Pu4þ in aqueous solutions. Nd-YAG-pumped dye laser system has been used by Russo et al. [54] to record PA spectra of aqueous solutions containing different concentrations of Pr3þ and Am3þ. Kim [55] has used PA spectroscopy to study the problem of actinide colloid generation in groundwater, which plays a critical role in geochemical interaction and migration of actinides. The onsite monitoring of traces of organic pollutants is of great importance in quality control of ground and drinking water, and PA spectroscopy is an appropriate technique in view of its capability of remote application using optical fiber. Adelhelm et al. [56] have used UV excitation laser beam in conjunction with a multimode optical fiber to carry out concentration studies of several pesticides and PAHs. PAS of DNP (dinitrophenol) at 359 nm exhibits a linear increase of PA signal with increase in concentration from 25 to 200 mg/L. DNP is an environmental contaminant, and acute oral exposure to it results in nausea, dizziness, headache, and loss of weight. Similar calibration curves have been obtained for organic PAH pollutants anthracene and benzo/gh/perylene at several laser wavelengths between 250 and 400 nm. As described earlier, PAHs are very injurious to human beings, some of them cause cancer.

5. HYPERSPECTRAL IMAGING AND PHOTOACOUSTIC SPECTROSCOPY Hyperspectral imaging (HSI) is a combination of spectroscopy and imaging so it is also known as “imaging spectroscopy.” The spatial information, in such an image, provides location of the scene, whereas the spectral information identifies the materials in the scene. This technology has developed over a period of four decades, but during the past 10 years it has come into great prominence due to applications in many diverse fields, such as remote sensing of earth resources, agriculture, biological sciences, medicine, crime detection, etc. HSI can gather spectral and spatial information using wavelengths ranging from UV to long-wave IR. The acquired image of the target contains characteristic spectral features of materials present in different locations in the scene, and this digital image is stored in the form of numbers. This imaging concept produces a data cube comprising of 2D spatial information and the third dimension providing a full spectrum for each pixel in the imaged scene [57]. The data obtained in the process of imaging are so massive and complex that its analysis requires the most modern methods of computer data processing. The end result of analyzing a hyperspectral image is akin to a superhuman eye that can see and distinguish materials, at different locations, on the basis of data obtained through electromagnetic waves that are invisible to human eye. The importance of hyperspectral image analysis is that it can distinguish objects reflecting or emitting UV (l < 380 nm) or IR (l > 780 nm) in addition to the visible wavelengths. This versatile technology makes use of the knowledge of spectroscopy and digital imaging for acquiring the information and that of computer data processing for analyzing it. High-resolution spectral imaging is a preferred means of remote earth sensing and mapping the location of “spectral objects” such as pollution, vegetation, minerals, and insect infection on agricultural farms. The same methodology has been adopted in medical field of histopathology with the goal of enhancing objective assessments on diagnostic accuracy. The application of spectral imaging in medicine with a histopathologist looking into a biological sample can be compared with that of an earth scientist looking into a region with vegetation and dry soil. Although the imaging spectrometer for collecting the data may be the same for both earth sensing and histopathology, the algorithms needed for data processing are quite different. This difference can be illustrated by considering the examples of spectral imaging of a tree leaf and that of a histological section. The spectrum of a tree is dominated by its leaf when acquired from a distance, and will be dominated by a single spectrum (that of chlorophyll). But zooming into a single leaf may reveal spatial differences in the color distribution due to localized areas of disease or insect attack. This composite spectral signature of the entire leaf can be analyzed as a linear mixture of the spectral components because each area of different color (spectral curve) is independent of all other colored areas. The spectral curves for green leaf and dry soil are shown in Fig. 13.29. Spectral curves, of the type shown in Fig. 13.29 (leaf and soil), are needed for constructing the images obtained from the spectral imaging systems, and such curves are available for a large number of materials at USGS Digital Spectral Library [58]. Fig. 13.30 has been obtained with the processing of hyperspectral data by the computer. The colors in such images are artificial and are determined by the relative intensities of different wavelengths that contribute to the image in each pixel. Thus the image of a diseased or infected leaf can be analyzed in terms of the spectral curves of the diseased matter and that of chlorophyll. However, in the case of a stained histological section, one cannot assume a linear mixture of the individual components. In this case, two or more stains are mixed and interact with biological material; the individual stains will undergo spectral change as a result of chemical interactions. In such a situation, it is reasonable to assume that

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FIGURE 13.29 Reflectance curves of dry soil (A) and green leaf (B). Their relative presence in an image pixel may vary from 0% to 100%. A typical mixed spectral curve (C) with 60% of dry soil spectrum and 40% of green leaf spectrum is also shown. Adapted from R.B. Smith, Introduction to Hyperspectral Imaging, Microimages Inc. 2012.

FIGURE 13.30 Hyperspectral image of the confluence of a river (black) and a polluting stream (blue (dark gray in print versions)). Artificial colors adjusted on the basis of scattered sunlight from different regions. Clean water reflects much less than the suspended particles in the polluted stream water, whereas reflection from white sand is the largest. Courtesy Prof. K.N.P. Raju, BHU.

two or more colocalized color centers are the result of a nonlinear mixture of the components. To employ the powerful algorithm necessary to handle nonlinear spectral data, a high spectral resolution is needed to ensure digital reconstruction of the field of view. The PA imaging differs from the HSI in the replacement of optical detectors by acoustic detectors. The concept of spectral imaging shown in Fig. 13.31 introduces a light dispersing element (prism or grating), between the incident light and the image forming lens, to obtain spectral image of a scene. Thus, a tiny spot in the scene is simultaneously imaged in all spectral colors of the visible light, but the brightness of the images depends on the relative proportion of lights of their corresponding wavelengths. Using a high-resolution dispersing device, the number of such tiny images can be increased to hundreds or thousands depending on the resolution of the imaging system. An array of tiny CCD detectors record the spectral image in the form of numbers that depend on the strength of electric signal generated in each of them. The final image is obtained with the help of computer, where the combinations of these numbers are converted into artificial colors according to a predecided code. In the case of PA imaging, a single or an array of piezoelectric-based acoustic detectors record the spectral images of the sites in the scene where the absorbed optical energy got converted into heat.

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FIGURE 13.31 Each pixel on a horizontal grid (y-direction) is imaged as a spectrum in a vertical grid (z-direction) corresponding to the electromagnetic wavelengths emanating from the source. The spectral image appears as a cube of numbers where the “x” and “y” give the spatial information and “z” gives the spectral information to be analyzed by the computer. With permission from F. Dell’Endice, J. Nieke, B. Koetz, M.E. Schaepman, K. Itten, ISPRS Journal of Photogrammetry and Remote Sensing 64 (2009) 632.

5.1 Photoacoustic Imaging PA imaging is mostly used in biology and medicine. It is a noninvasive technique that produces structural, functional, and molecular images of internal organs in small animals (usually mouse and rat). When the sample is illuminated by a short pulse of laser light, its local absorption is followed by rapid heating, which subsequently leads to thermal tissue expansion and generates ultrasonic waves. The outgoing ultrasonic waves are recorded with adequate transducers outside of the sample to form the image that reveals the initial absorbed energy distribution in the sample. Thus PA imaging is a hybrid technique making use of optical absorption and ultrasonic wave propagation as schematically shown in Fig. 13.32. It has the advantage of high contrast of optical imaging and high resolution of ultrasonic imaging. To efficiently generate PA signal, two conditions “thermal confinement” and “stress confinement” in the sample must be satisfied. The condition of thermal confinement requires that the laser pulse duration sp should be shorter than the temporal duration sth of thermal diffusion from the volume heated by the laser pulse. This condition implies that there is

FIGURE 13.32 Schematic presentation of the principle and processes involved in photoacoustic imaging.

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negligible heat diffusion during the excitation laser pulse. The thermal diffusion length during the laser pulse is given by dT ¼ 2(spDT)½ where DT is the thermal diffusivity of the sample. A typical value of DT ¼ 1.4  103 cm2/s for most tissue [60] leads to dT ¼ 0.05 mm for a laser pulse of sp ¼ 5 ns. The duration of thermal diffusion is given by sth ¼ L2/4DT where L is the radius of spherical region of heat propagation in the sample. Thus for L ¼ 15 mm we get sth ¼ 0.4 ms. Similarly, the condition of stress confinement requires that time ss for the stress to transit the heated volume should be larger than sp. If c is the velocity of sound in the sample, the time taken by the stress to transit a sample length L in the heated region is given by ss ¼ L/c. Thus assuming c ¼ 1.5 mm/ms and DT ¼ 1.4  103 cm2/s, we would achieve a spatial resolution of 15 mm in the PA image if ss ¼ 10 ns and sth ¼ 0.4 ms. Thus we find that in the descriptions of this and the previous paragraph, for the process of heat generation, a laser pulse of 5 ns duration would satisfy the two conditions for generating PA signals efficiently. There are two major implementations of PA imaging: photoacoustic microscopy (PAM) and photoacoustic tomography (PAT). In the case of PAT, the whole sample is illuminated by an expanded laser beam, and laser photons absorbed at various points in the sample create pressure changes due to thermoelastic expansion, leading to generation of ultrasonic waves. An ultrasonic transducer placed outside the sample detects the PA signal, which can be measured either by moving a single transducer around the sample or by using an array of transducers, and the PA image is obtained from the data set of PA signals by using suitable reconstruction algorithms in the computer. The resolution of PAT is determined by the duration of the excitation laser pulse and the bandwidth of piezoelectric transducers. The attenuation of ultrasound is caused by losses due to absorption and scattering, but ultrasound waves of a few megahertz suffer very little attenuation and can penetrate deep into the soft tissue. The attenuation, however, increases with increasing frequencies, and 3 MHz might be the maximum frequency for a 15 cm penetration. The most commonly used ultrasound detectors for imaging are piezoelectric based with low thermal noise, high sensitivity, and a wide band up to 100 MHz. In the case of PAM, the laser beam is focused into a small volume, and ultrasound waves are launched only from this localized region. For a 3D image the sample is scanned in two dimensions. The axial resolution of the image can be as good as the optical resolution (<1 mm) where the depth information is determined by the runtime of the ultrasonic waves. Fluorescence microscopy is an effective tool in thin biological samples such as single-celled organisms, but with slightly thicker samples it becomes difficult to know from where exactly the fluorescence originates. Even with advanced techniques such as confocal microscopy, it has been difficult to image deeper than 1 mm in transparent samples and matters become worse with diffuse samples. In more complex organisms, such as zebra fish, it is crucial to image deeper and deeper while keeping the samples alive. Because of multiple scattering of fluorescence light in the tissue, one loses information on its origin and propagation path, giving rise to blurred images and destroying the spatial resolution. PA detection of lightematter interaction circumvents these limitations because acoustic signals travel through the diffuse biological media with much less distortion than light. With PA imaging it has been possible to effectively resolve genetically expressed agents (fluorescent proteins) deep in intact living animals with high spatial resolution and high sensitivity. A typical experimental setup used for PAM by Harrison et al. [61] is shown in Fig. 13.33A. A tunable laser beam L is diverted toward the 45 degrees reflecting cone R, by the prism P, and a 45 degrees polished face reflects the horizontal light downward along the longer sides of the ultrasound probe (see Fig. 13.33A). The emerging light is focused about 10.5 mm below the bottom of the probe. The position of ultrasound transducer (T) is adjusted vertically to match its focus with the laser focus by maximizing the PA signal from a carbon fiber sample [61]. The combined ultrasound and laser light probe is mounted on a voice coil stage, which is driven by a programmed motor controller to achieve up to 10 Hz oscillations over about 9 mm and providing up to 20 images per second. This imaging system can be run in three different modes: (1) ultrasound mode, (2) PA mode, and (3) interlaced PAeultrasound mode. This system can provide images of microvascular structures to depths of 2e3 mm in vivo, and this feature is illustrated in the imaging of zebra fish in Fig. 13.33B. The image shown in Fig. 13.33B, however, was obtained by a different experimental setup where two-dimensional sections were recorded by keeping the position of laser fixed and rotating the platform holding the sample through 360 degrees [62]. The location of fluorescent protein mCherry is clearly seen in the image of zebra fish brain at the top of Fig. 13.33B. Five transverse image slices of zebra fish hindbrain are shown in the lower half of Fig. 13.33B, where each slice is separated by a depth of 0.5 mm inside the tissue.

5.2 Photoacoustic Tomography Hemoglobin is a very prominent molecular constituent of tissue, and its absorption spectrum is changed when it is bound to oxygen molecule. Oxygenated hemoglobin (HbO2) has strong absorption up to 600 nm at which point it decreases by almost two orders of magnitude and remains low till it increases again in the near-IR region (Fig. 13.34A). The absorption

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FIGURE 13.33 (A) Schematic presentation of a combined photoacoustic (PA) and ultrasound imaging system. (B) In vivo PA image of a section of zebra fish brain that expresses the fluorescent protein mCherry at the top, and five transverse image slices through the hindbrain separated by 0.5 mm. L, laser; LDP, light delivery probe; P, prism; R, reflective cone; T, ultrasound transducer; VC, voice coil. (A) With permission from T. Harrison, J.C. Ranasinghesagara, H. Lu, K. Mathewson, A. Walsh, R. Zemp, Optics Express 17 (2009) 22041. (B) With permission from D. Razansky, M. Distel, C. Vinegoni, R. Ma, N. Perrimon, R.W. Koster, V. Ntziachristos, Nature Photonics 3 (2009) 412.

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FIGURE 13.34 Absorption spectra of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the visible region (A) and absorption spectra of HbO2, Hb, and melanin in the near-infrared region (B). The isosbestic wavelengths around 584 and 800 nm can be clearly seen.

in deoxygenated hemoglobin (Hb), however, does not drop as steeply with increasing wavelengths as in HbO2, but it does go on decreasing progressively even in the near-IR region (see Fig. 13.34B). There are two isosbestic wavelengths, 584 nm in the visible and around 800 nm in near-IR, where the molar absorption coefficients of the two forms of hemoglobin are identical. Hemoglobin does not fluoresce and PA spectroscopy is an excellent technique to study its interaction with light. The PA signal is sensitive to the total concentration of hemoglobin at the isosbestic wavelengths but insensitive to the oxygenation of hemoglobin. Tuning the laser wavelength where the two forms of hemoglobin have different molar absorptions provides a second measurement, and the two measures are combined to compute the saturation of hemoglobin. The oxygen saturation of hemoglobin is related closely to the metabolic state of the imaged organ. Rapidly growing cancer cells need additional blood, and they develop a dense microvascular network around them to continue tumor growth. PA image can be used to deduce certain physiological parameters to quantify the hallmark of cancer and thereby help early cancer detection. Melanin has a broad spectral absorption in visible and near-IR wavelengths (see Fig. 13.34B), which have strong tissue penetration properties. Multiwavelength PA tissue imaging helps to distinguish between relative presence of HbO2, Hb, and melanin, and this feature has been used in monitoring the effect of drugs on growth of xenograft tumor in mice [63].

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Hand eczema is an obnoxious disease that exhibits the development of vesicles on the fingers and palm, which are extremely itchy and painful. The imaging of vessels under the thick skin of the palm helps in monitoring the progress of medication. In a typical imaging system, a laser can generate optical pulses with pulse energy of 100 mJ with pulse duration of 10 ns or shorter, which can excite PA signals at high frequency up to 100 MHz in a large area of soft tissue with good SNR. A focused ultrasound transducer scans along the tissue surface, and analogous to an ultrasound A-scan, each detected time-resolved signal on a pulsed laser excitation can be converted into a 1D image along the acoustic axis of the transducer. Combining multiple A-scan images acquired sequentially from various positions on the same plane forms cross-sectional images. The axial resolution along the acoustic axis is dependent on both the width of the laser pulse and width of the impulse response of the transducer. The lateral resolution is determined by the focal diameter of the ultrasonic transducer and the center frequency of the received PA signal. In this imaging configuration, the imaging zone is limited by the focal zone of the transducer. An alternative configuration of PA scanning is analogous to the C-scan mode in ultrasonography, in which a cross-sectional image at a certain image depth is formed, and then slices imaged at different depths can be stacked together to form a 3D image. The experimental setup used in recording PA images of Fig. 13.35 is slightly different from that shown in Fig. 13.33A. In this case the laser light is delivered by an optical fiber to a conical lens, to form a donut-shaped beam with a hole around the center. This beam geometry reduces the PA signals from the palm surface. The laser beam is focused into the skin tissue, whereas the ultrasonic transducer focuses coaxially into the same region for PA detection. The time-of-flight PA signal can be recorded at each lateral location of the ultrasound transducer. The multiplication of time-of-flight with the speed of sound provides a 1D, depth-resolved, image, where the concentration of blood vessels in stratum corneum, epidermis, and dermis is indicated in Fig. 13.35B. The focusing of the ultrasound transducer determines the lateral resolution and linear or raster scanning over the tissue yields 2D or 3D tomographic images. The cross-sectional PA image reveals the vasculature inside the palm surface as shown in Fig. 13.35C. In these experiments the laser wavelength was fixed at 584 nm and the broadband ultrasonic detector with a numerical aperture of 0.44, and a center frequency of 50 MHz produced lateral and axial resolutions of 45 and 15 mm, respectively [64].

FIGURE 13.35 Photoacoustic tomography of human palm: (A) photograph of palm with the imaged area in red square: 8 mm  8 mm; (B) axial view of the skin along the dotted vertical line shown in image (C), exhibiting stratum corneum, epidermis, dermis, epidermisedermis junction and subpapillary blood vessels; and (C) image of transverse section of palm corresponding to maximum photoacoustic signal using 584-nm laser excitation. With permission from C.P. Favazza, O. Jassim, L.A. Cornelius, L.V. Wang, Journal of Biomedical Optics 16 (2011) 016015.

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5.3 Combined Photoacoustic TomographyeOptical Coherence Tomography Imaging In optical coherence tomography (OCT), low coherence or short pulse light is split into two armsda sample arm containing the sample, and a reference arm that contains a mirror. The combination of reflected light from the sample arm and the reference arm produces an interference pattern, but this happens only when the distance traveled by the light from both arms does not differ by more than the coherence length of the source. Any light that is outside the short coherence length will not interfere. By scanning the mirror in the reference arm, a reflectivity profile of the sample is obtained. This reflectivity profile, called A-scan, contains information about the spatial dimensions and locations of structures within the sample of interest. A cross-sectional tomograph (B-scan) is obtained by laterally combining a series of the axial depth scans (A-scans). OCT was first applied for imaging in the eye, and it has had the largest clinical impact in ophthalmology. The first in vivo tomograms of the human optic disc and macula were demonstrated in 1993 [65]. Both PAT and OCT acquire depth-dependent information by time-of-flight measurements of the respective acoustic and optical waves. Combining PA imaging and OCT, the absorption-based spectroscopic contrast of PA imaging can be exploited to reveal the structure and oxygenation status of the microvasculature to depths of several millimeters, whereas the scattering-based contrast of OCT can reveal surrounding tissue microstructure. Unlike PA imaging, however, OCT is not background free and suffers from sensitivity to speckle, polarization changes, and scattering losses that alter the signal components spectrally. The diagnostic utility of combining PA imaging and OCT derives from the complementary contrast of each modality, which makes it possible to visualize vascular anatomy and tissue micromorphology. This complementary information has shown to be clinically useful for assessing the scar tissue in dyshidrotic hand eczema [65].

6. CONCLUSION In this chapter we have covered the developments in PAS based on detection of acoustic waves, resulting from nonradiative transitions of atoms and molecules in gases, liquids, and solids. The detection of nonradiative decay using optical probes has not been discussed here. The great advantage of this technique is its capability to carry out spectroscopy of materials in any form with minimal sample preparation. During the past four decades of its rediscovery, with the advent of tunable lasers, PAS has made very rapid progress due to developments in the fields of electronics and computer science. While laboratory-based investigations continue to reveal newer aspects of molecular structure and molecular dynamics, there has been a significant trend toward field-based experiments in industry, health care, and environment, including security environment. The availability of QCL systems, in the mid-IR region, has greatly added to new kind of applications of this versatile technique. PA experiments on nanoparticles are likely to benefit the developments in pharmaceutical industry and medicine. Because of its ability to perform studies of opaque or highly diffusive material, PAS has become a very widely used technique in the biomedical field. PA imaging has emerged as an advantageous modality with high resolution and deep tissue penetration. It has shown potential for molecular imaging that enables visualization of biological processes with systematically introduced contrast agents. The visualization of tumor morphology and microvessel distribution is likely to help early diagnosis and improved therapy monitoring of various diseases, especially cancer. PA imaging is the only available imaging modality that can provide high-resolution structural, functional, and molecular imaging of cells, tissues, and organs in vivo. PA technology is moving fast toward preclinical and clinical studies and is anticipated to be a powerful tool for both fundamental and clinical studies.

ACKNOWLEDGMENTS I am thankful to Professor V.P. Gupta for invitation to write this chapter. I wish to record my gratitude to Dr. M.B. Robin who helped me start photoacoustic experiments in 1978, with a gift of aluminum-coated mylar film along with the design for the microphone. I am grateful to all the authors and scientists whose results have been cited to make the presentation meaningful. This work would not have been possible without the excellent support from my family members. My daughters, Dr. Punam Rai in Varanasi and Dr. Vineeta Singh in California, have taken excellent care of my health. Sudheer and Sangeeta have helped me solve the difficulties I faced with the computer while writing this chapter. My daughterin-law, Michelle, took care of most of my needs during the course of writing, and my little grandchildren, Leo and Mia, kept me greatly entertained with their innocent inquiries.

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