Raman Spectroscopy: Introductory Tutorial Daniel T. Schwartz Department of Chemical Engineering Box 351750 University of Washington Seattle, WA 98195-1750 [email protected]Goal of the tutorial is to answer the questions, “What is Raman spectroscopy and can the new Raman microscope at NTUF help with my sample?”
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“What is Raman spectroscopy and can the new Raman microscope at NTUF help with my sample?”
Raman Spectroscopy: Some Sources
General Principles and Instrumentation: Principles of Instrumental Analysis, by Douglas A. Skoog, F. James Holler, Timothy A. Nieman
Inorganic:Infrared and Raman Spectra of Inorganic and Coordination Compounds : Theory and Applications in Inorganic Chemistry (Volume A) by Kazuo Nakamoto
Infrared and Raman Spectra of Inorganic and Coordination Compounds : Applications in Coordination, Organometallic, and Bioinorganic Chemistry (Volume B) by Kazuo Nakamoto
Organic:The Handbook of Infrared and Raman Characteristic Frequencies of Organic Moleculesby Daimay Lin-Vien, et al
Raman Spectroscopy: Overview
• A vibrational spectroscopy- IR and Raman are the most common vibrational spectroscopies for assessing molecular motion and fingerprinting species
- Based on inelastic scattering of a monochromatic excitation source
- Routine energy range: 200 - 4000 cm–1
• Complementary selection rules to IR spectroscopy- Selection rules dictate which molecular vibrations are probed
- Some vibrational modes are both IR and Raman active
• Great for many real-world samples- Minimal sample preparation (gas, liquid, solid)
- Compatible with wet samples and normal ambient
- Achilles Heal is sample fluorescence
Raman Spectroscopy: General
• IR and Raman are both useful for Fingerprinting
• Symmetry dictates which are active in Raman and IR
• Group assignments identify characteristic vibrational energy
Raman Spectroscopy: General
Raman Spectroscopy: Classical Treatment
• Number of peaks related to degrees of freedomDoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms
• Energy related to harmonic oscillator
• Selection rules related to symmetry Rule of thumb: symmetric=Raman active, asymmetric=IR active
Raman: 1335 cm–1
IR: 2349 cm–1
IR: 667 cm–1
CO2
σ or ∆σ =c
2πk(m1 + m2)
m1m2
Raman + IR: 3657 cm–1
Raman + IR: 3756 cm–1
RamanRamanRaman + IR: 1594 cm–1
H2O
ElectronicGround State
1st ElectronicExcited State
Exci
tatio
n En
ergy
, σ(c
m–1
)
Vib.states
4,000
25,000
0 IR
2nd ElectronicExcited State
σ
σ σemit
fluor
esce
nce
Impu
rity
σemit
fluor
esce
nce
UV/VisFluorescence
σemitσ
ElasticScattering(Raleigh)
Main Optical Transitions: Absorption, Scattering, and Fluorescence
ElectronicGround State
1st ElectronicExcited State
Exci
tatio
n En
ergy
, σ(c
m–1
)
Vib.states
4,000
25,000
0 IRσ
σ σemit
2nd ElectronicExcited State
Raman∆σ=σemit–σ
σ ±∆σ ∆σ
Resonance Raman∆σ=σemit–σ
Raman Spectroscopy: Absorption, Scattering, and Fluorescence
Stokes Anti-Stokes
Excitation Energy, σ (cm–1)
Raman Spectroscopy: At NTUF, you pick the Laser ExcitationIn
tens
ity
11,000 13,000 15,000 17,000 19,000 21,000
Near IR785 nm
Visible514 nm
–∆σ +∆σ –∆σ +∆σStokes Anti-Stokes
Stokes Anti-Stokes
ElectronicGround State
1st ElectronicExcited State
Exci
tatio
n En
ergy
, σ(c
m–1
)
Vib.states
4,000
25,000
0
fluor
esce
nce
IRσ
σ σemit
2nd ElectronicExcited State
Raman∆σ=σemit-σ
σ ∆σflu
ores
cenc
eIm
purit
y
Fluorescence = Trouble
Raman Spectroscopy: Absorption, Scattering, and Fluorescence
Stokes Anti-Stokes
1000 2000 3000
Raman Shift (cm-1)
Ram
an In
tens
ity
Without Bleaching
After 2 hours Bleaching
Poly (diallyl phthalate)
λex = 514.5 nm
Raman Spectroscopy: Coping w/ Fluorescence
1. Use NTUF 785 nm laser line (excites many fewer fluorophores)
2. Photobleach with long exposure laser irradiation.
Raman Spectroscopy: Summary
1. Raman is a vibrational spectroscopy akin to IR- Good for fingerprinting, probing molecular symmetry
2. Scattering-based, not transmission/reflection- Means no need for fancy sample preparation…gas, liquid, or solid- Virtually always use anti-Stokes lines due to stronger signal
3. You need to pick excitation energy (laser line)- 785 nm: Fluorescence less probable; Lower Raman signal- 514 nm: Fluorescence more probable; Resonance more likely; Higher signal
4. Other things not talked about- SERS: Surface Enhanced Raman Spectroscopy- Quantum origins of selection rules and scattering cross-section
Raman Spectroscopy: Dan’s trip to NTUF
Fuel Cell Materials --- picked because I had little Raman experience with these materials
From http://www.fueleconomy.gov/feg/fc_pics/fuel_cell_still.gif
Raman Spectroscopy: Dan’s trip to NTUF
Flow Field Plate - Graphite
From early literature on graphitic materialsTuinstra and Koenig, J. Chem Phys. 53, 1126 (1970).
l (Å) = 44Ig
Id
⎛
⎝ ⎜
⎞
⎠ ⎟
= 175Å
Ig
Id
⎛
⎝ ⎜
⎞
⎠ ⎟ = 3.98
Nanocrystalline graphite has graphitic (g) and disorder (d) peaks. The characteristic dimension of graphitic domains is given by:
Raman Spectroscopy: Dan’s trip to NTUF
Gas Diffusion Layers (graphite paper)
l (Å) = 76 Å
l (Å) = 161Å
Raman Spectroscopy: Dan’s trip to NTUF
Gas Diffusion Layers (Woven Fibers)
l (Å) = 55 Å
Raman Spectroscopy: Dan’s trip to NTUF
Nafion
Fluorescence problems!Maybe try photobleachingas next option.