Femtosecond time-resolved, fully resonant electronically enhanced coherent anti-Stokes Raman scattering (FREECARS) spectroscopy, incorporating a two-color ultraviolet excitation scheme, is used to demonstrate chemically selective and sensitive detection of gas-phase species, including nitric oxide (NO) and the hydroxyl (OH) radical. The observed time-dependent, spectrally resolved CARS signal contains rich structure that depends both on the rovibronic states accessed within the bandwidth of the initial (pump) excitation pulse and the Raman-active rovibrational levels within the vibrationally excited ground electronic state that are accessed following interaction with the second (Stokes) excitation pulse. By comparing experimental spectra to computational simulations, therefore, this approach also allows simultaneous determination of local temperature associated with the thermal distribution of initial states under singlelaser- shot conditions. For OH radical detected in a reacting flow, spectral resolution of the emitted FREE-CARS signal allows simultaneous single-shot detection of relative OH mole fraction and temperature in a laminar ethylene–air flame at 1-kHz repetition rates. By comparison to previously reported OH concentration and temperature measurements, we demonstrate excellent single-shot temperature accuracies (~2% deviation from adiabatic flame temperature) and precisions (~2% standard deviation), with simultaneous relative OH concentration measurements that demonstrate high detection sensitivity (100–300 ppm).
Zeolite catalysis has been exploited by the petrochemical industry since the 1940’s for catalytic cracking reactions of long chain hydrocarbons. The selectivity of zeolites strongly depends on a pore size, which is controlled by the chosen structure-directing agent (SDA) and by the SDA decomposition/removal process. Although zeolites are composed of micron-sized crystals, studies of zeolite materials typically focus on bulk (i.e., ensemble) measurements to elucidate structure-function information or to optimize catalysts and/or process parameters. To examine these phenomena on the microscale, non-linear optical microscopy is used to provide real-time imaging of chemical reactions in zeolites at temperatures exceeding 400°C. The template decomposition mechanism is studied, as elucidation of the mechanism is critical to understanding the relationship between the decomposition chemistry and the nanoscale features of the zeolite (topology, Si/Al ratio, added dopants). Forward stimulated Raman scattering (SRS), forward coherent anti-Stokes Raman scattering (CARS) and epi two-photon fluorescence (TPF) modalities are acquired simultaneously providing video-rate structural and chemical information. A high-temperature cell with gas inlet system is used for the study of reactions under various temperatures and gas environments. Examining the decomposition process with single-particle resolution enables access to ensemble-level and spatially-resolved behavior. Parallel experiments on bulk zeolite powders are conducted to enable comparison of ensemble and single-particle behavior during template decomposition. Our multi-technique approach has high potential for gaining insight into the link between nanoscale structure and catalytic activity and selectivity of zeolitic materials.