Image-potential-state effective mass controlled by light pulses

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

2. Introduction 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 10-100 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

3. Introduction 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

4. 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)

5. z Image Potential States Two dimensional electron gas Bound solution in the z direction Electrons are quasi-free in the surface plane Interactions may result in a modified electron mass m* m*

6. ToF Linear vs non-linear photoemission Angle Resolved LINEAR PHOTOEMISSION (hn>F) band mapping of OCCUPIED STATES Angle (and time) RESOLVED MULTI-PHOTONPHOTOEMISSION (hn<F) band mapping ofUNOCCUPIED STATES and ELECTRON SCATTERING PROCESSES

7. Amplified Ti:Sa laser system Experimental Set-up Pulse width: 100-150 fs Repetition rate: 1 kHz Average Power: 0.6 W Tunability: 750-850 nm Second harmonic: hn = 3.14 eV Third harmonic: hn = 4.71 eV Fourth harmonic: hn = 6.28 eV Traveling-wave optical parametric generation (TOPG) Average power: 30 mW Tunability: 1150-1500 nm (0.8-1.1 eV) Fourth harmonic: hn = 3.2-4.4 eV Non-collinear optical parametric amplifier (NOPA) Pulse width: 20 fs Tunability: 500-1000 nm (1.2-2.5 eV) Second harmonic: hn = 2.5-5 eV

8. sample detector PS1 PS2 PS3 PS4 PC Preamplifier Discriminator GPIB Multiscaler FAST 7887 stop start Laser Experimental Set-up • m-metal UHV chamber • base pressure < 2·10-10 mbar • residual magnetic field < 10 mG • electron energy analyzer: Time of Flight (ToF) spectrometer Acceptance angle:  0.83° Energy resolution: 30 meV @ 5eV Detector noise: <10-4 counts/s G. Paolicelli et al. Surf. Rev. and Lett. 9, 541 (2002)

9. Two-photon photoemission from Cu(111) VL FL projected band structure of Cu(111) with the non-linear photoemission spectrum collected with photon energy = 4.71 eV. Light grey spectrum: R. Matzdorf, Surf. Sci. reports,30 153 (1998)

10. Two-photon photoemission from Cu(111) Effective mass of n=1 IPS on Cu(111) measured with angle resolved 2PPE in the literature G. D. Kubiak, Surf. Sci. 201, L475 (1988), m*/m=1.0+-0.1, hv=4.38eV M. Weinelt, Appl. Phys. A 71, 493 (2000) on clean Cu(111) @ hv=4.5eV+1.5eV, m*/m=1.3+-0.1 Hotzel, M. Wolf, J. P. Gauyacq, J. Phys. Chem. B 104, 8438 (2000) on 1ML N2 / 1ML Xe/ Cu(111) @ hv=4.28eV+2.14eV, m*/m=1.3+-0.3 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 Cu(111) @ hv=3.14eV m*/m = 1.28+-0.07 Effective mass of n=0 SS on Cu(111) measured with high resolution angle resolved photoemission in the recent literature F. Forster, G. Nicolay, F. Reinert, D. Ehm, S. Schmidt, S. Hufner, Surf. Sci. 160, 532 (2003), SS m*/m=0.43+-0.01, binding energy: 434 meV

11. Two-photon photoemission from Cu(111) IPS and SS dispersion on the same data set ips k|| ss S. Pagliara, G. Ferrini, G. Galimberti, E. Pedersoli, C. Giannetti, F. Parmigiani, Surf. Sci. 600, 4290 (2006) IPS Binding energy=Ek-hv-Fsp , Fsp= 0.9-1 eV

12. IPS photoemission from Cu(111)

13. Two-photon photoemission from Cu(111) IPS m/m*=1.28+-0.07 VL m/m*=2.2+-0.07 IPS 4.71 eV SS FL

14. Two-photon photoemission from Cu(111) IPS m/m*=1.6+-0.07 VL IPS 4.28 eV SS FL

15. Two-photon photoemission from Cu(111) VL IPS 4.28 eV 4.71 eV 3.14 eV SS FL control point: one-photon photoemission SS

16. Two-color photoemission from Cu(111) IPS probe 3.14 eV static limit VL IPS pump SS 4.71 eV SS FL 1.3 1010ph/pulse= 10 nJ/pulse at 4.71 eV fluence 10 mJ/cm2

17. Two-color photoemission from Cu(111) 4.71eV+3.14 eV 4.71eV+4.71 eV

18. Two-color photoemission from Cu(111) How many electrons do we pump into the bulk bands? From band structure: 4·1018 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) ? VL 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)] IPS pump 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 1018 cm−3, a substantial fraction of the sp-bulk unoccupied states SS FL

19. Phase shift model: Cu(111) T. Fauster, W. Steinmann, “Two Photon Photoemission Spectroscopy of Image States” 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

20. 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.

21. People Fulvio Parmigiani (U Trieste) Stefania Pagliara (UCSC) Claudio Giannetti (UCSC) Gianluca Galimberti (UCSC) Thank you Emanuele Pedersoli (ALS-LBNL)