VUV Spectroscopy of Atoms, Molecules and Surfaces

114 Chapter 5. Femtosecond VUV core-level spectroscopy ...
sufficient statistics for a core-level spectrum within one hour of measurement
time.
For the fs laser-induced desorption studies, the almost co-propagating
laser pump- and harmonic probe beams will be incident on the surface at
a45◦ angle through a hole in the parabolic grids drilled by the laser beam.
The angle between the two beams should be kept small in order to reduce the
temporal walk-off and the harmonic beam should be horizontally polarized
since the photoelectrons are preferentially emitted in the polarization direction
[143]. The pump beam will be focused on the sample by an external lens
while the VUV beam profile from the gas jet will be imaged to a slightly astigmatic
∼50 µm diameter spot by the spherical multilayer mirror. The target
chamber is equipped with standard equipment for surface preparation, i.e. a
sputter gun, LEED apparatus, a quadropole mass spectrometer (QMS) and a
pulsed valve (General Valve) for gas dosing during desorption measurements.
The temperature can be measured with an Alumel-Chromel thermocouple attached
to the sample holder which can be liquid-nitrogen cooled to −170◦ C
and radiatively heated from behind by a tungsten filament. For systems like
O/Al where the adsorption layer does not saturate at a given coverage, some
sort of feed-back mechanism will have to be invented to ensure a constant
coverage between successive desorbing laser pulses. Alternatively, the experiments
may be performed in a single-shot mode where the sample is moved
between successive laser pulses. The UHV environment (∼10−10 Torr) of the
target chamber is separated from the ∼10−6 Torr vacuum of the VUV light
generation part by a ∼1000 ˚A thick polyimide foil (from Luxel) mounted on
a VAT valve. The polyimide foil can withstand a pressure difference of a
few Torr, is transparent in the visible and has a 80 % transmission around
10 nm. A ∼500 ˚A Zr foil foil with a 80–90 % transmission above ∼60 eV
can be inserted into the beam path to prevent the remaining fundamental
laser light of the harmonic beam from propagating through the system. By
switching a plane gold mirror into the VUV beam path between the gas
jet and the multilayer mirror, the harmonic beam can be directed into the
McPherson VUV spectrometer via the toroidal gold mirror (two mirrors are
needed for a reasonable reflectivity in the VUV). This facilitates the optimization
and characterization of the VUV light source without the need for
a re-arrangement of the experimental equipment. An MCP detector placed
in the target chamber in the VUV beam direction will be used for aligment
of the VUV beam and possibly for a relative measurement of the VUV flux.
The temporal overlap between the pump- and probe pulses can be found
by replacing the metal target with a non-linear crystal and looking for secondharmonic
light generated by the simultaneous presence of the visible parts of
the two beams. Alternatively, one could look in the core-level photoelectron