High-temperature rotational-vibrational O2 -CO2 coherent Raman spectroscopy with ultrabroadband femtosecond laser excitation generated in-situ

We present ultrabroadband two-beam femtosecond/picosecond coherent Raman spectroscopy on the ro-vibrational spectra of CO2 and O-2 , applied for multispecies thermometry and relative concentration measurements in a standard laminar premixed hydrocarbon flame. The experimental system employs fs-laser-induced filamentation to generate the compressed supercontinuum in-situ, resulting in a-24 fs full-width-at-half-maximum pump/Stokes pulse with sufficient bandwidth to excite all the rovibrational Raman transitions up to 1600 cm(-1) . We report the simultaneous recording of the ro-vibrational CO2 Q-branch and the ro-vibrational O-2 O-, Q- and S-branch coherent Stokes Raman spectra (CSRS) on the basis of a single-laser-shot. The use of filamentation as the supercontinuum generation mechanism has the advantage of greatly simplifying the experimental setup, as it avoids the use of hollow-core fibres and chirped mirrors to deliver a near-transform-limited ultrabroadband pulse at the measurement location. Time-domain models for the ro-vibrational Q-branch spectrum of CO2 and the ro-vibrational O-, Q- and S-branch spectra of O 2 were developed. The modelling of the CO2 Q-branch spectrum accounts for up to 180 vibrational bands and for their interaction in Fermi polyads, and is based on recently available, comprehensive calculations of the vibrational transition dipole moments of the CO2 molecule: the availability of spectroscopic data for these many vibrational bands is crucial to model the high-temperature spectra acquired in the flue gases of hydrocarbon flames, where the temperature can exceed 20 0 0 K. The numerical code was employed to evaluate the CSRS spectra acquired in the products of a laminar premixed methane/air flame provided on a Bunsen burner, for varying equivalence ratio in the range 0.6-1.05. The performance of the CO(2 & nbsp;)spectral model is assessed by extracting temperatures from 40-laser-shots aver-aged spectra, resulting in thermometry accuracy and precision of-5% and-1%, respectively, at temper-atures as high as 2220 K. (C)& nbsp;2021 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute.

Automated summary

This study presents the application of ultrabroadband two-beam femtosecond/picosecond coherent Raman spectroscopy on the ro-vibrational spectra of CO2 and O-2 for thermometry and concentration measurements. The experimental system utilizes fs-laser-induced filamentation to generate a compressed supercontinuum, enabling the excitation of all rovibrational Raman transitions. The study also develops time-domain models to simulate the ro-vibrational spectra and successfully measures CO2 and O-2 in the flame products.