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Raman spectroscopy
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Raman spectroscopy
- technique for probing vibrational properties of materials (solids, fluids, gases)
- inelastic light scattering spectroscopy
Subject of this course:
”Classical” Raman technique as discovered by Raman
”Spontaneous non-coherent Raman spectroscopy”
Examples of other Raman techniques
- coherent RS
- stimulated RS
- resonant RS
- surface-enhanced RS
- x-ray RS (utilizing synchrotron source)
C.V. Raman
Nobel prize in physics (1930)
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Raman spectroscopy - major research technique
- provides information on strength of bonds, crystal symmetries, chemical composition,
presence of impurities and defects, texture and more
Broad spectrum of applications
Materials science & Geoscience (geochemistry, mineralogy, hydrology, paleontology, glaciology)
Environmnetal studies - pollution monitoring and control
Telecommunication (stimulated Raman effect in optical fibers - Raman amplifiers)
Medicine, pharmaceutics
Food industry (quality check)
Security (e.g. screening for explosives and drugs at the airports)
Use of Raman Spectroscopy in Geosciences and Materials Science
- identification of materials and phases (minerals, rocks, fluids, gases)
- non-destructive study of fluid inclusions
- in situ study of phase transitions, mapping of phase diagrams
- Raman data used for calculation of thermodynamic and physical-chemical properties
- Raman imaging: phases, textures and chemistry revealed with a high spatial resolution
- carbon detection in paleontology
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Raman spectroscopy
An ideal tool for probing materials at
extreme conditions characteristic of
the deep Earth’s and planetary
interiors
Raman spectroscopy in diamond anvil cell
CO2 laser – heating laser
“Green” laser – Raman excitation laser
Scattered light
contains Raman signal
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Phase diagram of quartz studied by Raman spectroscopy
Sequence of Raman spectra
revealing quartz-to-coesite
phase transformation
Phase diagram of SiO2
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Example of application of Raman spectroscopy: studying fluid inclusions in minerals and rocks
CO2-CH4-H2O fluid inclusions in quartz
Study of fluid inclusions by Raman spectroscopy
Retreiving formation history (e.g. pressure-temperature paths)
of minerals and rocks
Raman spectrum from CO2 inclusion
Calculation of density of CO2
Formation pressure and temperature of olivine
Raman applications for environmental analyses and in biotechnology
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Raman mapping
(Source: Horiba website)
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Raman imaging
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Raman spectroscopy
- technique for probing vibrational properties of materials (solids, fluids, gases)
- vibrational properties depend on the nature (e.g. ionic, covalent), strength and
symmetries of chemical bonds in studied materials; hence Raman spectroscopy
can reveal these bonding properties
In a Raman experiment, an incident monochromatic laser beam is passed
through a sample and the inelastically scattered light is analyzed by a
spectrometer and detector.
Inelastic scattering : an energy exchange
occurs between crystal lattice vibrations
and incoming photons from a laser
laser light
Studied
material
inelastically
scattered light
spectrometer
detector
computer
Principle of Raman Effect
Monochromatic source of light (laser)
The re-radiation by the dipole redistributes the energy in new directions.
This re-radiation is often called “scattering” because part of the original
energy moving to the right has been “scattered” into other directions
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Principle of infrared absorption (IR) and Raman scattering
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Energy transitions between vibrational levels in crystal and Raman spectrum
Rayleigh peak – corresponds to the original energy of incoming photons from laser; reference energy - set to zero on the energy scale
Stokes peaks – correspond to photons which give energy to lattice vibrations in the course of inelastic scattering
Anti-Stokes peaks – correspond to photons which receive energy from lattice vibrations in the course of inelastic scattering
Stokes and anti-Stokes peaks belonging to the transitions between the same energetic levels (e.g. n=1) peaks are positioned symmetrically
with respect to the Rayleigh peak.
Raman spectrum of magnesite (MgCO3)
(laser excitation wavelength = 514.52 nm)
Vibrational
energy levels
in a crystal
10000000/514.52 = 19435.6
Spectrum measured
at the detector
19435.6 – 19106.6
= 329
Energy scale
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Units in Raman spectroscopy
Standard unit:
- Wavenumber: number of waves per unit length: if λ is wavelength then wavenumber is 1/λ
- In spectroscopy, wavenumbers are given in inverse centimeters cm-1
- In Raman spectroscopy relative wavenumbers are used: relative with respect to the position of
laser, position of which is set = 0 cm-1)
Wavelength λ in optics is usually given in nanometers (nm).
Calculation of absolute wavenumber in cm-1: wavenumber(cm-1) = 107/λ(nm)
Example:
Laser excitation is at 785 nm, i.e. 12738.8 waves/cm = 12738.8 cm-1
Peak position found at 909.1 nm, i.e. at 11000 cm-1
Raman shift (relative to laser) ∆ν = 12738.8 cm-1 - 11000 cm-1 = 1738.8 cm-1
Calculation of energies from wavenumbers:
Energies of excitations probed by Raman spectroscopy
Energy change of photon (in unit of Joules)
∆E = h*c/λ1 − h*c/λ2 = h*c*(1/λ1-1/λ2) = h*c*∆ν*100
c - speed of light; h - Planck’s constant
Range of shifts observed in Raman spectra
∆ν from 10 to 4500 cm-1
∆Ε from 1 to 550 meV
(meV = mili-electron-volt; 1 meV = 8.0663 cm-1)
Energies of incoming photons:
532 nm ~ 2333 meV; 785 nm ~1581 meV
Link to conversion of units eV-nm:
http://www.highpressurescience.com/onlinetools/conversion.html
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Magnitude of energies involved in Raman scattering
Energies of incoming photon from various excitation lasers
Laser wavelength
Energy
Blue 488 nm
Green 532 nm
Red 785 nm
Near-IR 1064 nm
2.54 eV
2.33 eV
1.58 eV
1.17 eV
Magnitude of energy exchange between photon
and phonon (=lattice vibration) in Raman scattering
Entire possible range 0.001 - 0.55 eV (10-4500 cm-1)
Typical range of energy exchange:
~ 0.01 - 0.15 eV (100 - 1200 cm-1)
1 eV = 8066.3 cm-1
Initial
state
Excited
state
Initial
state
De-excited state
Quantum - mechanical picture
In Raman scattering instantaneous electric dipole moment is first
induced by the incident light beam E
µ
ind
=αE
E - electric field vector
α - polarizibility (defines how easy is to deform an electronic cloud)
Intensity of Raman scattering is given by
 dα 
E∫ Ψ 
 ∆rΨm dr
 d∆r 
*
n
Ψ vibrational wave function; m – initial state, n – final state
Selection rules for Raman activity
1. change of polarizibility during the vibration
2. n = m ± 1; Stokes transition: m=0 n=1; anti-Stokes transition: m=1
n=0
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Change of polarizibility during vibration required for Raman activity
 dα 
E ∫ Ψn* 
 ∆rΨm dr
 d∆r 
Generally, there is a large change of polarizibility of electron cloud during vibration
of symmetric molecules (e.g. strong Raman scattering from H2 molecule)
2e2e∆r
∆r
H
H
H H
polarizibility = measure of resistance against change of shape of electron cloud
induced by the electric field of incoming laser photons.
A vibration can be Raman active only when there is a change of polarizibility
in the course of atomic movements (when ∆r changes)
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Both Raman and infrared (IR) spectroscopies probe atomic/molecular vibrations in materials as such they represent a class of vibrational spectroscopy. In centrosymmetric structures
(i.e. structures possessing center of symmetry), Raman and infrared activities are mutually
exclusive. Thus, spectra become complementary to each other.
Principle setup for infrared absorption experiment
Radiation from IR source
Sample
Polychromatic (broadband) IR source (lamp)
range of wavelengths 700 nm <λ> 10000 nm (even up to100 um)
Dipole
Vibration
Infrared
Detector
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Change of electric dipole moment µ during vibration required for infrared activity
 dµ 
 ∆rΨm dr
Intensity of IR absorption given by ∫ Ψ 
 d∆r 
*
n
Ψ
µ
wave function; m – initial state, n – final state
Electric dipole moment
Q x d (charge multiplied by distance between charges)
Selection rules:
1. change of dipole moment during the vibration
2. n = m ± 1
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IR absorption
change of dipole moment during the vibration
 dµ 
∫ Ψ  d∆r  ∆rΨmdr
*
n
Raman scattering
change of polarizibility during the vibration
 dα 
E∫ Ψ 
 ∆rΨm dr
 d∆r 
*
n
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Raman and IR activity
Raman activity: change of polarizibility during vibration required
Infrared (=IR) activity: change of dipole moment during vibration required
Raman active
Infrared inactive
(no change of dipole
moment during vibration)
Dipole
Raman inactive
Infrared active
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IR and Raman spectra of calcite
O2C4+
O2-
O2-
CO32- - triangular planar structure
in all carbonates
Strong bonds between carbon and coordinated
oxygens exist within the triangle; vibrations between
C and Os are termed internal vibrations and
their wavenumbers are characteristic for carbonates
External vibrations – involve movement of entire
CO32- group and cations (corresponding peaks
located at low wavenumbers in spectra)
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Vibrations of water molecule
3 atoms x 3 coordinates
9 vibrations in total
3 vibrations correspond to rotations of whole molecule (thus do not involve relative motion between H and O)
3 vibrations correspond to translations of whole molecule (thus do not involve relative motion between H and O)
3 remaining vibrations are both Raman and infrared active (both polarizib. and dipole moment change during vibrations)
Symmetric
stretch
Bending mode
Anti-symmetric
stretch
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Raman spectra of high-pressure H2O ices
Peaks in the low wavenumber region
corresponding to vibrations of entire water
molecules (external (=lattice) modes)
Peaks in the high wavenumber region corresponding to
vibrations between H and O within water molecules
(internal modes)
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Probing the long wavelength limit of vibrations
Using lasers with wavelengths of photons in visible range of the optical spectrum (4000 – 7000 Å) results in
Raman scattering at k ~ 0 cm-1
k – wave vector; k = 2 π / λ
(when λ is large, k is small, ~ 0 cm-1)
~ 2000 unit cells
λ laser = 5145 Å (Ångström)
a~Å
unit cell dimension a is very small
compared to the wavelength of laser
λ vibrational wave ~ λ laser
Vibrations in adjacent unit cells probed by lasers operating in the visible range of spectrum are
nearly in phase (= have almost the same amplitude of motion from equilibrium position)
Equilibrium position
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Probing the long wavelength limit of vibrations
We do not probe this range because k >> 0 cm-1
Wavenumber axis
(positions of peaks
in spectrum)
Acoustic branches (=sound waves)
are not probed by Raman and IR
spectroscopy
k=2π/λ
Raman probes this range of k ~ 0 cm-1
Advantages of micro-Raman spectroscopy
Micro-Raman: excitation laser focused to a small spot (typically just a few microns in diameter)
- in-situ technique (e.g. at high T, P…)
- high spatial resolution (axial and radial) ~1-2 µm
(confocal Raman spectroscopy – designed particularly for high spatial resolution)
- high sensitivity: detection limit 0.1% (ppm levels achievable by special techniques)
- temporal resolution 0.01sec to hours
(from fast combustion processes to astronomy)
- experimentally simple technique (e.g. no or minimal sample preparation, position detector
at convenient location (scattered signal present “everywhere”)
- only small amount of sample required (as little as few cubic microns is sufficient amount)
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Parallel processes competing/interfering with Raman scattering process
- Absorption (sample heating) – topic pf IR absorption spectroscopy
- Luminescence (electronic transitions)
- Elastic scattering (strong Rayleigh line)
Absorption of excitation laser beam by sample
- may causes significant heating of sample and even its possible degradation, for example, by
oxidation or decomposition
- results also in high thermal background, broadening of peaks, and shift of wavenumbers to
lower wavenumbers
Counter-measures
- use of low laser power
- cooling of sample
- inert atmosphere
Fluorescence
- electronic transition: intrinsic as well as extrinsic origin
from defects in sample, stress-induced (e.g. under pressure)
- strong and often interferes with Raman spectrum,
may complicate interpretation of results
- can be combined with vibrational transition
vibrational transition + electronic transition = vibronic transition
quantum of vibronic transition: vibron
Example: ruby fluorescence (utilized as a pressure sensor in high pressure studies)
ruby: chromium-doped Al2O3
Elimination/decrease of fluorescence in Raman experiment
- use of longer excitation wavelength (i.e. lower energies of photons) - near-IR lasers
use, e.g. Ti: sapphire laser at 785 nm; ruby fluorescence at pressure of 1 bar ceases when using laser with λ > 695
nm. Photons lack energy to cause electron excitation to a higher electronic level
- use of pure defect-free sample (if available)
Often, fluorescence peaks are broad and more intense than Raman peaks.
For some materials, however, these peaks may look similar.
How to distinghuish between Raman and fluorescence?
Take new Raman spectrum using different excitation wavelength
- Raman peaks will remain at the same relative wavenumber with respect to the new excitation line
- fluorescence peaks will shift in accord with the excitation wavelength shift (i.e. their absolute
wavenumber remain constant)
Schematic illustration of difference between fluorescence and Raman scaterring
Raman scattering
probing vibrations between atoms, molecules
Fluorescence
involves electronic transitions
Broad fluorescence background
Example of narrow fluorescence peaks
of Cr-containing ruby (Cr3+:Al2O3)
Samples for Raman studies
Raman study of single crystals
- oriented crystals typically studied – relative orientation of crystal’s
crystallographic axes with respect to the direction of electric vector in the
laser beam is either pre-set (using e.g. results of an earlier x-ray study;
crystal’s morphology), or can be determined in the Raman study
- polarized Raman spectra of single crystals reveal symmetry of
vibrational modes
c-axis (trigonal crystal system)
Plane polarized laser beam
Electric vector E vibrates in one
single direction (plane), perpendicular
to the direction of beam propagation
quartz
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Examples of polarized Raman spectra
of single crystal of magnetite (Fe3O4)
- Illustrating orientational dependence of Raman spectra of single crystals
Polarized Raman spectra of single crystal
of magnetite in 4 different orientations
Dipole moment induced by the electric field E of laser
µ
ind
= αE
Polarizibility tensors α for Ag, Eg, T2g vibrational Raman
modes of magnetite
z
x
Dipole moments µind corresponding to vibrations Ag, Eg, T2g
change for varying orientations of E with respect to the
crystallographic axes. This results in varying
peak intensities.
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Raman study of powders
- simple sample preparation
- ideally, particle sizes should be much smaller than the spot sampled
by Raman technique; observed peak intensities are then
reproducible and correspond closely to true scattering intensities
- nano-powders - particle sizes of few tens of nm and below
- new qualitative features in Raman spectra as compared to bulk samples
- surface effects become important
Raman spectrum of powdered magnetite
Polycrystals:
Intermediate case between fine powders and
single crystals. For crystalline sizes comparable
to the size of laser spot, relative intensities of
peaks in the Raman spectrum may vary from spot
to spot.
Polycrystalline quartz