4-1 chap. 7 (optical instruments), chap. 8 (optical atomic spectroscopy) general design of optical...
TRANSCRIPT
4-1
Chap. 7 (Optical Instruments), Chap. 8 (Optical Atomic Spectroscopy)
• 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
4-2
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
4-3
Apparatus
4-4
Sources of radiation
• Materials Transparent
windows
4-5
Sources of Radiation
• Continuum source Emission over a
large range Intensity can vary
with wavelength
• Line Source Intense emission of
discrete lines
4-6
Light Sources
4-7
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
4-8
Laser Process
4-9
Laser Process• Pumping
Excitation of lasing materialCrystal (ruby)Semiconducter (GaAs)DyeGas (Ar)
Spontaneous EmissionEmission of radiation in random direction
Stimulated EmissionExcited laser species interact with emitted radiation* Deexcitation of excited species
Photon emission energy same as spontaneous emitted photonCoherent emission
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Laser Dyes
4-11
Population Inversion and Amplification
Need to highly populate excited state
4-12
Three and four level transitions
Excitation to high state, transition to metastable state
4-13
0
10
20
30
Wav
enu
mb
er (
103 c
m-1
)Absorption and fluorescence process of Cm3+
Optical Spectra
HGF
7/2A
Z 7/2
Fluorescence Process
Excitation
EmissionlessRelaxation
FluorescenceEmission
4-14
4-15
Wavelength Selectors
• Quality of selected wavelength based on full with at half maximum
4-16
Filters
• Absorption filter Visible region Colored glass
or dye act as the filter
4-17
Filters
• Interference filters Combination of constructive and
destructive interference Filter wavelength based on properties of
filter
Dielectric layer determines wavelength
4-18
Filters• Constructive interference equations
n = 2dsin 90°, sin 1 n = 2d air = glass × = refractive index
n is order of interference
n
d 2
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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
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Monochromators
Grating are more common in modern equipment Linear dispersion= variation in along plane ABD=Fdr/d, F= focal lengthD-1=d/nF=[nm/mm]
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Monochromator
• Can calculate 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
)sin(sin ridn
4-22
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
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Monochromator Slit
• Can calculate slit width based on experimental consideration Resolution difference of
wavelength to be examined
• Theoretical calculation Actually need narrower slit
width due to imperfections
11
)(*5.0
DDw resolutioneff
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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
4-25
Phototube and Photomultiplier
105-107 electrons/photon
4-26
Optical Atomic Spectroscopy• Optical Atomic Spectroscopy• Atomization Methods• Sample Introduction
• Optical Spectroscopy Elements converted to gaseous atoms or ions Measurements of atomic species
FluorescenceUV-Visible absorptionEmission
• Calculations can be made based on electron energy diagrams Transition between states
4-27
Na and Mg energy levels
4-28
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
4-29
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
4-30
Atomic Line Widths
• Broadening due to differing effects Uncertainty
vt• Line width due to Hg with lifetime of 2E-8s at
253.7 nm
4-31
Line Widths
• Doppler Atom moves during radiation interaction
4-32
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
472.1)2500*2338.1
)1937.3exp(
2
61
E
KJKE
JE
N
N
o
j
4-33
4-34