Effects of pressure on fluorescence and intersystem crossing in benzene vapor

Abstract

Relative quantum yields of fluorescence have been measured from benzene vapor excited by the 2537-Å mercury line and at total pressures sufficiently low that the lifetime of the fluorescing 1B2u state was much shorter than the interval between collisions. At 84°, the yield and intensity distribution of emission are invariant over benzene pressures from 0.0055 to 0.183 Torr and are not affected by addition of cyclohexane at pressures up to 0.27 Torr. Resonance emission is observed in this pressure region. The yield begins to decrease (and the character of the emission spectrum to change) at a cyclohexane pressure between 0.27 and 0.45 Torr and reaches a limiting value of about 0.6 of that at lowest pressures when cyclohexane pressure is above 10 Torr. The behavior with added CO2 is similar. A benzene sensitized isomerization of cisbutene-2 is observed in the 2537-Å photolysis of benzene-butene mixtures with each gas at 0.05 Torr. Correlation with previous data shows that in the low-pressure region of constant fluorescence yield, collisions do not influence decay of 1B 2u benzene molecules, the absolute fluorescence yield is less than unity, and a triplet state is formed. The mechanism of the processes involved is discussed.