Investigation of the broadening effect of ammonia on the S-branch Raman linewidth of nitrogen with time-domain rotational coherent anti-Stokes Raman scattering.

Ammonia (NH3) emerges as a promising carbon-free, sustainable fuel, but its integration into technical combustion processes requires accurate, spatially, and temporally resolved diagnostics, obtained through non-invasive techniques like rotational coherent anti-Stokes Raman scattering (RCARS). However, the lack of spectroscopic data for ammonia-containing gas mixtures hinders its integration into advanced RCARS spectral models for data evaluation. In this study, we experimentally investigated the broadening effect of NH3 on the N2 S-branch Raman linewidths using the picosecond time-resolved RCARS technique. Experiments were conducted on N2-NH3 binary mixtures containing up to 50% NH3, at atmospheric pressure and temperatures up to 870 K close to the ammonia-air autoignition temperature. The peak intensities of the rotational spectrum of NH3 are weak compared with the N2 peak intensity and its rotational coherences decay more rapidly than those of N2, limiting the direct application of NH3-based RCARS diagnostics in the presence of intense scatterers and complicating its linewidths determination. Nevertheless, NH3 was found to induce significant broadening of the N2 Raman lines, which must be quantified to avoid diagnostic errors in RCARS. The magnitude of the S-branch Raman linewidth, obtained from the coherence decay time constant, increases linearly with NH3 concentration, indicating a more efficient collisional energy transfer in N2-NH3 compared to N2-N2 collisions. This behavior enables direct determination of N2-NH3 broadening coefficients, providing key spectroscopic parameters for RCARS spectral modeling. We further demonstrate that the inclusion of the newly determined linewidth data for nitrogen thermometry prevents temperature errors from inaccurate linewidth approximations. Thus, the obtained N2-NH3 broadening coefficients provide the spectroscopic basis for extending RCARS thermometry and species diagnostics to ammonia-fueled combustion and reactive flow environments.