Photosensitized production of singlet oxygen and factors governing its decay in xenon and carbon dioxide supercritical fluids

The photosensitized generation and subsequent decay of singlet oxygen in supercritical fluid xenon has been studied as a function of pressure and temperature. It has been found that the rate constant for quenching of singlet oxygen by ground state oxygen, kqO, increases as the pressure increases and decreases as the temperature increases. At 298 K, the value of kqO increases from (1.27 to 1.76) × 103 dm3 mol−1 s−1 as the pressure increases from 9.8 to 39.2 MPa; at 355 K the values of kqO drop to 6.2 × 102 and 1.54 × 103 dm3 mol−1 s−1 at these same pressures. It has also been found that the fractional contribution of the oxygen quenching to the overall singlet oxygen decay rate increases with increasing pressure, showing greater variations at high temperatures, and decreases with increasing temperature. The measured volume of activation was found to decrease with increasing pressure, and shows a small but systematic decrease with decreasing temperature, particularly at lower pressures.