Watching Electrons Do Chemistry in Liquids: Time-Resolved Soft X-Ray Spectroscopy of Solvated Molecules
Ultrashort laser pulses can follow chemical reaction kinetics in real time, but extracting quantitative information on the evolving molecular and electronic structure from optical measurements remains a major challenge. Therefore, ultrafast spectroscopy seeks new probes to provide a deeper understanding of chemical reactions at the level of atoms and electrons.
Ultrashort x-ray pulses are such new probes, and in recent years time-resolved laser spectroscopy and established x-ray methods have been combined to create new tools to directly probe the local electronic and molecular structure in time and energy. Hard x rays are typically used to probe atomic arrangements through scattering and K-edge spectroscopy, while soft x rays are sensitive to valence-charge distributions and hold tremendous potential for following the formation and dissolution of chemical bonds in real time. The information gleaned from ultrafast x-ray probes is essential to advance our understanding of the cooperative relationship between electronic charge distributions, atomic rearrangement, and the formation of new molecular structures. It is particularly effective for understanding molecular dynamics in solution, where much important chemistry occurs and where the solvent environment substantially influences reaction dynamics through interaction with the valence charge distribution.
Left: Schematic of the experiment with a green pump pulse and the soft x-ray probe pulse. The Fe(II) complex exhibits an elongation along the Fe–N bond upon optical excitation. Right: Transmission interferograms of a laser beam imaged through the cell indicate the liquid film thickness.
Solvated transition-metal complexes are of fundamental interest due to the strong interaction between electronic and molecular structure. In particular, the octahedral ligands in Fe(II) complexes effectively couple optical charge-transfer excitations to subtle changes of the molecular structure, leading to rapid spin-state interconversion. The scientific motivation for time-resolved x-ray studies is to understand this important coupling. The spin-crossover transition in Fe(II) is closely related to electron transfer reactions in heme proteins and is of practical interest for opto-magnetic storage via light-induced excited spin-state trapping.
We have applied time-resolved soft x-ray spectroscopy at the Fe L3 edge, using the recently commissioned Beamline 6.0.2, to reveal the electronic dynamics of an ultrafast Fe(II) spin transition in solution. Critical to such liquid-phase studies has been the development of a sample cell in which a liquid film is held between two 100-nm silicon nitride membranes while the interior cell pressure is balanced against the pressure of the sample chamber, thereby controlling the liquid film thickness with sub-200-nm accuracy.
Left: Absorption spectra of the Fe(II) L3 edge for the ground state with low-spin (blue) and excited high-spin states (red) at 5-ns delay. Inset: Time delay scans at the absorption maxima of the low- and high-spin states. Right: Simplified level scheme and the soft x-ray probe transition.
We have measured the absorption of the low- and high-spin states after the photo-induced metal-to-ligand charge transfer. These are to our knowledge the first time-resolved transmission spectra of solvated molecules ever recorded in the soft x-ray region. They reveal a red-shift of the Fe L3 edge of the high-spin state by 1.6 eV, which corresponds to a reduction in ligand-field splitting of the Fe(II) d-orbitals. The time delay scans track the laser-initiated decrease of the low-spin-state absorption and the increase of the high-spin-state absorption. These delay scans reflect the 70 ps width of the x-ray probe pulses and demonstrate that the Fe(II) compound reaches its meta-stable high-spin state within the duration of the probe pulse.
With the emergence of high-flux ultrafast soft x-ray sources, details on interplay between atomic structure, electronic states, and spin contributions will be revealed. Our experimental approach opens the door to femtosecond soft x-ray investigations of liquid phase chemistry that have previously been inaccessible.
ALSNews Vol. 292