EFFECTIVE MASS AND MOMENTUM RESOLVED INTRINSIC LINEWIDTH OF

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. INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

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 timedomain 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). INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

LASER APPARATUS Ti: Sapphire laser system: Amplified Ti: Sapphire oscillator Tunability: 750 -850 nm Pulse width: 150 fs Rep. rate: 1 k. Hz Average Power: 0. 5 W Travelling-wave optical parametric generation (TOPG): Tunability: 1150 -1500 nm (0. 8 -1. 1 e. V) 4 th (3. 2 -4. 4 e. V) Average power 30 m. W INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

ULTRA-HIGH-VACUUM SYSTEM § m-metal UHV chamber § B<10 m. G § Base pressure <2 x 10 -10 mbar § photoemitted electrons detector: time of flight spectrometer (TOF) TOF: To. F length = 432 mm Temporal resolution = 0. 5 ns Acceptance angle = 2. 6° Energy resolution about 30 me. V @ 2 e. V PS 1 PS 2 GPIB Laser start PS 3 PS 4 PC Preamplifier Discriminator Multiscaler FAST 7887 stop INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

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

DISPERSION OF IS Band structure of Ag(100) En: binding energy m*: effective mass INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

IS SPECTRA Ebin= 0. 5 e. V 2 -photon photoemission Ekin = h -Ebin Ag(100) hn=4. 32 e. V n=1 FWHM: 62 me. V n=2 FWHM: 52 me. V Efermi RADIATION: Polarization: P Incident angle: 30° INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

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

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

EFFECTIVE MASS n=2 n=1 Measured values n=1 n=2 0. 99± 0. 02 0. 97± 0. 02 1. 15± 0. 1* 1. 06± 0. 09 1. 03± 0. 06 Calculated values n=1 n=2 0. 95* 1. 03** * K. Giesen et al. , PRB 35 975 (1987). ** Z. Li and S. Gao , PRB 50 15349 (1994). INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

IS LIFETIMES FITTING: Gaussian-Lorentzian convolution Gaussian: experimental resolution FWHM 45 me. V Lorentzian: intrinsic linewidth 14 me. V @ k//=0 LIFETIME Measured values (fs) n=1 n=2 Calculated values (fs) n=1 n=2 47± 7 55± 5# 55## ≥ 55 160± 10# 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 L 1217 (1997). Linewidth dependance on k// INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

FIRST OBSERVATION OF IMAGE STATES OUT OF RESONANCE e- E EV hn=3. 14 e. V DE 0. 5 e. V n=1 e- DE=3. 8 e. V EF intensity Fermi edge LOG LIN SCALE Ekin (e. V) Ekin= hn-Ebin= (3. 14 -0. 5)e. V 2. 7 e. V INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

DISPERSION AND SELECTION RULES Fermi edge n=1 m*/m 0. 95 Ekin= hn-Ebin Dhn=0. 39 e. V hn=3. 15 e. V hn=3. 54 e. V EV n=1 e- EF INFM DMF INFMeeting, Genova 23 -25 Giugno 2003 Ebin

SELECTION RULES Ag(100) 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. 1 GW/cm 2) INFM DMF 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 hn E 1 -Efermi. In this case the imagepotential states can be populated and photoemitted also in S-polarization. INFM DMF INFMeeting, Genova 23 -25 Giugno 2003

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