Ultrafast X-Ray Absorption Near Edge Structure Reveals Ballistic Excited State Structural Dynamics

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B-12, cyanocobalamin (CNCbl), in the alpha beta-band at 550 nm and the gamma-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 angstrom is followed by significant elongation of the axial bonds (>0.25 angstrom) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S-1 excited state structure within 500 fs of excitation.