Measurement of Excitation-Emission Matrix of Shock-heated PAHs using a Multi-wavelength Laser Source

Abstract

Measurements of Excitation-Emission Matrix (EEM) of shock-heated vapors of polycyclic aromatic hydrocarbons (PAHs) at high temperature (750-1500K) and high pressure (0.3-1.3MPa) conditions were conducted using a multi-wavelength excitation laser in order to demonstrate the potential of the single-measurement EEM fluorometry for investigation of soot precursors. Argon-diluted vapors of naphthalene and pyrene, as PAH model compounds, were heated in an optically accessible shock tube. The PAH vapors were excited by a coherent multi-wavelength rainbow laser light generated by converting the 4th harmonic (266nm) of a pulsed Nd:YAG laser using a Raman cell frequency converter filled with high-pressure (2MPa) methane-hydrogen mixture. The multi-wavelength laser spectrum, when pumped by a 50mJ pulse, consisted of the pump laser and its Stokes and Anti-Stokes shifted coherent lights of up to the 4th order and covered a spectral range between 250 and 360nm with spectral gaps between different wavelengths less than 20nm. The multi-wavelength laser was diffracted by a triangular prism and focused into separate focusing points, for each different wavelength, in the shock tube. The LIF from the shock-heated PAHs at the separate focusing points was simultaneously captured using a spectrometer and an ICCD camera as an EEM. The measured EEM showed that the intensity of the LIF of naphthalene and pyrene vapors excited by 266nm significantly decreased with the increasing temperature. The normalized LIF spectra of naphthalene and pyrene vapors were broader and less structured than the spectra of liquid-phase samples at room temperature. The measurement results presented in this paper demonstrate the potential of the single-measurement EEM with a multi-wavelength laser source and provide a part of reference EEM of PAHs which is necessary for our future investigation of soot precursors using the EEM technique.