Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption
J. A. Wolf, Thomas; Heilemann Myhre, Rolf; P. Cryan, James; Coriani, Sonia; Squibb, Richard; Battistoni, Andrea; Berrah, Nora; Bostedt, Christoph; Bucksbaum, Phil; Coslovich, Giacomo; Feifel, Raimund; J. Gaffney, Kelly; Grilj, Jakob; Martinez, Todd J.; Miyabe, Shungo; Moeller, Stefan P.; Mucke, Melanie; Natan, Adi; Obaid, Razid; Osipov, Timur; Plekan, Oksana; Wang, Song; Koch, Henrik; Gühr, Markus
Journal article, Peer reviewed
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Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.