Experimental Methods
We use a variety of spectroscopies to study the
local structure of matter. Perturbed angular correlation (PAC),
and also Mössbauer effect (ME) and high-field solid-state nuclear
magnetic resonance (NMR), are used to measure
interactions
of nuclear moments with fields produced by nearby charges and spins in
solids.
These nuclear hyperfine interactionsfall
off
rapidly with distance, with interaction strengths sensitive to the
kinds of neighoring atoms and their geometrical arrangements in the
closest few atomic
shells around probe atoms. With sufficient resolution, measured
interactions "flag" the various local environments of the probe
atoms. Our primary method is PAC, and we are one of very few
groups in North America that uses PAC to study solids. We
identify lattice locations of
probe atoms and the presence (or absence) of point
defects such as impurity atoms or lattice vacancies next to probe atoms
via the nuclear quadrupole interaction. Measurements of nuclear
relaxation allow us to determine jump frequencies of vacancy defects
and jump frequencies of probe atoms themselves. PAC, ME and
NMR bring
one as close as possible to "seeing" lattice locations of probe atoms
in solids.
Other methods we can apply are positron annihilation, for
detecting
vacancy
defects,
and XAFS, for determining lattice locations. PAC,
ME
and positron lifetime spectrometers are located in the group's
laboratory in
Webster
Hall, the university's NMR
Center
is across the street, and XAFS measurements can be carried out at the
Advanced Photon Source at Argonne
National
Laboratory.
Nuclei spin like tops in electronic fields produced by nearby charges
and spins.
PAC allows direct measurement of the nuclear precessions in the time
domain. Precession frequencies "flag" each local environment, and
differ substantially when probe atoms are on different lattice sites
or have atomic defects localized
within 1-2 atomic
shells around the probes. We have two four-detector spectrometers
with BaF2 crystals and ovens for measurements up to ~1200 C
under vacuum, as well as a cryostat for measurements down to 20
K. This
is our primary experimental method. More
information
about PAC.
Other Methods
Atomic fluorescence has its counterpart in transitions between excited
and ground-state nuclear
energy levels. When transition energies are lower than about 30
keV, there is
a significant probability that transitions that take place are recoilless,
that is, without creation or annihilation of quantized lattice
vibrations (phonons).
The energy resolution is then determined solely by the lifetime
of the
excited state, of the order of 10-7
eV for a 100 ns lifetime. In that situation, a scanning
spectroscopy becomes possible
in which one varies the energy of gamma rays emitted from a radioactive
source using
the Doppler shift to sweep over the range of hyperfine energy
levels,
looking for resonant absorption or scattering of nuclear
radiations. Our instrumentation for such measurements is
state-of-the-art, including a Ranger 1200 spectrometer with built-in
laser interferometer for
absolute
velocity calibration and cryostat for measurements down to 20
K. More information about ME.
High-Field Solid-State Nuclear Magnetic
Resonance (NMR)
Radio-frequency absorption measurements can be used to identify lattice
locations of NMR-active isotopes via nuclear
quadrupole interactions, chemical shifts and, in metallic systems,
Knight shifts. The capability exists at WSU to make NMR
measurements using a 9.4 Tesla Bruker DRX400 spectrometer with
magic-angle spinning capability located across the street in the NMR Center..
X-ray absorption by atoms in a solid produces energetic
photoelectrons that
scatter
among neighboring atoms. Scattering of
photoelectron-waves
leads to constructive or destructive interference that can
enhance or reduce the absorption.
Roughly, the fourier transform of the absorption as a
function
of the wavevector of the photoelectron gives a radial distribution
function
for the surroundings of the absorbing atom. We have made
preminary studies to use XAFS to measure lattice locations of solutes
in intermetallic
compounds that had been studied also by PAC or ME. Measurements
have been carried out on beamline 20 of the Advanced
Photon Source at Argonne National Laboratory, but are currently
inactive. More
information about XAFS. Results to date have been ambiguous.
Positron Annihilation (e+)
Positrons annihilate with electrons, their antiparticles. Roughly, the
lifetime of a positron varies inversely with the density of electrons
at
its location. Since positrons tend to trap in open spaces in crystals,
such as
lattice vacancies, where the electron density is low, they live
longer than when they annihilate in a perfect solid. Thus,
measured lifetimes can be used to "flag" lattice locations
of the positrons and, in particular, to detect vacancies and measure
their concentrations. We have one lifetime spectrometer using
thin BaF2 scintillators, but our use of this method is
currently inactive.
October 2009, Gary S. Collins, Hyperfine Interactions
Group, Washington State University. Back to our group's
home page.