| abstract |
Method and apparatus for high-sensitivity optoacoustic (OA) spectroscopy of condensed matter in thin layer form. The layer to be investigated is to be in intimate contact with a solid substrate that is essentially transparent to the probe radiation, and that is capable of transmitting ultrasonic vibrations. Intermittent probe radiation, focused to a diameter typically 10 -3 -1 cm when incident on the layer, cause local heating and dimensional change in the irradiated region of the layer. The dimensional change of that region causes deformation of the adjacent substrate material, the irradiated layer region thereby becoming a source of ultrasonic waves that are spreading out in the substrate from the source region, and which can be observed with appropriate detection means at a location remote from the source. The method is applicable to liquids, solids suspended in liquids, gels, continuous or discontinuous solid films, powders, monolayers of strongly absorbing atoms, surfaces, and layerlike distributions of absorbing atoms within an essentially transparent matrix. Possible choices of probe radiation are not only the conventional ones of visible, near-UV, and near-infrared electromagnetic radiation, but also, for instance, X-rays, vacuum UV, and infrared, and matter beams, such as electron, ion, or neutral atom beams. Typical means for detecting the ultrasonic signal are ultrasonic bulk wave or surface wave transducers. For sufficiently short probe pulses, the amplitude of the ultrasonic signal is directly proportional to the absorption of the layer material at the frequency of the probe radiation, but longer pulses, such as, for instance, are obtained from mechanical choppers, can be used also. Apparatus is disclosed that minimizes interference with the measurement due to scattered light and transducer ringing, and that permits the measurement of fractional absorption down to 10 -6 or less. |