N<inf>2</inf> rotational temperature analysis by laser induced fluorescence following resonant enhanced multiphotonion ization

Abstract

It has been demonstrated that the rotational temperature of molecular nitrogen at atmospheric pressure can be determined by direct optical probing of the N 2 (X,v=0) ground state with subsequent analysis of the rotational state distribution. A tunable probe laser was scanned over resonant enhanced multi-photon ionization transitions initiating from various N 2 (X,v=0, J) states. At atmospheric pressure, the laser photo-ionization also induced nitrogen fluorescence bands. Analysis of the relative fluorescence as a function of laser wavelength produced a calculated N 2 (X,v=0, J) rotation state distribution and the assignment of a rotational temperature.This laser-based temperature probe may be more accurate and applicable in more environments than optical emission spectroscopy (OES), which is commonly applied as a passive remote temperature measurement technique. OES most often involves the analysis of the N 2 (C-B) emission bands, or the second positive system, where the rotational temperature is typically determined from a contour analysis of the unresolved rotational structure. OES analysis can be acceptable, but requires that (i) the nitrogen gas is electronically excited so that optical emission is observable and (ii) the rotational distribution of the N 2 (C) state is a Boltzmann distribution and it has the same distribution as the N 2 (X,v=0) ground state. The rotational temperature measurement technique introduced in this work involves remote sensing with a laser probe and can be applied to a more broad variety of nitrogen systems in thermal equilibrium or non-equilibrium, such as hot gases, flames, or atmospheric plasmas.1.