Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces Kazuya Watanabe Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan and PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan Ken-ichi Inoue and Ikuyo F. Nakai Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan Masanori Fuyuki* The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan Yoshiyasu Matsumoto † Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan; and Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan Received 18 February 2009; revised manuscript received 3 July 2009; published 4 August 2009 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 mono- layer ML, TRSHG traces show the oscillatory component with a frequency of 3.05  0.05 THz. The ampli- tude of this component decreases as coverage increases higher than 0.35 ML, whereas another oscillating component with a frequency of 1.26  0.03 THz grows. Both components are ascribed to K-Cu stretching motion. The spectral changes with coverage suggest that the overlayer structure varies with lateral compres- sion. The fast transient peak in TRSHG traces at around zero delay changes its sign from negative to positive when coverage exceeds 0.22 ML. Since the quantum-well state QWS is partly filled at around this cover- age, 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 vibra- tion. Consequently, the QWS plays a major role in the electronic and nuclear dynamics induced by pump pulses at h =2.2 eV. DOI: 10.1103/PhysRevB.80.075404 PACS numbers: 78.47.J-, 68.35.Ja, 68.43.Pq, 82.53.St I. INTRODUCTION Electron confinement is one of the central issues in nano- scale physics and chemistry. Even at a metal surface, elec- trons can be trapped in a potential well between the surface and the vacuum if the metal has a local band gap in the direction normal to the surface. This electron confinement gives rise to quantum-well states QWSs that form two- dimensional 2D electron gases. The interactions between quasiparticles in the QWS, different from those of bulk met- als, strongly influence physical and chemical properties of metal surfaces. Therefore, knowledge of QWSs on metal sur- faces has a great importance in surface science and nanotech- nology. QWSs at alkali-metal overlayers on metals have been ex- tensively studied experimentally 1–11 and theoretically. 12–21 A metallic alkali monolayer introduces new surface electronic bands. The bands most relevant to bonding at the alkali over- layer are those originating from the free-electron-like bands of a freestanding close-packed alkali-atom monolayer: the s-like lowest and the p-like second lowest bands. 22 These bands maintain the integrity when the monolayer is placed at a metal surface. On a metal substrate with an s , p-inverted projected band gap such as Cu111, a two-dimensional po- tential well is formed between the substrate and the vacuum barrier. Since the p-like band is located in the L band gap of Cu111, it turns out to be a QWS; the wave function is strongly localized at the surface. The QWS band, unoccupied in low coverages, is stabilized and partly filled as alkali cov- erage increases. In contrast, because the s-like band is lo- cated below the lower edge of the L band gap, this band becomes a surface resonance and is denoted as an overlayer resonance OR. In addition to these bands, a series of unoc- cupied image potential states IPSs is pinned to the vacuum level. Since these electronic states have wave functions confined more or less at the surface where alkali atoms are located, charge-density fluctuations in the surface bands strongly af- fect the vibrational motions of alkali atoms. This was dem- onstrated in recent theoretical studies; 16,17 holes generated at the occupied QWS of Na/Cu111 preferentially decay by inelastic electron-phonon scattering. This strong electron- phonon coupling was also noted in other metallic quantum well systems: Ag/V100Ref. 23 and Ag/Fe100. 24 Thus, electron-phonon coupling at the metallic quantum well sys- tems plays an important role in the temporal evolution of quasiparticles and nuclear dynamics at the surfaces. The electron-phonon coupling in the QWSs has been mainly ex- plored through analysis of spectral linewidths in photoemis- sion and scanning tunneling spectroscopy. 4,15–18,20,21 How- ever, little attention has been paid on how the vibrational PHYSICAL REVIEW B 80, 075404 2009 1098-0121/2009/807/07540410 ©2009 The American Physical Society 075404-1