High-resolution absorption cross sections of formaldehyde at wavelengths from 313 to 320 nm

Abstract

Absorption cross sections have been measured for the 2²₀4³₀ and 2³₀4¹₀ vibrational bands of the ùA₂–¹A₁ electronic transition of formaldehyde in the wavelength range 313–320 nm. Accurate values are of considerable importance for atmospheric monitoring and to understand the photochemistry of this compound. The 0.10 cm⁻¹ FWHM wavenumber resolution of the experiments is determined by the bandwidth of the ultraviolet laser used, and is a factor of 10 or more higher than any previously reported data. The absorption cross section data are thus obtained at a spectrometer resolution close to the Doppler broadening limit at 294 K of 0.07 cm⁻¹ FWHM, for isolated rotational lines, but lifetime broadening effects contribute a further ∼0.5 cm⁻¹ of width. Our spectral resolution is thus higher than required to resolve the sharpest spectral features and, as a consequence, the cross sections peak at greater values than previous studies of these structured rovibronic bands conducted at much lower spectrometer resolutions. Previous data can be quantitatively reproduced by convolution of the newly obtained spectra with lower-resolution instrument functions. Pressure broadening of regions of the spectra in the presence of up to 500 Torr of N₂ is examined and the effects on peak absorption cross sections are very small. The influence of reduced temperature on the spectrum is also explored through experimental measurements and spectral simulations.