Applied Physics A manuscript No. (will be inserted by the editor) A time-of-flight spectrometer for angle-resolved detection of low energy electrons in two dimensions Patrick S. Kirchmann ? , Laurenz Rettig, Dhananjay Nandi, Uwe Lipowski, Martin Wolf, Uwe Bovensiepen Freie Universit¨ at Berlin,Fachbereich Physik, Arnimallee 14, DE-14195 Berlin-Dahlem, Germany January 21, 2008 Abstract We developed a time-of-flight photoelectron spectrometer that simultaneously analyzes low energy electrons photoemitted from solid surfaces in an energy- and angle-resolved manner. To achieve this, a field free drift tube with an acceptance angle of 22 ◦ is combined with two-dimensional position-sensitive detection of pho- toelectrons, which is realized by a microchannel plate stack and a delay-line anode for position encoding. Here, we present the design considerations and principles of operation including analysis of multiple events per light pulse. The performance of the spectrometer is demon- strated by photoemission from a Cu(111) single crys- talline surface by UV femtosecond laser pulses at 6.2 eV photon energy. 1 INTRODUCTION Angle-resolved photoelectron spectroscopy using ultra- violet radiation is a well established experimental tech- nique to study the bulk and surface electronic structure of solids [1,2]. Femtosecond time- and angle-resolved two- photon photoemission (2PPE) spectroscopy has contributed significantly during the last two decades to the under- standing of elementary scattering processes and relax- ation dynamics in excited electronic states on solid sur- faces [3]. In particular, 2PPE has proven to be a power- ful tool for the study of ultrafast intra- and inter-band relaxation dynamics in image potential states of clean and adsorbate-covered metal surfaces [4–7]. Moreover, the band dispersion of localized and delocalized excited states and the respective energy relaxation dynamics have been probed at metal-molecule interfaces by angle- resolved 2PPE spectroscopy directly in the time domain [8–10]. ? Corresponding author. Web: www.physik.fu- berlin.de/∼femtoweb E-mail: patrick.kirchmann@physik.fu- berlin.de Usually, such angle-dependent studies are performed by rotating the sample with respect to the spectrometer axis, which is only applicable to spectrometers with suf- ficiently small acceptance angles. This approach yields an individual spectrum per photoemission angle, i.e. for one particular electron momentum parallel to the sur- face [2]. Two commonly used experimental approaches for the angle-resolved energy analysis of photoelectrons are electrostatic energy analyzers, which are compat- ible with quasi-continuous light sources, and time-of- flight (TOF) electron spectrometers [11,12], which re- quire pulsed sources, respectively. Electrostatic analyz- ers enable very efficient simultaneous angle- and energy- resolved measurements as the angular distribution of the photoelectrons can be imaged onto a position-resolved two-dimensional (2D) detector after energy selection in the hemisphere. The electrostatic lens system used for the imaging may be adapted to electrons of low kinetic energies which makes these imaging-type hemispherical analyzers very well suited for angle-resolved 2PPE spec- troscopy [7]. In this approach, the dispersion of photo- electrons along one of the two independent directions of the 2D detector represents the kinetic energy. The dispersion along the second direction refers to the an- gular dependence of the photoemission yield along one in-plane direction x of the sample surface and represents thus the dispersion of the electronic structure with elec- tron momentum p x = ~k x . However, to analyze the sec- ond in-plane electron momentum component ~k y it is necessary to rotate the sample surface in the azimuthal direction and to record a series for various values of ~k y . Here, we combine such two-dimensional imaging de- tectors with the conventional TOF concept to a two- dimensional position-sensitive TOF electron spectrome- ter, hence abbreviated pTOF. This instrument analyzes the kinetic electron energy E kin along both in-plane elec- tron momenta p x and p y of electronic Bloch states in a solid. This spectrometer allows to map out the respective band dispersion of occupied and unoccupied electronic states in direct photoemission and 2PPE. Since both in-