Verónica Slezak

Blue light-emitting diode-based, enhanced resonant excitation of longitudinal acoustic modes in a closed pipe with application to NO2

We present a new, very compact resonant photoacoustic system based on a simple closed pipe, transversally illuminated by an array of blue light-emitting diodes uniformly distributed along the tube to produce an acoustic signal from NO2–N2 samples. The illumination is modulated in a particular way as to produce amplification of the acoustic resonance corresponding to the second longitudinal mode. The linearity of the system for NO2 trace detection is studied. This arrangement provides a simple, compact, and cheap setup, useful for both measuring emissions from diesel engines and teaching photoacoustic spectroscopy in gases.

Simple model for the optogalvanic effect in a neon negative glow discharge

Abstract A model based on rate equations is proposed to describe the optogalvanic signal induced in a neon hollow cathode discharge by pulsed laser irradiation at 594.5 nm. Good fitting between the experimental and theoretical signal is achieved when a “diffusion like” loss term for the electrons is included.

CHOPPED CW LASER-INDUCED OPTOGALVANIC EFFECT IN A NEON HOLLOW CATHODE DISCHARGE

A detailed model for the optogalvanic effect in a neon hollow cathode discharge irradiated by a chopped CW dye laser is presented. A rate equation formalism is used to calculate the evolution of the first and second electronic configuration populations coupled by the laser and of the electric charges number density. Processes as ambipolar-like electrons loss, electronic collisional coupling of level populations and electron emission by the cathode due to VUV radiation from the 1s2 resonant level are taken into account and further discussed.The transients and steady-state magnitude of the optogalvanic signal are calculated, compared with experimental data and related to population changes. We predict sign changes of the optogalvanic signal when the laser is tuned over transitions originating from the resonant level with respect to transitions involving the metastables states. The optogalvanic signal is shown to be basically determined by the laser-induced variations of the excited-state populations while changes in the electron temperature, due to laser energy transfer by collisions between electrons and excited atoms, play a negligible role.

CO2-laser-induced multiphoton absorption of CF2Cl2

Energy absorption by CF2Cl2 from an intense TEA CO2 laser pulse is measured as a function of the pressure of CF2Cl2 and the pressure of Ar bath gas for different laser energy fluences. The fraction of the molecules excited by the laser field and the average energy of the molecular ensemble are determined by a simple experimental method.

Photoacoustic beam profiling of pulsed lasers

We introduce a beam profiler of pulsed lasers based on the photoacoustic technique. The method assumes that the initial pressure distribution inside the acoustic cell follows the laser intensity pattern if absorbed energy relaxes rapidly. This initial pressure condition can be described as a superposition of acoustic modes of different amplitudes and phases. We analyze how to reconstruct the intensity profile of the laser beam from the recorded acoustic signals. Finally, we present preliminary results obtained with a frequency doubled Nd:YAG laser that excites NO(2)-CF(2)Cl(2) mixtures.

Novel optical method for background reduction in resonant photoacoustics.

We report a new way of reducing the background originated by window absorption in resonant photoacoustics. The technique employs a secondary light source that is absorbed by the window but not by the gas sample. This auxiliary source is modulated 180 degrees apart from the one used to probe the gas. This way the window is heated almost uniformly during each cycle, thus lessening the associated background signal. We tested the scheme on a photoacoustic setup, conceived to measure NO(2), which is excited by an array of blue light-emitting diodes. Another array of near-infrared, light-emitting diodes served as the secondary light source. With the addition of this cancelling method, the detection limit was decreased to 4% of the previous reached without it.

Recovery of noisy pyroelectric-detector signals through neural-network processing

We introduce a neural-network-based filter devised to extend the dynamic range of pyroelectric detectors which otherwise would only be useful for medium-to-high energy measurements. To accomplish this task, we trained a multilayer perceptron through the back-propagation method using the theoretical signal derived from the detector equivalent electric circuit. We tested the performance of the neural-network filter both numerically and experimentally. In the former case we recovered theoretical signals corrupted with white and impulse noise and compared the results with those obtained through the use of standard filtering methods. In the latter case, we applied the filter to measure pulses from a Nd:YAG laser whose energy was below the detector noise-equivalent energy. With this processing technique in a standard PC we have been able to accurately measure laser energy values as low as one-tenth the detector’s noise-equivalent energy and at 10–20Hz repetition rate.

Influence of the neon 1s configuration on the optogalvanic effect at the 594.5-nm (1s 5 –2p 4 ) and 585.2-nm (1s 2 –2p 1 ) lines

A model based on rate equations has been developed to describe the optogalvanic signal observed in the negative-glow region of a hollow-cathode discharge when a pulsed dye laser is tuned to the 1s5–2p4 and 1s2–2p1 neon transitions (Paschen notation). We include an ambipolarlike term of charge loss that is determined empirically and a term that takes into account the variation of photoelectrons produced by changes in the 1s2 population on laser irradiation. The change of sign of the signal for transitions from the 1s2 level with respect to those from the 1s5 state, the amplitude, and the time evolution are well predicted.

Vibration-translation relaxation rates of vibrationally excited CF2Cl2

Abstract V-T energy transfer rates in multiphoton excited CF2Cl2, pure and in mixtures with Ar, were measured studying laser induced fluorescence, without spectral resolution, in the regions 1100-960 cm−1 and 920-700 cm−1

Digitally processed, phase-reading fibre gyroscope

Abstract We present a simple digital processing method that provides a significant improvement for the performance of a phase-reading open-loop fibre-optic gyroscope. The phase-reading gyroscope measures the angular speed in terms of a phase difference between the two counter-propagating beams in the Sagnac interferometer. Instead of electronic phase meters we use a very simple PC-based digital process which computes the Sagnac phase. With this method we measure rotation rates down to 100°h−1. The errors of this scheme are studied for different signal-to-noise ratios and the number of bits of the analog-to-digital converter.