The effect of standing acoustic waves on the formation of laser-induced air plasmas.

The expected location of an air plasma produced by a focused YAG laser pulse has been found to be influenced by the acoustics of the surrounding environment. In open air, the expected location of a laser-induced air plasma is centered close to the focal point of the lens focusing the laser beam. When confining the same beam coaxially along the interior of a quartz tube, the expected location of the air plasma shifts away from the focal point, toward the focusing lens, in a region of less laser fluence. This shift is caused by an interaction between standing acoustic waves (formed from sound waves produced by previous laser-induced plasmas) and the impinging laser pulse. Standing acoustic waves in a tube produce areas (antinodes) of slightly higher and slightly lower pressure than ambient atmospheric conditions, that in turn have a noticeable affect on the probability of creating an air plasma at a given location. This leads to two observed phenomena: Increased probability of air plasma formation before the optical focal point is reached, and the formation of distinct (separate) air plasmas at the antinodes themselves.

Utilization of the coherence function with Welch's method for signal analysis in low resolution laser-induced breakdown spectroscopy.

This work presents a technique by which a low resolution ( approximately 1 nm) fiberoptic spectrometer may be used to definitively identify elements and molecular fragments in laser-induced breakdown spectroscopy. Commercial laser-induced breakdown spectroscopy (LIBS) spectrometers have high resolution in the area of spectral interest, and software is used to identify elements via a look-up table containing known spectral lines. When analyzing spectra from a lower resolution fiber-optic spectrometer, software based on look-up tables can produce erroneous results, reporting elements absent from the sample. As a solution to this problem, an analysis using the coherence function in conjunction with Welch's method is used to compare sample spectra with a library of reference spectra, which contain peaks primarily from a single element. The analysis has proved to be adept at identifying specific elemental signatures in multi-component samples. The technique leverages the increased information content of concomitant atomic emission lines, which are easily collected with a low resolution broadband (200-1100 nm) fiber-optic spectrometer. This technique alleviates the need for the user to visually verify the vicinity of individual peaks during testing. While Pearson's method is generally used for this type of analysis, we show that Welch's method has the advantage of being less susceptible to problems caused by continuum background.