Concentration and pressure scaling of CH₂O electronic-resonance-enhanced coherent anti-Stokes Raman scattering...

Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde (CH₂O) 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 (ν₁) vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the 101403 vibronic transition between the ground (X~¹A₁) and first excited (A~¹A₂) electronic states. This allows detection of CH₂O at concentrations as low as 9×10¹⁴molecules/cm³ (55 parts per million) in a calibration cell with CH₂O and N₂ at 1 bar and 450 K with 3% uncertainty. The measurements show a quadratic dependence of the signal with CH₂O 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 CH₂O 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 CH₂O in reactive environments and shows promise for application in high-pressure combustors.