1. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 EFFECTIVE MASS AND MOMENTUM RESOLVED INTRINSIC LINEWIDTH OF IMAGE-POTENTIAL STATES ON Ag(100) INFM and Università Cattolica del Sacro Cuore Dipartimento di Matematica e Fisica, Via Musei 41 Brescia. Claudio Giannetti

2. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 Femtosecond Laser and tunability of photon energy Non-Linear Photoemission Study of image-potential states on Ag(100) at different photon energies: effective masses and lifetimes. INTRODUCTION WHY IMAGE POTENTIAL STATES? Relaxation of electrons into bulk states can be studied directly in the time- domain with pump-probe techniques. U. Höfer et al., Science 277, 1480 (1997). W. Berthold et al., Phys. Rev. Lett. 88, 056805 (2002). It is a many body interesting problem. A.García-Lekue et al., Phys. Rev. Lett. 89, 096401 (2002). J. Kliewer et al., Science 288, 1399 (2000). Interplay between image-potential states and dynamics of molecules at surfaces. A. D. Miller et al., Science 297, 1163 (2002).

3. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 LASER APPARATUS Ti:Sapphire laser system: Tunability: 1150-1500nm (0.8-1.1 eV) 4th (3.2-4.4 eV) Average power 30mW Travelling-wave optical parametric generation (TOPG): Amplified Ti:Sapphire oscillator Tunability: 750-850nm Pulse width: 150fs Rep. rate: 1kHz Average Power: 0.5W

4. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 ULTRA-HIGH-VACUUM SYSTEM -metal UHV chamber B<10 mG Base pressure <2x10 -10 mbar photoemitted electrons detector: time of flight spectrometer (TOF) PC GPIB Multiscaler FAST 7887 PS1PS2PS3PS4 start stop Preamplifier Discriminator Laser TOF: ToF length = 432 mm Temporal resolution = 0.5 ns Acceptance angle = 2.6° Energy resolution about 30 meV @ 2eV

5. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 IMAGE-POTENTIAL STATES ON METALS Coulomb potential due to electronic image-charge: V(z) 1/z Image-charge of an electron at a metal surface. U. Hofer, I.L. Shumay, Ch. Reuß, U. Thomann, W. Wallauer, Th. Fauster, Science 277, 1480 (1997). forbidden gap in bulk states 2-D electron gas Ag(100) n=1 n=2 Rydberg series:

6. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 DISPERSION OF IS Band structure of Ag(100) E n : binding energy m*: effective mass

7. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 IS SPECTRA E kin = h -E bin 2-photon photoemission E bin = 0.5eV RADIATION: Polarization: P Incident angle: 30° h =4.32 eV Ag(100) n=1 FWHM: 62meV n=2 FWHM: 52meV E fermi

8. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 DIPOLE SELECTION RULES: J=0 in S-polarization J0 in P-polarization J is the current density associated to image-potential states Ag(100) SELECTION RULES

9. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 IS DISPERSION 2-Dimensional electron gas: DISPERSION Ag(100) G.Ferrini, C.Giannetti, D.Fausti, G.Galimberti, M.Peloi, G.P.Banfi, F.Parmigiani, PRB 67, 235407 (2003). n=1 n=2 Intensity (log scale) E kin (eV)

10. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 EFFECTIVE MASS Measured values Calculated values n=1n=2n=1n=2 0.99±0.02 1.06±0.09 0.97±0.021.03±0.06 1.15±0.1 * 0.95 * 1.03 ** * K.Giesen et al., PRB 35 975 (1987). ** Z.Li and S.Gao, PRB 50 15349 (1994). n=1 n=2

11. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 Gaussian: experimental resolution FWHM 45meV Lorentzian: intrinsic linewidth 14meV @ k // =0 IS LIFETIMES FITTING: Gaussian-Lorentzian convolution LIFETIME Linewidth dependance on k // Measured values (fs) Calculated values (fs) n=1n=2n=1n=2 47±7 55 55±5 # 160±10 # 55 ## 132 ## # # I.L.Shumay et al., PRB 58 13974 (1998). ## A.García-Lekue et al., Phys. Rev. Lett. 89, 096401 (2002). ### E.V Chulkov et al., Surf. Sci 391 L1217 (1997).

12. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 h eV E FIRST OBSERVATION OF IMAGE STATES OUT OF RESONANCE E EVEV EFEF n=1 e-e- E=3.8 eV 0.5 eV E kin = h -E bin = (3.14-0.5)eV 2.7eV e-e- LIN SCALE Fermi edge intensity E kin (eV) LOG SCALE Fermi edge intensity E kin (eV)

13. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 DISPERSION AND SELECTION RULES m*/m 0.95 n=1 Fermi edge E kin = h -E bin h =0.39eV h =3.15eVh =3.54eV EVEV EFEF n=1 e-e- E bin

14. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 SELECTION RULES DIPOLE SELECTION RULES: J=0 in S-polarization J0 in P-polarization Ag(100) WHICH IS THE POPULATION AND PHOTOEMISSION MECHANISM? We can exclude: Role of surface roughness Ponderomotive and tunnel effects at the surface, because the radiation intensity is too low. (I~0.1GW/cm 2 )

15. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 CONCLUSIONS Non-linear photoemission on image-potential states in Ag(100). Improvement of the precision in the measurement of the effective masses and lifetimes. Observation of image-potential states also when h E 1 -E fermi. In this case the image- potential states can be populated and photoemitted also in S-polarization.

16. INFMDMF INFMeeting, Genova 23-25 Giugno 2003 Responsibles: F. Parmigiani, G. Ferrini. Co-workers: F. Banfi, D. Fausti, G. Galimberti, S. Pagliara, M. Peloi.