System of pulsed photoacoustics based on a TEA CO2_2 laser applied to SF6_6-N2_2 mixtures

A transversal single-mode TEA CO$_2$ laser, whose cavity design is a diffraction coupled resonator, tuned at the 10P(16) line, is used for photoacoustic detection of SF$_6$ traces in N$_2$. The acoustic cavity, resonant at a longitudinal mode, is designed on the basis of an electric analog model of a one-dimensional resonator. The acoustic signal from SF$_6$-N$_2$ and the signal from a laser energy detector enter simultaneously into a PC through its sound port. The photoacoustic signal, proportional to the average number of absorbed photons per molecule, is found to increase with the laser energy as a power of 0.5. The constant of the setup, previously measured from pulsed photoacoustics on NO$_2$-N$_2$ mixtures excited with the second harmonic of a Q-switched Nd:YAG laser, allows to quantitatively determine the SF$_6$ absorption cross section as a function of the CO$_2$ laser fluence. The calibration of this system is performed down to 150 ppbV of SF$_6$ in N$_2$ at atmospheric pressure for different values of laser energy

Study of velocity-changing collisions by time resolved saturated absorption

By use of two stabilized CW dye lasers and of electrooptic choppers we have developed an experimental set-up which combines the advantages of high frequency resolution and time resolved spectroscopy. We use this set-up to study the velocity thermalization of 1s3 metastable neon atoms. The collision kernel appears to be constituted of two components. The first one is broad and asymmetric, the second one is narrow and symmetric.L'usage de deux lasers à colorant continus, stabilisés, et de modulateurs électrooptiques permet de combiner les avantages de la spectroscopie à haute résolution et de la spectroscopie résolue en temps. Ce montage est utilisé pour l'étude de la thermalisation des vitesses d'atomes métastables de néon (1s3). Le noyau de collision apparaît formé de deux composantes, l'une large et asymétrique, l'autre étroite et symétrique

Improved pulsed photoacoustic detection by means of an adapted filter

We present a numerical and experimental study of two adapted filters devised to the quantitative analysis of weak photoacoustic signals. The first one is a simple convolution-type one and the other is based on neural networks of the multilayer perceptron type. The theoretical signal used as one of the inputs in both filters is derived from the solution of the transient response of the acoustic cell modeled with a simple transmission-line analogue. The filters were tested numerically by using the theoretical signal corrupted with white noise. After 500 iterations it was possible to define an average error for the returned value of each filter. Since the neural network outperformed the convolution-type, we assessed its performance by measuring SF$_6$ traces diluted in N$_2$ and excited by tuned TEA CO$_2$ laser. The results show the use of the neural network filter allows recovering a signal with poor signal-to-noise ratio without resorting to extensive averaging, thus reducing the acquisition time while improving the precision of the measurement

Pulsed photoacoustic detection of isotopic mixtures of chloroform

Photoacoustic spectroscopy is applied to the detection of small concentrations of CDCl$_3$ in CHCl$_3$-N2 using a TEA CO$_2$ laser tuned at the 10P(48) line. At this laser line both species H/D are excited but the ratio between absorbances is such that a relatively higher excitation of the deuterated species is expected compared to other available laser wavelengths. The photoacoustic signal is digitized by an oscilloscope and, at the same time, acquired through the sound port of a PC, pulse by pulse, for comparison of both acquisition methods. The signal processing is performed through the calculation of the standard deviation of each transient. For a fixed mixture the processed signal shows a very good linear dependence on laser energy around 1 mJ. Different mixtures of CDCl$_3$ in a fixed proportion of CHCl$_3$ / N$_2$ are prepared for studying the capability of the photoacoustic technique of distinguishing the deuterated component from the hydrogenated background. The results show that the sensitivity of this setup allows to measure smaller quantities of CDCl$_3$ in CHCl$_3$ than those corresponding to natural abundance (150 ppmV)

Estudio de un Lidar de gran alcance en base a laser de Nd: Yag para mediciones de parametros atmosfericos

Consiglio Nazionale delle Ricerche (CNR). Biblioteca Centrale / CNR - Consiglio Nazionale delle RichercheSIGLEITItal