Improvement of the coherent model function for S-branch Raman linewidth determination in oxygen.

Abstract

Determination of S-branch Raman linewidths of oxygen from picosecond time-domain pure rotational coherent anti-Stokes Raman spectroscopy (RCARS) measurements requires consideration of coherence beating. We present an optimization of the established model for fitting the coherence decay in oxygen, which leads to an improvement in Raman linewidth data quality, especially for the challenging small signal intensity and decay constant regime, enabling the application for low oxygen concentrations. Two modifications to the fitting procedure are discussed, which aim at reliably fitting the second coherence beat properly. These are evaluated statistically against simulated decay traces, and weighing the data by the inverse of the data magnitude gives the best agreement. The presented temperature dependent ${{\rm O}_2} {-} {{\rm O}_2}$ S-branch Raman linewidth from the modified model shows an improved data quality over the original model function for all studied temperatures. ${{\rm O}_2} {-} {{\rm N}_2}$ linewidths of oxygen in air for the temperature range from 295 K to 1900 K demonstrate applicability to small concentrations. Use of the determined RCARS ${{\rm O}_2} {-} {{\rm O}_2}$ S-branch linewidth instead of regularly used Q-branch derived linewidths leads to a lowering in evaluated RCARS temperature by about 21 K, thereby, a much better agreement with thermocouple measurements.