Probing ultrafast \pi\pi*/n\pi* internal conversion in organic   chromophores via K-edge resonant absorption
  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. 
T. J. A. Wolf, R. H. Myhre, J. P. Cryan, S. Coriani, R. J. Squibb, A.   Battistoni, N. Berrah, C. Bostedt, P. Bucksbaum, G. ...
physics.chem-ph