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

Methoden und Instrumentierung Poster: Mi., 14:00–16:30 M-P59
Photoemission Spectroscopy Using Flash
Mitsuru Nagasono 1 , Alexander Föhlisch 1 , Edlira Suljoti 1 , Annette
Pietzsch 1 , Wilfried Wurth 1
1 Institut für Experimentalphysik, Universität Hamburg
The vacuum ultraviolet free electron laser (FEL) Flash is a self amplified spontaneous
emission (SASE) type 4th generation light source with high peak brilliance (10 12 ∼10 13
photons/pulse), short pulses (20∼200 fs) and high coherence. The photon energy
regime is 30∼200 eV. We present first results of photoelectron spectroscopy for gases
and solids using Flash The experiments were carried out with our transportable spectroscopy
UHV system at the monochromator beamline PG2. Operation mode of the
VUV-FEL was single bunch or 8 bunch-train with a repetition rate of 2 Hz. The
analyzer chamber of the system is equipped with a hemispherical electron analyzer,
SES2002, which is rotatable around the optical axis of the FEL beam. In this study,
the analyzer was fixed at magic angle with respect to the polarization vector of the
FEL. The detector of the electron analyzer was synchronized with the FEL, so that
photoelectron spectra could be detected for individual pulses or for a bunch-train. Incoming
photon flux was measured by a MCP placed between the undulator device and
the experimental hall. For the gas study, the intensity of the transmitted FEL beam
was measured with fluorescence intensity of a Ce-Yag crystal using a CCD synchronized
with the FEL. For the solid study, sample current was measured. High-harmonics of
the VUV-FEL were cut by an Al filter with thickness of 700 nm. Gases were dosed by a
nozzle with 0.2 mm diameter. Samples were He atoms and N2 molecules for gas phase
experiments and a Cu single crystal for solid experiments. The photoelectron yield
differs for each bunch because the photon flux of the FEL has a broad intensity distribution
due to its statistical nature. The photoelectron yield of He is a linear function
of the photon flux, but that of N2 shows a saturation effect. This is due to a difference
of the photoionization cross section between He and N2. The N2 photoelectron spectrum
at high flux differs from that at low flux. This may be related to differences in
partial cross-section. In the Cu crystal photoelectron spectra at high photon flux we
observed photoelectrons with higher kinetic energy than the incoming photon energy.
This is considered to be due to the coulomb repulsion among photoelectrons which are
emitted simultaneously from the Cu.