Differential chirped-pulse dual-comb spectroscopy for complex line shape fitting the R(6) manifold of the 2ν3 band of methane

Abstract

We use a dual electro-optic comb system (EOOFC) and a differential chirped-pulse down conversion technique to study the R(6) manifold of the 2ν₃ band (1645 nm) of methane at low pressure (< 1 kPa). In the frequency domain, the EOOFC method magnifies of the temporal dynamics of the system and due to of the 6-component asymmetric line shape, is shown to give unique amplitude and phase spectra that depend on the chirp direction. We model the frequency domain complex response function using chirped-pulse multiplication and convolution with the Kramers-Kronig transfer function of the molecular system. We then compare how the predicted Voigt line shape with parameters from HITRAN2012 and HITRAN2020 databases and the Hartmann-Tran profile (HTP) perform for fitting both the temporally magnified magnitude and phase profiles in the frequency domain and following the removal of these effects from back transformation, the traditional (natural) line shape components in the time domain. The complex responses in the two domains give unique insight into the physical nature of time dependent absorption processes that may help guide development of molecular models to improve their predictive behavior of both amplitude and phase.