Excited State Absorption of Uracil in the Gas Phase: Mapping the Main Decay Paths by Different Electronic Structure Methods

We present a computational study of the one-photon and excited-state absorption from the two lowest-energy excited states of uracil in the gas phase: an n?pi* dark state
(1n) and the lowest-energy bright ??? pi-pi* state (1?pi). The predictions of six di?fferent linear response electronic structure methods, namely TD-CAM-B3LYP, EOM-CCSD,
EOM-CC3, ADC(2), ADC(2)-x and ADC(3) are critically compared. In general, the spectral shapes predicted by TD-CAM-B3LYP, EOM-CCSD, EOM-CC3 and ADC(3) are fairly similar, though the quality of TD-CAM-B3LYP slightly deteriorates in the high energy region. Computing the spectra at some key structures on the di?fferent potential energy surfaces (PES), i.e. the Franck-Condon point, the 1n minimum,
and structures representative of di?fferent regions of the 1? PES, we obtain important insights into the shift of the excited-state absorption spectra, following the motion of the
wavepacket on the excited state PES. Though 1pi ? has larger excited-state absorption than 1n, some spectral regions are dominated by these latter signals. Aside from its
methodological interest, we thus obtain interesting indications to interpret transient absorption spectra to disentangle the photoactivated dynamics of nucleobases.