EMA 5015 Characterization (5)
Optical Spectroscopy
Light
• "There are three stages in the life of a light beam: it is
created, it travels through space, and it is destroyed...light
is created and destroyed only via its interaction with
matter, from glowing gases in the sun to rhodopsin in the
eye." (Michael Sobin in "Light", p. 73, University of
Chicago Press,1987).
• Light travels in waves - electromagnetic waves.
• These waves are vibrations of electric and magnetic fields
that pass through space.
• It is the interaction of light with electrons that is
responsible for its creation and destruction.
Light
h
h
• Particles (photons) λ = =
p mv
• Waves
Light
• p = h/λ
λ (matter also)
• p = E/c
• E = hf = hc/λ
λ
– Period, Frequency, Velocity, Wavelength,
Wavenumber (σ, the number of waves per
centimeter), Power, Intensity.
– Energy of Waves: As frequency increases, energy
increases; As wavelength increases, energy decreases
When two or more waves traverse the same
space, a displacement occurs which is the
sum of the displacements caused by the
individual waves.
• Constructive vs. Destructive Interference
– based on phase difference between
waves.
• Fourier transform based on fact that any
wave motion, regardless of complexity,
can be described by a sum of simple sine
or cosine terms.
• Diffraction – process in which a parallel
beam of radiation is bent as it passes a
sharp barrier or through a narrow
opening. A consequence of interference.
• Refractive Index: n = c/v
where c is the speed of light in a vacuum
Refraction of light from
less dense medium into
more dense medium.
Velocity is lower in more
dense medium.
Optical Spectroscopy Methods
•
•
•
•
Absorption
Emission
Luminescence
Scattering
Source: Skoog, Holler, and Nieman,
Principles of Instrumental Analysis, 5th
edition, Saunders College Publishing.
Two electron energy levels in a solid: a) electron occupying the lower
level and the upper level empty, b) photon is absorbed by the electron
which moves to the upper level, c) photon is created by electron moving
from upper to lower level with energy = E2-E1
Light and Electrons
• The outermost electrons in silicon have a binding energy of
3 electron volts. This is at the upper end of the visible light
energy spectrum. Violet light has an energy close to 3.1 eV
with a wavelength of 400 nanometers.
• In solid silicon the outermost electrons are shared between
atoms and these electrons are bound to the solid by
energies of 1.1 electron volts. This means that visible light
is absorbed by silicon and infrared light of energy less
than 1.1 eV and wavelengths greater than 1100 nanometers
can pass through silicon without absorption.
• Electrons in atoms and in matter - from gases to solids are held in place and require a boost in energy to remove
them. In atoms the negatively charged electrons are held
(bound) by the positively charged protons. From the
quantum theory, we know that the electrons are bound to
the atom with fixed energies, the binding energies.
• In the semiconductor element silicon the binding energies
for the innermost electrons are nearly 2000 electron volts
(eV) - more exactly, 1.839 kiloelectron volts (keV). This
means that an energy of at least 1.839 keV must be given
to an innermost electron to remove it from the silicon
atom. These 1 keV energies lie in the x-ray portion of the
spectrum.
Optical Light Emitting
•
•
•
Light is emitted over the whole energy region of visible light by the heating of
a tungsten filament in a light bulb.
With electric discharge in gases light can also be produced in a spectrum of
narrow (also called sharp) lines distributed in energy across the visible
spectrum. Each line is characterized by the element atom which emits the light.
Sodium emits two lines fairly close together in the yellow region of the
spectrum, cadmium emits a strong red and a strong green line, and mercury
emits several strong lines.
•the energy-level
diagram for mercury
and the wavelengths of
light emitted by the
transition of electrons
from upper to lower
levels.
Scattering of Radiation
Rayleigh Scattering – scattering by molecules or aggregates
of molecules with dimensions significantly smaller than the
wavelength of radiation. Intensity related to wavelength,
dimensions of scattering particles, and polarizability.
Raman Scattering – part of the scattered radiation suffers
from quantized frequency changes as a result of vibrational
energy transitions occurring in a molecule as a consequence
of the polarization process.
Absorption of Radiation
Selective removal of certain frequencies by transfer of energy
to atoms or molecules.
Particles promoted from lower-energy (ground) states to
higher energy (excited) states.
Energy of exciting photon must exactly match the energy
difference between the ground state and one of the excited
states of the absorbing species.
Emission of Radiation
Electromagnetic radiation is
produced when excited
particles return to lowerenergy levels or the
ground state.
Energy-level diagrams
showing emission from
atoms (left) and
molecules (right).
Energy-level diagram of molecular fluorescence
Source: Rubinson and Rubinson, Contemporary Instrumental Analysis, Prentice Hall Publishing.
Components of Spectroscopic
Instruments
Stable source of radiant energy
In emission spectroscopy, sample is radiation
source
Transparent container to hold sample
Device to isolate restricted region of
spectrum for measurement
Radiation detector or transducer
Signal processor
Filters
Monochromators
Grating
Prism
Spectrometer
Optical instruments are used to measure the intensities and
wavelengths of the visible region of the electromagnetic
spectrum. A spectroscope is a "spectrum-observing"
instrument and a spectrometer is for "spectrum
measurement". A spectrometer measures the wavelength of
light.
A simple prism spectrometer
Czerney-Turner
Grating Monochromator
Bunsen Prism
Monochromator
Diffraction from an Echellette-type
grating
Advantages of Grating
Monochromators
Wavelength independence of dispersion.
Fixed dispersion makes it easy to scan an entire spectrum at
constant bandwidth after initial adjustment of slitwidth.
Better dispersion for same size of dispersing element.
Can disperse radiation in far UV and infrared regions where
absorption prevents use of prisms.
Disadvantages of Grating
Monochromators
Produce great amounts of stray radiation.
Produce more high-order spectra.
Both of these disadvantages can be minimized
with filters.
Types of Photon Detectors
Photovoltaic Cells (or Barrier-Layer Cells) – the radiant
energy generates a current at the interface of a
semiconductor layer and a metal.
Phototubes – radiation causes emission of electrons from a
photosensitive solid surface.
Photomultiplier Tubes – contain a photoemissive surface as
well as several additional surfaces that emit a cascade of
electrons when struck by electrons from the photosensitive
area.
Raman Spectroscopy
•
Raman spectroscopy is a spectroscopic technique based on inelastic scattering of
monochromatic light, usually from a laser source.
•
Inelastic scattering means that the frequency of photons in monochromatic light
changes upon interaction with a sample. A small fraction of light (approximately 1
in 107 photons) is scattered at optical frequencies different from, and usually lower
than, the frequency of the incident photons.
•
Photons of the laser light are absorbed by the sample and then reemitted.
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Frequency of the reemitted photons is shifted up or down in comparison with
original monochromatic frequency, which is called the Raman effect.
•
This shift provides information about vibrational, rotational and other low
frequency transitions in molecules. Chemists are concerned primarily with the
vibrational Raman effect. We will use the term Raman effect to mean vibrational
Raman effect only.
•
Raman spectroscopy can be used to study solid, liquid and gaseous samples.
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
Scattering Process
The difference in energy between the incident photon and the
Raman scattered photon is equal to the energy of a vibration of
the scattering molecule.
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
H2O
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
σ
4,000
σemit
σemit
Vib.
states
σ
0
σemit
fluorescence
Elastic
Scattering
(Raleigh)
fluorescence
Excitation Energy, σ (cm–1)
Main Optical Transitions: Absorption, Scattering, and Fluorescence
IR
Electronic
Ground State
UV/Vis
Fluorescence
Raman Spectroscopy: 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
1st Electronic
Excited State
∆σ
IR
Raman
σ
∆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:
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:
Summary
1. Raman is a vibrational spectroscopy akin to IR
-
Good for fingerprinting, probing molecular symmetry
2. Scattering-based, not transmission/reflection
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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)
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785 nm: Fluorescence less probable; Lower Raman signal
514 nm: Fluorescence more probable; Resonance more likely; Higher signal
4. Other things not talked about
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SERS: Surface Enhanced Raman Spectroscopy
Quantum origins of selection rules and scattering cross-section