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The possibility of using a minimal-volume photoacoustic cell to perform spectroscopy of samples is discussed. It is shown that this alternative signal-to-noise-enhanced photoacoustic configuration allows one to obtain both absorption and transmission spectra with minimal experimental arrangement and cell machining requirements. The theoretical model is presented, the use of which is exemplified by a complete optical and thermal characterization of leaves.

Using two different photoacoustic techniques for a two-layer system of variable thickness, we show that the thermal diffusivity and the thermal conductivity are completely determined, based upon the efFective-sample model widely used in heat-transfer problems. A procedure to establish a standard photothermal technique for measuring both the thermal diffusivity and the thermal conductivity is also discussed.

The band-gap energies of the CdS semiconductor are obtained by a photoacoustic spectroscopy (PAS) technique over a range of temperature of thermal annealing (TTA), in which the evolution of the sample structure is characterized by x-ray diffraction patterns. The PAS experiment gives a set of data for the band-gap shift in the region of the fundamental absorption edge. With increasing TTA the band-gap shift increases up to a critical TTA when its slope decreases in a roughly symmetrical way. It is suggested that at this temperature a cubic to hexagonal-lattice transition occurs.The use of photoacoustic spectroscopy (PAS) has become well established in the past few years mainly due to its importance as a guide in the study of optical properties of semiconductors.'-" PAS can lead, for instance, to the value of the band-gap energy, which is an important parameter in electronic and optoelectronic design.'-" It is worth saying that less attention has been paid to applications of this technique to investigate the band-gap shift (BGS) of intrinsic and extrinsic semiconductors. In particular, the CdS semiconductor, which presents a highly stable hexagonal structure (y_CdS,= can also be obtained in the metastable cubic phase ,8-CdS.'2-'5 Cardona et al. by means of reflectivity measurements at room temperature found the optical band gap of the two phases of CdS thin films, and they could not infer any other conclusions except that the energy difference between the cubic and hexagonal CdS energy gap differs less than 0

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