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Eingeladener Vortrag
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The Radon transform tomography is used for reconstruction of the emissivity distribution in single- (SP) and double-pulse (DP) laser induced plasmas in orthogonal geometry. The orthogonal DP plasma is intrinsically asymmetric and thus suitable for the Radon reconstruction. The DP plasma consists of two plasmas separated by a short time interval of ~ 1 µs. The first plasma is created in air near the surface of a Si wafer and is followed (pre-ablation mode) or preceded (post-ablation mode) by the second plasma induced on this surface. A spectrometer moves in a semi-circular path around the plasma keeping the plasma in the rotation center. The optical detection is arranged so that a thin plasma layer parallel to the target surface is monitored. The axial symmetry of the SP plasma is investigated by comparing data from the Abel inversion taken at different angles and Radon reconstruction. The multi-angle measurements are used to estimate errors of the Abel reconstruction due to asymmetries of the plasma. Time-resolved Radon reconstruction in white light is performed for the DP plasma in both pre- and post-ablation modes. In the former case, the effect of ablated aerosol on the formation of the air plasma is monitored. In the latter case, a formation of an asymmetric compression shock created by the target plasma inside the air plasma is visualized. This observation is supported by computer simulations. An interaction of the two plasmas is studied by spectrally resolved Radon reconstruction revealing a complex distribution of target and ambient species inside the plasma at all studied delay times. Overall, it is demonstrated that Radon-based tomography is an informative tool to study transient asymmetric laser induced plasmas.
The authenticity of objects and artifacts is often
the focus of forensic analytic chemistry. In document fraud
cases, the most important objective is to determine the
origin of a particular ink. Here, we introduce a new
approach which utilizes the combination of two analytical
methods, namely Raman spectroscopy and laser-induced
breakdown spectroscopy (LIBS). The methods provide
complementary information on both molecular and elemental
composition of samples. The potential of this hyphenation
of spectroscopic methods is demonstrated for ten blue
and black ink samples on white paper. LIBS and Raman
spectra from different inks were fused into a single data
matrix, and the number of different groups of inks was
determined through multivariate analysis, i.e., principal
component analysis, soft independent modelling of class
analogy, partial least-squares discriminant analysis, and
support vector machine. In all cases, the results obtained
with the combined LIBS and Raman spectra were found to
be superior to those obtained with the individual Raman or
LIBS data sets.
An emission tomography of laser-induced plasmas employed in the laser induced breakdown spectroscopy (LIBS) requires signal integration times in a microsecond range during which the LIBS plasma cannot be considered stationary. Consequently, the use of the data for reconstructing the plasma properties under the assumption that the latter does not change significantly during the integration time leads to inaccurate results. To reduce the integration time, it is proposed to measure a plasma absorption in parallel rays using a scanning rectangular aperture whose dimension Δ along the scanning direction is about a characteristic size of plasma plumes (Δ ~ 1 cm) and the other dimension Δp is of the order of a uniformity length of plasma parameters (Δp ~ 10 µm). The aperture is moved step by step along the scanning direction and the total energy of photons coming through the aperture is measured during time T at each position of the aperture. Owing to the large size of the aperture, the integration time T is reduced by a factor ~ Δp/Δ. A numerical data processing is proposed to restore the spatial resolution of the plasma absorption along the scanning direction. It is determined by the scanning step Δs ≤ Δp. Another advantage of the proposed procedure is that inexpensive linear CCD or non-discrete (PMT, photodiode) detectors can be used instead of costly 2-dimensional detectors.
We studied the confocal double-pulse laser-induced plasma in the very beginning of its life. It was found that the second laser pulse fired 0.7 to 5 µs after the first pulse produces plasma which, during the first 0 to 20 ns, resembles solar configuration. There is a very hot and compact plasma core that radiates a broad continuum spectrum and a much larger and cooler outer shell. The light from the hot core passes through the cold outer shell and is partly absorbed by atoms and ions that are in ground (or close to ground) states. This produces absorption lines that are similar to Fraunhofer lines observed in the sun spectrum. The possibility to use these absorption lines for new direct and calibration free laser-induced breakdown spectroscopy analytical applications, both in laboratory and industrial conditions, is proved.