UV absorption spectrum of the C2 Criegee intermediate CH₃CHOO

The UV spectrum of CH₃CHOO was measured by transient absorption in a flow cell at 295 K. The absolute absorption cross sections of CH₃CHOO were measured by laser depletion in a molecular beam to be (1.06 ± 0.09) × 10⁻¹⁷ cm² molecule⁻¹ at 308 nm and (9.7 ± 0.6) × 10⁻¹⁸ cm² molecule⁻¹ at 352 nm. After scaling the UV spectrum of CH₃CHOO to the absolute cross section at 308 nm, the peak UV cross section is (1.27 ± 0.11) × 10⁻¹⁷ cm² molecule⁻¹ at 328 nm. Compared to the simplest Criegee intermediate CH₂OO, the UV absorption band of CH₃CHOO is similar in intensity but blue shifted by 14 nm, resulting in a 20% slower photolysis rate estimated for CH₃CHOO in the atmosphere