I. Delgadillo

On the thermal characterization of two-layer systems by means of the photoacoustic effect

In this work, the problem of the thermal characterization of two-layer systems by means of the photoacoustic technique is discussed. For a two-layer system under rear-side illumination conditions, we have applied the Rosencwaig and Gersho model for calculating the pressure fluctuation in the photoacoustic gas chamber. The limiting cases in which both layers are thermally thin, thermally thick and one layer is thermally thin and the other is thermally thick are discussed. When both layers are thermally thin, a consistent equation for the heat capacity is obtained and an effective thermal diffusivity equation is derived when both layers are thermally thick. In order to test our theoretical results, we apply them to two-layer systems consisting of AlGaAs layers of different Al concentrations, grown by liquid phase epitaxy on GaAs substrates. The results of our measurements are in good agreement with the theoretical predictions. Our results show the general character of the expression for the effective thermal diffusivity of two-layer systems reported by Mansanares et al (1990 Phys. Rev. B 42 4477).

On the use of the photoacoustic technique for corrosion monitoring of metals: Cu and Zn oxides formed in tropical environments

Samples of electrolytical copper and zinc were exposed for nine months to a naturally occurring marine environment of the Yucatan Peninsula in Mexico and were analysed without any additional pretreatment. The study is mainly based upon the measurements of the samples thermal properties by using the non-destructive photoacoustic technique. The thermal diffusivity may determine with a conventional open photoacoustic cell and the thermal effusivity using a new experimental configuration.

Photoacoustic thermal characterization of spark‐processed porous silicon

A non‐separation approach to determine the spark‐processed porous silicon thermal parameters is presented. This thermal characterization was performed through application of the photoacoustic technique, in combination with compositional models for spark‐processed porous silicon samples. The thermal parameters obtained are in agreement with existing studies about the composition of this material. This approach opens the possibility of performing the thermal characterization of other porous semiconductors and analogous materials.

Photoacoustic determination of recombination parameters in CdTe/glass system

Using the open photoacoustic cell technique we analyze, for a polycrystal CdTe/glass system, the dependence upon the photoacoustic signal phase of both the rear and front semiconductor carrier surface velocities and the bulk recombination time.

Tunneling lifetime broadening on quantum well in an electric field

The effect of an electric field on the electron energy and resonant width in a quantum well have been investigated theoretically. By considering the tunneling of electrons under the influence of an electric field in the quantum well, we obtain a shifted and broadened energy level spectrum from the tunneling of electrons. The lifetime broadening due to the rapid tunneling escape of electrons is related to the energy width of the resonant level by the uncertainty principle. Such lifetime decreases when the electric field increases.

PHOTOACOUSTIC TECHNIQUE FOR MONITORING THE THERMAL PROPERTIES OF POROUS SILICON

The use of the photoacoustic technique to monitor the ther- mal properties of materials that can be obtained only as parts of multi- component samples is illustrated by performing the thermal character- ization of two porous materials: porous silicon obtained from n-type crystalline silicon through the spark process and that obtained through the electrochemical etching method. This nonseparative, and hence nondestructive, approach makes use of an effective thermal diffusivity treatment based on the analogy between thermal and electrical resis- tances, in combination with simplified compositional models for the cor- responding multicomponent systems. The thermal parameters obtained are in agreement with existent studies concerning the composition of these materials. This approach offers the possibility of performing the thermal characterization of other porous semiconductors and analogous materials.

On the use of the photoacoustic technique for the measurement of the thermomechanical properties of semiconductor two-layer systems

Abstract The photoacoustic technique is used in semiconductor two-layer systems for the determination of thermal properties and thermal expansion coefficient. The two-layer systems studied were amorphous silicon-glass and Al0.2Ga0.8AsGaAs. Our results show that the proposed method is a reliable technique for the characterization of other semiconductor two-layers systems.

Lattice vibration effects on electronic structure and photoemission

Abstract A theoretical approach to the electronic structure of solids at finite temperature has been developed on the basis of the adiabatic approximation. For any given temperature, correlated ion core displacement configurations on a large cluster are determined, which are consistent with experimental phonon dispersion relations. The electron Bloch spectral function, i.e. a generalized band structure, and photoemission intensities are obtained by a tight-binding recursion method for each configuration followed by a configurational average. Calculations for the 3d-band region of Cu yield the vibration-induced hole lifetime broadening (typically about 0.35 eV at 900 K) and real energy shifts (up to ±0.1 eV). Due to a compensation by thermal lattice dilation, the d-band width changes only marginally with T. Comparison with experimental photoemission data from Cu(1 1 0) shows good agreement for the temperature dependence of peak height, line shape and energy shift.

Modelling and analysis of experimental photothermal beam deflection signals in gases

Abstract Photothermal beam deflection is a powerful tool for studying molecular dynamics in gases. We present a new theoretical model that extends the versatility of the technique substantially. The model yields excellent agreement with experimental temporal signal shapes obtained with a continuously tunable CO 2 laser and C 2 H 4 or SF 6 gas buffered in Ar or He/Ar mixtures. In agreement with experimental data, the derived first-order rate coefficient k e or the molecular relaxation time τ relax are strongly affected by the change of the Ar content in the buffer gas, an experimental regime that could not be treated with previous models. Finally, we present a concept that permits to find out the shortest τ relax that can be determined from experimental signal shapes with a specified accuracy and signal-to-noise ratio, or vice versa, to quantify optimum pump beam radii and pump–probe beam distance in a certain experiment.

Photothermal studies of laser-excited molecules in gas mixtures

Photothermal studies of molecular dynamics in gases and gas mixtures have been performed by means of the photothermal beam deflection technique. The experimental setup employs a continuously tunable high pressure CO2 laser with a linewidth of only 0.017 cm−1 and energy fluences up to 100 mJ cm−2 for exciting the molecules to specific energy levels, and a collinear HeNe laser as probe. Measurements have been performed for different gases, gas pressures, pump beam diameters and distances between pump and probe beams. The measured signals are analyzed numerically with a theoretical model that is valid for all time regimes and generally applicable. From such analyses nonradiative molecular deactivation rates as well as thermal conductivity and sound speed data of the gases can be determined simultaneously. Furthermore, we present a new criterion which is suited to define optimum experimental conditions (mainly pump beam radius) for the determination of gas relaxation times with a given accuracy by the numeric...