PC Laboratory – Raman Spectroscopy
Schedule:Week of September 5-9: Student presentationsWeek 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.
Spectroscopic measurements
X‐Ray crystal diffraction (Röntgen, Laue, Bragg, Watson‐Crick, …)
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!
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
4Molecular Spectroscopy Overview
electronic / nuclear spinsin mag. field
rotationsvibrations
electronic transitions
inner‐shell spectroscopy
ionization
The energy scale of molecular transitions
Photons with an energy exceeding 7‐10 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.
‐ ‐
‐
‐‐
‐ ‐
‐‐
Fermicoupling
Paulirule
Fermicoupling
H He- e-
wavelength
MHz GHz THz PHz EHz
mkm mm m nmradiowaves infrared ultravioletmicrowaves X‐ray
frequencycm-1 eV
J/mol
Eh(log scale)
5Infrared Spectroscopy
electronic / nuclear spinsin mag. field
rotationsvibrations
electronic transitions
inner‐shell spectroscopy
ionization
‐ ‐
‐
‐‐
‐ ‐
‐‐
Fermicoupling
Paulirule
Fermicoupling
H He- e-
wavelength
MHz GHz THz PHz EHz
mkm mm m nmradiowaves infrared ultravioletmicrowaves X‐ray
frequencycm-1 eV
J/mol
Eh(log scale)
= 3–30 m; = 14–100 THz; E = 6–40 kJ/mol; E =60-400 meV; ῶ = 500–3500 cm-1; ~
Infrared spectroscopy investigates the vibrational structure of molecules by observing the interaction with electromagnetic radiation (light) in the near infrared wavelength region (typically 180‐800 nm).
6
E
h
Einitial
Efinal
phot
on e
nerg
y
quantized state energies of a molecule
Energy
Absorption spectrum of the molecule
Absorption Spectroscopy
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.
sample
In absorption spectroscopy, photons of an incoming light beam are absorbed.
Iout < IinIin
7Infrared spectroscopy
Light source
sample
dispersive element
wavenlengthselection mask
detectordata acquisition and analysis
‐
+
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 IR‐active vibrational modes.
The interaction with molecular dipoles is exploited in vibrational spectroscopy to identify the characteristic frequencies of molecular vibrations.Scheme of a simple IR spectrometer:
E‐fie
ld
time / spaceoscillating force
8
Some photon important photon properties:Relation between photon energy E and frequency : E = h∙(with the Planck constant h = 6.626∙10-34 Js)Relation between photon frequency and wavelength : c = ∙(with the speed of light c = 2.998 ∙108 m/s)The inverse wavelength is often used as pseudo‐energy unit:( is called wavenumber and is given in the unit cm-1)
)/(/1~ hcE ~~
Absorption Spectroscopy
9
In Raman spectroscopy, photons are not absorbed, but scattered.
Raman Spectroscopy
Ener
gy
Raman spectrum
EinitialEfinal
photon energy
exci
tatio
n ph
oton
Ram
an photon
0
Ram
an s
hift
0
incident light
scatt
ered l
ight 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.
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.
10Raman spectroscopy
‐
+E‐fie
ld
time / space(2) oscillating force
‐
+
(1) induce dipole
The Raman interaction is weak and a very high light intensity is required to observe Raman effects.
11Raman Spectroscopy
532 nm laserdichroic532 nm mirror
microscope objective
sample cuvette
filter(s)
Spectrometer
lens
glass fiberLasers can be focused to very high intensities and allow to observe Raman signals.
You will set up a Raman experiment and identify an unknown compound by its Raman spectrum.
12Raman Spectroscopy
Resources:
• The lab course guidelines• IR correlation table• Reference Raman spectra• Teaching assistant 이종찬
Relativ
e Sign
al (%
)
100
0
Wavenumber (cm‐1)4000 3000 2000 1500 500 01000
Reference spectrum for Nitrobenzene
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
