Factors affecting the rate of decay of the first excited singlet state of molecular oxygen O-2(a(1)Delta(g)) in supercritical fluid carbon dioxide

The photosensitized production and subsequent decay of singlet oxygen in su percritical fluid carbon dioxide has been studied as a function of pressure and temperature. The rate of decay is shown to be a function of the fluid density, demonstrating a quenching effect by the solvent. The observed rate constant for decay is also found to depend on the oxygen concentration, as a result of quenching of singlet oxygen, O-2(a(1)Delta (g)) by ground stat e oxygen. The rate constant for quenching by oxygen is found to be 1.9 x 10 (4) dm(3) mol(-1) s(-1), at 41 degreesC and at a pressure of 150 kg cm(-2), which is compared with previous findings in conventional solvents. The lif etime of singlet oxygen in supercritical fluid carbon dioxide, under these conditions, extrapolated to the value in the absence of ground state molecu lar oxygen, was found to be 5.1 ms (at 150 kg/cm(2) and 41 degreesC). The f actors determining the lifetime of O-2(a(1)Delta (g)), in supercritical flu id carbon dioxide are discussed.