SO 2 scavenging and self-reaction of CH 2 OO were utilized for the decay of CH 2 OO to extract the absorption spectrum of CH 2 OO under bulk conditions. Absolute absorption cross sections of CH 2 OO at 308.4 and 351.8 nm were obtained from laser-depletion measurements in a jet-cooled molecular beam. The peak cross section is (1.23 AE 0.18) Â 10 À17 cm 2 at 340 nm.Ozonolysis is a major removal mechanism in the troposphere for unsaturated hydrocarbons which are emitted in large quantities from both natural and human sources. Now it is generally accepted that ozonolysis of alkenes proceeds via Criegee intermediates, highly reactive species postulated in 1949 by Rudolf Criegee. 1,2 In the troposphere, Criegee intermediates are involved in several important atmospheric reactions, 3 including reactions with SO 2 and NO 2 , 4-7 or can be photolyzed by near UV light, 7-10 as shown for CH 2 OO in (R1)-(R4).The formation of SO 3 and NO 3 , as in (R2) and (R3), plays an important role in atmospheric chemistry, 11,12 including aerosol and cloud formation. The formation of O( 1 D), as in (R4), will result in OH formation through (R5).Because CH 2 OO absorbs strongly at wavelengths longer than 300 nm, 7-9 tropospheric photolysis of CH 2 OO would be quite efficient with an effective photolysis lifetime on the order of 1 second. 8 As a result, the OH formation of (R4) + (R5) may contribute significantly to the atmospheric OH concentrations.Despite their importance, the direct detection of Criegee intermediates was not realized until recently. 4,13 Welz et al. 4 reported an efficient way to prepare Criegee intermediates. For example, CH 2 OO can be prepared via (R6) + (R7a). Welz et al. 4 also demonstrated the direct detection of CH 2 OO by using vacuum UV photoionization mass spectrometry. The parent ion CH 2 O 2 + was observed when the photon energy exceeded the ionization energy of CH 2 OO (10.0 eV); other isomers like dioxirane and formic acid were excluded due to their different ionization energies. 4 At low pressure, the yield of (R7a) is close to unity, 14,15 while the adduct formation (R7b) may dominate at near atmospheric pressures. 14The kinetics of CH 2 OO reactions with SO 2 and NO 2 were investigated by Welz et al. 4 and by Stone et al. 5 by observing the disappearance of CH 2 OO and by detecting the H 2 CO products, respectively. The rate coefficients of these reactions were found to be unexpectedly rapid and imply a substantially greater role of Criegee intermediates in models of tropospheric sulfate and nitrate chemistry.Beames et al. 8 recorded the UV spectrum of CH 2 OO through observing its depletion in a molecular beam upon laser irradiation (an action spectrum). Based on their laser pulse energy and spot size, Beames et al. 8 roughly estimated the peak absorption cross section to be 5 Â 10 À17 cm 2 (at 335 nm with FWHM B40 nm). Lehman et al. 10 measured the angular and velocity distributions of the O( 1 D) photoproduct arising from UV excitation of CH 2 OO in the 300-365 nm range. From the observed

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) cm(2) molecule(-1) at 308 nm and (9.7 ± 0.6) × 10(-18) cm(2) 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) cm(2) 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.