Photochemistry of formaldehyde under tropospheric conditions

Abstract

We report new results on the absorption cross sections and photochemical quantum yields for radical (H + HCO) production from formaldehyde in the wavelength interval from 308–320 nm, obtained at resolutions of better than 0.1 nm. The absorption cross sections, measured at resolutions close to the limit for Doppler broadening of HCHO, show rotationally resolved fine structure, with maximum values higher than those obtained in previous, lower resolution, studies. In this wavelength region, absorption cross sections peak at 2.3 × 10⁻¹⁹ cm² molecule⁻¹, but band-integrated values are in excellent accord with previous measurements. HCO absorption coefficients, measured by cavity ring-down spectroscopy following UV photolysis of HCHO at wavelengths across the 2¹₀5¹₀, 2²₀4³₀ and 2³₀4¹₀ bands of the ùA₂–¹A₁ transition, generally mimic the parent absorption band profiles. Division of these absorption coefficients by the high resolution parent absorption cross sections gives relative quantum yields for the H + HCO radical product channel that can be put on an absolute scale using single-wavelength literature values. These quantum yields are observed to show some variation with parent excitation wavelength (and thus with excited vibronic level). Addition of 200 Torr of N₂ increases the HCO quantum yields.