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Detection methods for Photo-Acoustic Spectroscopy
Photo-Acoustic Spectroscopy (PAS) is a useful technique that measures the non-radiative transitions of a material. It is particularly beneficial for samples of low fluorescence, reflectivity, or transmission, which can make conventional spectroscopic techniques difficult. PAS stems from the photoacoustic effect, which is the formation of acoustic waves that are emitted from a sample illuminated by modulated light. The non-radiative transitions resulting from the electron excitation cause the sample to heat (and expand) then cool (and contract) periodically. This expansion and contraction, if coupled with the surrounding gas, cause acoustic pressure waves (sound).
This sound has traditionally been measured using conventional microphones, either capacitive or piezoelectric. Evan Bieske’s research group at the University of Melbourne has shown the use of microcantilevers as novel detectors of photoacoustic waves in gaseous mixtures (http://www.ions.org.au/page4/). Current research is investigating whether this method can be used to detect photoacoustic waves from solids. This is being done by using a photoacoustic cell (obtained from the Bieske research group) and a modulated laser beam to excite the sample, then measuring the deflection of the reflected laser beam on a Position Sensitive Detector (PSD). The amplitude of the deflection of the microcantilever corresponds to the intensity of the photoacoustic waves, and hence the non-radiative transitions occurring in the sample.
Other methods to measure and characterise the non-radiative transitions in materials is also being researched. This is done by attempting to measure the expansion of the sample directly, using a conventional Atomic Force Microscope (AFM) set up. By performing an AFM scan of a surface of known morphology, then another scan of the surface while the sample is being illuminated by a modulated laser beam, the difference in the surface can be deduced and contributed directly to the non-radiative transitions.