The first images of a Au grid were obtained on 10/302010
The first electrons round the system and onto the image detector
showed that there were severe distortions in the system. These
distortions came from two main sources.
The most severe was a magnetic inhomogeneity due to the machining
of a stainless steel flange. This was the most complex flange in the
microscope and could not be simply replaced. A soft iron ring was
placed around the flange to even out the magnetic field. This changed
the electron optics but sucessfully removed the distortion.
The second source of distortion was the stray field from the field
termination. This was 15 gauss at the CHA, A set of coils was added to
the system to buck out the fields.
The first image of the Au grid was then obtained on the 30 October 2010
These are white light images. The U4A beamline was a bending
magnet UV beam line focused to a 2-3 mm spot size incident at 15o
to the microscope axis.
The U4A spot size was comparatively large for microscopy. The UV
white light photon flux was estimated at approximately 10^13 photons/s,
and therefore the photon flux density is 5x10^6 photons/micron^2 /s.
UV white light imaging using the photo-electron true secondary
electrons was straightforward, and effectively real time. Within the
spectrometer energy window 100% of the photoelectrons emitted into
the forward 2pi steradians are collected.
The images shown here used a fixed energy window of 1 eV, at the low
energy peak of the secondary electron
The first image of a grid from 30 October 2010 is shown here. A short
mpeg of several snapshots put together can be found at the link
These results are of a 600 mesh Au grid, 42 micron spacing. The
microscope field of view is circular and approximately 80 microns. It
was necessary to attach the Au grid above a hole in the microscope
sample stub so that there was no background from the large depth of
focus. In fact, our first images were of a Au grid above a Au foil, and very
confusingly only the 15 degree shadows were seen.
These images have significant optical bloom associated with the early
version of the detector and reflections from the vacuum window.
600 line Au grid 42 micron spacing
Uncoated single silk thread.
This is an image of a Au grid on top of a borosilicate glass fiber.
This image shows some of the features of VPPEM.
First there is a very large depth of focus.
Second because the secondary electrons are trapped in the magnetic field
there is a natural voltage compensation for charging effects.
Au grid on top of a uncoated
borosilicate glass fiber
The energy of the electrons that are imaged can be changed by altering the
sample bias. The secondary electron distribution can be sampled at different
These are two images of an Au gird on an Al foil. By changing the detection
energy the difference in workfunction can be imaged as a change in contrast.
Au grid on top of an Al foil