Photoacoustic effect

[1][page needed][2] The photoacoustic effect is quantified by measuring the formed sound (pressure changes) with appropriate detectors, such as microphones or piezoelectric sensors.

It is useful for substances in extremely low concentrations, because very strong pulses of light from a laser can be used to increase sensitivity and very narrow wavelengths can be used for specificity.

The discovery of the photoacoustic effect dates back to 1880, when Alexander Graham Bell was experimenting with long-distance sound transmission.

Through his invention, called "photophone", he transmitted vocal signals by reflecting sun-light from a moving mirror to a selenium solar cell receiver.

[3] As a byproduct of this investigation, he observed that sound waves were produced directly from a solid sample when exposed to beam of sunlight that was rapidly interrupted with a rotating slotted wheel.

[4] He noticed that the resulting acoustic signal was dependent on the type of the material and correctly reasoned that the effect was caused by the absorbed light energy, which subsequently heats the sample.

In 1938 Mark Leonidovitch Veingerov revived the interest in the photoacoustic effect, being able to use it in order to measure very small carbon dioxide concentration in nitrogen gas (as low as 0.2% in volume).

The primary universal mechanism is photothermal, based on the heating effect of the light and the consequent expansion of the light-absorbing material.

In detail, the photothermal mechanism consists of the following stages: The main physical picture, in this case, envisions the original temperature pulsations as origins of propagating temperature waves ("thermal waves"),[11] which travel in the condensed phase, ultimately reaching the surrounding gaseous phase.

[1][page needed][2][12] This property of the thermal wave confers unique features to the detection of light absorption by the photoacoustic method.

There, the electric field which is formed in the reaction center, following the light induced electron transfer process, causes a micro electrostriction effect with a change in the molecular volume.

10–10000 Hz) and the modulated photoacoustic signal is analyzed with a lock-in amplifier for its amplitude and phase, or for the inphase and quadrature components.

In this case the time scale is between less than nanoseconds to many microseconds [1][page needed][2][22][23] The photoacoustic signal, obtained from the various pressure sensors, depends on the physical properties of the system, the mechanism that creates the photoacoustic signal, the light-absorbing material, the dynamics of the excited state relaxation and the modulation frequency or the pulse profile of the radiation, as well as the sensor properties.

In this case the ordinary method of absorption spectroscopy, based on difference of the intensities of a light beam before and after its passage through the sample, is not practical.

A typical parameter which governs the signal in this case is the "thermal diffusion length", which depends on the material and the modulation frequency and ordinarily is in the order of several micrometers.

[1][page needed][12] The signal is related to the light absorbed in the small distance of the thermal diffusion length, allowing the determination of the absorption spectrum.

[29] Another application of the photoacoustic effect is its ability to estimate the chemical energies stored in various steps of a photochemical reaction.

As noted above, the photoacoustic signal from wet photosynthesizing specimens (e.g. microalgae in suspension, sea weed) is principally photothermal.

The photoacoustic signal from spongy structures (leaves, lichens) is a combination of photothermal and photobaric (gas evolution or uptake) contributions.

Energy storage and the intensity of the photobaric signal are related to the efficiency of photosynthesis and can be used to monitor and follow the health of photosynthesizing organisms.