This website is a source of information about Vector Potential Photoelectron Microscopy (VPPEM).
The proof-of principle version 1 VPPEM
instrument at U4A beamline NSLS
The large blue box is 0.5" mild steel that
acts as the ferromagentic shield.
A helium plume from the superconducting
magnet can be seen at the top right.
This is an image of the reaction zone
between calcium and aluminum. The
sample is a high distortion strengthened
metal matrix wire for overhead power
transmission. When annealed the Ca
and Al react forming a brittle zone of
intermetallics, and impurity phases
(unassigned black areas).
As the photoelectrons exit the field, conservation of the angular momentum
from the vector potential field creates an angular image. This figure shows a
numerical simulation of the cross section through the model above.
The diverging photoelectron trajectories
leaving the magnetic field are focused into
the aperture of a concentric hemispherical
analyzer (CHA). The CHA is double
focusing, and images a monochromatic
angular image onto the output aperture.
The monochromatic angular image is
focused as a real image onto a
multichannel plate and phosphor.
New results demonstrate much higher
spatial resolution than expected on some
We need to design a version 2 prototype to
investigate the new imaging phenomena.
A small shielded permanent magnet which
is flux condensed to give a 1.8 Tesla field is
being used for these experiments.
We are now setting up at the NIST soft and tender beamlines (100 eV -10 KeV x-rays) at NSLS II 7-ID.
We are now setting up a version 2 VPPEM at NSLS II beamline7-ID
With the new shielded fixed magnet assembly shown
at the left the microscope is very compact. This is
the test arrangement with the CHA at beamline 7ID.
The x-ray energy is scanned across
the characteristic core energy levels of
the atoms in the sample. The resulting
spectral images can be analysed, and
imaged as chemical images.
A sample sits at the center of a strong magnetic field from a solenoid.
Photoelectrons are emitted from the sample surface using x-ray illumination from a synchrotron. The
photoelectrons travel down axis of the magnet along the field lines in cyclotron orbits.
The magnet field is terminated by a ferromagnetic shield. The electrons then exit the shield into field free
vacuum though an aperture.
Mesoscale chemical imaging of technical materials
How the VPPEM works.
X-ray photon spectroscopy is valuable in a wide range of technology areas
from semiconductor device to catalysis. X-ray photoelectron spectra give
elemental compositions, chemical state, local atomic coordination,
semiconductor band structure, magnetic properties, and other details of
The field of applications is still broadening with the rapid development of
specific techniques enabled by the availability of extended spectral range
photons from the third generation of synchrotron light sources. These
instrumental developments come at an opportune time as we have a
technology driver. There is a pressing need for chemical state analysis on the
mesoscale for which VPPEM microscopy could make a significant contribution.
VPPEM is a new class of microscope being developed at the National
Synchrotron Light Source (NSLS II), Brookhaven National Laboratory.
VPPEM images X-ray photoelectron spectra from the surface of a sample
irradiated with a synchrotron X-ray source. VPPEM images give the
composition and chemistry of the sample surface.
Results show that the microscope has high sensitivity, and high spatial
resolution, with a very high depth of focus.