Photoacoustic opaque samples

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

Photoacoustic IR spectroscopy is particularly recommended for investigation of opaque samples, i.e., samples showing very low transmittance. This technique is... 

Applications of visible/UV Fourier transform spectroscopy will continue to include those topics already mentioned. Additional areas of expansion will probably include amplitude spectroscopy5 55 which has proved useful in the past by providing both absorption magnitude and phase information on transmitting samples, photoacoustic spectroscopy which can determine the composition of opaque samples, time-resolved chemical kinetic studies, and optical probing of combustion processes. 

Such unusual phenomena as reabsorption lines of molecular oxygen and water require a confirmation by an additional independent technique. The best way is to try to detect the corresponding absorption lines by UV-visible spectroscopy. The problem is that sedimentary apatite samples are in the form of non-transparent powder, which is not suitable for optical spectroscopy. The photoacoustic spectroscopy (PAS), which allows measurement of absorption spectra of powdered opaque samples, has been chosen for this purpose (Gaft et al. 1998). PAS works as follows ... 

Microphonic Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has emerged as a useful tool for characterizing fractions of a monolayer of organic species adsorbed on opaque, high surface area samples. Such a study of calcined and sulfided hydrodesulfurization catalysts will be discussed. 

The photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s (27), but not applied to FTIR spectroscopy until a century later (28,29). Significant advantages of FTIR photoacoustic spectroscopy (PAS) include (a) spectra may be acquired on opaque materials (commonly found in pharmaceutical formulations), (b) minimal sample preparation is necessary, and (c) depth profiling is possible. 

The interaction between the thermal diffusion length, optical absorption length, and sample thickness (/) is used to describe the photoacoustic properties of a sample. Thus, a sample is described as opaque if the optical absorption length is less than the sample thickness, and light cannot be transmitted through the sample. However, when ii and /is are greater than I the sample is described as optically and thermally thin, respectively, and when jj, and /is are less than I the sample is described as optically and thermally thick, respectively. 

ABSTRACT. The construction and operation of a Michelson interferometer that permits Fourier transform photoacoustic spectroscopy of opaque and partially transparent samples at visible wavelengths is described. Multiplexing and throughput advantages are considered. A visible spectrum of Nd(III) doped laser glass is reproduced and potential kinetic applications are described. 

Two FT-IR sampling methods are commonly employed to measure IR spectra of mixture components separated by thin layer chromatography (TLC). Diffuse reflectance and photoacoustic IR spectroscopy are techniques that can be employed when sample materials are opaque. 

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