Drew A. Meyer

Tracking excited-state charge and spin dynamics in iron coordination complexes

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

Lost in reciprocal space? Determination of the scattering condition in spot profile analysis low-energy electron diffraction

The precise knowledge of the diffraction condition, i.e., the angle of incidence and electron energy, is crucial for the study of surface morphology through spot profile analysis low-energy electron diffraction (LEED). We demonstrate four different procedures to determine the diffraction condition: employing the distortion of the LEED pattern under large angles of incidence, the layer-by-layer growth oscillations during homoepitaxial growth, a G(S) analysis of a rough surface, and the intersection of facet rods with 3D Bragg conditions.

Detectors for Ultrafast X-ray Experiments at SPPS

The paper describes two detectors designed specifically for the SPPS ultrafast x‐ray source, one of which provides 2‐D position and intensity information, and the other acts as a low‐noise point detector for diffraction experiments. The beam position monitor (BPM) was used as a reference detector for most of the experiments performed there, and the point detector was used for pump‐probe experiments involving phonons in bismuth and on photosensitive metal‐organic crystals. The schedule for development of these detectors was extremely tight, so as much as possible we used available designs for amplifiers etc.

Femtosecond lattice dynamics in photoexcited bismuth: Ultrafast optical and x-ray measurements

We investigate high amplitude coherent optical phonon dynamics in bismuth through femtosecond X-ray and optical scattering. From these experiments, we present the first detailed measurements of changes in the interatomic potential following high-density photoexcitation.

Ultrafast Optical and X-Ray Measurements of Femtosecond Lattice Dynamics in Photoexcited Bismuth

We present the first detailed measurements of the interatomic potential of photoexcited bismuth using ultrafast optical and x-ray scattering. Our results show that electronic softening is responsible for the strong chirp in phonon frequency.