Pulsed indirect photoacoustic spectroscopy: application to remote detection of condensed phases

Abstract

The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO(2) laser (tunable within the 9-11-microm region) provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.

Standoff Chemical Detection Using Laser Absorption Spectroscopy: A Review

“…Since the 21st century, LPAS has been gradually used in open-path detection, but it was subjected to the large size, heavy-weight, MIR laser sources, i.e., high power OPO or CO 2 lasers, etc. [139,140]. With the development of laser technology in recent years, especially the availability of commercialized QCLs and ECQCLs, LPAS has been becoming a research hotspot to achieve standoff measurement by using QCL/ECQCL as a light source and deforming traditional measurement structures [141].…”

Deep Ultraviolet Standoff Photoacoustic Spectroscopy of Trace Explosives

“…The evolution of heat at the solid-air interface relies on sample properties including the absorption coefficient and the thermal diffusivity. 7,[9][10][11] The absorption coefficient determines the heated depth and temperature rise of the solid sample. The thermal diffusivity (a ¼ K/pC p ) determines the heat flow through the system, where K is the thermal conductivity, C p is the heat capacity, and p is the solid density.…”

“…Moreover, the photoacoustic signal decays by 1/ r with detection distance, compared with 1/ r 2 for other spectroscopic techniques. 7,8…”

Chemical and explosive detections using photo-acoustic effect and quantum cascade lasers

“…One earlier study, which the author named PADAR (photoacoustic detection and ranging), demonstrated standoff detection and ranging for gas vapors using PA effect 6 . In later years PA spectroscopy was also applied to condensed matter and liquids for standoff detection and sensing 7,8 .…”