Capturing reactions on surfaces in real time with site-specificity is a challenge relevant to many important processes. Surfaces host important chemical reactions that occur in the presence of a catalyst that itself remains unchanged at the end of the reaction. Catalytic reactions often occur via by rapid transfers of electron charge. An in-depth understanding of this behavior would open the door to optimizing catalysis, electrochemical processes, and solar energy conversion. Such optimization could have enormous scientific, technical, and economic impact.
As yet, direct methods for monitoring the details of catalytic processes have not been developed. However, this topic is well suited for the application of ultrafast soft x-ray spectroscopy. A short laser pulse initiates electronic or other surface behaviors. Then x-rays generated from high harmonics are used to probe the resultant charge transfer or phase transition with femtosecond-to-attosecond time resolution.
The Kapteyn/Murnane group has used time-resolved soft x-ray photoelectron spectroscopy to capture the fastest electronic dynamics at surfaces. In 2006, using a laser-assisted version of the original photoelectric effect (LAPE) discovered by Einstein, the group was able to follow how fast an electron moved from one state to another on a solid surface. Since then, the group has shown that LAPE can be used to measure the coupling between electronic states of a chemical attached to a surface, called an adsorbate, and the surface on which the adsorbate resides. Such a measurement is fundamental to the understanding of many surface interactions.
The group has used LAPE to observe the decay of excited states of a xenon atom bound to a surface. In collaboration with Michael Bauer, the group combined high-harmonic generation (HHG) with photoemission studies to capture the collapse of long-range order due to hot electron screening in a charge-density wave material. The fast dynamics showed that electronic order can collapse before the periodic lattice distortion becomes perturbed in a system with hot electrons present.
Nanoscale High-Frequency Acoustic Measurements
Surface acoustic waves, or SAWs, are elastic oscillations confined on the surfaces of some materials. SAWs are interesting because of their sensitivity to the mechanical properties of the material in which they propagate. Their penetration depth corresponds to a fraction of the wavelength of excitation, making them an appealing tool studying the properties of thin films, including the mechanical response of nanostructures deposited on, or embedded within, the surface.
The study of tiny nanosized surfaces requires the generation and detection of surface-confined SAWs with short wavelengths, making coherent high-harmonic beams an ideal tool for their investigation. The Kapteyn/Murnane group demonstrated their use recently in investigations of the shortest wavelength SAWs measured to date. The experiment was sensitive to picometer surface displacements and films as thin as 2 nm.