The ro-vibrational nu(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 nu(2) 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 nu(2) CRS spectrum, including all five ro-vibrational branches allowed by the selection rules delta v = 1, delta J = 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 (O-2), carbon dioxide (CO2), and molecular hydrogen (H-2), along with CH4. Fundamental physicochemical processes, such as H-2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v(2) 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 H-2 production.