High-temperature rotational-vibrational O_2^[sbnd]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 O₂, 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 ro-vibrational Raman transitions up to 1600 cm^-1. We report the simultaneous recording of the ro-vibrational CO₂ Q-branch and the ro-vibrational O₂ 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 O₂ 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 2000 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 CO₂ spectral model is assessed by extracting temperatures from 40-laser-shots averaged spectra, resulting in thermometry accuracy and precision of ∼5% and ∼1%, respectively, at temperatures as high as 2220 K.