Eur. Phys. J. D (2021) 75: 17
https://doi.org/10.1140/epjd/s10053-020-00017-z

Regular Article - Topical Issue

Excited state dynamics of protonated keto uracil: intersystem crossing pathways in competition

Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405, Orsay, France

^d gilles.gregoire@universite-paris-saclay.fr

Received: 24 July 2020
Accepted: 16 November 2020
Published online: 21 January 2021

Abstract

The relaxation dynamics of protonated keto uracil has been investigated through cryogenic UV photodissociation spectroscopy. Steady-state spectroscopy and time-resolved photochemistry, including pump-probe photodissociation and kinetics of appearance of photofragments, are monitored over 10 orders of magnitude as a function of excess energy imparted in the bright ${}^1\mathrm{\pi }\mathrm{\pi }$* state. Although photofragments are produced in the ground electronic state after internal conversion, the non-radiative decay mechanism abruptly changes with a slight increase of excess energy in the ${}^1\mathrm{\pi }\mathrm{\pi }$* state. At the band origin, a three-step decay involving electronic couplings to the charge transfer ${}^1{\text{n}}_{\text{o}}\mathrm{\pi }$* state and the triplet ${}^3\mathrm{\pi }\mathrm{\pi }$* state with lifetimes in the range of $10\mathrm{\mu }\text{s}$ and 2 ms, respectively, is proposed. However, the pathway through the charge transfer state closes a few hundreds of wavenumbers above the band origin. From this excess energy, the excited state population is transferred through a low energy barrier towards a region of the ${}^1\mathrm{\pi }\mathrm{\pi }$* potential energy surface where a triple crossing with the ${}^3\mathrm{\pi }\mathrm{\pi }$* state and the ground state is located. The experimental results are assigned with the help of ab initio calculations at the spin-component scaled coupled-cluster level.