Raman spectroscopy Lecture

Transcription

1 Raman spectroscopy Lecture Licentiate course in measurement science and technology Spring Antti Kivioja

2 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

3 Introduction What is Raman spectroscopy? Raman spectroscopy is the measurement of the wavelength and intensity of inelastically scattered light from molecules. Raman scattering of light by molecules may be used to provide information on a sample's chemical composition and molecular structure. Surface enhanced Raman spectroscopy is a type of Raman spectroscopy. chemistry.allinfoabout.com/features/spectroscopy.html Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. en.wikipedia.org/wiki/raman spectroscopy

4 The electromagnetic spectrum IR near-ir visible UV Increasing energy vibrational energy levels electronic energy levels

5 Vibrational modes of H-C-H H 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

6 The Raman effect Excited electronic state Laser energy ν 0 ν 0 - ν 0 ν 0 + ν 0 Virtual state hν 0 Rayleigh scattering Stokes scattering Anti-Stokes scattering Ground electronic state (vibrational levels)

7 Raman spectrum of CCl 4 Rayleigh scattering Stokes lines Anti-Stokes lines

8 What is Raman spectroscopy?

9 Raman spectrum 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. 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.

10 IR and Raman IR: Transition of a molecule from a ground state to a vibrationally excited state by absorption of infrared radiation. Raman: : The radiation is not absorbed or emitted, but shifted in frequency. ency. In Raman spectroscopy, UV, Vis or NIR lasers can be used as light t source. -In IR spectroscopy, the transitions must have a change in the molecular dipole moment. -In Raman spectroscopy, the change has to be in the polarizability of the molecule. -These characteristics are inversely related. -Water disturbs in IR spectroscopy but not in Raman spectroscopy.

11 FTIR and Raman spectra of thermomechanical pulp

12 Fluorescence Fluorescence is an optical phenomenon that often disturbs in Raman spectroscopy. Excited electronic state Fluorescence is most disturbing when visible light wavelengths are used in excitation. Fluorescence is less intense when UV or NIR is used. Fluorescence emission Ground electronic state

13 Fluorescence Sample : kaolin coating with 785 nm excitation Arbitrary / Raman Shift (cm-1)

14 Fluorescence suppression by 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.

15 Kerr-gated resonance Raman spectrometer

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

17 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

18 Confocal Principle in dispersive spectrometer

19 UV-Raman spectrometer Renishaw 1000 UV

20 The most important components in a dispersive Raman instrument in Renishaw 1000 UV Renishaw RM Series Raman microscope microscope Mikroscope holographic confocal diffraction filters notchfilter slit slit grating stage motor grating CCD CCD-detector detector sample Laser entrance System 1000 general imaging filter (laser filter and laser attenuation filters) 15 Copyright Renishaw plc 1999

21 UV-Raman spectrometer Renishaw 1000 UV

22 Kaiser Hololab Raman 785 nm

23 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

24 Inside Kaiser spectrometer

25 WITec alpha 300- instrument Pinhole Beam splitter Objective AFM-tip Scan stage Sample

26 Principle of AFM - Raman Light source (laser) E o E o + hν E o Scattered light Inelastic Segmented photodiode RAMAN sample -electromagnetic interaction process -Raman spectrum: - Intensity vs. energy difference -gives information of chemical structures E o -hν Laser Combined AFM- Confocal Raman Cantilever AFM -gives information of surface properties Both chemical & structural features can be analysed simultaneously

27 Applications

28 Information from Raman Spectroscopy and what can be used for Mapping characteristic Raman frequencies composition of material e.g. MoS 2, MoO 3 changes in frequency of Raman peak stress/strain state e.g. Si 10 cm -1 shift per % strain parallel perpendicular 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 intensity of Raman peak amount of material e.g. thickness of transparent coating

29 Applications of Raman spectroscopy in wood, pulp and paper research Carbohydrates Fibril orientation Crystallinity of cellulose Differecnt cellulose types I and II etc.. Hexenuronic acid content Lignin Guaiasyl/syringyl ratio Extractives

30 Preparation of cross-section section 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

31 Structure of wood cells S3 S2 S1 P

32 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 (a) 1598 (b) 1095 Lignin Cellulose

33 Raman microscopy Interesting location in sample is selected Spectra in regular intervals are recorded every single point in image contain one spectrum

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

35 Lignin/cellulose ratio in pine

36 Lignin/cellulose ratio in spruce

37 Total Internal Reflection Raman Spectroscopy (TIR)

38 Total Internal Reflection Raman Spectroscopy (TIR)

39 Total Internal Reflection Raman Spectroscopy (TIR)

40 Total Internal Reflection Raman Spectroscopy (TIR)

41 Total Internal Reflection Raman Spectroscopy (TIR)

42 Total Internal Reflection Raman Spectroscopy (TIR)

43 Coating layer characterization by TIR- Raman spectroscopy

44 Applications of vibrational 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

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48 Development of TIR-Raman Raman Schematic diagram of TIR-Raman

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

50 New sample holder

51 Possibilities of TIR-Raman Raman Due to total internal reflection surface sensitivity is remarkably improved compared to confocal Raman spectroscopy.

52 Possibilities of TIR-Raman Raman

53 Possibilities of TIR-Raman Raman

54 Possibilities of TIR-Raman Raman

55 Possibilities of TIR-Raman Raman

56 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

57 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.

58 Thank you for your attention!

59 Excursion to Raman lab on (week 19)