Interaction of Light with Matter
Light interact with bulk matter in three ways:
• Reflection: The angle of incidence equals the angle of reflection.
Specular reflection: flat surface and θi = θr
Diffuse reflection: light reflected in a variety of directions
• Transmission: A medium is transparent to a given particular wavelength(s) of
light. The speed of light is slower than it is in a vacuum, and varies with
wavelength.
• Absorption:
Eabs = Ef – Ei
• The conservation law of energy:
I0 = IR + IT + IA
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Index of Refraction
• Index of refraction = the ratio of the speed of light in a vacuum, c, to the
speed of light in a medium, c’:
n = c/c’
• Refraction: the change in direction of propagation of light
•
Snell’s law:
n1sinθ1 = n2 sinθ2
• Indices of refraction are temperature-dependent and wavelength-dependent.
Flint glass is optical glass
that has relatively high
refractive index and low
Abbe number (high
dispersion).
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Index of Refraction
Application: Achromatic lens
Chromatic aberration of a single
lens causes different
wavelengths of light to have
differing focal lengths.
An achromatic doublet brings red
and blue light to the same focus,
and is the earliest example of an
achromatic lens.
• One element is a negative (concave) element made out of flint glass such as F2,
which has relatively high dispersion, and the other is a positive (convex)
element made of crown glass such as BK7, which has lower dispersion.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Internal Reflection
• Total internal reflection can occur when the incident light does not leave
the first the first medium. It happens under conditions that
θ1 > θc = Asin (n1 / n2) = sin-1 (n1 / n2)
-
•
-
θc: critical angle
Application:
Fiber optics transmission
Polarizers
Attenuated total reflectance (ATR) spectroscopy
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Internal Reflection
Application: Fiber optics transmission
Application:
Attenuated total
reflectance (ATR)
spectroscopy
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Dichroism
•
•
Original definition: Any optical device which can split a beam of light into two
beams with differing wavelengths.
In spectroscopy, dichroism occurs when a material absorbs left circular
polarized light in different amounts than right circularly polarized light.
(1) Analysis of chiral molecules – Track the chirality in a sample
 Optical rotation dispersion (ORD) at a single wavelength: It determines
concentration for chiral molecules and chiral purity compared to a known
standard.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Circular Dichroism Spectroscopy
(2) A material in which light in different polarization states travelling through it
experience a varying absorption.
(= A material absorbs left circular polarized light in different amounts than right
circularly polarized light.)
•
Far-UV (ultraviolet) CD spectrum of proteins: characteristics of the secondary
structure (alpha-helix, beta-sheet, beta-turn, or some other conformation),
changes in conformation.
•
Near-UV CD spectrum (>250 nm) of proteins: information on the tertiary
structure = structural information on the nature of the prosthetic groups in
proteins
•
Visible CD spectroscopy is a very powerful technique to study metal–protein
interactions and can resolve individual d–d electronic transitions as separate
bands. CD spectra in the visible light region are only produced when a metal ion
is in a chiral environment, thus, free metal ions in solution are not detected
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Circular Dichroism Spectroscopy
(2) A material in which light in different polarization states travelling through it
experience a varying absorption.
Circular dichroism spectra are typically displayed as difference
spectra – the absorption of one polarization minus the absorption of
the other polarization.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
CD Spectroscopy
Signal/Noise =
detector quantum efficiency 1/2
 signal intensity
 acquisition time
Data quality一定要夠好!
(以下例子只計算intensity count數的random error,尚未包括儀器的不穩定度
(light intensity, detector stability, instrument stability etc.), calibration uncertainty,
sample concentration uncertainty, etc.)
CD measurement, intensity
I0
100
1000
10000
100000
1000000
10000000
100000000
1000000000
10000000000
IL
IR
99
98
990
980
9898
9800
98980
98000
989800
980000
9898000
9800000
98980000
98000000
989800000 980000000
9898000000 9800000000
Uncertainty (%) =
sqrt(intensity)/intensity
Absorbance = ln (I0/I)
CD Results
I0
IL
IR
ln(I0/IL) StDev(L) ln(I0/IR) StDev(R) delta CD StDeV
10.0% 10.1% 10.1% 0.010
0.142
0.020
0.142 9.95E-03 2.01E-01
3.2%
3.2%
3.2%
0.010
0.045
0.020
0.045 9.95E-03 6.35E-02
1.0%
1.0%
1.0%
0.010
0.014
0.020
0.014 9.95E-03 2.01E-02
0.3%
0.3%
0.3%
0.010
0.004
0.020
0.004 9.95E-03 6.35E-03
0.1%
0.1%
0.1%
0.010
0.001
0.020
0.001 9.95E-03 2.01E-03
0.03% 0.03% 0.03% 0.010
0.000
0.020
0.000 9.95E-03 6.35E-04
0.01% 0.01% 0.01% 0.010
0.000
0.020
0.000 9.95E-03 2.01E-04
0.003% 0.003% 0.003% 0.010
0.000
0.020
0.000 9.95E-03 6.35E-05
0.001% 0.001% 0.001% 0.010
0.000
0.020
0.000 9.95E-03 2.01E-05
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
S/N
Ratio
0.0
0.2
0.5
1.6
5.0
15.7
49.6
156.7
495.6
Circular Dichroism Spectroscopy
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Circular_Dichroism
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
http://www.photophysics.com/tutorials/circular-dichroism-cd-spectroscopy
CD Spectrometer
A double quartz prism monochromator
http://www.photophysics.com/tutorials/circular-dichroism-cdspectroscopy/5-cd-spectrometer-performance
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
• What actually passes through the intermediate slit is a mixture of horizontally
polarized light of the selected wavelength and vertically polarized light of a different
wavelength.
• The second prism separates out the contaminating vertically polarized beam, as well
as straylight that has made it through the first monochromator.
• The unique combination of the two polarized quartz prisms provides a wide
separation of polarized components, allowing wider slit widths, and so more light, to
be used.
• The polarized light is passed through a photoelastic modulator (PEM) which
converts the beam into alternating left and right circularly polarized light.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Continuum Sources in UV-VIS
(1) QTH (quartz tungsten halogen lamp) - An incandescent lamp with a tungsten
filament contained within an inert gas and a small amount of a halogen such as
iodine or bromine.
- As voltage is reduced, total output is reduced and the peak wavelength shifts only
slightly to the red.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Continuum Sources in UV-VIS
(2) Arc lamp
- Inert gas: high-pressure Xenon, high-pressure mercury, highpressure mercury-xenon
- Hydrogen or deuterium arc lamps (more in the UV region)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Continuum Sources in IR
SiC light source (globars):
- A silicon carbide rod electrically heated up to 1,000 to 1,650 °C
 radiation from 4 to 15 μm.
- Used as thermal light sources for infrared spectroscopy
Wavelength (nm)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Line sources
• Light – emitting diodes
http://www.spectrecology.com/uploads/Lumileds_data_sheet.pdf
Grace H. Ho, Department of Applied Chemistry, National
University of Kaohsiung
Light Sources
Line sources
• Atomic analysis
by hollow-cathode lamp (metal-vapor discharge lamps)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Line sources
• Energy Calibration by mercury lamp (Hg lamp)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Line sources
• Energy calibration lines:
Kr
Ne
Ar
Xe
Hg(Ne)
Hg(Ar)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources – Other incoherent sources
• Discharge of high-pressure noble gases:
He: 105 –400 nm
Mechanisms:
Ar: 107 –165 nm
Kr: 124 –185 nm
Xe: 147 –225 nm
• Discharge of low-pressure noble gases:
He I (21.2 eV)
He II (40.8 eV)
• Flash lamp:
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
He + e  He*
He* + He  He2*
He2*  He + He + hv
(excimer continuum)
Light Sources – Materials for lens, windows and prisms
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources – Synchrotron Radiation
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources
Laser - Light Amplification by Stimulated Emission of Radiation
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources - Laser
Requirements of laser action:
(1) an active medium with correct optical
properties
(2) population inversion
(3) stimulated emission to generate light
amplification.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources - Laser
(3) Stimulated emission to generate light amplification – An excited state is
stimulated to emit a photon by radiation of the same frequency. The more
photons that are present, the greater the probability of the emission.
or
building up
then pulsing
** CW (continuous wave) laser: (1) It is possible to sustain a population inversion
and the laser operates continuously. (2) When the heat dissipation is not an issue.
** Pulse laser: (1) The amplification builds up and is discharged in pulses and then
builds up again. (2) When the heat dissipation is an issue. (3) When a lot of power
concentrated in a brief pulse is required.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources - Laser
** Light amplification conditions: N = 2(cavity length)
** Characteristics of laser :
(1) photons of particular frequency, (2) directional light path, (3) linear polarization
 Coherent light = spatial coherent + temporal coherent
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources - Laser
** Q switch - a method of obtaining high-peak-power, short-duration laser pulses by
controlling the loop gain of the optical cavity of the laser.
(1) Mechanical Q-switch
(2) Electro-Optic
Q-switch
(Pockel or Kerr cell)
- Polarization
(3) Acousto-Optic
Q-switch
- Diffraction
(4) Bleachable –Dye Q-switch
- Saturation
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Light Sources - Laser
(2) Electro-Optic Q-switch, Pockel cell
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung