Bell Telephone Laboratories

Andrew C Tam

Chandra K Patel

Method and apparatus for high-sensitivity (.alpha..ltorsim.10.sup.-6 cm.sup.-1) optoacoustic (OA) spectroscopy of substantially transparent bulk condensed matter. Part of the sample to be investigated is irradiated by pulses of radiation, typically from a pulsed dye laser. The small amount of energy absorbed by the sample causes thermal expansion of the irradiated region, resulting in cylindrical stress waves being radiated from that region. These stress waves can be observed with appropriate detection means at a location remote from the source. The method is applicable to essentially transparent liquids, and solids, and to absorptive solids suspended in liquids, and the like. Possible choices of probe radiation are not only the conventional ones of visible, near UV, and near infrared electromagnetic radiation, but also, for instance, .gamma.-rays, x-rays, vacuum UV, and far infrared. Two conditions are shown to exist that relate pulse duration, beam size, and various material parameters, observation of which results in optimization of the sensitivity of the method, and the sensitivity is typically highest for pulses having duration of about 10.sup.-7 sec-10.sup.-4 sec.

Optoacoustic spectroscopy of condensed matter in bulk form

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.sup.-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.sup.-6 or less.

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Sunnyvale, CA, United States of America

Andrew C Tam