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EFFECTIVE MASS AND MOMENTUM RESOLVED INTRINSIC LINEWIDTH OF IMAGE-POTENTIAL STATES ON Ag(100)

EFFECTIVE MASS AND MOMENTUM RESOLVED INTRINSIC LINEWIDTH OF IMAGE-POTENTIAL STATES ON Ag(100). Claudio Giannetti. INFM and Università Cattolica del Sacro Cuore Dipartimento di Matematica e Fisica, Via Musei 41 Brescia. INTRODUCTION. Femtosecond Laser and tunability of photon energy

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EFFECTIVE MASS AND MOMENTUM RESOLVED INTRINSIC LINEWIDTH OF IMAGE-POTENTIAL STATES ON Ag(100)

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

  2. INTRODUCTION • 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. • 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. Travelling-wave optical parametric generation (TOPG): • Tunability: • 1150-1500nm • (0.8-1.1 eV) • 4th (3.2-4.4 eV) • Average power 30mW LASER APPARATUS Ti:Sapphire laser system: Amplified Ti:Sapphire oscillator Tunability: 750-850nm Pulse width: 150fs Rep. rate: 1kHz Average Power: 0.5W

  4. ToF length = 432 mm Temporal resolution = 0.5 ns Acceptance angle =  2.6° Energy resolution about 30 meV @ 2eV TOF: PS1 PS2 PS3 PS4 PC Preamplifier Discriminator GPIB Multiscaler FAST 7887 stop start Laser ULTRA-HIGH-VACUUM SYSTEM • m-metal UHV chamber • B<10 mG • Base pressure <2x10-10 mbar • photoemitted electrons detector: • time of flight spectrometer (TOF)

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

  6. DISPERSION OF IS Band structure of Ag(100) En: binding energy m*: effective mass

  7. Ag(100) hn=4.32 eV n=1 FWHM: 62meV n=2 FWHM: 52meV Efermi RADIATION: Polarization: P Incident angle: 30° IS SPECTRA 2-photon photoemission Ebin= 0.5eV Ekin = h-Ebin

  8. Ag(100) SELECTION RULES DIPOLE SELECTION RULES: J=0 in S-polarization J≠0 in P-polarization J is the current density associated to image-potential states

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

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

  11. Measured values (fs) Calculated values (fs) n=1 n=2 n=1 n=2 47±7 ≥55 55## 132### 55±5# 160±10# # 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). Linewidth dependance on k// IS LIFETIMES FITTING: Gaussian-Lorentzian convolution Gaussian: experimental resolution FWHM45meV Lorentzian: intrinsic linewidth 14meV @ k//=0 LIFETIME

  12. Fermi edge LOG SCALE intensity Fermi edge LIN SCALE Ekin (eV) intensity Ekin (eV) FIRST OBSERVATION OF IMAGE STATES OUT OF RESONANCE e- hn=3.14eVDE E EV 0.5 eV n=1 e- DE=3.8 eV   EF Ekin= hn-Ebin= (3.14-0.5)eV 2.7eV

  13. hn=3.15eV hn=3.54eV DISPERSION AND SELECTION RULES Fermi edge n=1 m*/m0.95 Ekin= hn-Ebin Dhn=0.39eV EV Ebin n=1 e- EF

  14. Ag(100) SELECTION RULES DIPOLE SELECTION RULES: J=0 in S-polarization J≠0 in P-polarization 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/cm2)

  15. 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 hnE1-Efermi. In this case the image-potential states can be populated and photoemitted also in S-polarization.

  16. Responsibles: F. Parmigiani, G. Ferrini. Co-workers: F. Banfi, D. Fausti, G. Galimberti, S. Pagliara, M. Peloi.

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