**2. The history of photoacoustic effect definition**

The photoacoustic effect was first investigated in the 1880 by Alexander Graham Bell. During his experiments with the "photophone", which carried an acoustic signal with a beam of sunlight that was reflected by an acoustic modulated mirror, he noticed that a

Photoacoustics: A Potent Tool for the Study of Energy Fluxes in Photosynthesis Research 259

reinforcements were constantly occurring, which became more and more marked as the true

As shown in Fig.3 the light pulse was absorbed by the sample of matter, and then converted to energy equivalents. The resulting energy will be partially radiated as heat (generation of wave), consequently a pressure wave can be detected by an acoustic sensor. The pressure waves are characteristic of the sample and are used to determine composition,

"When at last the frequency of interruption corresponded to the fundamental frequency of the resonator, the sound produced was so loud that it might have been heard by an

Some time later Viengerov (1938, 1940) used the photoacoustic effect for the measurements of light absorption in gases and obtained quantitative estimates of concentration in gas mixtures based on signal magnitudes. He used blackbody infrared sources, for radiation input and a microphone to detect the acoustic signal. However, his results were affected by the relatively low sensitivity of his microphone as well as undesired photoacoustic effects

In 1946 Gorelik suggested the use of the photoacoustic effect for the determination of energy transfer rates between vibrational and translational degrees of freedom of gas molecules. When a sample of gas in a photoacoustic cell is irradiated by photons, which it absorbs, the absorbed energy is used to excite to vibrational or vibrational-rotational energy state in the infrared, visible or ultraviolet ranges of the electromagnetic spectrum. (Rosencwajg, 1980). Between 1950 and 1970 the photoacoustic gas analyzer employing a conventional light source gave way to the more sensitive gas chromatography technique. Similarly, the

The development of the laser in the early 1970s had critical implications for photoacoustic spectroscopy. Lasers provided high intensity light at a tunable frequency, which allowed an increase in sound amplitude and sensitivity. In 1968 Kerr and Atwood were the first to apply a CO2 laser to illuminate a photoacoustic cell (Kerr and Atwood, 1968). More interest in the method was generated when Kreuzer (1971) demonstrated part-per-billion (ppb) detection sensitivities of methane in nitrogen using a helium-neon laser excitation source, and later (Kreuzer 1972) sub-ppb concentrations of ammonia and other gases in mixtures,

Later, in the 1980s, Patel and Tam (Patel and Tam, 1981, Tam, 1986) have established not only the modern technological basis of the method, by using pulsed lasers as the light source and piezoelectric transducers as the photoacoustic detectors, but also provided the complete theoretical description of the photoacoustic phenomenon, based on the original concepts of Landau and Lifschitz (Landau and Lifschitz, 1959). Since then the photoacoustic method has been adapted and further developed by several groups (Rothberg 1983,

from the glass chamber, a problem that persists in modern photoacoustic analysis.

spectrophone gave way to the more versatile infrared spectrophotometer.

pitch of the resonator was neared.

concentration, and other thermophysical properties.

audience of hundreds of people." (Bell, 1881).

using infrared CO and CO2 lasers.

Braslavksy 1986).

Fig. 3. The schematic of sound signal detection as used by Bell (Bell, 1881).

rapidly interrupted beam of sunlight focused on a solid substance produces an audible sound. He observed that the resulting acoustic signal is dependent on the composition of the sample and correctly conjectured that the effect was caused by absorption of the incident light.

Recognizing that the photoacoustic effect had applications in spectroscopy, Bell developed the "spectrophone," essentially an ordinary spectroscope equipped with a hearing tube instead of an eyepiece. (Fig. 1). Samples could then be analyzed by sound when a source of light was applied.

Fig. 1. Historical setup used by Bell (Bell, 1881).

As noted by Bell, "the ear cannot of course compete with the eye for accuracy", when examining the visible spectrum. Bell published the results in a presentation to the American Association for the Advancement on Science in 1880 (Bell 1880).

In his paper, Bell described for the first time the resonant photoacoustic effect: "When the beam was thrown into a resonator, the interior of which had been smoked over a lamp, most curious alternations of sound were observed. The interrupting disk was set rotating at a high rate of speed and was then allowed to come gradually to rest. An extremely musical tone was at first heard, which gradually fell in pitch as the rate of interruption grew less." (Fig. 2) The loudness of the sound produced varied in the most interesting manner. Minor

Fig. 2. Schematic setup of "photophone" used by Bell. As light source the sun (or a conventional radiation source) was employed. The acoustic signal was detected with a hearing tube and the ear (Bell, 1881).

rapidly interrupted beam of sunlight focused on a solid substance produces an audible sound. He observed that the resulting acoustic signal is dependent on the composition of the sample and correctly conjectured that the effect was caused by absorption of the incident

Recognizing that the photoacoustic effect had applications in spectroscopy, Bell developed the "spectrophone," essentially an ordinary spectroscope equipped with a hearing tube instead of an eyepiece. (Fig. 1). Samples could then be analyzed by sound when a source of

As noted by Bell, "the ear cannot of course compete with the eye for accuracy", when examining the visible spectrum. Bell published the results in a presentation to the American

In his paper, Bell described for the first time the resonant photoacoustic effect: "When the beam was thrown into a resonator, the interior of which had been smoked over a lamp, most curious alternations of sound were observed. The interrupting disk was set rotating at a high rate of speed and was then allowed to come gradually to rest. An extremely musical tone was at first heard, which gradually fell in pitch as the rate of interruption grew less." (Fig. 2) The loudness of the sound produced varied in the most interesting manner. Minor

Fig. 2. Schematic setup of "photophone" used by Bell. As light source the sun (or a conventional radiation source) was employed. The acoustic signal was detected with a

light.

light was applied.

Fig. 1. Historical setup used by Bell (Bell, 1881).

hearing tube and the ear (Bell, 1881).

Association for the Advancement on Science in 1880 (Bell 1880).

reinforcements were constantly occurring, which became more and more marked as the true pitch of the resonator was neared.

As shown in Fig.3 the light pulse was absorbed by the sample of matter, and then converted to energy equivalents. The resulting energy will be partially radiated as heat (generation of wave), consequently a pressure wave can be detected by an acoustic sensor. The pressure waves are characteristic of the sample and are used to determine composition, concentration, and other thermophysical properties.

Fig. 3. The schematic of sound signal detection as used by Bell (Bell, 1881).

"When at last the frequency of interruption corresponded to the fundamental frequency of the resonator, the sound produced was so loud that it might have been heard by an audience of hundreds of people." (Bell, 1881).

Some time later Viengerov (1938, 1940) used the photoacoustic effect for the measurements of light absorption in gases and obtained quantitative estimates of concentration in gas mixtures based on signal magnitudes. He used blackbody infrared sources, for radiation input and a microphone to detect the acoustic signal. However, his results were affected by the relatively low sensitivity of his microphone as well as undesired photoacoustic effects from the glass chamber, a problem that persists in modern photoacoustic analysis.

In 1946 Gorelik suggested the use of the photoacoustic effect for the determination of energy transfer rates between vibrational and translational degrees of freedom of gas molecules. When a sample of gas in a photoacoustic cell is irradiated by photons, which it absorbs, the absorbed energy is used to excite to vibrational or vibrational-rotational energy state in the infrared, visible or ultraviolet ranges of the electromagnetic spectrum. (Rosencwajg, 1980).

Between 1950 and 1970 the photoacoustic gas analyzer employing a conventional light source gave way to the more sensitive gas chromatography technique. Similarly, the spectrophone gave way to the more versatile infrared spectrophotometer.

The development of the laser in the early 1970s had critical implications for photoacoustic spectroscopy. Lasers provided high intensity light at a tunable frequency, which allowed an increase in sound amplitude and sensitivity. In 1968 Kerr and Atwood were the first to apply a CO2 laser to illuminate a photoacoustic cell (Kerr and Atwood, 1968). More interest in the method was generated when Kreuzer (1971) demonstrated part-per-billion (ppb) detection sensitivities of methane in nitrogen using a helium-neon laser excitation source, and later (Kreuzer 1972) sub-ppb concentrations of ammonia and other gases in mixtures, using infrared CO and CO2 lasers.

Later, in the 1980s, Patel and Tam (Patel and Tam, 1981, Tam, 1986) have established not only the modern technological basis of the method, by using pulsed lasers as the light source and piezoelectric transducers as the photoacoustic detectors, but also provided the complete theoretical description of the photoacoustic phenomenon, based on the original concepts of Landau and Lifschitz (Landau and Lifschitz, 1959). Since then the photoacoustic method has been adapted and further developed by several groups (Rothberg 1983, Braslavksy 1986).

Photoacoustics: A Potent Tool for the Study of Energy Fluxes in Photosynthesis Research 261

These phenomena include states of the oxygen evolving complex in leaf tissue (Canaani et al. 1988) and the earliest steps of photosynthetic electron transport in photosystems (Arata

In the work of da Silva (da Silva et al, 1995) the photoacoustic method has been demonstrated to be suitable, efficient and reliable technique to measure photosynthetic O2

The O2 evolution in intact undetached leaves of dark adapted seedlings was measured during photosynthesis with the objective to detect genetic differences (da Silva et al., 1995). Photoacoustic method can also measure state photosynthesis in intact cells and leaf tissue if the measuring pulses are given in combination with continuous background light

A simple technique based on photoacoustic measurements allowed us to determine the biomass, as well as the efficiency of photosynthesis, for different taxa of phytoplankton in

The experimental system is shown schematically in Figure 5 and 6 (Dubinsky et al. (1998),

Fig. 5. Photoacoustic phytoplankton cell (Dubinsky et al., 1998). The reddish algal culture is

The laser pulse is incident upon the suspension of algae, whose pigments absorb part of the laser beam. A variable fraction of the absorbed light pulse is stored in the photochemical products of photosynthesis. The remainder of the absorbed light is converted to heat, producing an acoustic wave that is intercepted by a detector (for details see Pinchasov *et al*. 2005). The signal contains a noisy background and later reflections from the walls of the

and Parson 1981; Delosme et al. 1994, Edens et al. 2000).

Mauzerall et al. (1998), and Pinchasov et al. (2005)).

of *Porphyridium cruentum* and the laser pulse at 560nm.

vessel as well as from impedance mismatch within the detector.

evolution in leaves.

(Kolbowski et al. 1990).

situ (Dubinsky et al., 1998).

**5. Phytoplankton** 
