Ultrafast Surface Science

Figure: (left) Capturing reactions on surfaces in real time with site-specificity is a scientific grand challenge relevant to many important processes. (right) Laser-assisted Auger decay from atoms on a surface. The few-femtosecond delay between when the direct photoelectrons and the inner-shell excited Auger electrons are emitted can be directly measured in the time domain, making possible a better understanding of how the adsorbate and substrate are electronically coupled.

Figure: (left) Laser-assisted photoemission (LAPE) from atoms. Soft x-ray harmonics photoionize the atoms and, in the presence of the laser field, additional visible photons can be absorbed or emitted, leading to sidebands or streaking in the photoelectron spectrum that are present only when the two fields are overlapped in time. (right) Laser-assisted photoemission from surfaces, showing that the laser field can dress the photoelectron and delayed Auger electrons.

A large fraction of chemical reactions of interest to technology and the environment are catalytic, with the reaction happening on a surface. These reactions are often mediated by electron charge transfer dynamics that occur on extremely fast time scales, and understanding these dynamics is critical for optimizing catalysis, electrochemical processes and solar energy conversion. Currently, direct methods to monitor detailed microscopic mechanism are lacking.

However, this topic is particularly well suited for the application of ultrafast soft x-ray spectroscopy. A short laser pulse (UV/visible/infrared) can initiate electronic or vibrational dynamics on a surface, with x-rays generated from high harmonics used to probe the resultant charge transfer or phase transition with femtosecond-to-attosecond time resolution. Optimization of heterogeneous catalytic reactions can have enormous scientific, technical and economic impact. Until now, catalyst development has been generally empirical, because of the experimental challenges associated with accessing the short timescales of chemical reactions, intermediates and transition states.

In recent work, we used time-resolved soft x-ray photoelectron spectroscopy as a powerful probe for capturing the fastest electronic dynamics at surfaces. In 2006, we made the first observation of laser-assisted photoemission (LAPE) from a solid. This process represents the laser-assisted version of the original photoelectric effect, first explained by Einstein. LAPE can be used to directly measure the coupling between electronic states of an adsorbate and the surface on which it resides – a measurement that is fundamental to the understanding of many surface interactions. More recently, we used LAPE to observe in real-time the decay of inner-shell excited states of the Xenon atom bound to a surface. Many other groups have adopted the LAPE technique for attosecond time-scale measurements. Other recent work in collaboration with Michael Bauer at Kiel combined HHG with time- and angle-resolved photoemission (ARPES) to capture the collapse of long-range order due to hot electron screening in a charge density wave material. The very fast dynamics show that electronic order can collapse before the periodic lattice distortion has a chance to be perturbed, at least in an excited system where there are hot electrons present.

Research Category: