| Abstract
| - The decay paths on the singlet excited-state surface of 9H-adenine and the associated energybarriers have been calculated at the CAS−PT2//CASSCF level. There are three fundamental paths for thephotophysics: two paths for the 1Lb state which are virtually barrierless at the present level of theory andcorrespond to formation of the (n,π*) intermediate and direct decay to the ground state and a third path forground-state decay of the (n,π*) state with an activation barrier of approximately 0.1 eV. The 1La state,which has the largest oscillator strength, either decays directly to the ground state or contributes indirectlyto the excited-state lifetime by populating the two other states. The results are used to interpret thephotophysics in terms of an excited-state plateau for the 1Lb state that corresponds to the short-lived excited-state component (approximately 0.1 ps) and a well (i.e., a proper minimum) for the (n,π*) state that givesrise to the long component (1 ps or more). The direct decay to the ground state of the 1Lb state is probablythe decay channel invoked to explain the experimental wavelength dependence of the relative amplitudesof the two components. In addition to that, the excited-state component in the nanosecond range detectedin the time-resolved photoelectron spectrum is proposed to be a triplet (π,π*) state formed after intersystemcrossing from the singlet (n,π*) state.
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