Cavity-enhanced transient absorption spectroscopy of 2-thiouracil

The ultrafast dynamics of 2-thiouracil after excitation by UV light, and their comparison to those of uracil, have garnered significant interest recently in the context of understanding the photostability of nucleobases. However, previous gas-phase time-resolved photoelectron spectroscopy (TRPES) and solution-phase transient absorption spectroscopy (TAS) experiments have reported very different excited-state dynamics [1]. The competition between relaxation pathways involving internal conversion and intersystem crossing impedes theoretical treatments of 2-thiouracil. TAS is an excellent probe for studying intersystem crossing due to the unique spectral profiles of the triplet states, but previous TAS studies have been restricted to the solvated molecule.

We present new visible transient absorption spectra of gas-phase, jet-cooled 2-thiouracil after S₂ excitation obtained with a unique cavity-enhanced femtosecond transient absorption spectrometer [2]. We compare our results to TRPES data and ab initio theory that directly calculates our observables. Our results elucidate the contribution of the probe process to the observed dynamics, separating the underlying molecular dynamics from artifacts of the measurement method.

[1] J.A. Sánchez-Rodríguez et al., Phys. Chem. Chem. Phys. 19, 19756 (2017).

[2] M.C. Silfies et al., Phys. Chem. Chem. Phys. 23, 9743 (2021).