Direct Measurement of Core-Level Relaxation Dynamics on a Surface-Adsorbate System

The coupling between electronic states in a surface-adsorbate system is fundamental to the understanding of many surface interactions. In this Letter, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By comparing laser-assisted photoemission from a substrate with a delayed Auger decay process from an adsorbate, we measure the lifetime of the $4d^{-1}$ core level of xenon on Pt(111) to be $7.1\pm 1.1\mathrm{fs}$. This result opens up time-domain measurements of surface dynamics where energy-resolved measurements may provide incomplete information.

Figure 1

Schematic of the experiment. Ultrafast SXR and IR pulses are focused onto a $\mathrm{Xe}/\mathrm{Pt}(111)$ surface, where the kinetic energy of the photoemitted electrons is measured by a time-of-flight detector. An electron from the Xe ${\mathrm{N}}_4$ ($4d$) shell is ejected by the SXR, followed by filling of the core hole by an ${\mathrm{O}}_{2,3}$ ($5p$) shell electron and ejection of a secondary (Auger) electron from the ${\mathrm{O}}_{2,3}$ shell. In the presence of the IR beam, sidebands appear in the photoelectron spectrum of both Xe and Pt. By comparing the sideband amplitude vs time delay for the Pt and Xe, we can extract the lifetime of the Xe $4d$ core hole.

Figure 2

Extraction of the LAPE/LAAD sideband amplitudes for $\mathrm{Xe}/\mathrm{Pt}(111)$ at zero time delay between the IR and SXR beams. (a) Photoelectron spectra of the Pt $d$ band; (b) of the Xe Auger NOO bands; and (c) of the Xe $4d$ peaks. Red (black) lines represent the spectra with (without) the IR pulse present. The blue lines show the fit to the $\mathrm{SXR}+\mathrm{IR}$ curve when the LAPE response function from Eq. (1) is convolved with the SXR-only spectrum. The insets show the extracted LAPE/LAAD response function associated with the fit.

Figure 3

Photoelectron spectra from $\mathrm{Xe}/\mathrm{Pt}(111)$ as a function of time delay. Negative time delays denote that the SXR pump pulse arrives after the IR probe pulse. The time evolution of the redistribution of electrons due to the IR dressing at the Pt $d$ band and at the Xe Auger photoelectron peaks can be observed. The spectra shown in the lineouts correspond to the photoelectron spectrum at zero (red) and at $-50$ (black) fs time delay between the SXR and the IR pulses.

Figure 4

Sideband height vs time delay for the Pt $d$-band and Xe Auger photoelectrons. The sideband height for the Pt $d$ band (red curve) corresponds to a cross correlation between the SXR and the IR pulses. In contrast, the Auger sideband height (black curve) is clearly shifted by $5.9\pm 0.2\mathrm{fs}$ with respect to time zero, and is also broadened with respect to the Pt $d$-band curve. The shift and broadening are due to the inherent lifetime of the Xe $4d$ core hole. From these curves we extract a Xe $4d$ core-hole lifetime of $7.1\pm 1.1\mathrm{fs}$.