Analytische Chemie
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The stimulated emission (SE) in aluminum laser-induced plasma pumped in resonance with the 3s²3p-3s²4s aluminum transition at 266.04 nm is modeled. A collisional-radiative plasma model based on kinetic equations is proposed to explain the creation of the population inversion and lasing. The model predicts fast depopulation of the ground 3s²3p state by the absorption of resonant laser light at 266 nm and very fast population of the excited 3s²4s state by the cascade transitions from the laser-pumped level, which is driven optically and by collisions. The SE of the 3s²3p-3s²4s transition at 396.15 nm is studied and possible SE at 1.3 and 2.1 μm is predicted. It is confirmed by calculations that the population inversion between the 3s²3p and 3s²4s states can be created by weak pumping at several microjoule–millijoule pulse energies and results in high gain.
Spatial heterodyne spectroscopy (SHS) is used for quantitative analysis and classification of liquid samples. SHS is a version of a Michelson interferometer with no moving parts and with diffraction gratings in place of mirrors. The instrument converts frequency-resolved information into spatially resolved one and records it in the form of interferograms. The back-extraction of spectral information is done by the Fast Fourier transform. A SHS instrument is constructed with the resolving power 5000 and spectral range 522 - 593 nm. Two original technical solutions are used as compared to previous SHS instruments: the use of a high frequency diode pumped solid state (DPSS) laser for excitation of Raman spectra and a microscope-based collection system. Raman spectra are excited at 532 nm at the repetition rate 80 kHz. Raman shifts between 330 cm-1 and 1600 cm-1 are measured. A new application of SHS is demonstrated: for the first time it is used for quantitative Raman analysis to determine concentrations of cyclohexane in isopropanol and glycerol in water. Two calibration strategies are employed: univariate based on the construction of a calibration plot and multivariate based on partial least square regression (PLSR). The detection limits for both cyclohexane in isopropanol and glycerol in water are at a 0.5 mass% level. In addition to the Raman-SHS chemical analysis, classification of industrial oils (biodiesel, poly(1-decene), gasoline, heavy oil IFO380, polybutenes, and lubricant) is performed using their Raman-fluorescence spectra and principal component analysis (PCA). The oils are easily discriminated as they show distinct non-overlapping patterns in the space of principal components.