Abstract

Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde (${{\rm{CH}}_2}{\rm{O}}$) concentrations in reacting and nonreacting conditions. The three-color scheme utilizes a 532 nm pump beam and a scanned Stokes beam near 624 nm for Raman excitation of the C–H symmetric stretch (${\nu _1}$) vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the $1_0^14_0^3$ vibronic transition between the ground (${\tilde X}{^{{1}}{{\rm{A}}_1}}$) and first excited (${\tilde A}{^{{1}}{{\rm{A}}_2}}$) electronic states. This allows detection of ${{\rm{CH}}_2}{\rm{O}}$ at concentrations as low as ${{9}} \times {{10}^{14}}\;{\rm{molecules}}/{\rm{cm}}^3$ (55 parts per million) in a calibration cell with ${{\rm{CH}}_2}{\rm{O}}$ and ${{\rm{N}}_2}$ at 1 bar and 450 K with 3% uncertainty. The measurements show a quadratic dependence of the signal with ${{\rm{CH}}_2}{\rm{O}}$ number density. Pressure scaling experiments up to 11 bar in the calibration cell show an increase in signal up to 8 bar. We study pressure dependence up to 11 bar and further apply the technique to characterize the ${{\rm{CH}}_2}{\rm{O}}$ concentration in an atmospheric premixed dimethyl ether/air McKenna burner flame, with a maximum concentration uncertainty of 11%. This approach demonstrates the feasibility for spatially resolved measurements of minor species such as ${{\rm{CH}}_2}{\rm{O}}$ in reactive environments and shows promise for application in high-pressure combustors.

© 2021 Optical Society of America

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