The ro-vibrational $\nu_2$ mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy

TL;DR

This paper demonstrates the first use of ultrabroadband coherent Raman spectroscopy to analyze methane's ro-vibrational spectrum, enabling in situ detection and thermometry of methane and related species in combustion environments.

Contribution

It introduces a novel ultrabroadband CRS method for methane spectroscopy, including a comprehensive time-domain model and validation in a flame environment.

Findings

Successful detection of methane, O2, CO2, and H2 in a flame front

Validation of CRS thermometry against CO2 measurements

Observation of H2 production via methane pyrolysis

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 $\Delta v = 1$, $\Delta J = 0$, $\pm1$, $\pm2$; 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.