PC Laboratory Raman Spectroscopy

Transcription

1 PC Laboratory Raman Spectroscopy Schedule: Week of September 5-9: Student presentations Week of September 19-23:Student experiments Learning goals: (1) Hands-on experience with setting up a spectrometer. (2) Individual measurement and analysis of spectroscopic data. (3) Training of presentation, scientific data analysis, and report writing.

2 Spectroscopic measurements XRay crystal diffraction (Röntgen, Laue, Bragg, WatsonCrick, ) Nuclear Magnetic Resonance Spectroscopy (Bloch, Purcell, Ernst, Wüthrich) Ultrafast Spectroscopy (Eigen, Zewail) Most of what we know about molecules is derived from spectroscopic or spectrometric measurements!

3 Spectroscopy: looking at matter Light can be dispersed, e.g. with a prism. When molecules absorb or emit some light frequencies, the dispersed Spectrum shows corresponding bands. Pictures from Astro-Canada Website

4 Molecular Spectroscopy Overview 4 The energy scale of molecular transitions MHz GHz cm -1 THz ev PHz EHz km m J/mol mm m nm radiowaves microwaves infrared ultraviolet Xray E h frequency wavelength (log scale) electronic / nuclear spins in mag. field rotations vibrations electronic transitions ionization innershell spectroscopy H e - e - Fermi Pauli coupling rule H Fermi coupling Photons with an energy exceeding 710 ev ionize matter and can be used for ionizing spectroscopy. Photons with much larger energy (and wavelength down to be picometer range) are used for diffraction experiments.

5 Infrared Spectroscopy 5 Infrared spectroscopy investigates the vibrational structure of molecules by observing the interaction with electromagnetic radiation (light) in the near infrared wavelength region (typically nm). MHz GHz cm -1 THz ev PHz EHz km m J/mol mm m nm radiowaves microwaves infrared ultraviolet Xray E h frequency wavelength (log scale) electronic / nuclear spins in mag. field rotations vibrations electronic transitions ionization innershell spectroscopy H e - e - Fermi Pauli coupling rule H Fermi coupling = 3 30 m; = THz; E = 6 40 kj/mol; E = ~ mev; ῶ = cm -1 ;

6 Absorption Spectroscopy 6 In absorption spectroscopy, photons of an incoming light beam are absorbed. I in sample I out <I in Molecules and atoms have quantized energetic states. Only photons with an energy that corresponds to the energy difference between two molecular states can be absorbed or emitted. Energy E h E final photon energy Absorption spectrum of the molecule quantized state energies of a molecule E initial

7 Infrared spectroscopy 7 Charged particles and dipoles feel a periodic force in the electric field of light and their vibrational motion can be exited. Only molecular bonds with dipoles can be excited, they are IRactive vibrational modes. Efield time / space + oscillating force The interaction with molecular dipoles is exploited in vibrational spectroscopy to identify the characteristic frequencies of molecular vibrations. Scheme of a simple IR spectrometer: sample detector data acquisition and analysis Light source dispersive element wavenlength selection mask

8 Absorption Spectroscopy 8 Some photon important photon properties: Relation between photon energy E and frequency : (with the Planck constant h = Js) Relation between photon frequency and wavelength : (with the speed of light c = m/s) The inverse wavelength ~ is often used as pseudoenergy unit: ( ~ is called wavenumber and is given in the unit cm -1 ) E = h c = ~ 1/ E /( hc )

9 Raman Spectroscopy 9 In Raman spectroscopy, photons are not absorbed, but scattered. incident light scattered light The photon can gain or loose some energy upon scattering. The energy difference between incoming and outgoing photon must be equal to the energy difference between two molecular states. Raman 0 spectrum Energy excitation photon E initial E final Raman photon photon energy Raman shift 0

10 Raman spectroscopy 10 The electric field of light can induce a dipole moment even if the molecule has no intrinsic dipole moment. Interaction of the light field with the induced dipole moment excites the molecule in Raman spectroscopy. Efield time / space + (1) induce dipole + (2) oscillating force The Raman interaction is weak and a very high light intensity is required to observe Raman effects.

11 Raman Spectroscopy 11 Lasers can be focused to very high intensities and allow to observe Raman signals. glass fiber lens filter(s) Spectrometer 532 nm laser dichroic 532 nm mirror You will set up a Raman experiment and identify an unknown compound by its Raman spectrum. microscope objective sample cuvette

12 Raman Spectroscopy 12 Resources: The lab course guidelines IR correlation table Reference Raman spectra Teaching assistant 이종찬 100 Reference spectrum for Nitrobenzene Relative Signal (%) Wavenumber (cm 1 )

13 Report Please use the ACS template and follow the ACS guidelines. Important: Legible figures (font size, layout, colors) Precise description of the experiment (Can somebody else reproduce your work based on your description?) Present your own results (scientific honesty) Cite the literature Keep it short