Temperature sensitivity of molecular oxygen resonant-enhanced multiphoton ionization spectra involving the C 3Π g intermediate state

Abstract

The technique of measuring O2 rotational temperature by coherent microwave Rayleigh scattering from resonance-enhanced multiphoton ionization (Radar REMPI) has been studied to determine temperature sensitivity and range. The molecular oxygen Rydberg state of \(\left( {3s\sigma } \right)C{{}^{3}}\Pi_{g} \left( {v^{\prime } = 2} \right)\) has been selected as the intermediate state in the 2 + 1 REMPI process, which is known to provide a relatively strong REMPI signal. Rotational-resolved spectra representing the two-photon \(C{{}^{3}}\Pi_{g} \left( {v^{\prime } = 2} \right) \leftarrow \leftarrow X{{}^{3}}\varSigma _{g}^{ - } \left( {v^{\prime \prime } = 0} \right)\) transition have been obtained under several gas conditions including pure oxygen, air-like syngas, ambient air, and flame environments from room temperature (~300 K) to flame temperature (~1700 K). An O2 REMPI spectral model has been developed to simulate the experimental spectral line intensity distribution which is dependent on the O2 ground-state temperature. The model has been verified at a low-temperature condition (~5 K) and then applied to various oxygen environments over an extended temperature range with an overall error of less than ±10 %. The current O2 REMPI spectral model is an improvement over a previously reported version in both accuracy and the quantity of lines fit to provide rotational temperature measurements. This work details an optimized model that fits simulated spectra to full experimental spectral bands over various conditions with a wide temperature range, including both low temperature ( 1300 K).