Intersystem crossing and internal conversion in benzene vapour at low pressures

Abstract

The pressure dependence of triplet formation in C₆H₆ and C₆D₆ excited at 253.7 nm was studied using both but-2-ene sensitized isomerization and biacetyl phosphorescence. Pressures were varied between 0.1 and 20 Torr by addition of argon. The triplet yield decreases as the pressure decreases from 0.70 at high pressures ( 8 Torr) to 0.18 at 0.1 Torr in C₆H₆ and from 0.65 to 0.11 in C₆D₆. The results are interpreted in terms of a collisional assisted intersystem crossing S₁ [graphic omitted] → T₂ in the low pressure region. For pressures above 1 Torr the observed pressure dependence is attributed to a reversible intersystem crossing process S₁ [graphic omitted] → T₂. It is shown that a decrease in triplet lifetime with an increase in vibrational energy does not explain the present observations even at the lowest pressures used. The results show also that internal conversion S₁ [graphic omitted] → S₀ is an important channel of nonradiative decay in the lowest excited singlet state of C₆H₆ and C₆D₆.