Fast Laser Spectroscopy: Dynamical Absorption Line

Abstract

This paper presents analysis of the problems and principles of design of the remote, fast, and precise spectroscopic sensor, operating from the robotic platform. The typical errors of gas concentration measurement have been analyzed. It has been shown that in case of fast tunable laser spectroscopy (a sample registration time – 100ns or less) the line shape is, in principle, unrepeatable, asymmetric, and chaotically modulated at each new scan. The final composite line profile is sharper at the top as compared with a Lorentzian one and wider near the bottom. Three reasons for such specific line shape formation were considered: 1) The gas absorption line shape has been simulated by the summation of Lorentzian lines of different linewidth. The Maxwellian distribution of the molecular speeds has been used to find the molecular collisional energy distribution and related linewidths with taking into account probable number of molecules with specific impact energy; 2) Appearance of the spectral satellites when the absorption process is suddenly interrupted; 3) Spectral presentation of the collision correlation function that is modulated by massive collisions at free-flight termination. To calculate the gas concentration properly the data received from the independent sensors of pressure, temperature and humidity have been used.