Photolysis of CH3C(O)CH3 (248 nm, 266 nm), CH3C(O)C2H5 (248 nm) and CH3C(O)Br (248 nm): pressure dependent quantum yields of CH3 formation

Abstract

The formation of CH(3) in the 248 or 266 nm photolysis of acetone (CH(3)C(O)CH(3)), 2-butanone (methylethylketone, MEK, CH(3)C(O)C(2)H(5)) and acetyl bromide (CH(3)C(O)Br) was examined using the pulsed photolytic generation of the radical and its detection by transient absorption spectroscopy at 216.4 nm. Experiments were carried out at room temperature (298 +/- 3 K) and at pressures between approximately 5 and 1500 Torr N(2). Quantum yields for CH(3) formation were derived relative to CH(3)I photolysis at the same wavelength in back-to-back experiments. For acetone at 248 nm, the yield of CH(3) was greater than unity at low pressures (1.42 +/- 0.15 extrapolated to zero pressure) confirming that a substantial fraction of the CH(3)CO co-product can dissociate to CH(3) + CO under these conditions. At pressures close to atmospheric the quantum yield approached unity, indicative of almost complete collisional relaxation of the CH(3)CO radical. Measurements of increasing CH(3)CO yield with pressure confirmed this. Contrasting results were obtained at 266 nm, where the yields of CH(3) (and CH(3)CO) were close to unity (0.93 +/- 0.1) and independent of pressure, strongly suggesting that nascent CH(3)CO is insufficiently activated to decompose on the time scales of these experiments at 298 K. In the 248 nm photolysis of CH(3)C(O)Br, CH(3) was observed with a pressure independent quantum yield of 0.92 +/- 0.1 and CH(3)CO remained below the detection limit, suggesting that CH(3)CO generated from CH(3)COBr photolysis at 248 nm is too highly activated to be quenched by collision. Similar to CH(3)C(O)CH(3), the photolysis of CH(3)C(O)C(2)H(5) at 248 nm revealed pressure dependent yields of CH(3), decreasing from 0.45 at zero pressure to 0.19 at pressures greater than 1000 Torr with a concomitant increase in the CH(3)CO yield. As part of this study, the absorption cross section of CH(3) at 216.4 nm (instrumental resolution of 0.5 nm) was measured to be (4.27 +/- 0.2) x 10(-17) cm(2) molecule(-1) and that of C(2)H(5) at 222 nm was (2.5 +/- 0.6) x 10(-18) cm(2) molecule(-1). An absorption spectrum of gas-phase CH(3)C(O)Br (210-305 nm) is also reported for the first time.

Rate coefficients for the reactions CH3 + Br2 (224–358 K), CH3CO + Br2 (228 and 298 K), and Cl + Br2 (228 and 298 K)

“…To measure the rate coefÞcient for the reaction of CH 3 CO with Br 2 , we made use of the fact that acetone photolysis at 266 nm (R1) results in formation of one CH 3 and one CH 3 CO radical. Our previous studies [6,26] have showed that at 60 Torr of N 2 , both the overall CH 3 C(O)CH 3 photodissociation quantum yield and the yield of CH 3 are close to unity, and that negligible amounts of the CH 3 CO formed decompose thermally to CH 3 + CO. The rate expression for Br atom formation is then…”

OH yields from the CH3CO+O2 reaction using an internal standard

“…Khamaganov et al [21] have observed a weak pressure dependence of the quantum yield for acetyl formation which decreases slightly with increasing N 2 bath gas pressures. Applying a correction factor to the OH yields in Consequently, at a given total pressure a smaller OH yield is obtained in N 2 than in…”

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

“…Higherresolution absolutely-calibrated photoabsorption measurements of the longest-wavelength absorption features (Cameron et al 2002;Khamaganov et al 2007; 200 nm to the photoabsorption A105, page 26 of 62 Extrapolation of Gans et al (2010) threshold at 220 nm) allow for a rough calibration of the other photographic features based on their apparent saturation. An absolute photoionisation cross section is recorded in the neighbourhood of the 126 nm ionisation threshold (Gans et al 2010), which was extrapolated here to shorter wavelengths.…”