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

Abstract

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B, cyanocobalamin (CNCbl), in the αβ-band at 550 nm and the γ-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 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S excited state structure within 500 fs of excitation.

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

“…Oscillatory wavepacket motions have been clearly measured in time-resolved WAXS [19,27,32], however the effects of purely nuclear dynamics in ultrafast Kα/Kβ XES has not been observed [10][11][12][13][14]. This contrasts with other X-ray spectroscopic methods that directly involve valence orbitals, such as X-ray absorption [33][34][35][36][37][38] and resonant inelastic X-ray scattering [39][40][41], which are sensitive to both electronic and nuclear structure. The development of ultrafast XES requires understanding the impact of nuclear dynamics on Ka and Kb XES spectra.…”

Sub-100 Fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES

“…Similar fast dynamics have been observed in cobalamins, where simultaneous ballistic expansion along multiple vibrational coordinates was shown to rapidly dissipate vibrational energy in less than 500 fs. 49,50 The <100 fs formation of the 3 In that work, the initial (π, π*) state evolved into a Ni(I) ligand-to-metal charge transfer state in 1 ps, followed by back-electron transfer to form a…”

Resolving structures of transition metal complex reaction intermediates with femtosecond EXAFS

“…15 XFEL based XANES was first used at the Linac Coherent Light Source (LCLS) to study the ultrafast spin switching in the model systems [Fe(bpy) 3 ] 2+ 16 and [Fe(phen) 2 (NCS) 2 ]. 17 Since then, femtosecond XANES has found numerous applications: the photoactivation and the subsequent ultrafast structural and electronic dynamics of the active sites of several biomolecules have been investigated, including carbonmonoxy myoglobin, 18 vitamin B12, 19,20 and in model Ni-porphyrins. 21 Next to TM complex systems, fs timeresolved XAS has been applied to condensed matter charge carrier dynamics; at the SACLA XFEL, the charge-carrier dynamics of anatase TiO 2 nanoparticles in an aqueous solution were investigated via Ti K-edge XANES, which revealed trapping of the conduction band electrons in B100 fs.…”