Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces

Abstract

Electron and coherent phonon dynamics at potassium-covered Cu(111) surfaces have been studied by using femtosecond time-resolved second-harmonic generation (TRSHG). At the coverages from 0.22 to 0.35 monolayer (ML), TRSHG traces show the oscillatory component with a frequency of $3.05\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{THz}$. The amplitude of this component decreases as coverage increases higher than $\ensuremath{\sim}0.35$ ML, whereas another oscillating component with a frequency of $1.26\ifmmode\pm\else\textpm\fi{}0.03\text{ }\text{THz}$ grows. Both components are ascribed to K-Cu stretching motion. The spectral changes with coverage suggest that the overlayer structure varies with lateral compression. The fast transient peak in TRSHG traces at around zero delay changes its sign from negative to positive when coverage exceeds $\ensuremath{\sim}0.22$ ML. Since the quantum-well state (QWS) is partly filled at around this coverage, electrons in the QWS are principally responsible for the transition of the electronic response. Furthermore, the excitation photon energy dependence of TRSHG traces indicates that the excitation of substrate $d$-band electrons, giving rise to rapid charge fluctuations in the QWS, generates the coherent K-Cu stretching vibration. Consequently, the QWS plays a major role in the electronic and nuclear dynamics induced by pump pulses at $h\ensuremath{\nu}=2.2\text{ }\text{eV}$.