Instrumentelle Analytik in den Geowissenschaften (PI)

Transcription

1 VU MA-ERD-2 Instrumentelle Analytik in den Geowissenschaften (PI) Handoutmaterial zum Vorlesungsteil Spektroskopie Bei Fragen bitte zu kontaktieren: Prof. Lutz Nasdala, Institut für Mineralogie und Kristallographie der Universität Wien UZA2 Raum 2A251 / Telefon / lutz.nasdala@univie.ac.at

2 Aims of the spectroscopy part: The students know the interactions of electromagnetic radiation with matter and are familiar with spectroscopic methods used in Earth Sciences. They know the physical and chemical basics of the light-spectroscopy techniques that are used in Earth sciences research at Universität Wien: Fourier-transform infrared absorption, Raman, optical absorption and photoluminescence (emission and excitation) spectroscopy. By means of laboratory demonstrations, students get basic information on how to perform analyses, reduce data and interpret results. Examples for the application of analyses (including spectroscopic imaging techniques) are known. Suggested literature: Beran & Libowitzky (Eds.), Spectroscopic Methods in Mineralogy. EMU Notes in Mineralogy, vol.6 (2004), European Mineralogical Union. Chapters 2 (luminescence) und 7 (Raman) are available for download ( there under ). Further textbooks: Putnis, Introduction to Mineral Sciences. Cambridge University Press (1992) Kuzmany, Solid-State Spectroscopy. An Introduction. Springer (2nd edition 2009)

3 Solid-state spectroscopy: Analytical techniques used to obtain spectra Probe the structure of minerals (and other matter), complemetary to diffraction techniques Probe the short-range order (in contrast to diffraction techniques that probe the long-range order / periodicity of the lattice) Probe electronic, nuclear, spin-state, vibrational or other transitions in minerals and other matter. Provide information on the energy and intensity of interactions between electromagnetic radiation and matter. Terminology: Spectroscopy: Generalities - Spectroscopy = information obtained with the eye - Spetrography = information obtained graphically - Spectrometry = information obtained in digital form Note: Even though spectrometers are used, it has become commonplace to call it spectroscopy.

4 Spectrum: Spectroscopy: Generalities Plot of signal intensity (or intensity of the interaction) versus energy (of the electromagnetic radiation) Problem I: Types of interactions of electromagnetic radiation with various atomic / electronic processes Various types of spectroscopic techniques Problem II: Quantification of the energy of the signal detected Energy units used to quantify the energy of electromagnetic radiation: Photon energy (ev) Frequency (Hz) Wavelength (nm) Wavenumber (absolute cm 1 ) Wavenumber (relative cm 1 ) Other units used: ppm (NMR: chemical shift) mms/s (Mössbauer: velocity)

5 Spectroscopy: Interactions and techniques Electromagnetic radiation: Overview Energy Wavelength Radiation Interaction and technique (ev) (m) radio waves Nuclear spin resonance: NMR spectroscopy microwaves Electron spin resonance: EPR spectroscopy infrared light Molecular vibrations: FTIR visible and UV light Electronic transitions: Optical absorption and emission (luminescence) spectroscopy X-rays Core-electron transitions: X-ray spectroscopy, Photoelectron spectroscopy (XPS) Gamma rays Nuclear transitions: Mössbauer spectroscopy

6 Spectroscopy: Interactions and techniques NMR: Nuclear Magnetic Resonance (in German: Kernspinresonanz) Probes energy differences between allowed spin states of atomic nuclei Based on the resonant interaction among the magnetic momenta of nuclei All nuclei with a nuclear spin of I 0 have a magnetic momentum (examples 1 H; 6 Li; 13 C; 15 N; 17 O; 29 Si; 43 Ca) No external magnetic field: all spin states have uniform energy (degenerate) With strong external field: splitting ( Zeeman effect ) Energy differences E are in the radio frequency range Transitions among spin states can be induced by applying a radio-frequency field (to the sample in a strong magnetic field H 0 ) Theory: All nuclei of the same isotope have the same resonance frequency Reality: The exact resonance frequency varies with the local environment of the nucleus Measured as chemical shift relative to a standard (in ppm)

7 Spectroscopy: Interactions and techniques NMR: Nuclear Magnetic Resonance (in German: Kernspinresonanz) MAS-NMR = Magic-angle spinning Spinning at a magic angle of reduces the dipolar interaction between nuclei Improves signal quality in the NMR analysis of solids (anisotropic interaction of nuclei in solids lead to line broadening, with is reduced by MAS)

8 Spectroscopy: Interactions and techniques EPR: Electron Paramagnetic Resonance (in German: Elektronenspinresonanz, ESR) Probes energy differences between allowed spin states of electrons (whose energies are in the microwave range) Principle similar to that of NMR: No external magnetic field: all spin states are equal (S = 1 / 2 ) With strong external field: splitting (S = + 1 / 2 and S = - 1 / 2 ) (here: EPR spectra are obtained by keeping the microwave frequency constant and varying the magnetic field until resonance is reached) Effect of the (spinning) nucleus on unpaired electrons: hyperfine coupling tensor, leads to hyperfine splitting (magnetic nucleus with spin I will split electron resonance in 2I+1 lines whose inter-distance corresponds to the hyperfine coupling tensor) Effect of interaction of more than one unpaired electrons (ESR fine structure) In a spherical environment all of them have the same energy (degenerate) In a non-spherical, distorted environment (crystal field), spin-energy levels are split

9 Spectroscopy: Interactions and techniques Electronic spectroscopy: Optical absorption (also electronic absorption, also UV-VIS-NIR spectroscopy), Emission spectroscopy (luminescence techniques) Probe energy differences between electronic levels (energies correspond to light in the near infrared to ultraviolet range) band-band transitions or transitions among introduced levels (activators) Isolators: Band gap > 4-5 ev (higher than visible light), therefore colourless and non-luminescent if pure Isolators: Introduced electronic levels within the forbidden band gap (activators / defects / colour centres) may lead to colouration and/or visible emission

10 Spectroscopy: Interactions and techniques X-ray spectroscopy: here: X-ray absorption spectroscopy (XAS) (X-ray emission spectroscopy: XRF, EPMA) XANES = X-ray absorption near-edge structure spectroscopy EXAFS = Extended X-ray absorption fine structure spectroscopy both are affected by the local environment of the atom X-ray Photoelectron spectroscopy (XPS): Electrons are ejected as caused by incident external irradiation Probes effects of the nearest-neighbouring environment on electronic levels (energy differences in the X-ray range)

11 Spectroscopy: Interactions and techniques Mössbauer spectroscopy (in German: Mößbauer-Spektroskopie): Combination of Mössbauer effect (= recoil-free emission or absorption of a gamma-quantum by a nucleus) and Doppler effect (= temporary compression or extension of a signal due to temporary change of distance) Probes energy differences between nuclear states (gamma-ray energies) Modulation of gamma ray by movement of sample (re-absorption of gamma energy is only possible if two cores approach each other with two times the recoil velocity) Isomer shift (or chemical shift) δ, provides information on the energy difference between ground and excited state Quadrupole splitting, provides information on the energy difference between ground and excited state (ion in non-cubic environment) Magnetic splitting, provides information on the additional splitting due to a magnetic field acting on the nucleus

12 Energy of light (1) Corpuscular consideration (particle model): Photons (light quanta) = smallest pieces of light energy (E = h ν) (2) Electromagnetic wave consideration (wave model): - characterised by the direction of propagation - perpendicular: polarisation plane of the electric field vector (E) - perpendicular to both: polarisation plane of the magnetic field vector (H) Energy of light: Parameter Symbol UV-visible boundary Visible-NIR boundary Wavelength Wavenumber Frequency Quantum energy λ ν ν E 400 nm cm s ev 750 nm cm s ev

13 Energy of light Wavelength (λ): Distance between successive wavefronts of like phase (i.e., from peak to peak or from trough to trough). _ Wavenumber (ν or ν): The reciprocal of the wavelength. Number of waves per unit distance in the direction of propagation. In spectroscopy, wavenumbers are usually expressed in reciprocal centimetres (cm -1 ; per centimetre) Frequency (ν): Rate of oscillation. Units: 1 cycle per second = 1 Hz (Hertz) 1 MHz = 10 6 s -1 ; 1 GHz = 10 9 s -1. Frequency of waves: Number of like phase (peaks, troughs) wave-fronts passing a given point in a unit of time.

14 Electron volt (ev): Energy of light Energy acquired by a charged particle carrying the unit electronic charge when it falls through a potential difference of one volt. One electron volt = ± J (joule) Multiples of this unit are also in common use: the kilo-, million-, and billion electron volt: 1 kev = 1000 ev; 1 MeV = 10 6 ev; and 1 GeV = 10 9 ev. Note: An ev is associated through the Planck constant with a photon of wavelength λ = µm. Planck's constant (h): A universal constant of nature which relates the energy of a quantum of radiation to the frequency of the oscillator which emitted it. It has the dimensions of action (energy time). h = ev s Expressed by E = h ν, where E is the energy of the quantum and ν is its frequency. Its numerical value is (36) J sec.

15 Vibrational spectroscopy: energetic consideration IR (1) E Infrared absorption E = h c ν Infrared absorption: Light with a quantum energy corresponding to the energy difference between two vibrational states is absorbed to excite a vibration.

16 Vibrational spectroscopy: energetic consideration IR (2) Infrared absorption: The energy of an incoming photon (light quantum) corresponds to the energy difference of two energetic states of the sample. The photon can be absorbed to excite a vibration.

17 Vibrational spectroscopy: energetic consideration IR (3)

18 Vibrational spectroscopy: energetic consideration Raman (1) Raman scattering (Stokes type): A fraction of the photon energy (light quantum) is used to excite a phonon (vibrational quantum). Through scattering, the light photon loses some energy and is therefore redshifted in the electromagnetic spectrum.

19 Vibrational spectroscopy: energetic consideration Raman (2) Raman scattering (Stokes type): A fraction of the photon energy (light quantum) is used to excite a phonon (vibrational quantum). Through scattering, the light photon loses some energy and is therefore red-shifted in the electromagnetic spectrum.

20 Vibrational spectroscopy: energetic consideration Raman (3)

21 Vibrational spectroscopy: Raman spectrum (1) Principal components of a Raman spectrum (example: Raman spectrum of silicon).

22 Vibrational spectroscopy: Raman spectrum (2) Note: Bands I a Raman spectrum are NOT necessarily Raman bands. Example: Spectrum obtained from monazite with red laser excitation.

23 Vibrational spectroscopy: Raman spectrum (3) Principal components of a Raman spectrum (crocoite), shown at different energy scales.