The ro-vibrational ν2 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 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, Δ 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 (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.

An ultrafast algorithm for ultrafast spectroscopy

“…3a and Fig. 3b, matching the coherence beating of the unresolved, Coriolis-split ro-vibrational transitions 26 . This shows that the dominating error components in the simulated CRS spectra arise from the coalescence of the approximation errors of the individual unresolved lines.…”

“…PAHs are also the precursors of soot in hydrocarbon combustion and are thus the subject of widespread laboratory research. The application of CRS in the fingerprint region 26 of the Raman spectrum can combine the high chemical specificity required to identify and measure different PAHs 41 , with the ability to resolve their specific vibrational dynamics in the ground electronic state, thus providing important complementary information to time-resolved XUV-IR spectroscopy 42 .…”

“…For both cases, a Python and C++ implementation (-py and -cpp respectively) is tested; more details are available in the Methods section. The MeCaSDa spectral database 25 is rescaled according to the procedure of Mazza et al 26 to calculate the weights 𝑊 𝑖 of the ~10 7 individual lines in the H-C-H bend (ν2 mode) CRS spectrum of CH4. The computation time for an increasing number of spectral lines included in the spectrum is measured for the different numerical implementations.…”

“…They furthermore employed a high-performance computing cluster to calculate the resulting spectra 24 . Mazza et al used the same database to compute the H-C-H bend spectrum of CH4: when employing the whole line list (~11 M lines), the computation of a single spectrum required more than 8 hours on a commercial laptop 26 . This being the state of the art, the widespread use of CRS for quantitative spectroscopy on large polyatomics, especially in hot environments where one would possibly need to consider many billions (B) of spectral lines 21 , is demonstrably hindered by the lack of fast spectral algorithms.…”

“…The algorithm is based on the discrete integral transform developed by van den Bekerom and Pannier 27 , who demonstrated a 2-3 orders-of-magnitude reduction in the computational cost of simulating the absorption spectrum of CO2, as compared to the state-of-the-art spectral code RADIS 28 . The accuracy and computational performance of the ultrafast algorithm are assessed against the numerical code of Mazza et al, which models the spectrum of CH4 in the dyad region of its Raman spectrum 26 . We furthermore discuss the potential implications of these results for the application of time-resolved CRS to investigate the gas-phase chemistry of large polyatomic molecules.…”