We report the observation and systematic investigation of the space charge effect and mirror charge effect in photoemission spectroscopy. When pulsed light is incident on a sample, the photoemitted electrons experience energy redistribution after escaping from the surface because of the Coulomb interaction between them (space charge effect) and between photoemitted electrons and the distribution of mirror charges in the sample (mirror charge effect). These combined Coulomb interaction effects give rise to an energy shift and a broadening which can be on the order of 10 meV for a typical third-generation synchrotron light source. This value is comparable to many fundamental physical parameters actively studied by photoemission spectroscopy and should be taken seriously in interpreting photoemission data and in designing next generation experiments.

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi 2 Sr 2 CaCu 2 O 8+δ . The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy -the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink and associated weight transfer are robust to a possible breakdown of the sudden approximation. 74.72.Hs, 74.00.00, 74.25.Jb, 73.90.+f High T c superconductivity has been at the forefront of solid state physics research since its discovery in 19861 . Tunneling spectroscopy 2 3 and angle resolved photoemission spectroscopy (ARPES) 4 5 have been among the key techniques for studying the electronic structure of the cuprates in the quest to understand the many-body interactions responsible for high T c superconductivity. Unfortunately, both of these techniques are surface sensitive, making unclear their detailed applicability to bulk physics such as superconductivity. Here we introduce laser-based ARPES for studies of superconductors, which is expected to have significantly greater bulk sensitivity and which also offers superior energy and momentum resolution.The bulk-sensitivity of ARPES is limited by the electron mean free path in the solid, which depends on the electron kinetic energy in a roughly universal way 6 . Typically, there is a broad minimum of the mean free path in the 20-50 eV kinetic energy range with a sharp increase at lower energy and a slower increase at high energy, which is thought to hold true for Bi2212 7 . With the current interest in using ARPES to study bulk-physics such as superconductivity, the surface sensitivity has become a real hindrance. In order to improve the bulk-sensitivity of ARPES, one may go to very high photon energy such as a few thousand eV, though photoelectron cross sections decrease, and it becomes prohibitively difficult to obtain high energy and momentum resolution. Moving to low energy is thus a more attractive rout to increase bulk sensitivity, though there are some limitations to the extent of k-space that can be accessed.A critical question for ARPES, especially at low photon energy, is whether the sudden approximation, in which one assumes that the electron leaves the sample prior to relaxation of the created photo-hole, is valid 7 8 9 . If this is so, then the photoelectron spectrum should be directly proportional to the spectral function A(k,ω) which is in principle calculable using many-body techniques. This is consistent with a recent theoretical calculation which argues that the energy at which the adiabatic-sudden transition occurs should be a function of the type of excitation -in particular the transition is expected to occur at lower energies for more localized excitations 9 .We have built a high resolution ARPES system centered around a Scienta 10 SES...

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi(2)Sr(2)CaCu(2)O(8+delta) reveals a distinct oxygen isotope shift near the electron-boson coupling "kink" in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, and are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches.

A laser-based angle resolved photoemission (ARPES) system utilizing 6 eV photons from the fourth harmonic of a mode-locked Ti:sapphire oscillator is described. This light source greatly increases the momentum resolution and photoelectron count rate, while reducing extrinsic background and surface sensitivity relative to higher energy light sources. In this review, the optical system is described, and special experimental considerations for low-energy ARPES are discussed. The calibration of the hemispherical electron analyzer for good low-energy angle-mode performance is also described. Finally, data from the heavily studied high T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) is compared to the results from higher photon energies.

Using angle-resolved photoemission, we have observed sharp quasiparticlelike peaks in the prototypical layered manganite La(2-2x)Sr(1+2x)Mn(2)O(7) (x=0.36,0.38). We focus on the (pi,0) regions of k space and study their electronic scattering rates and dispersion kinks, uncovering bilayer-split bands, the critical energy scales, momentum scales, and strengths of the interactions that renormalize the electrons. To identify these bosons, we measured phonon dispersions in the energy range of the kink by inelastic neutron scattering, finding a good match in both energy and momentum to the oxygen bond-stretching phonons.