Raman Spectroscopy
Contact: Rilana Maeser and Benjamin Schulz
Raman spectroscopy investigates inelastically scattered light that originates from atoms,
molecules or quasi-particles in solids such as phonons, magnons, plasmons or electronic excitations.
Thus, it reveals valuable information on the properties of matter. When light hits matter, most of the light
is scattered elastically or reflected directly, i. e. without changing energy. A small part of the
light is scattered inelastically, i. e. it changes its energy by creating or annihilating an excitation.
Raman spectroscopy is applied to many fields from solid state physics to biophysics or
chemistry. Since the inelastically scattered light is very weak and has often only a
very small energy difference to the excitation energy, there is the need for sophisticated
spectroscopic instruments that offer high energy resolution and good suppression of the stray light
(elastically or quasi-elastically scattered light that originates from sample defects such as grain boundaries,
tiny surface scratches or deposits on the sample).
A good stray-light rejection enables measurements close to the Rayleigh line that represents the
elastically scattered light.
We are able to perform Raman spectroscopy from the visible spectral range to the upper vacuum ultraviolet (VUV).
Our future VUV-Raman spectrometer at the VUV-FEL (vacuum-ultraviolet free electron laser) of
HASYLAB/DESY will extend the
Raman spectroscopy to the lower VUV and XUV range.
UV-VUV Raman spectrometer
The UV-VUV Raman spectrometer UT-3 (ultimative triple of the third generation) provides
a broad wavelength range from 165 to 1000 nm and an energy resolution of 10-5.
The extremely versatile setup includes
two lasers (Krypton and Argon) and a frequency doubler that generate over 50 wavelengths
from 207 to 800 nm and thus enable to study resonance behavior very closely.
The samples may be solids, liquids or gases. For example,
we perform measurements on high-temperature superconductors or water.
Figure 1: Setup of the UV-VUV-Raman spectrometer with the two lasers
and the frequency doubler. The beam path in the front leads to the
spectrometer visible in the back.
Figure 2: The left picture shows the triple monochromator shortly after the delivery.
The right picture gives a closer look on the entrance optics with the two large parabolas
which have a diameter of 25 cm.
The design of the UV-VUV Raman spectrometer UT-3 was developed by our group in collaboration
with Miles V. Klein (University of Illinois at Urbana-Champaign), who designed the prototype UT-1
for the visible spectral range, and the firm McPherson.
Important parameters such as focus sizes, focal-plane angles and mirror curvatures were optimized by ray-tracing simulations.
The main part of the
spectrometer is a specially designed triple monochromator that leads to the name "ultimative triple".
It consists of three gratings and six off-axis parabolas as well as a spherical and an ellipsoidal mirror
in the last stage. "Off-axis" means that the segments do not include the optical center of the beam,
so that the central part cannot obscure the beam. Parabolic mirrors offer many advantages. For example,
they are free of spherical aberration and thus focus a parallel beam to a point.
All mirrors are handcrafted.
The entrance objective has a Cassegrainian design with two large handcrafted parabolic mirrors.
The fully reflective design enables accurate and quick resonance studies.
The UT-3 offers high energy resolution and good stray-light rejection, so that it is possible
to study low-energy regions.
Figure 3: Ray-tracing image of the triple monochromator UT-3.
VUV-Raman spectrometer at the VUV-FEL of HASYLAB/DESY
The vacuum-ultraviolet free electron laser (VUV-FEL) offers unparalleled possibilities for the investigation
of condensed matter. Examples for the outstanding properties are high spectral intensity, high brilliance and high
time resolution. The energy range of the photon pulses reaches from 10 to 200 eV corresponding to a wavelength range
from 120 to 6 nm. We are going to establish a novel VUV-Raman spectrometer that will make excellent use of
the specific features provided by the VUV-FEL.
Figure 4: Experimental hall at HASYLAB/DESY where the VUV-Raman spectrometer is going to be established.
The techniques for spectroscopy in the VUV spectral range differ from those in other spectral ranges.
Conventional spectrometers from the visible to the upper VUV range use mirrors in normal incidence in order to limit
abberations. By the use of double or triple monochromators a high energy resolution and a high suppression
of stray light can be achieved. With the development of synchrotron radiation sources, inelastic light scattering
has been extended from the VUV to the hard-X-ray range. But compared to conventional lasers, even third generation
synchrotron radiation sources have much weaker brilliances. In order to achieve a sufficient photon flux, the
bandwidth of the incident photons must be large. This limits the energy resolution. Moreover spectroscopy in the
VUV (below 165 nm) and XUV requires grazing incidence optics. Thus, so far only the use of single stage
instruments with limited stray-light rejection and resolution has been possible. The high brilliance of the
VUV-FEL enables the use of double monochromators.
The VUV-Raman spectrometer will be the first double-stage instrument for inelastic light scattering
in the VUV and XUV range. It will consist of a novel high-resolution double monochromator with
parabolic mirror segments. This guarantees optimal stray-light rejection, ultimative resolution and high
throughput. Various fields of physics, chemistry, biology and medecine will benefit from the new research
possibilities. For example, functional materials such as high-temperature superconductors, systems with colossal
magnetoresistance, glasses, and bio-organic matter can be studied on shortest time and length scales.
The VUV-Raman spectrometer is funded by the BMBF (Federal Ministry for Education and Research) and will be set up
within the
Helmholtz
Research Center "Nanostructure Research on Functional Materials at the VUV-FEL".
Further information on the spectrometer can be found on the web pages of the Helmholtz Research Center.
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