  Organic chromophores with heteroatoms possess an important excited state
relaxation channel from an optically allowed {\pi}{\pi}* to a dark
n{\pi}*state. We exploit the element and site specificity of soft x-ray
absorption spectroscopy to selectively follow the electronic change during the
{\pi}{\pi}*/n{\pi}* internal conversion. As a hole forms in the n orbital
during {\pi}{\pi}*/n{\pi}* internal conversion, the near edge x-ray absorption
fine structure (NEXAFS) spectrum at the heteroatom K-edge exhibits an
additional resonance. We demonstrate the concept with the nucleobase thymine, a
prototypical heteroatomic chromophore. With the help of time resolved NEXAFS
spectroscopy at the oxygen K-edge, we unambiguously show that
{\pi}{\pi}*/n{\pi}* internal conversion takes place within (60 \pm 30) fs.
High-level coupled cluster calculations on the isolated molecules used in the
experiment confirm the superb electronic structure sensitivity of this new
method for excited state investigations.
