Yurii N. Ponomarev

High-sensitivity laser photoacoustic leak detector

A photoacoustic (PA) leak detector on the base of a compact CO2 laser is proposed for the detection of hydrogen leaks from cooling systems of electric power generators. Leak detection is based on a trace gas marker SF6 registration, which is added to the gas carrier within the leakage area. Experimental results of SF6 impurity detection in weak gas leakage using a PA detector with a differential ring cavity flow type are sited. The threshold sensitivity of the detector is measured at ~7.5·10−10 cm−1. The effect of gas-flow rate through the detector is studied. The minimum reliably detectable flow is estimated to be ~1·10−10 Pa·m3/s, which exceeds the sensitivity of commercial helium and halogen leak detectors. Description and specifications of the developed model of a portable laser leak detector, designed in accordance with experimental results, are given.

Investigation of spectral dependence of shortwave radiation absorption by ambient aerosol using time-resolved photoacoustic technique

The method of time-resolved photoacoustic spectroscopy is applied to study the weak aerosol absorption of shortwave radiation in the atmospheric air. Using synchronous measurements of aerosol light absorption coefficients by a pulsed photoacoustic spectrometer and mass concentration of black carbon by an Aethalometer, we determine the values of the efficiency σ of aerosol light absorption in the atmospheric air in the shortwave spectral range for three wavelengths: σ (0.532 µm)=(5.49±3.57)m 2 g –1 , σ(0.694 µm)=(4.46±2.61)m 2 g –1 , and σ(1.064 µm)=(2.87±1.84)m 2 g –1 . The experimental results are approximated by the spectral dependence σ(λ)=3.1λ– 0.92 , where wavelength λ is expressed in micrometers. We discuss the calibration procedure for the pulsed photoacoustic spectrometer in studying the aerosol absorption.

Fourier Transform and Photoacoustic Absorption Spectra of Ethylene within 6035–6210 cm-1: Comparative Measurements

Measurements of ethylene absorption spectra with Fourier Transform (FT) and Photoacoustic (PA) spectrometers within 6035–6210 are described. The methodology used for building the frequency scale for both spectrometers is presented. The methane absorption spectrum, included into the HITRAN database, was used in both cases to calibrate the frequency scale. Ethylene absorption spectra were obtained with the two recording methods; a coincidence of the measured line center positions was obtained with an accuracy of 0.0005 .

Non‐linear Absorption and Ionization of Gases by Intense Femtosecond Laser Pulses

Recent experiments on applications of high‐intensity femtosecond pulses for studying multi‐photon and tunnel ionization of different gases, including air, are discussed. Mechanisms of non‐linear absorption and ionization of pure atomic argon and molecular nitrogen gases by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic technique. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive up‐shift of the first ionization potential of argon atoms and nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity. The photoacoustic technique was also applied for studying the effect of tunnel ionization of air by IR femtosecond laser pulses with sub‐critical peak power in the range of intensity ∼0.5–20 PW/cm2. Saturation of ultrasonic signals at near‐atomic laser fields, which is well described by ADK model, is observed.

An analysis of methods of atmospheric gas concentration retrieving from diode laser measurements

Atmospheric gas tunable diode laser (TDL) monitoring scheme is sensitive, local, real-time and portable. The traditional spectrophotometric methods have more performances for gas analysing, but are slow in response in high spectral resolution scheme and depend on influences by different gas species. Local measurements of small atmospheric gas components concentration (CH4, CO, etc.) with diode-laser spectrometers are widely used in various of science and technical applications. An inverse task is usuallu solved by the correlation method (using all the measurement wavelengths) or other methods (for example, the method of fitting of the recorded spectrum under modelling). Each of these approaches has restrictions on retrieving connected with the features of measurement methods used in practice. This report, the results of the different inverse methods for retrieving the methane concentration from the data of a diode-laser spectrometer working in the NIR spectral range are analyzed. The methane diode-laser detector has a reference cell (with the known methane concentration) and a cross-flow cell (atmospheric air) as shown in Fig.1. The program of control and data processing of measurements is written with help of LabView software.

Using TEA CO2 laser radiation harmonics in helicopterborne lidar for controling leakage of toxic and dangerously explosive gases

The characteristics of a helicopter-borne lidar based on tunable TEA CO2 laser and its third harmonic designed for remote detecting the methane and ammonia leakages from pipe-lines are analyzed numerically. The spectral range near 3 micrometers is shown to be most promising for remote sensing of methane emissions. Parameters of radiation of the tunable pulse-periodic mini-TEA CO2 laser and generators of harmonics to be utilized in the helicopter-borne differential absorption lidar are estimated. Emissions of different gases intensity are analyzed for possible detectability at a distance up to 1 km. The use of the third harmonic of the TEA CO2 laser allows methane emissions from a pipe-line to be detected and measured with mean measurement error from 10 to 15% for methane concentrations varying from the background level to the explosion-hazardous one. The optimal pair and possibilities of the ammonia remote sensing on the base of the first harmonic of TEA CO2 laser was determined as well.

Photo-acoustic spectrometer with Ti:sapphire laser

A dual-channel photo-acoustic (PA) spectrometer using a single mode 1 W Ti-Sapphire laser with 50 kHz linewidth is described. The value of minimal detectable absorption coefficient equals of 10-8cm-1 was determined from the measurements of water absorption lines with well-known intensities. The PA spectrometer was applied to record weak absorption spectra of water, HDO, SiH4, H2Se, AsH3 in 11,300 - 12,800 cm-1 region. The photo-acoustic (PA) method based on the measurements of an optical radiation absorption by the substance under study. When the pulsed optical radiation passes through the cell containing an absorbing substance (molecular gas in our case) the last one heats due to the absorption. The heat of the gas in a closed volume of the PA cell produces the pulse of the pressure (Delta) p, which was detected by a sensitive microphone proportional optical radiation energy W and absorption coefficient of gas k(v).

Using of laser spectroscopy and chemometrics methods for identification of patients with lung cancer, patients with COPD and healthy people from absorption spectra of exhaled air

The results of application of the joint use of laser photoacoustic spectroscopy and chemometrics methods in gas analysis of exhaled air of patients with chronic respiratory diseases (chronic obstructive pulmonary disease and lung cancer) are presented. The absorption spectra of exhaled breath of representatives of the target groups and healthy volunteers were measured; the selection by chemometrics methods of the most informative absorption coefficients in scan spectra in terms of the separation investigated nosology was implemented.

Nonlinear ionization of pure atomic and molecular gases by intense UV femtosecond laser pulses

Mechanisms of nonlinear absorption and ionization of pure atomic argon and molecular nitrogen gases by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of argon atoms and nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity.

Spectroscopic lidar technology for small space apparatus

The development of the techniques for spaceborne detecting gas anomalies in the lower atmosphere is very important. The results of the Kioto protocol, an extended use of hydrocarbon raw material, the occurrence of new chemical emissions call for further realization of global control over gaseous contaminations in the atmosphere. A spaceborne location of sensors is very promising for solving this problem. In this case the light automatic satellites, oriented to a limited area of application, are the most promising. As a rule, for such satellites the orbits at 500 km altitude and more are selected. This altitude and small mass of a satellite impose severe requirements on the efficiency of the techniques and apparatus realizations. Taking account of the last-named fact, the paper describes the use of the differential absorption method with a reflection from the Earths surface for global monitoring of gaseous contaminations. The experiments were performed to assess lidar detection of ground anomalies of hydrocarbons in the 3-5 micrometers transmittance window. It is shown that, as applied to a spaceborne platform MKA- 200, this technique provides for localization of the background concentration excess of gases of hydrocarbon cycle with an error from 15% to 25%.