method and apparatus for high - sensitivity optoacoustic 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 , γ - 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 - 7 sec - 10 - 4 sec .