Download Raman Spectroscopy: Introductory Tutorial

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?”
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 Molecules
by 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
Raman Spectroscopy:
General
• Group assignments identify characteristic vibrational energy
Raman Spectroscopy:
Classical Treatment
• Number of peaks related to degrees of freedom
DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms
• Energy related to harmonic oscillator
σ or ∆σ =
c
2π
k(m1 + m2 )
m1m2
• Selection rules related to symmetry
Rule of thumb: symmetric=Raman active, asymmetric=IR active
CO2
Raman: 1335 cm–1
IR: 2349 cm–1
IR: 667 cm–1
H2 O
Raman + IR: 3657 cm–1
Raman + IR: 3756 cm–1
Raman
Raman + IR: 1594 cm–1
2nd Electronic
Excited State
Impurity
25,000
1st Electronic
Excited State
σ
4,000
σemit
σemit
σemit
Vib.
states
σ
0
σ
fluorescence
Elastic
Scattering
(Raleigh)
fluorescence
Excitation Energy, σ (cm–1)
Main Optical Transitions: Absorption, Scattering, and Fluorescence
IR
Electronic
Ground State
UV/Vis
Fluorescence
Excitation Energy, σ (cm–1)
Raman Spectroscopy: Absorption, Scattering, and Fluorescence
2nd Electronic
Excited State
25,000
1st Electronic
Excited State
Stokes
σ
Anti-Stokes
σemit
4,000
σ
0
±∆σ
IR
σ
Raman
∆σ=σemit–σ
Vib.
states
∆σ
Electronic
Ground State
Resonance Raman
∆σ=σemit–σ
Raman Spectroscopy: At NTUF, you pick the Laser Excitation
Visible
514 nm
Intensity
Near IR
785 nm
Stokes
–∆σ
11,000
Stokes
Anti-Stokes
+∆σ
13,000
Anti-Stokes
–∆σ
15,000
17,000
19,000
Excitation Energy, σ (cm–1)
+∆σ
21,000
Impurity
25,000
σ
Anti-Stokes
σemit
4,000
σ
∆σ
IR
σ
1st Electronic
Excited State
Raman
∆σ=σemit-σ
fluorescence
Stokes
0
2nd Electronic
Excited State
fluorescence
Excitation Energy, σ (cm–1)
Raman Spectroscopy: Absorption, Scattering, and Fluorescence
Vib.
states
Fluorescence
= Trouble
Electronic
Ground State
Raman Spectroscopy:
Coping w/ Fluorescence
1. Use NTUF 785 nm laser line (excites many fewer fluorophores)
2. Photobleach with long exposure laser irradiation.
Raman Intensity
Poly (diallyl phthalate)
λex = 514.5 nm
Without Bleaching
After 2 hours Bleaching
1000
2000
3000
Raman Shift (cm-1)
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
⎛ Ig ⎞
⎜ ⎟ = 3.98
⎝ Id ⎠
Nanocrystalline graphite has graphitic (g)
and disorder (d) peaks. The characteristic
dimension of graphitic domains is given by:
⎛ Ig ⎞
l (Å) = 44 ⎜ ⎟
⎝ Id ⎠
= 175Å
From early literature on graphitic materials
Tuinstra and Koenig, J. Chem Phys. 53, 1126 (1970).
Raman Spectroscopy:
Gas Diffusion Layers
(graphite paper)
l (Å) = 161Å
l (Å) = 76 Å
Dan’s trip to NTUF
Raman Spectroscopy:
Gas Diffusion Layers
(Woven Fibers)
l (Å) = 55 Å
Dan’s trip to NTUF
Raman Spectroscopy:
Dan’s trip to NTUF
Nafion
Fluorescence problems!
Maybe try photobleaching
as next option.