Chap. 7 (Optical Instruments), Chap. 8
(Optical Atomic Spectroscopy)
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General design of optical instruments
Sources of radiation
Selection of wavelength
Sample containers
Radiation Transducers
Instruments
• Optical instruments fundamental methods
 Absorption
 Fluorescence
 Phosphorescence
 Scattering
 Emission
 Chemical Luminenscence
4-1
Optical methods
• Similarities for differing methods over wavelength
range
 Stable source of radiation
 Transparent sample holder
 Isolation of region of interest
 Radiation detector
Transducer
• Signal processor
• Variations in setup depend upon detection of light
 Linear for absorbance
 90 degrees for fluorescence
 Emission and chemiluminescence source and
sample are same
4-2
Apparatus
4-3
Sources of radiation
• Materials
 Transparent
windows
4-4
Sources of Radiation
• Continuum source
 Emission over a
large range
 Intensity can vary
with wavelength
• Line Source
 Intense emission of
discrete lines
4-5
Light Sources
4-6
Laser Sources
• Laser properties
 light amplification by stimulated emission
of radiation
High intensity
Narrow wavelength
Coherent
* Can very pulse energy, wavelength
* Combined with laser system
electronics for short lifetime
measurements
4-7
Laser Process
4-8
Laser Process
• Pumping
 Excitation of lasing material
Crystal (ruby)
Semiconducter (GaAs)
Dye
Gas (Ar)
 Spontaneous Emission
Emission of radiation in random direction
 Stimulated Emission
Excited laser species interact with emitted
radiation
* Deexcitation of excited species
Photon emission energy same as
spontaneous emitted photon
Coherent emission
4-9
Laser Dyes
4-10
Population Inversion and Amplification
Need to highly populate excited state
4-11
Three and four level transitions
Excitation to high state, transition to metastable state
4-12
Absorption and fluorescence process of Cm3+
Optical Spectra
Fluorescence Process
30
Wavenumber (10
3
-1
cm )
H
G
F
Emissionless
Relaxation
20
A
7/2
Excitation
10
Fluorescence
Emission
4-13
0
Z
7/2
4-14
Wavelength Selectors
• Quality of selected wavelength based on full
with at half maximum
4-15
Filters
• Absorption filter
 Visible region
 Colored glass
or dye act as
the filter
4-16
Filters
• Interference filters
 Combination of constructive and
destructive interference
 Filter wavelength based on properties of
filter
Dielectric layer determines wavelength
4-17
Filters
• Constructive interference equations
 nl = 2dsin q
 q90°, sin q1
 nl = 2d
 lair = lglass ×h
 h= refractive index
2 dh
l
n
 n is order of interference
4-18
Monochromators
• Allow selection of specific wavelengths over a scanned range

IR, Visible, Ultraviolet
• Similar components

Entrance slit
Rectangular optical image

Collimating lens
Parallel beam of radiation

Prism or grating
Selection of wavelength

Focus element
Reforms image and places on focal plan

Exit slit
Isolates desired wavelength
4-19
Monochromators
Grating are more common in modern equipment
Linear dispersion= variation in l along plane AB
D=Fdr/dl, F= focal length
D-1=d/nF=Dl/D(AB) [nm/mm]
4-20
Monochromator
• Can calculate l
 i is incident
 r is reflection
• i is known
• d is from grating in
nm
 i.e., 2000
lines/mm needs
to be converted
to nm/line
• n is generally 1
• Angle r must be
defined to find l
nl  d (sin i  sin r )
4-21
Monochromator Slit
• Parameter that can be set
• Controls light input
• Resolution can be affected by slit width
 Wavelength to be examined is considered
 Wider slits less resolution but may have
better signal
4-22
Monochromator Slit
• Can calculate slit width based on
experimental consideration
 Resolution difference of
wavelength to be examined
w
Dleff
D
1
0.5 * (Dlresolution)

1
D
• Theoretical calculation
 Actually need narrower slit
width due to imperfections
4-23
Radiation Transducers
• Photon Transducers
 Photovoltaic cells
 Phototubes
e- emission from phosphor
 Photomultiplier
Cascade of electrons
 Photoconductors
 Photodiodes
 Charge-transfer
Si crystal collects charge due to absorption
4-24
Phototube and Photomultiplier
105-107 electrons/photon
4-25
Optical Atomic Spectroscopy
• Optical Atomic Spectroscopy
• Atomization Methods
• Sample Introduction
• Optical Spectroscopy
 Elements converted to gaseous atoms or ions
 Measurements of atomic species
Fluorescence
UV-Visible absorption
Emission
• Calculations can be made based on electron energy
diagrams
 Transition between states
4-26
Na and Mg energy levels
4-27
Electronic Energy Symbols
• 2S+1LJ
• S is spin from unpaired e +½
 L is written as S, P, D
 J=L+S
• Li= 1s22s1
 L=0, S =+ ½
 2S1/2
4-28
Atomic Emission Spectra
• Excitation of electrons
 Short lived
 Relaxation to ground state
Emission of photon
* Visible range
* Possible multiple lines
• Absorption spectroscopy
 Resonance due to transitions from ground
to excited state
• Fluorescence can also occur
4-29
Atomic Line Widths
• Broadening due to differing effects
 Uncertainty
DvDt
• Line width due to Hg with lifetime of 2E-8s at
253.7 nm
4-30
Line Widths
• Doppler
 Atom moves during radiation interaction
4-31
Thermal effects
• Boltzmann equation
• Calculate Na atoms in 3p excited states to
ground as 2500 K
• 3s to 3p transition is 3.37E-19J
• P based on quantum states
 3s has 2, 3p has 6
6
3.37 E  19 J )
 exp(
)  1.72 E  4
1
No 2
1.38E  23JK * 2500 K
4-32
Nj
4-33
4-34