Experimental Determination of the Temperature-Dependent Broadening Effect of Water Vapour on the S-Branch Raman Linewidths of Nitrogen

Abstract

This work reports the S-branch Raman linewidths of N₂ broadened by H₂O vapour at temperatures 870–1900 K determined using the picosecond time-resolved pure rotational coherent anti-Stokes Raman scattering (RCARS) approach. The coherent dephasing time constant of nitrogen in a binary mixture was determined from the time trace fitting of the RCARS signal. Raman linewidths of N₂ for four different binary mixture compositions up to 20% H₂O were obtained for each temperature and the N₂-H₂O broadening coefficient was determined. The decay of nitrogen broadened by water vapour is faster than that of self-broadened nitrogen for all temperatures leading to a higher Raman linewidth. Respectively, the S-branch Raman linewidths of N₂-H₂O deviate significantly in magnitude and slope from the Q-branch Raman linewidth calculated by the modified energy gap (MEG) law. Therefore, taking into account the broadening effect of water molecules in nitrogen has the potential to significantly improve thermometry and species concentration determination. The determined S-branch Raman linewidths and broadening coefficient will therefore find application in combustion and reactive flow diagnostics.