Raman spectroscopy Raman spectroscopy LectureLecture

Licentiate course in measurement science and technologyLicentiate course in measurement science and technology

Spring 2008Spring 2008

10.04.200810.04.2008

Antti Kivioja

ContentsContents

- 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 andRaman spectroscopy is the measurement of the wavelength andintensity of intensity of inelasticallyinelastically scattered light from molecules.”scattered light from molecules.”www.aboutremediation.com/techdir/tech_definitions_al.aspwww.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 informationinformationon 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.htmlchemistry.allinfoabout.com/features/spectroscopy.html

“Raman spectroscopy is a spectroscopic technique used“Raman spectroscopy is a spectroscopic technique usedin condensed matter physics and chemistry to study in condensed matter physics and chemistry to study vibrationalvibrational, , rotational, and other lowrotational, and other low--frequency modes in a system.” frequency modes in a system.” en.wikipedia.orgen.wikipedia.org/wiki/Raman spectroscopy/wiki/Raman spectroscopy

IntroductionIntroduction

The electromagnetic spectrumThe electromagnetic spectrum

IR near-IR visible UV

vibrational energy levels electronic energy levels

Increasing energy

VibrationalVibrational modes of Hmodes of H--CC--H H groupgroup

a) Symmetrical stretchingb) Asymmetrical stretchingc) Wagging or out-of-plane bending

d) Rocking or asymmetrical in-plane bendinge) Twisting or out-of-plane bendingf) Scissoring or symmetrical in-plane bending

The Raman effectThe Raman effect

Ground electronic state (vibrationallevels)

Excited electronic state

Virtual state

Rayleighscattering

ν 0

Stokes scattering

ν 0-∆ν 0

Anti-Stokes scattering

ν 0+∆ν 0

hν0

Laser energy

Raman spectrum of CClRaman spectrum of CCl44Stokes lines

Anti-Stokes lines

Rayleigh scattering

What is Raman spectroscopy ?What is Raman spectroscopy ?

Raman spectrumRaman 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.

RamanRaman: 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 lighIn 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 momentmoment..

--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 RamanIR and Raman

FTIR and Raman spectra of FTIR and Raman spectra of thermomechanicalthermomechanical pulppulp

FluorescenceFluorescenceFluorescence 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

Fluorescenceemission

FluorescenceFluorescence

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 / Raman Shift (cm-1)

Sample : kaolin coating with 785 nm excitation

Fluorescence suppression by Fluorescence suppression by Kerr gate systemKerr 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.

KerrKerr--gated resonance Raman gated resonance Raman spectrometerspectrometer

Raman spectra of semiRaman spectra of semi--bleached bleached pulp with and without the Kerr gatepulp with and without the Kerr gate

Raman instrumentsRaman instruments

• UV–Raman spectrometer Renishaw 1000 UV

• Kaiser Raman Hololab series 5000spectrometer

• WITec alpha 300 combined confocal Ramanmicroscope and atomic force microscope

ConfocalConfocal Principle inPrinciple indispersive spectrometerdispersive spectrometer

UVUV--Raman spectrometer Raman spectrometer RenishawRenishaw 1000 UV1000 UV

System 1000general

15 Copyright Renishaw plc 1999

Renishaw RM SeriesRaman microscope

microscope holographicfilters

(laser filter andlaser attenuation

filters)

CCDdetector

diffractiongrating stage

confocalslit

CCD-detector

The most important components in a dispersive Raman The most important components in a dispersive Raman instrument in instrument in RenishawRenishaw 1000 UV1000 UV

Mikroscope

notchfilter slit motor grating

imaging filter

sample

Laser entrance

UVUV--Raman spectrometer Raman spectrometer RenishawRenishaw 1000 UV1000 UV

Kaiser Kaiser HololabHololabRaman 785 nmRaman 785 nm

Raman microscopeRaman 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 spectrometerInside Kaiser spectrometer

Pinhole

Objective

Scan stage

Beam splitter

AFM-tip

Sample

WITecWITec alpha 300alpha 300-- instrumentinstrument

Eo Eo

Eo- hν

Eo+ hνLight source(laser)

Scattered light

Inelastic

sample

Segmentedphotodiode

Laser

CantileverRAMAN

AFMCombined AFM- Confocal Raman-electromagneticinteraction process

-gives information of chemical structures

-Raman spectrum:

- Intensity vs. energy difference

-gives informationof surfaceproperties

Both chemical & structural features canbe analysedsimultaneously

Principle of AFM Principle of AFM -- RamanRaman

Applications Applications

Information from Raman Spectroscopy Information from Raman Spectroscopy –– and what and what can be used for Mappingcan be used for Mapping

characteristic Raman frequencies

composition of material

e.g. MoS2, MoO3

changes in frequency of Raman peak

stress/strainstate

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 researchpaper research• Carbohydrates

– Fibril orientation– Crystallinity of cellulose– Differecnt cellulose types I and II etc..– Hexenuronic acid content

• Lignin– Guaiasyl/syringyl ratio

• Extractives

Preparation of crossPreparation of cross--section section samplessamples

• 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

StructureStructure of of woodwood cellscells

S1S1

S2S2

PP

S3S3

Raman spectroscopyRaman 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

15981095(a) (b)

Lignin Cellulose

Raman microscopyRaman microscopy•Interesting location in sample is selected•Spectra in regular intervals are recorded

every single point in image contain one spectrum

Raman microscopyRaman microscopy•Baseline of the spectra is corrected•Certain feature is chosen and the image is drawn according to the intensity

Lignin/cellulose ratio in pineLignin/cellulose ratio in pine

Lignin/cellulose ratio in spruceLignin/cellulose ratio in spruce

TotalTotal InternalInternal ReflectionReflectionRamanRaman SpectroscopySpectroscopy (TIR)(TIR)

TotalTotal InternalInternal ReflectionReflectionRamanRaman SpectroscopySpectroscopy (TIR)(TIR)

TotalTotal InternalInternal ReflectionReflectionRamanRaman SpectroscopySpectroscopy (TIR)(TIR)

TotalTotal InternalInternal ReflectionReflectionRamanRaman SpectroscopySpectroscopy (TIR)(TIR)

TotalTotal InternalInternal ReflectionReflectionRamanRaman SpectroscopySpectroscopy (TIR)(TIR)

TotalTotal InternalInternal ReflectionReflectionRamanRaman SpectroscopySpectroscopy (TIR)(TIR)

Coating layer characterization by Coating layer characterization by TIRTIR-- Raman spectroscopyRaman spectroscopy

Applications of vibrational Applications of vibrational spectroscopyspectroscopy

• 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 TIRDevelopment of TIR--Raman Raman Schematic diagram of TIR-Raman

Development of TIRDevelopment of TIR--Raman Raman Benefits of hemisphere shape crystal

New sample holderNew sample holder

Possibilities of TIRPossibilities of TIR--Raman Raman Due to total internal reflection surface sensitivity is remarkably improvedcompared to confocal Raman spectroscopy.

Possibilities of TIRPossibilities of TIR--Raman Raman

Possibilities of TIRPossibilities of TIR--Raman Raman

Possibilities of TIRPossibilities of TIR--Raman Raman

Possibilities of TIRPossibilities 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

ConclusionsConclusions

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.

ThankThank youyou for for youryourattentionattention!!

Excursion to Raman labExcursion to Raman labon (week 19)on (week 19)