Most aromatic ketones containing first-row elements undergo unexpectedly fast intersystem crossing in few
tens of picosecond and a quantum yield close to unity. Among them, xanthone (9H-xanthen-9-one) possesses
one of the fastest intersystem crossing rates of ~1.5 ps, despite containing only first-row elements. The exact
mechanism of this unusually fast singlet-triplet transition is still under debate. Here, we perform a complete
wavepacket dynamics simulation of the internal conversion and intersystem crossing reactions of xanthone
in the gas phase. We show that xanthone follows El-Sayed’s rule for intersystem crossing. From the second
singlet excited state, the mechanism is sequential: (i) an internal conversion between singlets 1pipi*-1npi* (~0.14 fs), (ii) an intersystem crossing 1npi*-3pipi* (~1.8 ps), and (iii) an internal conversion between triplets
3pipi*-3npi* (~27 ps). Each transfer finds its origin in a barrierless access to electronic state intersections.
These intersections are close to minimum energy structures, allowing for an efficient radiationless transition from
1pipi* to 3npi*.