Photothermal spectroscopy

Abstract: The theory for a sensitive spectroscopy based on the photothermal deflection of a laser beam is developed. We consider cw and pulsed cases of both transverse and collinear photothermal deflection spectroscopy for solids, liquids, gases, and thin films. The predictions of the theory are experimentally verified, its implications for imaging and microscopy are given, and the sources of noise are analyzed. The sensitivity and versatility of photothermal deflection spectroscopy are compared with thermal lensing and photoacoustic spectroscopy.

Abstract: Absorption, Energy Transfer, and Excited-State Relaxation. Hydrodynamic Relaxation: Heat Transfer and Acoustics. Optical Principles for Photothermal Spectroscopy. Temperature Change and Optical Elements in Homogeneous Samples. Photothermal Spectroscopy in Homogeneous Samples. Analytical Measurement and Data Processing Considerations. Analytical Applications. Photothermal Spectroscopy of Heterogeneous Samples. Index.

Abstract: WHEN a material absorbs a photon, a fraction of the energy may be transformed into heat. A measurement of photothermal heating as a function of wavelength can provide an absorption spectrum of the material. We have recently1,2 developed a micromechanical sensor capable of detecting heat changes of the order of picojoules (10-12 J). The instrument incorporates a bilayer cantilever of micrometre dimensions which bends in response to heating. Here we show that this device can be used for photothermal spectroscopy with a power sensitivity of 100 pW—two orders of magnitude better than the sensitivity of conventional photothermal deflection spectroscopy3. The small size of the sensor allows picogram quanti-ties of material to be studied, opening up the possibility of spectro-scopic studies on individual cells and bacteria. Being based on silicon technology, the sensor should be compatible with micro-electronic circuitry.

Abstract: Optical beam deflection near a heated surface was recently introduced as a method of photothermal spectroscopy. Photothermal spectroscopy (PTS) is closely related to photoacoustic spectroscopy (PAS) in its ability to measure optical properties of opaque solids and liquids. This paper develops a theory of this effect and compares this theory with extensive experimental observations. Both are in excellent agreement. The relationship between this form of PTS and PAS is explicitly developed. Applications to the measurement of thermal diffusivity of gases is described.

Abstract: We present a highly sensitive and simple photothermal scheme for determining optical absorptions in condensed matter samples. {alpha}l values as low as 10{sup -7} and 10{sup -8} were measured for thin films and coatings and for liquids, respectively. A comparison with thermal lens effect is given, and the experimental factors limiting our sensitivity are discussed.