The ro-vibrational ν 2 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 ν 2 mode spectrum of methane (CH 4 ). 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 CH 4 ν 2 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 CH 4 chemistry is demonstrated in a laboratory CH 4 /air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O 2 ), carbon dioxide (CO 2 ), and molecular hydrogen (H 2 ), along with CH 4 . Fundamental physicochemical processes, such as H 2 production via CH 4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH 4 v 2 CRS thermometry, and we validate it against CO 2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH 4 -rich environments, e.g., in plasma reactors for CH 4 pyrolysis and H 2 production.