Infrared spectrum of the simplest Criegee intermediate CH₂OO at resolution 0.25 cm^-1 and new assignments of bands 2 ν ₉ and ν ₅

The simplest Criegee intermediate CH₂OO is important in atmospheric chemistry. It has been detected in the reaction of CH₂I + O₂ with various spectral methods, including infrared spectroscopy; infrared absorption of CH₂OO was recorded at resolution 1.0 cm^-1 in our laboratory. We have improved our system and recorded the infrared spectrum of CH₂OO at resolution 0.25 cm^-1 with rotational structures partially resolved. Observed vibrational wavenumbers and relative intensities are improved from those of the previous report and agree well with those predicted with quantum-mechanical calculations using the MULTIMODE method on an accurate potential energy surface. Observed rotational structures also agree with the simulated spectra according to theoretical predictions. In addition to derivation of critical vibrational and rotational parameters of the vibrationally excited states to confirm the assignments, the spectrum with improved resolution provides new assignments for bands 2ν₉ at 1234.2 cm^-1 and ν₅ at 1213.3 cm^-1; some hot bands and combination bands are also tentatively assigned.