Photothermal spectroscopy

Photothermal spectroscopy is a group of high sensitivity spectroscopy techniques used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is the change in thermal state of the sample resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature thereby influencing the sample thermodynamic properties. Measurement of the temperature, pressure, and/or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic measurements.

As with photoacoustic spectroscopy, photothermal spectroscopy is an indirect method for measuring optical absorption. Indirect methods do not measure light transmission or emission but rather measure an effect of sample absorption. The term "indirect" applies to the optical measurement, not the optical absorbance.

There are several methods and techniques used in photothermal spectroscopy. Each of these has a name indicating the specific physical effect measured.
* Photothermal lens spectroscopy (PTS or TLS) measures the thermal blooming that occurs when a beam of light heats a transparent sample. It is typically applied for measuring minute quantities of substances in homogeneous gas and liquid solutions.
* Photothermal deflection spectroscopy (PDS), also called the mirage effect, measures the bending of light due to optical absorption. This technique is particularly useful for measuring surface absorption and for profiling thermal properties layered materials.
* Photothermal diffraction, a type of four wave mixing, monitors the effect of transient diffraction gratings "written" into the sample with coherent lasers. It is a form of real-time holography.
* Photothermal emission measures an increase in sample infrared radiance occurring as a consequence of absorption. Sample emission follows Stefan's law of thermal emission. This methods is used to measure the thermal properties of solids and layered materials.

References

* J. A. Sell Photothermal Investigations of Solids and Fluids Academic Press, New York 1989
* D. P. Almond and P. M. Patel Photothermal Science and Techniques Chapman and Hall, London 1996
* S. E. Bialkowski Photothermal Spectroscopy Methods for Chemical Analysis John Wiley, New York 1996