Background light Nonmodulated white light used to saturate the photochemistry of the photosystems.

Depth-profile Depth specific characterization of a sample based on the frequency dependence of the thermal sample.

Modulation frequency The amplitude of the measuring light beam chopped at required frequency to generate the photoacoustic signal at the same frequency.

Optical absorption length Penetration depth of the incident light flux.

Phase angle Phase angle shift of photoacoustic signal relative to the modulation frequency.

Thermal deactivation Relaxation pathway of an optically excited molecule, wherein the excitation energy is converted into heat.

Thermal diffusion length The thermal diffusion length is defined by fis = Jnf where a is the thermal diffusivity of the sample and f is frequency.

This work is dedicated to Professor Danuta Frackowiak, Institute of

Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan,

Poland, on the occasion of her 75th birthday.

Thermal diffusivity The thermal diffusivity is given by a = pc where k is the thermal conductivity, p is the density, and c is the thermal capacity.

THE PHOTOACOUSTIC effect is based on the conversion of absorbed light energy into heat by means of radi-ationless deexcitation processes. On irradiation of a solid or liquid sample by intensity modulated light, a heat wave of the same frequency is generated in the sample. The heat wave is transferred by diffusion to the surface of the sample, where oscillatory thermal effects are generated in the coupled gas. These can be detected as an acoustic signal. This technique is known as gas-coupled photoacoustic spectroscopy.

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