Raman spectroscoopy. Matti Hotokka Department of Physical Chemistry Åbo Akademi University

Transcription

1 Raman spectroscoopy Matti Hotokka Department of Physical Chemistry Åbo Akademi University

2 The phenomenon Electronically excited state Virtual level, light is not absorbed Resonance Raman Vibrational levels in electronic ground state Rayleigh scattering Rayleigh- Stokes scattering Rayleigh-anti- Stokes scattering Electronic ground state, vibrational ground state

3 Scattering intensity Selection rule: discussed earlier. Intensity grows as the fourth power of the frequency. The Bolzmann term is the dominant one. If the intensity of the incident light is 1 then the intensity of the Rayleigh scattered light is 10-4 and the intensity of the Raman scattered light is 10-7.

4 Raman spectrum Stokes Rayleigh Anti- Stokes

5 Spectrometer Incident laser beam Source Sample Detector The scattered signal is observed at 90 or 180 angle to the incident beam.

6 Observation geometry The numerical values refer to nitrogen Observation at 90 Raman to Rayleigh ratio is higher Simpler optical arrangement Observation at 180 More scattered light totally Only one side of the sample is needed

7 Spectrometer technologies FT-Raman Popular like in FTIR but not that dominant Typically an accessory to FTIR Dispersive Quite common in Raman spectroscopy Has some advantages as compared to FT- Raman

8 Demands on spectrometer Intensity of the incident light Both the incident light and the Rayleigh signal must be removed Old instruments: monochromator as filter Modern instruments: notch filter The filter must have a high filtering efficiency and narrow band in order to reach low vibration frequencies

9 Filtering Intensity of the Rayleigh band must be reduced by a factor of ten thousand Wavelength ( µ m) Notch filter efficiency

10 Old instruments Multi-stage monochromators McPherson Mc Triple LE The two first stages 20 cm Third stage 67 tai 133,5 cm f/4.8 or f/9.4

11 FT-Raman Mirror scanner Filter is the most central component Input beam Filter D 2 Interferometer Raman sample 1064 nm S 1 Sample S 2 YAG laser D 1 Exit beam Raman detector

12 Modern dispersive CCD detector Transmission grating Filter Raman signal Raman and Rayleigh signal A standard monochromator can be used when combined with a notch filter.

13 Modern dispersive spectrometer Higher sensitivity A 100 times higher sensitivity than in FT-Raman Lower noise level Lower detection limit Change of incident wavelength Both dispersive and FT-Raman require change of the notch filter Helps avoiding fluorescence Scattering cross section is higher at high energy

14 Interpretation of Raman spectra Same vibrations, different intensities

15 Raman and IR spectra CCl 4 Γ = 2T 2 Γ = A 1 +E+2T

16 Raman and IR spectra Benzene

17 Interpretation of Raman spectra Raman is simpler than IR Often only fundamentals are visible Same principles as in IR Use correlation tables for Raman! Often bands below 400 cm -1 can be observed Inorganic compounds Lattice vibrations

18 Interpretation of Raman spectra Lattice vibrations Ammonium perrheate, NH 4 ReO 4 δ (ReO 4 ) ν (ReO 4 ) NH 4 ReO 4 Lattice vibrations ν (NH 4 ) Wavenumber (cm -1 ) Wavenumber (cm -1 )

19 Interpretation of Raman spectra Raman spectrum as a measure of crystallinity The crystal lattice lines appear when crystallinity increases

20 Interpretation of Raman spectra Depolarization Sample cell Optical axis Analyser I I Spectrometer Depolarization ratio E Laser in Symmetric component: a Asymmetric component: γ Totally symmetric vibrations: ρ = 0; asymmetric vibrations: ρ = 3/4

21 Interpretation of Raman spectra Parallel and perpendicular polarization; CCl 4 I I

22 Interpretation of Raman spectra Three central advantages Water is poor scatterer. Samples containing water (e.g., biological samples) can be studied Selection rule is based on polarizability. Symmetrical molecules (such as N 2 ) can be observed Low frequencies can be observed

23 Interpretation of Raman spectra Raman spectrum of air

24 Interpretation of Raman spectra Isotope substitution CH 3 CD 3 and CD 3 CD 3 3N-6=18 Point groups CD 3 CD 3 : D 3d CH 3 CD 3 : C 3v

25 Spetroscopic techniques Problems: Fluorescence Change wavelength Illuminate for a long time (bleaching) Remove chemically the trace compounds

26 Spetroscopic techniques Problems: Pyrolysis Change the laser so that the radiation is not absorbed Use lower laser power, shorter measuring time Defocus

27 Spetroscopic techniques Resonance Raman: para-ethyl phenol

28 Spetroscopic techniques SERS: BPE on AgFON Wavenumber (cm -1 )

29 Spetroscopic techniques CARS Dye laser Fixed laser Sample b> Scattered beam a> ν 2 ν CARS ν 1 ν Stokes k Stokes k CARS 1> k 1 k 2 0>