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The phenomenon of photochemical nucleation of particles (also called laser snow) has been observed by many workers, for example, in metal vapor system1 and organic vapors.2,3 Although this laser-snow effect seems to occur generally under the right excitation, temperature, and pressure conditions, the phenomenon remains poorly understood. One problem is lack of method for detecting the onset of photonucleation of particles, which has up to now been detected, e.g., by light scattering1,2 or electrostatic deflection.3

We have developed a single-ended pulsed radiometry technique for the remote measurement of thermal diffusivity, absolute absorption coefficients, thickness, or layered structure of thin films, which can be free-standing or on a substrate. Our technique relies on the use of a short-pulsed N2 laser beam to heat up rapidly the sample surface (e.g., by a few degrees Celsius), and the detection of the shape of the transient IR thermal radiation that is backscattered from the sample surface. No sample damage occurs, since the laser pulse (< 1-mJ energy and 8-nsec duration) is gently focused to a spot size of ~4-mm diam at the sample. We have developed exact analytical solutions to permit us to analyze the observed IR signal profile to derive the desired parameters of the sample. We have applied this single-ended pulsed photothermal radiometry (PPTR) technique to various cases of metal films, polymer films, and paper (including paper currency) and have demonstrated several important advantages of the PPTR technique for material testing applications; namely, it is noncontact, fast, nondestructive, and applicable to materials with an inaccessible backside. This should be compared with traditional methods1 of measuring thermal diffusivity; these methods are usually double-ended (i.e., requiring transmission probing) or require the attachment of electrical or thermocouple leads to the sample.

A pulsed pump beam provides photo-thermal labelling of flowing aerosols and vapors. The labelling is detected by transient refraction or Mie scattering variations in a displaced probe beam, providing a new technique for flow and spectroscopic measurements. Authors keywords: Aerosol, flow, photo-thermal, spectroscopy, thermal lens, Mie scattering, laser applications.