Absolute photodissociation cross section of anti-CH₃CHOO

Criegee intermediates are highly reactive species that play important roles in atmospheric chemistry. The absolute values of absorption cross sections are key parameters for determining solar photolysis rates and for quantifying concentrations via optical methods. Over the years, the absolute absorption cross sections have been measured for a few small Criegee intermediates, including CH₂OO, syn-CH₃CHOO, (CH₃)₂COO, methyl vinyl ketone oxide, and methacrolein oxide. However, a direct cross section measurement had remained unavailable for anti-CH₃CHOO due to complications arising from the co-existence of a more stable conformer syn-CH₃CHOO. The UV photodissociation quantum yields of small Criegee intermediates are known to be unity. Thus, their photodissociation cross sections would be equal to their absorption cross sections. In this work, we directly measured this quantity for the first time by measuring the concentration ratio of anti-CH₃CHOO with and without 352 nm photodepletion. The relative concentration of anti-CH₃CHOO was monitored by time-resolved UV absorption spectroscopy using a flow reactor. The signals of syn- and anti-CH₃CHOO were separated by introducing CH₃OH, which selectively scavenges anti-CH₃CHOO. The 352 nm laser fluence was calibrated via photodepletion of NO₂, for which the absorption cross section and photodissociation quantum yield are well-known. The cross section of anti-CH₃CHOO was determined to be (1.43 ± 0.18) × 10^-17 cm² at 352 nm. In addition, an updated anti-CH₃CHOO spectrum of improved signal-to-noise ratio was reported with a peak cross section of (1.51 ± 0.19) × 10^-17 cm² at 364 nm and a full width at half maximum of 83 nm. This spectrum allows us to determine the population ratio of anti-CH₃CHOO to syn-CH₃CHOO from the 352 nm photolysis of CH₃CHI₂ in the presence of O₂: [anti-CH₃CHOO]/[syn-CH₃CHOO] = (0.32 ± 0.05) ([anti-CH₃CHOO]: [syn-CH₃CHOO] = (0.24 ± 0.04): (0.76 ± 0.04)) at 51 Torr and 297 K.