High resolution quantum cascade laser spectroscopy of the simplest Criegee intermediate, CH₂OO, between 1273 cm^-1 and...

The region 1273-1290 cm^-1 of the ν₄ fundamental of the simplest Criegee intermediate, CH₂OO, has been measured using a quantum cascade laser transient absorption spectrometer, which offers greater sensitivity and spectral resolution (<0.004 cm^-1) than previous works based on thermal light sources. Gas phase CH₂OO was generated from the reaction of CH₂I + O₂ at 298 K and 4 Torr. The analysis of the absorption spectrum has provided precise values for the vibrational frequency and the rotational constants, with fitting errors of a few MHz. The determined ratios of the rotational constants, A'/A″ = 0.9986, B'/B″ = 0.9974, and C'/C″ = 1.0010, and the relative intensities of the a- and b-type transitions, 90:10, are in good agreement with literature values from a theoretical calculation using the MULTIMODE approach, based on a high-level ab initio potential energy surface. The low-K (=K_a) lines can be fitted extremely well, but rotational perturbations by other vibrational modes disrupt the structure for K = 4 and K ≥ 6. Not only the spectral resolution but also the detection sensitivity of CH₂OO IR transitions has been greatly improved in this work, allowing for unambiguous monitoring of CH₂OO in kinetic studies at low concentrations.