A Study on REMPI as a Measurement Technique for Highly Rarefied Gas Flows

A technique for measurement of thermodynamic variables with high sensitivity is necessary for analyses of highly rarefied gas flows, gas-surface interaction, and so on. REMPI (Resonantly Enhanced Multiphoton Ionization) is a powerful optical tool because of its high sensitivity even in highly rarefied gas flows and its ability to measure nonequilibrium amongst internal (translational, vibrational, and rotational) energy modes. In this paper, a REMPI system is developed and REMPI spectra are measured on the center line of a free-molecular flow. The fundamental properties of the REMPI signal are also described. A method using a Boltzmann plot with the spectral lines of multiple branches is proposed, and the ratio of the electronic transition dipole moments in the two-photon Hönl-London factors for the O and P branches is determined.