EXPRESS: Quenching-Independent Two-Photon Absorption Laser-Induced Fluorescence of Atomic Oxygen in High-Enthalpy Air-Carbon Gas-Surface Interaction.

Abstract

Understanding the abundance of atomic oxygen in the vicinity of carbon surfaces exposed to high-enthalpy flows is critical to accurate predictions of the gas-surface interaction. A novel approach for obtaining absolute number density measurements of atomic oxygen in high-enthalpy facilities with nanosecond laser pulses is described and demonstrated using photoionization-dominated, two-photon laser-induced fluorescence. In two-photon laser-induced fluorescence measurements, the depopulation of the excited state is typically dominated by electronic quenching, which depends on the temperature, pressure and gas composition. To account for the electronic quenching rate, the fluorescence lifetime can be measured by temporally resolving the fluorescence. This can prove challenging in high-temperature and/or high-pressure environments where the fluorescence lifetime can be less than a nanosecond. Instead, by increasing the laser intensity until photoionization dominates the depopulation of the excited state, we create a quenching-independent measurement that is proportional to absolute number density.This technique is demonstrated here in the reacting boundary layer of a graphite sample ablating in the 6000 K plume of an inductively-coupled plasma torch. The boundary layer possesses a large temperature gradient that varies from about 2000 K near the sample surface to the plume temperature of 6000 K in a span of approximately 2 mm. The photoionization-dominated technique is calibrated by using the freestream oxygen concentration assuming the torch plume is in local thermodynamic equilibrium. The spatial resolution of the measurements is 50 μm and we are able to measure the number density of atomic oxygen to within about 60 μm of the graphite sample.