ALBERT

Description: The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S 1 ( 1 ππ ∗ ) state. We investigate these ISC rates experimentally and theoretically as a function of S 1 state vibrational excess energy E exc . The S 1 vibronic levels are pumped with a ∼5 ns UV laser, the S 1 and triplet state ion signals are separated by prompt or delayed ionization with a second UV laser pulse. After correcting the raw ISC yields for the relative S 1 and T 1 ionization cross sections, we obtain energy dependent ISC quantum yields Q ISC corr = 1 % –5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates k ISC = 0.4–1.5 ⋅ 10 9 s −1 , the corresponding S 1 ⇝ S 0 internal conversion (IC) rates are 30–100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S 1 ; v = 0 state with k ISC = 3 ⋅ 10 9 s −1 to the T 1 ( 3 ππ ∗ ) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of 1 n O π ∗ character into the S 1 ( 1 ππ ∗ ) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below E exc = 550 cm −1 in the S 1 state, S 1 ⇝ S 0 internal conversion dominates the nonradiative decay with k IC ≥ 2 ⋅ 10 10 s −1 , (2) the calculated S 1 ⇝ T 1 ( 1 ππ ∗ ⇝ 3 ππ ∗ ) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S 1 ⇝ T 1 ISC is moderately fast ( k ISC = 3 ⋅ 10 9 s −1 ), and not ultrafast, as claimed by other calculations, and (4) at E exc ∼ 550 cm −1 the IC rate increases by ∼50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels.