Process acoustic emission spectroscopy

Emission spectroscopy (fluorescence, phosphorescence) of the excited species allows their lifetime and multiplicity to be evaluated and a term scheme to be set up 535>. The influence of varying concentrations of reactants and additives on the quantum yields of the luminescent processes as well as on the product distribution gives information about the reaction mechanism 436>. Radiationless processes can be directly observed by studying the optical-acoustic relaxation of periodically irradiated solutions 229>. 

Reliability of electronic devices is caused predominantly by failures which result from the latent defects created during the manufacture processes or during the operating life of the devices. A search for new nondestructive methods to characterise quality and predict reliability of vast ensembles became a trend in the last four decades (Saveli etal. 1984), (Hartler et al. 1992), (Vandamme 1994), (Hashiguchi et al. 1998). The most promising methods to provide a non-destructive evaluation are an analysis of the electron transport parameters. Experiments are based on the measurements of device VA characteristics, nonlinearity using the non-linearity index (NLl), electronic noise spectroscopy, electro-ultrasonic spectroscopy and acoustic emission. These ones apply to both active and passive components, i.e., bipolar devices and MOS structures, on one hand, and resistors and capacitors on the other. 

In a broad sense, spectroscopic methods applied in process analytics comprise widely used techniques like UVA IS, mid-IR, NIR, NMR and XRF, and less frequently used ones, such as Raman spectroscopy, fluorescence, chemiluminescence, acoustic emission and dielectric specfloscopy. Upcoming in-process analysis techniques are 2D-fluorescence, and laser absorption specfloscopy (LAS) with tuneable lasers and ppm level sensitivity. The availability of mini-spectrometers (e.g. UVA IS/NIR) is not highly relevant in plant environments where safety is of primary concern. 

In passive ultrasonics, which is usually referred to as acoustic emission, the source of the ultrasound is the process itself. Passive acoustic spectroscopy is a measure of the inherent acoustic output of a system or process. Physical processes producing acoustic emission (AE) include particle collisions, fracturing of solids, turbulent gas flow, gas evolution, fermentation, cavitation, boiling multiphase flow, and... 

After selective absorption of radiation, excited molecules may relax either by emission of radiation or by non-radiative processes (cf Chapter 3, Fig. 3.1). In photoacoustic measurements, the conversion of absorbed radiation into thermal energy is utilized. This type of conversion results in changes in the sample s thermodynamic parameters such as temperature or pressure. Changes in pressure generate acoustic waves, which eventually will be transferred to the surroundings of the sample (Fig. 5.12) where they can be measured by a sensitive microphone see Fig. 5.13 (photoacoustic spectroscopy (PAS)). 

Photoacoustic spectroscopy involves the modulation of IR radiation at a frequency in the acoustic range. The radiation strikes a sample and is subsequently turned into heat energy by nonradiative deexcitation processes (i.e., excitation is loss by thermal motion and not by the emission of energy). Heat-generated thermal waves then propagate to the sample surface and facilitate the rapid expansion and contraction of a carrier gas. A microphone is used to sense the change in pressure in the enclosed cell. 

---