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280061 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 4277-53220 / e-mail: [email protected] Ziel des Vorlesungsteils Spektroskopie: Die Studierenden besitzen generelle Kenntnis von den in den Erdwissenschaften eingesetzten Spektroskopie-Methoden. Sie kennen die physikalischen und chemischen Grundlagen der im Bereich Erdwissenschaften der Universität Wien betriebenen Methoden der Licht-Spektroskopie: Infrarot-Absorptions-spektroskopie, Ramanspektroskopie, optische Absorptionsspektroskopie und Lumineszenzspektroskopie. Im Rahmen von Gerätedemonstrationen werden Grundkenntnisse zur Durchführung von Analysen vermittelt. Anwendungs-beispiele (einschließlich spektroskopischer Bildmethoden) sind bekannt. Empfohlene Literatur: Beran & Libowitzky (Eds.), Spectroscopic Methods in Mineralogy. EMU Notes in Mineralogy, vol.6, European Mineralogical Union. Kapitel 2 (Lumineszenz) und 7 (Raman) stehen zum Download zur Verfügung (http://www.univie.ac.at/Mineralogie/studium_n.htm), dort unter 280077. Weitere Lehrbücher: Putnis, Introduction to mineral sciences Kuzmany, Solid-State Spectroscopy Spectroscopy: Generalities 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 = 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. Spectroscopy: Generalities Spectrum: 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) Spectroscopy: Interactions and techniques Electromagnetic radiation: Overview Energy Radiation Interaction and technique 10-10…10-6 104…100 radio waves Nuclear spin resonance: NMR spectroscopy 10-6…10-3 100…10-3 microwaves Electron spin resonance: EPR spectroscopy 10-3…100 10-3…10-6 infrared light Molecular vibrations: FTIR 100…102 visible and UV light Electronic transitions: Optical absorption and (eV) Wavelength (m) 10-6…10-7 emission (luminescence) spectroscopy 102…105 10-8…10-11 X-rays Core-electron transitions: X-ray spectroscopy, Photoelectron spectroscopy (XPS) 105…106 10-11…10-12 Gamma rays Nuclear transitions: Mössbauer spectroscopy Spectroscopy: Interactions and techniques NMR: Nuclear Magnetic Resonance (dt. 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 1H; 6Li; 13C; 15N; 17O; 29Si; 43Ca) 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 H0) → 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) Spectroscopy: Interactions and techniques NMR: Nuclear Magnetic Resonance (dt. Kernspinresonanz) → MAS-NMR = Magic-angle spinning → Spinning at a “magic” angle of 54.74° 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) Spectroscopy: Interactions and techniques EPR: Electron Paramagnetic Resonance (dt. 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 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 Spectroscopy: Interactions and techniques X-ray spectroscopy: here: X-ray absorption spectroscopy (X-ray emission spectroscopy: XRF, EPMA) XAS = X-ray absorption near-edge structure EXAFS = Extended X-ray absorption fine structure → 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) Spectroscopy: Interactions and techniques Mössbauer spectroscopy (dt. 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 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 Wavelength Wavenumber Symbol λ ν∼ UV-visible boundary 400 nm Visible-NIR boundary 750 nm 25000 cm-1 13333 cm-1 Frequency ν 7.5 × 1014 s-1 4 × 1014 s-1 Quantum energy E 3.1 eV 1.65 eV 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 = 106 s-1 ; 1 GHz = 109 s-1. Frequency of waves: Number of like phase (peaks, troughs) wave-fronts passing a given point in a unit of time. Energy of light Electron volt (eV): Energy acquired by a charged particle carrying the unit electronic charge when it falls through a potential difference of one volt. One electron volt = 1.60207 ± 0.00007 × 10-19 J (joule) Multiples of this unit are also in common use: the kilo-, million-, and billion electron volt: 1 keV = 1000 eV; 1 MeV = 106 eV; and 1 GeV = 109 eV. Note: An eV is associated through the Planck constant with a photon of wavelength λ = 1.2398 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 = 4.135667 × 10-15 eV s Expressed by E = h × ν, where E is the energy of the quantum and ν is its frequency. Its numerical value is 6.626176 (36) × 10-34 J sec. Vibrational spectroscopy: energetic consideration IR (1) Infrared absorption E ∆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. 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. Vibrational spectroscopy: energetic consideration IR (3) 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. 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. Vibrational spectroscopy: energetic consideration Raman (3) Vibrational spectroscopy: Raman spectrum (1) Principal components of a Raman spectrum (example: Raman spectrum of silicon). 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. Vibrational spectroscopy: Raman spectrum (3) Principal components of a Raman spectrum (crocoite), shown at different energy scales.