Probing ultrafast ??*/n?* internal conversion in organic chromophores via K-edge resonant absorption.

ABSTRACT: 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 ??* to a dark n?* state. The ??*/n?* 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 ??*/n?* 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 ??*/n?* 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.Many photo-induced processes such as photosynthesis occur in organic molecules, but their femtosecond excited-state dynamics are difficult to track. Here, the authors exploit the element and site selectivity of soft X-ray absorption to sensitively follow the ultrafast ??*/n?* electronic relaxation of hetero-organic molecules.

SUBMITTER: Wolf TJA 

PROVIDER: S-EPMC5481431 | biostudies-literature | 2017 Jun

REPOSITORIES: biostudies-literature