Chap. 7 (Optical Instruments), Chap. 8 (Optical Atomic
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Transcript Chap. 7 (Optical Instruments), Chap. 8 (Optical Atomic
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
q90°, sin q1
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 )
D 1
• 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
3.37E 19J )
exp(
) 1.72E 4
1
No 2
1.38E 23JK * 2500K
4-32
Nj
4-33
4-34