Femtosecond time resolved coherent anti-Stokes Raman spectroscopy: experiment and modelization of speed memory effects on H2-N2 mixtures in the collision regime

H2 chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering thermometry at high temperature and pressure

This article describes the development and application of hydrogen (H2) chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS) in high-temperature and high-pressure conditions. H2 fs CARS thermometry was performed in Hencken burner flames up to 2300 K, as well as in a heated gas-cell at temperatures up to 1000 K. It was observed that the H2 fs CARS spectra are highly sensitive to the pump and Stokes chirp. Chirp typically originates from optical components or systems such as windows, polarizers, and the optical parametric amplifier. As a result, the pump delay is modeled to provide a shift to the Raman excitation efficiency curve. With the updated theoretical model, excellent agreement was found between the simulated and experimental spectra. The mean error and precision are 2.8% and 2.3%, respectively. In addition, the spectral phase of the input pulses and the pump time delay determined from the experimental spectra closely align with the theoretical predictions. It is also found that pressure does not have significant effects on the H2 fs CARS spectra up to 5 MPa at 1000 K. The collision model provides excellent agreement with the experiment. This allows the use of low-pressure laser parameters for high-pressure thermometry measurements.

Coherent Raman spectroscopy on hydrogen with in-situ generation, in-situ use, and in-situ referencing of the ultrabroadband excitation

Time-resolved spectroscopy can provide valuable insights in hydrogen chemistry, with applications ranging from fundamental physics to the use of hydrogen as a commercial fuel. This work represents the first-ever demonstration of in-situ femtosecond laser-induced filamentation to generate a compressed supercontinuum behind a thick optical window, and its in-situ use to perform femtosecond/picosecond coherent Raman spectroscopy (CRS) on molecular hydrogen (H₂). The ultrabroadband coherent excitation of Raman active molecules in measurement scenarios within an enclosed space has been hindered thus far by the window material imparting temporal stretch to the pulse. We overcome this challenge and present the simultaneous single-shot detection of the rotational H₂ and the non-resonant CRS spectra in a laminar H₂/air diffusion flame. Implementing an in-situ referencing protocol, the non-resonant spectrum measures the spectral phase of the supercontinuum pulse and maps the efficiency of the ultrabroadband coherent excitation achieved behind the window. This approach provides a straightforward path for the implementation of ultrabroadband H₂ CRS in enclosed environment such as next-generation hydrogen combustors and reforming reactors.

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N₂), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H₂) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H₂ S-branch. We demonstrate H₂ thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H₂ S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm^-1. We present a pure-rotational H₂ CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N₂ spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H₂ and N₂ CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H₂ and N₂ temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H₂ CARS thermometry for probing combustion reactions.

Gas-phase thermometry using delayed-probe-pulse picosecond coherent anti-Stokes Raman scattering spectra of H2

We report the development and application of a simple theoretical model for extracting temperatures from picosecond-laser-based coherent anti-Stokes Raman scattering (CARS) spectra of H2 obtained using time-delayed probe pulses. This approach addresses the challenges associated with the effects of rotational-level-dependent decay lifetimes on time-delayed probing for CARS thermometry. A simple procedure is presented for accurate temperature determination based on a Boltzmann distribution using delayed-probe-pulse vibrational CARS spectra of H2; this procedure requires measurement at only a select handful of probe-pulse delays and requires no assumptions about sample environment.