HIGH-RESOLUTION PHOTOELECTRON EMISSION-SPECTROSCOPY OF SURFACE-STATESON NI(111)

One-photon and two-photon photoelectron emission (1PPE/2PPE) spectrosc opy is used to study surface states at and below the Fermi energy on N i(111) in <(Gamma)over bar> --> (K) over bar and <(Gamma)over bar> --> (M) over bar direction. Picosecond UV laser pulses at lambda=210.70 n m (<(nu)over tilde>=47 460.16 cm(-1)) and lambda=351.17 nm (<(nu)over tilde>=28 476.09 cm(-1)) are used for photoexcitation. The energy of t he emitted photoelectrons is analyzed in a time-of-flight spectrometer with an energy resolution of Delta E<20 meV. Two surface states locat ed below the Fermi energy and energetically separated by 120 meV are d etected at <(Gamma)over bar>. With increasing k(parallel to) the high- energy state disperses upwards, while the low-energy state disperses d ownwards. Furthermore, a theoretical investigation is performed in the framework of the relativistic one-step formalism. The measured surfac e-state dispersions are shown to be quantitatively described by the ca lculations. For the relative and absolute intensities of the two surfa ce states at and around <(Gamma)over bar> experiment and theory are no t in agreement. [S0163-1829(97)06147-X].