The ro-vibrational ν₂ mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν₂ mode spectrum of methane (CH₄). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000 cm^-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH₄ ν₂ CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH₄ chemistry is demonstrated in a laboratory CH₄/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O₂), carbon dioxide (CO₂), and molecular hydrogen (H₂), along with CH₄. Fundamental physicochemical processes, such as H₂ production via CH₄ pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH₄ v₂ CRS thermometry, and we validate it against CO₂ CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH₄-rich environments, e.g., in plasma reactors for CH₄ pyrolysis and H₂ production.