PRIMARY QUANTUM YIELD DETERMINATION BY INTERMITTENT ILLUMINATION IN THE REACTION OF METHANOL VAPOR WITH Hg 6(3P1) ATOMS

Abstract

The decomposition of methanol vapor by Hg 6(3P1) atoms has been studied under intermittent illumination conditions, in a static system at 25 °C. The behavior of the NO-inhibited reaction under intermittent illumination has also been examined.For the pure substrate at a pressure of 50 mm, the quantum yield of hydrogen formation, Φ(H2), was found to rise steadily toward unity with decreasing light period (tL). The maximum value of Φ(H2) at a given tD could only be obtained if the dark period (tD) exceeded 100 mseconds. At the shortest tL studied, 0.156 msecond, Φ(H2) was 0.89 compared with the steady illumination value, at zero extent of reaction, of 0.46. A linear relation was found to obtain between Φ(H2) and log (tL) at a given tD Extrapolation showed that Φ(H2) would be unity at tL = 0.040 msecond.For a mixture of 4.8 mm of NO and 50 m of CH3OH, Φ(CH3ONO) was found to rise from its steady illumination value of 0.14 to 0.27 at a sector speed of 300 r.p.m., with a dark/light ratio of 20. At higher speeds the quantum yield decreased again.From the study it is concluded that the primary process of H atom detachment is at least 89% efficient, and that the primary quantum yield is likely unity. The fact that the quantum yield under steady illumination is only 0.46 is attributed to the consumption of H atoms by radical addition processes of which the most probable is the addition of H to CH2OH. For the inhibited reaction the low value of Φ(CH3ONO) under steady illumination is ascribed to the abstractive attack of CH3O on NOH.The kinetic effects of intermittent illumination are discussed, in the light of the results. It is concluded that the technique should be of considerable value in primary process studies in mercury photosensitization.