Rotational Relaxation Analysis and Simulation of REMPI Spectra for Molecule Nitrogen

Abstract

The new DLR-High Vacuum Test Facility STG enables an experimental investigation of typical thruster plumes as prerequisite to determine disturbance effects of thruster firing like e.g. perturbing forces and moments, excessive heat loads, and contamination. Its main feature, a large liquid helium-driven cryopump allows the undisturbed plume gas expansion into the highly rarefied regime including the backflow. Because of the very low number densities in these regions, as an appropriate measurement technique the highly sensitive spectroscopie method REMPI (Resonance Enhanced Multi-Photon Ionization) is applied to obtain multiphoton spectra. This method delivers a precise determination of the most important plume flow characteristics, namely the temperature, the density, and gas velocity. In the first part of this study, the analytical technique of Gallagher and Fenn [?] is used to determine the characteristic rotational relaxation rate for nitrogen. The rates will then be discussed with those obtained by other methods. Further, a comparison between the experimental rotational temperature data measured in the STG and the analytical results will be carried out. In the second part, the simulation of the REMPI spectra is extended to consider the broadening of each absorption line including Doppler and collision broadening.