Download Spectroscopy - Universität Wien

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.