The importance of interstitial gas expansion in infrared photoacoustic spectroscopy of powders

Abstract

Model calculations and experiments have been performed to explore the modes of photoacoustic signal generation from highly porous powders. These indicate that two modes of signal generation are important: (i) a thermal mode resulting from heating of the sample and conduction to the transducer gas above the sample, and (ii) a pressure mode resulting from the volumetric expansion of the interstitial gas in the porous sample. For highly porous materials, such as silica, the pressure mode can dominate the photoacoustic signal. Under conditions where the pressure mode is dominant, the frequency dependence of the acoustic magnitude and phase is similar to that of a homogeneous sample in photoacoustic saturation, however, the photoacoustic signal will still scale with absorption coefficient in this regime. The pressure mode of signal generation can be attenuated by compaction of the powdered sample to reduce the porosity. Unambiguous determination of photoacoustic saturation is shown to require measurements as functions of both frequency and porosity. The experimental results show that for highly porous silica even the strongest absorption bands are not in photoacoustic saturation. The results also suggest that infrared light scattering by powders of submicron-sized particles is not of major significance. This suggests that photoacoustic spectroscopy is extremely well suited to the study of highly porous materials.

Photoluminescence of highly porous nanostructured Si-based thin films deposited by pulsed laser ablation

“…[32][33][34][35] The rapid reactions of the samples on 30 min of air exposure are clearly seen by photoacoustic FTIR ͑Fig. 5, which is separated into three frequency ranges to better show the peak intensities͒.…”

Application of infrared photoacoustic spectroscopy in catalysis

“…A successful application of this technique has been continuing for the investigations of powdered or porous samples [51][52][53], solids and surface species [54,[56][57][58][59][60][61], chemisorbed species [62], catalysis [63]. Gardella et al have demonstrated the capabilities of FT-IR/PAS to analyze catalytic systems, providing qualitative and quantitative analysis of both gas phase and adsorbed species.…”

“…Some of them contain general content of the use and application of FT-IR/PAS [22][23][24][25][26][27][28][29][30][31], others are more specific and are devoted to real-time PA parallel detection of products from catalyst libraries [32], step-scan and depth profiling analysis [33][34][35][36][37][38][39][40], the effect of particle size on FT-IR/PAS spectra [41][42][43][44], the temperature effect on PA signal [45], sample emission effects [46], and synchrotron IR/PAS [47]. During a PA measurement the sample is enclosed in a small, tightly closed sample compartment called PA cell [48][49][50][51][52][53][54]. Nowadays commercially available PA detector fits to most modern FT-IR spectrometers [55].…”

ESTIMATION Of PARTICLE SIZE bY DRIFTS and FTIR-PAS

“…Opaque and light scattering samples that cannot be measured by other IR techniques but can be easily analyzed by FTLR-PAS. FTIR-PAS has been used to study powder samples (Monchlin et al, 1984;Rockley et al, 1984;McGovern et al, 1985;Fateley, 1986a and1986b;Belton et al, 1987). It has been demonstrated that the intensity of FTIR-PAS spectra increases with a decrease in particle size.…”