Infrared and Raman Spectroscopy: From Concept to Experiment

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• Infrared and Raman Spectroscopy From Concept to
  Experiment
• Outline for Four Lectures
• Introduction to Vibrational Spectroscopy
• Different wavelength regions, instrumentation
  overview, absorption vs. scattering, group
  frequencies (SEPT 15)
• Infrared Spectroscopy FT vs. Dispersive
• Evolution from gratings to interferometry, Are
  we headed back to the future?, sensitivity to
  orientation, order and molecular configuration
• (SEPT 24)
• Raman spectroscopy FT vs. Dispersive
• Advantages of a non-invasive method,
  fluorescence good, bad or ugly?, changes in
  polarizability vs. changes in dipole moment
• (SEPT 29)
• Applications in Analytical Chemistry
• thin films and monolayers, surfaces, polymers,
  biopolymers, crystalline materials, prognosis for
  the future (OCT 8)

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PERSONAL HISTORY JOHN RABOLT

• EDUCATION
• B.S., PHYSICS, STATE UNIVERSITY OF NEW YORK (NEW
  YORK)
• Ph.D., PHYSICS, SOUTHERN ILLINOIS UNIVERSITY
  (ILLINOIS)---Far-IR Spectroscopy of polymers and
  surfaces
• EXPERIENCE
• 2 YRS. POST DOC, BIOPHYSICS RESEARCH DIVISION,
• UNIVERSITY OF MICHIGAN (MICHIGAN)--- Mid IR
  Spectroscopy of Biopolymes
• 2 YRS. POST DOC, POLYMERS DIVISION
• NATIONAL INSTITUTE OF STANDARDS AND
  TECHNOLOGY, U.S. DEPARTMENT OF COMMERCE
  (MARYLAND)---Raman Spectroscopy of Polymers
• 20 YRS. RESEARCH STAFF, IBM RESEARCH DIVISION
• SAN JOSÉ (CALIFORNIA)---Thin Organic Films and
  Monolayers
• 13 YRS. PROFESSOR AND CHAIR
• DEPARTMENT OF MATERIALS SICENCE AND
  ENGINEERING, UNIVERSITY OF DELAWARE
  (DELAWARE)---Real Time Characterization of
  Fiber Formation and Time Resolved Studies of
  Molecular Assembly

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• Why Vibrational Spectroscopy???
• Characterization of chemical, conformational and
  crystal structure.
• Correlation of structure/processing/properties
• (e.g., polymorphism)
• Process monitoring
• Quality control (batch-to-batch variations)

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Polymorphism can effect- physical
properties- solubility- stability-
bioavailability
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Raman Spectroscopy
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Electric and magnetic fields associated with a
propagating light wave
Oscillating dipoles can couple to the E field (or
the H field) but NOT along the propagation
direction.
A B-
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CO
CO
Infrared absorption If the vibrational
frequency is resonant with the light wave
frequency and if the orientation is correct the
light wave will transfer energy to the
vibration.
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• Evolution of Infrared Instrumentation

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1950s
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Introduction to Infrared Spectrometers
Evolution of Infrared Spectrometers
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How about Raman scattering?

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A A

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Electric fields polarize the electron cloud
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Classical Viewpoint The incident laser radiation
has an electric field intensity E E0
cos(wlt) This field can polarize the electron
cloud and induce an oscillating dipole moment,
P P a E where a is the polarizability of the
electron cloud. By the Born-Oppenheimer
approximation, electrons follow nuclei, so we can
expand the polarizability in a Taylor series
around the normal mode of vibration a a0
(da/dQ)Q0 cos(wmt) ...
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P (a0 a1 cos(wmt)) (E0 cos(wlt))
(a0E0 cos(wlt) (1/2)a1E0cos(wl wm)t
(1/2)a1E0cos(wl - wm)t Note that a1
(da/dQ)Q0 So for a transition to be Raman
allowed there must be a non-zero change in
polarizability at Q0
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Stokes
Anti-Stokes
w1
w1
w3
w3
w2
w2
n0 - w3
n0 - w1
n0 w3
n0 w1
n0
n0 w2
n0 - w2
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What does the classical description tell us? 1)
Both Rayleigh and Raman scattering are linear
in the field intensity. 2) The vibration must
cause a change in the polarizability to be
Raman active, da/dQ ? 0 3) Anti-Strokes
scattering is implied, but not properly
explained. 4) The scattered light will have at
least the same frequency bandwidth as the
perturbing E field. It may be broader due to
sample interactions.
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CHCl3
Intensity reduced by 108
19436
Lower frequency
Higher frequency
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A quantum picture explaines a little more 1)
relative intensities of Stokes and
anti- Stokes ni/no e -(Ei-E0)/kT Boltzman
distribution 2) Intensity proportional to n4
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Incident and scattered power in terms of
watts IR I0 s0 (no - nj)4 D dz where D
number density dz depth of sampling scatter
ing volume s0 cross-section in terms of
photons IPn PR P0 s n0 (n0 - nj)3 D
dz
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What determines the utility of a Raman scattering
spectrum for the identification of chemical
functionality or a group frequency? (
da/dQ)0 2
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Relevant vibrational modes for IR and Raman
spectroscopy but intensity will be governed by
changes in dipole moment vs. changes in
polarizability for a given normal mode 1)
backbone carbon-carbon vibrations 2)
unsaturated bonds CC, CN, etc. 3) breathing
modes aromatic rings 4) halogenated
substitutions 5) C-H stretches
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What do we need to obtain a Raman spectrum?
Raman scattering is weak 1/108 photons are
usually Raman scattered 1) high intensity
monochromatic source of photons. 2) ability to
distinguish and detect Raman photons in the
presence of Rayleigh photons. 3) a sensitive
detector for photons. 4) vibrations that exhibit
a change in polarizability
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• Evolution of Raman Instrumentation

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Double Monochromator
Photon Counting Control Computer
Photomultiplier
Laser
Sample
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Kaiser HoloSpec? Spectrograph
CCD Camera
Electronics Module
Notch Filter
Slit
Transmission Grating
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GE Ion Track StreetLab Raman Spectrometer
7.5 x 5 x 10 inches 6.9 lbs 60 seconds
measurement time internal calibration street
identification of narcotics, pharmaceuticals,
hazardous materials
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Hand held Raman instrument Field
deployable Hazmat, DEA, etc
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• Infrared and Raman Intensities

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Sampling gases problematic but not
impossible liquids or solids cap tubes or NMR
tubes pre-scan for fluorescence fibers,
pellets, molded parts 180 backscattering
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• Raman Revolution?

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• Motivation for
• FT-Raman
• 1986

Availability of CW YAG Laser
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Problems with Raman Scattering Measurements Every
thing goes back to the inherent weakness of Raman
scattering 1/108 photons Raman
scattered Weak spectra Fluorescence Hea
ting
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Answer to Fluorescence problems Move to the
red! FT-Raman Be aware of tradeoffs reduced
fluorescence accompanied by reduced sensitivity
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• FT-Raman provided an opportunity to study
  polymers and biomaterials that were previously
  uninvestigated due to the presence of fluorescent
  chromophores or impurities and is currently found
  in over 2000 laboratories world wide!!!!

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• Infrared Revolution?

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Introduction to Infrared Spectrometers
Evolution of Infrared Spectrometers
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PA-IR spectral images
Background image
Sample image
Spectral dimension
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Focal plane array
20th Century
21st Century
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PA-IR vs FT-IR System

• Construction of a PAIR grating system for
  spectroscopy over the 1725-800cm-1 range

38mm PS film
PAIR
FTIR
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• Advantage of Planar Array Infrared (PA-IR) is
  true double-beam operation and compensation for
  background (e.g., water vapor, liquid water,
  etc.)!!!!!

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Dual Beam

• Demonstration of dual-beam operation of the PA-IR
  spectrometer

Sample
IR Source
To FPA
Reference
Aqueous b-lactoglobulin, 10 w/w
Aqueous poly(acrylic acid) at 2, 1, 0.1, and
0.05 w/w
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PA-IR Spectroscopy

• PA-IR combines a monochromator with a focal plane
  array detector.
• Simple, no moving parts.
• High speed sub 100 µsec time resolution.
• Stable and rugged instrument field or on-site
  measurements.
• Scalable
• Excellent noise levels, row binning.
• Projection of different images on area of the
  camera.
• inhomogeneous or multiple samples, multiple beams
  ( p- and s-polarized).
• Real-time background correction.

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• FUTURE?????
• PA-IR provided an opportunity to do time
  resolved measurements of molecules and polymers
  that could not be previously investigated due to
  the lack of high speed IR (and Raman)
  instrumentation and is currently found in over
  10,000 laboratories world wide!!!!

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Gas phase spectra Sarin surrogate
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