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Studying Image-Potential States on Ag(100): Effective Masses and Lifetimes

This research explores the effective masses and intrinsic linewidth of image-potential states on Ag(100) utilizing femtosecond laser techniques and tunable photon energies. We investigate the dynamics of electrons transitioning to bulk states through pump-probe methods, revealing intricate details of image-potential states influenced by molecular interactions and nonlinear photoemission effects. Our findings enhance the understanding of electron behavior at metal surfaces and provide refined measurements for effective masses and lifetimes, with implications for future studies in the field.

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Studying Image-Potential States on Ag(100): Effective Masses and Lifetimes

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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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