The ro-vibrational ${\nu }_2$ mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy

Abstract

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ${\nu }_2$ 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${}_4$ ${\nu }_2$ CRS spectrum, including all five ro-vibrational branches allowed by the selection rules $\Delta v=1$, $\Delta J=0$, $\pm 1$, $\pm 2$; 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${}_4{\nu }_2$ 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.

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• Nonlinear optical spectroscopy of single, few, and many molecules; nonequilibrium Green's function QED approach
• Ultrafast-laser-absorption spectroscopy in the mid-infrared for single-shot, calibration-free temperature and species measurements in low- and high-pressure combustion gases