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J.K. and R.B. thank the Deutsche Forschungsgemein- schaft for support via Sonderforschungsbereich 341. S.C. acknowledges the support of the Engineering and Physical Sciences Research Council. P.M.E. acknowl- edges the Max Planck Research Award funds. We thank N. Lorente for discussions. 31 January 2000; accepted 5 April 2000 Real-Time Observation of Adsorbate Atom Motion Above a Metal Surface Hrvoje Petek,*†‡ Miles J. Weida, Hisashi Nagano, Susumu Ogawa The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10 –15 seconds) time resolution. Unusual changes in the surface electronic structure within 160 femtoseconds after excitation, observed by time-resolved two-photon photo- emission spectroscopy, are attributed to atomic motion in a copper–cesium bond-breaking process. Describing the change in energy of the cesium anti- bonding state with a simple classical model provides information on the me- chanical forces acting on cesium atoms that are “turned on” by photoexcitation. Within 160 femtoseconds, the copper–cesium bond extends by 0.35 angstrom from its equilibrium value. The observation of atomic and molecular dy- namics on surfaces is a long-standing goal in surface science (1–3). Traditional measure- ments of the energy and momentum deposit- ed in the gas-phase products provide only indirect information on surface processes (4, 5). However, time-resolved spectroscopies using femtosecond-duration laser pulses have finally allowed direct observation of surface dynamics in real time (6 ). For example, ele- gant time-resolved photoemission experi- ments on image potential states have charted coherent surface electron motion and electron trapping and localization by adsorbates (7, 8). Strong excitation-induced coupling between the electrons and nuclei also occurs on a femtosecond time scale; however, to date there has been no equivalent observation of nuclear dynamics on surfaces. The recent discovery of an unusually long-lived elec- tronic state for Cs on Cu(111) (9) makes it possible to record the nuclear motion upon electronic excitation. Here, time-resolved photoemission spectroscopy is used to take a “movie” (13.4 fs per frame) of the change in Cs/Cu surface electronic structure in the pro- cess of breaking the Cu–Cs bond. Inverting this electronic response reveals the real-time dissociative motion of an atom on a surface. A logical starting point for discussing the photodesorption dynamics is the potential en- ergy surfaces (PESs) for Cs on Cu (Fig. 1A). Despite the fundamental importance of alkali atom chemisorption in catalysis and thermi- onic emission, only selected aspects of the ground- and excited-state PESs along the Cu– Cs internuclear coordinate (R Cu-Cs ) are known (10, 11). The electronic character of the two lowest lying states can be understood from simple atomic orbital ideas. Near a met- al, the highest occupied and lowest unoccu- pied electron orbitals of highly polarizable Cs atoms combine to form a 6s–6p z bonding and 6s+6p z antibonding pair in which the elec- tron density is concentrated at the surface or vacuum side, respectively, of the Cs atom (12, 13). Optical coupling of the ground- and excited-state PESs induces significant charge redistribution about the alkali atom, consid- erably weakening the Cu–Cs bond (14 ). Be- cause photoexcitation is much faster than nu- clear motion, it projects the ground-state Cs atom probability distribution (wave packet) onto the repulsive wall of the excited state. The ensuing wave packet motion corresponds to the Menzel-Gomer-Redhead scenario for photodesorption (1, 2). The wave packet evolution can be observed because the 6s+6p z antibonding orbital forms a sharp resonance, which, according to a model calculation, decreases in energy as R Cu-Cs in- creases (13). This electronic change in response to the nuclear motion is recorded by time- resolved two-photon photoemission (2PP) (6, 9). This pump-probe method measures 2PP ex- cited by a pair of identical laser pulses with a Advanced Research Laboratory, Hitachi, Ltd., Ha- toyama, Saitama 350-0395, Japan. *To whom correspondence should be addressed. E- mail: petek@pitt.edu †Present address: Department of Chemical Physics, Fritz-Haber Institute, Faradayweg 4-6, D-14195 Ber- lin, Germany. ‡Permanent address: Department of Physics and As- tronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA. 6 4 2 0 Electronic energy (eV) Parallel momentum 2 1 0 -1 -2 Potential energy (eV) 8 6 4 2 R Cu-Cs (Å) Cu + Cs Cu – + Cs + Cs + Cs Pump Probe SS Φ Final state E F Pump Probe A (∆=0) A (∆>0) A B Fig. 1. (A) Schematic PESs are constructed fol- lowing the procedure in (9). The energy of adsorption of Cs + (1.9 eV ) and equilibrium R Cu-Cs (2.97 Å) for the ground state (red) are from thermal desorption (9) and theory (13). The excited state (blue) is constructed with a van der Waals potential for the Cs atoms and a repulsive term that gives the correct excitation energy (5). The asymptotic energies of the Cs + and Cs products are indicated by red and blue horizontal lines, respectively (9). The pump pulse projects the ground-state wave packet (represented by a blue gaussian distribution) onto the excited state, “turning on” the repul- sive forces. The evolving wave packet (green distribution) is detected by the delayed probe pulse–induced photoemission from the 6s+6p z antibonding state. (B) Band structure for Cu(111), indicating the band gap (unshaded area) and the 2PP excitation scheme. The color gradient for the final state conveys different energies for observation of the dynamics of A. The energy of A decreases approximately quad- ratically with the delay . R EPORTS 26 MAY 2000 VOL 288 SCIENCE www.sciencemag.org 1402