High-resolution photoelectron emission spectroscopy of surface states on Ni(111)

One-photon and two-photon photoelectron emission (1PPE/2PPE) spectroscopy is used to study surface states at and below the Fermi energy on Ni(111) in <(Gamma)over bar> --> (K) over bar and <(Gamma)over bar> --> (M) over bar direction. Picosecond UV laser pulses at lambda=210.70 nm (<(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 the emitted photoelectrons is analyzed in a time-of-flight spectrometer with an energy resolution of Delta E<20 meV. Two surface states located below the Fermi energy and energetically separated by 120 meV are detected at <(Gamma)over bar>. With increasing k(parallel to) the high-energy state disperses upwards, while the low-energy state disperses downwards. Furthermore, a theoretical investigation is performed in the framework of the relativistic one-step formalism. The measured surface-state dispersions are shown to be quantitatively described by the calculations. For the relative and absolute intensities of the two surface states at and around <(Gamma)over bar> experiment and theory are not in agreement. [S0163-1829(97)06147-X].