Mixed quantum-classical dynamics simulations show that intersystem crossing between singlet and triplet states in SO2 and in nucleobases takes place on an ultrafast timescale (few 100~fs), directly competing with internal conversion.
Cite
@article{arxiv.1703.09494,
title = {Ultrafast Intersystem Crossing in SO$_2$ and Nucleobases},
author = {Sebastian Mai and Martin Richter and Philipp Marquetand and Leticia González},
journal= {arXiv preprint arXiv:1703.09494},
year = {2021}
}