Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Methoden und Instrumentierung Poster: Mi., 14:00–16:30 M-P92
The FLASH high-resolution monochromator beamline
Michael Wellhöfer 1 , Michael Martins 1 , Jon Tobias Hoeft 1 , Martin Beye 1 ,
Florian Sorgenfrei 1 , Wilfried Wurth 1
1 Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761
Hamburg
FLASH at DESY is the first SASE Free electron laser user facility dedicated to delivering
extremely short VUV laser pulses of 20-200 fs duration with very high radiation
energy in the range of few ten microjoules per pulse. It is a powerful tool for molecular
and cluster dynamics investigations, ionization and fragmentation experiments. The
intrinsic energy resolution of the VUV-FEL is in the order of 1 %, which is not sufficient
for quite a number of applications. To make this machine available for a broader range
of questions two high resolution monochromator beamlines were prospected of which
one is in operation since June 2005.
The layout of the monochromator follows the design of Follath and Senf [1] employing a
plane grating monochromator using collimated light with an SX700 pre-mirror grating
optic. For all optical components Diamond-Like-Carbon (DLC) coatings are used to
withstand the high power density of FLASH. The monochromator is equipped with
two gratings (200 and 1200 lines/mm) to cover the energy range from 20 eV to 1000
eV photon energy. Hence, the monochromator covers the whole energy range from the
first harmonic up to the fifth harmonic of FLASH.
In particular for experiments on non linear processes in the soft x-ray region the
statistical nature of the SASE process is critical. Even when the total intensity of two
different FEL puls is similar, the peak intensity might differ significantly. However,
for non-linear processes the peak intensity and not the integrated intensity of the FEL
pulses is important.
Thus, a special monochromator option exists to use the zeroth order from the monochromator
grating for experiments and to analyse FEL pulses in first order. The zeroth
order, which normally is absorbed in the beamdump, can be used in a dedicated zeroth
order beamline PG0 simultaneously with the first order in PG2.
First experiments have been performed using this unique setup on the photoemission
of rare gas atoms. The photoelectron spectra have been measured using time-of-flight
(TOF) photo-electron spectrometers. In the linear region in both channels the same
FEL spectrum should be measured. Thus, in the case of a simple electronic structure,
e. g. the He 1s, the photoelectron spectrum can be used as a probe to measure the
SASE spectrum of each FEL puls. This method will also be suitable as an online
diagnostic tool for the upcoming X-FEL.
[1] R.Follath and F.Senf, Nucl.Instrum.Methods A390, 388 (1997)