The 5m-NIM beamline is dedicated to the education and of undergraduate and PhD students of the Humboldt University. The technique offered by the beamline is angle-resolved photoemission spectroscopy (ARPES), which is carried out in the low-photon energy regime, i.e. between 8 eV and 40 eV.
|Monochromator||5m-NIM (normal incidence monochromator equipped with two gratings)|
|Energy range||grating 1 from 7 eV to 15 eV; grating 2 from 24 eV to 35 eV|
|Energy resolution||0.6 meV beam resolution at 22 eV kinetic energy|
|Flux||2E11 photons per second peak @22 eV|
|Polarisation||only horizontal in the electron beam orbit plane|
|Divergence horizontal||8 mrad|
|Divergence vertical||5 mrad|
|Focus size (hor. x vert.)||50 x 200 μm^2 (hor. x vert.)|
|Fixed endstation||ARPES endstation with SES 2002 analyser HU 5mNIM endstation|
|Distance Focus/last valve||800 mm|
|Height Focus/floor level||1762.5 mm|
|Beam availability||24h/d (dedicated to Humboldt University researchers and students)|
The beamline receives the white radiation from a bending magnet and two mirror focus it in the two planes perpendicular to the light direction on the monochromator. To monochromatise the light two gratings can be selected, each optimised for a specific photon energy range, but both working in the normal-incidence geometry in order to achieve high transmission and high resolution. The choice of a normal incidence geometry, to achieve high energy resolution, naturally limits the available photon energy range, i.e. from 8 eV minimum to about 35 eV, although slightly higher energies can be used for core-level photoemission.
The monochromatic beam is directed to the last toroidal mirror which focusses the beam on the sample in the main chamber. This is equipped with a SES 2002 photoelectron energy analyser able to measure the electron kinetic energy and momentum at the same time. The sample temperature is controlled by a closed-cycle Helium cryostat reaching 10 K. The cryostat is mounted on a manipulator having 5 degrees of freedom and with the two rotations it is possible to freely choose the photoemission angles in order to measure ARPES spectra at any point of the reciprocal space.
The resolving power of the beamline is 36000 at 22 eV, measured from gas-phase photoemission, and the analyser resolution was less than 2 meV, measured at an experimental temperature of 15 K.
The samples are put in UHV with a fast-entry lock and prepared in a dedicated preparation chamber. There, they can be sputtered with Ar ions and annealed up to a temperature (currently) of 500°C. This allows measurements on a wide variety of material systems, among which elemental metals and many semiconductors. It is also possible to cleave samples in UHV at either room or low temperature directly on the manipulator in the main chamber. This is particularly interesting for those systems showing strong contamination, since the latter can be minimised by avoiding transfer between UHV chambers.