VUV XV, Berlin, 29 July -03 August 2007 Image-potential-state effective mass controlled by light pulses ELPHOS Lab UCSC (Università Cattolica del Sacro Cuore-Brescia) Dipartimento di Matematica e Fisica (Brescia, Italy) Gabriele Ferrini, Stefania Pagliara, Gianluca Galimberti, Emanuele Pedersoli, Claudio Giannetti, Fulvio Parmigiani DMF

VUV XV, Berlin, 29 July -03 August 2007 The study of the electron dynamics at surfaces is an important topic of current research in surface science. Experimental techniques that combine surface and band- structure specificity are essential tools to investigate these dynamics. Angle-resolved non-linear photoemission using short laser pulses is particularly suited for such experiments. In typical experiments a short laser pulse, with pulse widths of femtoseconds, is used to photoemit the electrons using multiple photon absorpion. Electrons are first excited into empty states below the vacuum level and then emitted by subsequent photon absorption Introduction

VUV XV, Berlin, 29 July -03 August 2007 A rather interesting system to study the electron dynamics at the metal surfaces is represented by Image Potential States (IPS) and Shockley States (SS). IPS are a 2-D electronic gas suitable to study band dispersion direct versus indirect population mechanisms polarization selection rules effective mass (in the plane of the surface) electron scattering processes and lifetime Introduction

VUV XV, Berlin, 29 July -03 August 2007 Image Potential States In most metals exists a gap in the bulk bands projection on the surface. When an electron is taken outside the solid it could be trapped between the Coulomb-like potential induced by the image charge into the solid, and the high reflectivity barrier due the band gap at the surface. P. M. Echenique, J. Osma, V. M. Silkin, E. V. Chulkov, J. M. Pitarke, Appl. Phys. A 71, 503 (2000)

VUV XV, Berlin, 29 July -03 August 2007 Two dimensional electron gas z Image Potential States Bound solution in the z direction Electrons are quasi-free in the surface plane Interactions may result in a modified electron mass m* m*m*

VUV XV, Berlin, 29 July -03 August 2007 Linear vs non-linear photoemission ToF Angle Resolved LINEAR PHOTOEMISSION (h >  ) band mapping of OCCUPIED STATES Angle (and time) RESOLVED MULTI-PHOTON PHOTOEMISSION (h <  ) band mapping of UNOCCUPIED STATES and ELECTRON SCATTERING PROCESSES

VUV XV, Berlin, 29 July -03 August 2007 Pulse width: fs Repetition rate: 1 kHz Average Power: 0.6 W Tunability: nm Second harmonic: h = 3.14 eV Third harmonic: h = 4.71 eV Fourth harmonic: h = 6.28 eV Amplified Ti:Sa laser system Traveling-wave optical parametric generation (TOPG) Average power: 30 mW Tunability: nm ( eV) Fourth harmonic: h = eV Non-collinear optical parametric amplifier (NOPA) Pulse width: 20 fs Tunability: nm ( eV) Second harmonic: h = eV Experimental Set-up

VUV XV, Berlin, 29 July -03 August 2007 Experimental Set-up   -metal UHV chamber  base pressure < 2· mbar  residual magnetic field < 10 mG  electron energy analyzer: Time of Flight (ToF) spectrometer Acceptance angle:  0.83° Energy resolution: 30 5eV Detector noise: <10 -4 counts/s PC GPIB Multiscaler FAST 7887 PS1PS2PS3PS4 start stop Preamplifier Discriminator Laser sampledetector G. Paolicelli et al. Surf. Rev. and Lett. 9, 541 (2002)

VUV XV, Berlin, 29 July -03 August 2007 projected band structure of Cu(111) with the non-linear photoemission spectrum collected with photon energy = 4.71 eV. Two-photon photoemission from Cu(111) VL FL Light grey spectrum: R. Matzdorf, Surf. Sci. reports, (1998)

VUV XV, Berlin, 29 July -03 August 2007 G. D. Kubiak, Surf. Sci. 201, L475 (1988), m * /m= , hv=4.38eV M. Weinelt, Appl. Phys. A 71, 493 (2000) on clean hv=4.5eV+1.5eV, m * /m= Hotzel, M. Wolf, J. P. Gauyacq, J. Phys. Chem. B 104, 8438 (2000) on 1ML N 2 / 1ML Xe/ hv=4.28eV+2.14eV, m * /m= S. Caravati, G. Butti, G.P. Brivio, M.I. Trioni, S. Pagliara, G. Ferrini, G. Galimberti, E. Pedersoli, C. Giannetti, F. Parmigiani, Surf. Sci. 600, 3901 (2006), theory m*/m = 1.1, exp. on clean hv=3.14eV m*/m = Effective mass of n=1 IPS on Cu(111) measured with angle resolved 2PPE in the literature Two-photon photoemission from Cu(111) F. Forster, G. Nicolay, F. Reinert, D. Ehm, S. Schmidt, S. Hufner, Surf. Sci. 160, 532 (2003), SS m*/m= , binding energy: 434 meV Effective mass of n=0 SS on Cu(111) measured with high resolution angle resolved photoemission in the recent literature

VUV XV, Berlin, 29 July -03 August 2007 S. Pagliara, G. Ferrini, G. Galimberti, E. Pedersoli, C. Giannetti, F. Parmigiani, Surf. Sci. 600, 4290 (2006) Two-photon photoemission from Cu(111) IPS Binding energy=Ek-hv-  sp,  sp = eV ss ips k || IPS and SS dispersion on the same data set

VUV XV, Berlin, 29 July -03 August 2007 IPS photoemission from Cu(111)

VUV XV, Berlin, 29 July -03 August 2007 SS IPS 4.71 eV VL FL IPS Two-photon photoemission from Cu(111) m/m*= m/m*=

VUV XV, Berlin, 29 July -03 August 2007 SS IPS 4.28 eV VL FL IPS m/m*= Two-photon photoemission from Cu(111)

VUV XV, Berlin, 29 July -03 August 2007 Two-photon photoemission from Cu(111) SS IPS 4.28 eV 4.71 eV VL FL control point: one-photon photoemission SS 3.14 eV

VUV XV, Berlin, 29 July -03 August 2007 Two-color photoemission from Cu(111) pump probe static limit SS IPS SS IPS ph/pulse= 10 nJ/pulse at 4.71 eV fluence 10  J/cm eV 3.14 eV VL FL

VUV XV, Berlin, 29 July -03 August eV+3.14 eV4.71eV+4.71 eV Two-color photoemission from Cu(111)

VUV XV, Berlin, 29 July -03 August 2007 ? pump How many electrons do we pump into the bulk bands? From band structure: 4·10 18 cm −3 states available in |k|<0.2 A −1, and in an energy interval of 300 meV from the upper edge of gap. (calculations courtesy of C.A. Rozzi, S3 INFM-CNR and UniMoRe) From scanning tunnel microscopy: SS constitute about 60% of the total surface electron density on (111) surfaces of noble metals. [L. Burgi, N. Knorr, H. Brune, M.A. Schneider, K. Kern, Appl. Phys. A 75, 141 (2002)] Assuming that the photons in the pump pulse are absorbed in the surface layer in proportion to the surface density of states and that the totality of the SS excited electrons are promoted to the empty bulk states at the bottom of the gap, we estimate an upper limit for the hot-electron gas density in the bulk bands of the order of cm −3, a substantial fraction of the sp-bulk unoccupied states Two-color photoemission from Cu(111) SS IPS VL FL

VUV XV, Berlin, 29 July -03 August 2007 T. Fauster, W. Steinmann, “Two Photon Photoemission Spectroscopy of Image States” Phase shift model: Cu(111) qualitative explanation: -Cu(111) IPS penetrates into the bulk because it is at the gap edge. -Excited e - density interacts with IPS wavefunction increasing dephasing processes and/or decreasing lifetime - Excited e - density push IPS wavefunction outside, decreasing binding energy preferentially at k || =0 -effective mass increases k || IPS dispersion

VUV XV, Berlin, 29 July -03 August 2007 Conclusions The effective mass of the Cu(111) IPS depends on the excited electron density generated by the laser pulses in the unoccupied sp-band. A qualitative explanation based on the phase shift model is given. Interest in these processes for controlling band structure and chemical reaction at surfaces.

VUV XV, Berlin, 29 July -03 August 2007 People Thank you Fulvio Parmigiani (U Trieste) Stefania Pagliara (UCSC) Claudio Giannetti (UCSC)Gianluca Galimberti (UCSC) Emanuele Pedersoli (ALS-LBNL)