Acetone photolysis at 248 nm revisited: pressure dependence of the CO and CO2 quantum yields

Abstract

Pressure dependent CO and CO2 quantum yields in the laser pulse photolysis of acetone at 248 nm and T = 298 K have been measured directly using quantitative infrared diode laser absorption. The experiments cover the pressure range from 50 to 900 mbar. It is found that the quantum yields show a significant dependence on total pressure, with Phi(CO) decreasing from around 0.5 at 20 mbar to approximately 0.3 at 900 mbar. The corresponding CO2 yields as observed when O2 exists in the reaction mixture, exhibit exactly the opposite behaviour. For the sum of both a value of 1.05(-0.05)(+0.02) independent of pressure is obtained, showing that the sum of (Phi(CO) + Phi(CO2)) is a measure for the primary quantum yield in the photolysis of acetone. In addition, CO quantum yields and corresponding pressure dependences were measured in experiments using different bath gases including He, Ar, Kr, SF6, and O2 as third body colliders. The theoretical framework in which we discuss these data is based on our previous findings that the pressure dependence of the CO yield is a consequence of a stepwise fragmentation mechanism during which acetone decomposes initially into methyl and a vibrationally 'hot' acetyl radical, with the latter being able to decompose promptly into methyl plus CO. The pressure dependence of the CO yield then originates from the second step and is modelled quantitatively via statistical dynamical calculations using a combination of RRKM theory with a time-dependent master equation (ME) approach. From a comparison of experiment with theory the amount of excess energy in the vibrationally hot acetyl radicals (E* approximately 65 kJ mol(-1)) as well as the characteristic collision parameters for interaction of acetyl with the different bath gases were derived. Values of 90, 280, 310, 545, 550 and 1800 cm(-1) for the average energy transferred per downward collision for the bath gases He, Ar, Kr, O2, N2, and SF6, respectively, are obtained. The calculations also considered different models for the energy transfer kernel P(E,E') and best fits were obtained with a rho-weighted exponential down model.

Pressure dependent OH yields in the reactions of CH3CO and HOCH2CO with O2

“…Calibration of the LIF-system was achieved by fixing a 1b at zero pressure to unity, which neglects to take into account the fact that the acetyl radical yield is pressure dependent as a significant (but variable) fraction thermally decomposes to CH 3 and CO, at least in nitrogen bath gas. [31][32][33] Blitz et al could thus have underestimated the value of KM/KD by about 16%. 3 Talukdar et al 34 used different photolytic schemes (photolysis of acetone, Cl + CH 3 CHO and OH + CH 3 CHO) for CH 3 CO generation coupled to OH-LIF to investigate OH-formation or modification of OH kinetics due to (R1) at experimental pressures between 27-800 mbar in He, N 2 and O 2 .…”

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

“…Photolysis of acetone is one of the most extensively studied photochemical reactions [118]. The CC bond dissociation, called "Norrish type I reaction" (CH 3 ) 2 CO → CH 3 CO + CH 3 , has been the main focus of many theoretical and experimental studies [119][120][121][122][123][124][125][126][127][128][129][130][131][132][133]. Roaming was also suggested experimentally [102], which was proposed to proceed on the S 0 PES as (CH 3 ) 2 CO → CH 3 CO · · · CH 3 → CO + CH 3 -CH 3 .…”

Pressure Dependent Product Formation in the Photochemically Initiated Allyl + Allyl Reaction

“…The model qualitatively explains the trends of pressure dependent end product yields of allene, propene and 1,5-hexadiene (see Figure 3, in order to guide the eye, simulated yields at three pressures (0.4, 4 and 500 mbar) are interpolated by B-splines). We note that the mechanism in Table 2 should be regarded as a tentative, empirical model because the treatment of pressure dependent, photochemical activation driven kinetics is difficult for single chemical steps [43]; here many such steps are involved and some kinetic assumptions may proof implausible by a more detailed analysis. Therefore, we did not try to model acetylene formation, which involves additional uncertain chemical steps.…”