Raman spectroscopy Raman spectroscopy
Lecture Lecture
Licentiate course in measurement science and technology
Licentiate course in measurement science and technology
Spring 2008 Spring 2008 10.04.2008 10.04.2008 Antti Kivioja
Contents Contents
- Introduction
- What is Raman spectroscopy?
- The theory of Raman spectroscopy - Fluorescence
- Fluorescence suppression by Kerr Gate system - Raman spectrometers (Renishaw, Kaiser, Witec) -Applications
- New method : TIR-Raman spectroscopy
- Discussion of excursion
What is Raman spectroscopy ? What is Raman spectroscopy ?
“Raman spectroscopy is the measurement of the wavelength and“Raman spectroscopy is the measurement of the wavelength and intensity of
intensity of inelasticallyinelastically scattered light from molecules.”scattered light from molecules.”
www.aboutremediation.com/techdir/tech_definitions_al.asp www.aboutremediation.com/techdir/tech_definitions_al.asp
“Raman scattering of light by molecules may be used to provide
“Raman scattering of light by molecules may be used to provide information
information
on a sample's chemical composition and molecular structure.
on a sample's chemical composition and molecular structure.
Surface enhanced Raman spectroscopy is a type of Raman Surface enhanced Raman spectroscopy is a type of Raman spectroscopy.”
spectroscopy.”
chemistry.allinfoabout.com/features/spectroscopy.html chemistry.allinfoabout.com/features/spectroscopy.html
“Raman spectroscopy is a spectroscopic technique used
“Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study
in condensed matter physics and chemistry to study vibrationalvibrational, , rotational, and other low
rotational, and other low--frequency modes in a system.” frequency modes in a system.”
en.wikipedia.org
en.wikipedia.org/wiki/Raman spectroscopy/wiki/Raman spectroscopy
Introduction
Introduction
The electromagnetic spectrum The electromagnetic spectrum
IR near-IR visible UV
vibrational energy levels electronic energy levels
Increasing energy
Vibrational
Vibrational modes of H modes of H - - C C - - H H group
group
a) Symmetrical stretching b) Asymmetrical stretching
c) Wagging or out-of-plane bending
d) Rocking or asymmetrical in-plane bending e) Twisting or out-of-plane bending
f) Scissoring or symmetrical in-plane bending
The Raman effect The Raman effect
Ground
electronic state (vibrational
levels) Excited
electronic state
Virtual state
Rayleigh scattering
ν0
Stokes scattering
ν0-∆ν0
Anti-Stokes scattering
ν0+∆ν0 hν0
Laser energy
Raman spectrum of CCl Raman spectrum of CCl 4 4
Stokes lines
Anti-Stokes lines Rayleigh scattering
What is Raman spectroscopy ? What is Raman spectroscopy ?
Raman spectrum Raman spectrum
• The shifts are independent of the frequency of the incident light.
• Usually the Stokes lines are studied, because they are more intense than the anti-Stokes lines.
• The Raman shifts correspond to those of infrared shifts, but the intensities are different.
• The sample is exposed to a monochromatic source of exciting photons.
• The frequencies of the scattered light are measured.
• The intensity of Raman scattered components is much lower than the Rayleigh-scattered component, because the probability of inelastic collisions is only ~10-8.
>
A highly selective monochromator and a very sensitive detector are needed.
IR:
IR: Transition of a molecule from a ground state to a Transition of a molecule from a ground state to a vibrationallyvibrationally excited state by excited state by absorption of infrared radiation.
absorption of infrared radiation.
Raman
Raman: The radiation is not absorbed or emitted, but shifted in frequ: The radiation is not absorbed or emitted, but shifted in frequency.ency.
In Raman spectroscopy, UV, Vis or NIR lasers can be used as ligh
In Raman spectroscopy, UV, Vis or NIR lasers can be used as light source.t source.
-
-In IR spectroscopy, the transitions must have a change in the moIn IR spectroscopy, the transitions must have a change in the molecular lecular dipole dipole moment
moment..
--In Raman spectroscopy, the change has to be in the In Raman spectroscopy, the change has to be in the polarizabilitypolarizability of the molecule.of the molecule.
--These characteristics are inversely related.These characteristics are inversely related.
--Water disturbs in IR spectroscopy but not in Raman spectroscopy.Water disturbs in IR spectroscopy but not in Raman spectroscopy.
IR and Raman
IR and Raman
FTIR and Raman spectra of FTIR and Raman spectra of
thermomechanical
thermomechanical pulp pulp
Fluorescence Fluorescence
Fluorescence is an optical Fluorescence is an optical
phenomenon that often disturbs in phenomenon that often disturbs in Raman spectroscopy.
Raman spectroscopy.
Fluorescence is most disturbing Fluorescence is most disturbing when visible light wavelengths are when visible light wavelengths are used in excitation.
used in excitation.
Fluorescence is less intense when Fluorescence is less intense when UV or NIR is used.
UV or NIR is used.
Excited electronic state
Ground electronic state
Fluorescence emission
Fluorescence Fluorescence
1084 1027 999.2 618.2 465.9 378.3 334.8
0 500 1000 1500 2000
3000 2500 2000 1500 1000 500 0
Arbitrary / Ram an S hift (cm -1)
Sample : kaolin coating with 785 nm excitation
Fluorescence suppression by Fluorescence suppression by
Kerr gate system Kerr gate system
• The Raman scattering is faster than the fluorescence emission (picoseconds vs.
nanoseconds).
• When the Kerr-gated system is used, only the light that is scattered immediately reaches the detector, while the slower fluorescence emission is blocked.
• Not a routine analysis, applied only once for pulp
samples.
Kerr Kerr - - gated resonance Raman gated resonance Raman spectrometer
spectrometer
Raman spectra of semi
Raman spectra of semi - - bleached bleached
pulp with and without the Kerr gate
pulp with and without the Kerr gate
Raman instruments Raman instruments
• UV–Raman spectrometer Renishaw 1000 UV
• Kaiser Raman Hololab series 5000 spectrometer
• WITec alpha 300 combined confocal Raman
microscope and atomic force microscope
Confocal
Confocal Principle in Principle in
dispersive spectrometer
dispersive spectrometer
UV UV - - Raman spectrometer Renishaw Raman spectrometer Renishaw 1000 UV 1000 UV
System 1000 general
15 Copyright Renishaw plc 1999
Renishaw RM Series Raman microscope
microscope holographic filters
(laser filter and laser attenuation
filters)
CCD detector diffraction
grating stage confocal
slit
CCD-detector
The most important components in a dispersive Raman The most important components in a dispersive Raman instrument in
instrument in Renishaw Renishaw 1000 UV 1000 UV
Mikroscope
notchfilter slit motor grating
imaging filter sample
Laser entrance
UV UV - - Raman spectrometer Renishaw Raman spectrometer Renishaw 1000 UV 1000 UV
Kaiser
Kaiser Hololab Hololab
Raman 785 nm
Raman 785 nm
Raman microscope Raman microscope
1. depth profiling
– lateral resolution: 2.5 µm – depth resolution: 4 µm
2. lateral bulk mapping
– lateral resolution: 10 µm – analysis depth: 6 µm
3. lateral surface mapping
– lateral resolution: 2.5 µm
– analysis depth: 1-2 µm
Inside Kaiser spectrometer
Inside Kaiser spectrometer
Pinhole
Objective
Scan stage
Beam splitter
AFM-tip
Sample
WITec
WITec alpha 300 alpha 300 - - instrument instrument
Eo Eo
Eo- hν Eo+ hν
Light source (laser)
Scattered light
Inelastic
sample
Segmented photodiode
Laser
Cantilever
RAMAN
Combined AFM- Confocal Raman AFM -electromagnetic
interaction process
-gives information of chemical structures -Raman spectrum:
- Intensity vs.
energy difference
-gives information of surface
properties
Both chemical &
structural features can be analysed
simultaneously
Principle of AFM
Principle of AFM - - Raman Raman
Applications
Applications
Information from Raman Spectroscopy
Information from Raman Spectroscopy – – and what and what can be used for Mapping
can be used for Mapping
characteristic Raman frequencies
composition of
material e.g. MoS2, MoO3
changes in frequency of Raman peak
stress/strain state
e.g. Si 10 cm-1 shift per
% strain
polarisation of Raman peak
crystal symmetry and orientation
e.g. orientation of CVD diamond grains
width of Raman
peak quality of crystal e.g. amount of plastic deformation
parallel perpendicular
intensity of
Raman peak amount of material e.g. thickness of transparent coating
Applications of Raman Applications of Raman
spectroscopy in wood, pulp and spectroscopy in wood, pulp and
paper research paper research
• Carbohydrates
– Fibril orientation
– Crystallinity of cellulose
– Differecnt cellulose types I and II etc..
– Hexenuronic acid content
• Lignin
– Guaiasyl/syringyl ratio
• Extractives
Preparation of cross
Preparation of cross - - section section samples
samples
• Samples are usually embedded in epoxy resin
• Pressure needs to be used in case of wood samples
• Epoxy block is cut with microtome
• Smoothness of the sample is extremely important for
good results
Structure
Structure of of wood wood cells cells
S1 S1 S2 S2
P P
S3 S3
Raman spectroscopy Raman spectroscopy
• Based on excitation of molecules to higher energy level
• IR and Raman spectroscopies yield similar data
• Unlike in IR, water does not disturb the Raman measurements
1598
(a) (b) 1095
Lignin Cellulose
Raman microscopy Raman microscopy
•Interesting location in sample is selected
•Spectra in regular intervals are recorded
Î every single point in image contain
one spectrum
Raman microscopy Raman microscopy
•Baseline of the spectra is corrected
•Certain feature is chosen and the
image is drawn according to the
intensity
Lignin/cellulose ratio in pine
Lignin/cellulose ratio in pine
Lignin/cellulose ratio in spruce
Lignin/cellulose ratio in spruce
Total
Total Internal Internal Reflection Reflection Raman
Raman Spectroscopy Spectroscopy (TIR) (TIR)
Total
Total Internal Internal Reflection Reflection Raman
Raman Spectroscopy Spectroscopy (TIR) (TIR)
Total
Total Internal Internal Reflection Reflection Raman
Raman Spectroscopy Spectroscopy (TIR) (TIR)
Total
Total Internal Internal Reflection Reflection Raman
Raman Spectroscopy Spectroscopy (TIR) (TIR)
Total
Total Internal Internal Reflection Reflection Raman
Raman Spectroscopy Spectroscopy (TIR) (TIR)
Total
Total Internal Internal Reflection Reflection Raman
Raman Spectroscopy Spectroscopy (TIR) (TIR)
Coating layer characterization by Coating layer characterization by
TIR TIR - - Raman spectroscopy Raman spectroscopy
Applications of vibrational Applications of vibrational
spectroscopy spectroscopy
• Latex migration (x-y-z)
• Interactions of coating components
• Print mottling analysis
– binder and pigment distribution – coat weight variation
• Colorant distribution in coating (x-y-z)
• Long-term permanence of printed
image
Development of TIR
Development of TIR - - Raman Raman
Schematic diagram of TIR-Raman
Development of TIR
Development of TIR - - Raman Raman
Benefits of hemisphere shape crystal
New sample holder
New sample holder
Possibilities of TIR
Possibilities of TIR - - Raman Raman
Due to total internal reflection surface sensitivity is remarkably improved compared to confocal Raman spectroscopy.
Possibilities of TIR
Possibilities of TIR - - Raman Raman
Possibilities of TIR
Possibilities of TIR - - Raman Raman
Possibilities of TIR
Possibilities of TIR - - Raman Raman
Possibilities of TIR
Possibilities of TIR - - Raman Raman
Future work
Develop better TIR-system to study forest products materials Main challenges:
- Find a optically high quality prism with broadband transparency - Get a good prism-sample contact
- Build convenient prism-sample holder
- Fit external TIR-Raman parts to commercial confocal Raman instrument
Conclusions Conclusions
Total internal reflection TIR-Raman technique remarkably improves sensitivity of Raman spectroscopy measurements
Preliminary experiments have proven the possibilities of analyzing paper and print samples.
