Insights into the evolution of silica nanoparticles forming in tetramethyl silane - doped H₂/O₂/Ar flames inferred via emission spectroscopy and quartz crystal microbalance diagnostics

Emission spectroscopy and quartz crystal microbalance (QCM) measurements were performed in tetramethyl silane (TMS)-doped, premixed, laminar, low-pressure (30 mbar) H₂/O₂/Ar flames. At selected flame conditions/locations particles were additionally characterized by transmission electron microscopy (TEM). The visible emission is dominated by a broadband continuum peaking near 400 nm that appears only upon TMS addition. The signal is strongest in the early particle formation region and decreases further downstream. Control experiments with large methane additions, spectral identification of CH* and SiH* under fuel-rich, high TMS loading conditions, and analysis of wavelength-dependent emission efficiencies demonstrate that the broadband continuum is unlikely to originate from broadband CO₂* chemiluminescence, narrowband molecular emission, or thermal radiation from hot particles. Instead, the observations are consistent with previously proposed non-thermal particle-associated luminescence. The results support interpretations invoking heterogeneous reactions on the surface of nascent silica particles, while particle luminescence induced by UV excitation cannot be entirely excluded. Both broadband emission and QCM measurements reveal two distinct particle formation zones separated by a pronounced dip. However, whereas the QCM signal exhibits pronounced renewed downstream growth, followed by a plateau, the broadband emission recovers only to a shallow secondary maximum, followed by decline. Together with TEM observations, these results indicate that particles formed in the early and late formation zones differ not only in abundance but also in physicochemical properties governing their luminescence. The combined emission and QCM measurements therefore provide complementary insight into transient silica nanoparticle evolution beyond condensed phase mass alone.