Detectors

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Lecture 8
Detectors Spectrophotometer design Background
correction Width of the spectral peak
Ch. 20 Harris
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Monochromator (filter, wavelength selector)
Light Source
Detector
Sample
Spectrometer
Data Processing
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A phototube
K/Cs/Sb
GaAs
-

Ag/O/Cs
90 V
Wavelength, nm
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Photomultiplier
Several electrons for each incident electron
Several electrons for each photon
Quarz envelope
Light, h?
Photoemissive cathode
Anode
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Photoelectromultiplier
dynode
dynode
electron
photon
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Single beam
Double beam
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Double beam with the beams separated in time
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Single beam design with 210-element area detector
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Flat background
Ipeak
I
Ipeak I0
Intensities at two different wavelengthes
Peak
I0
Baseline
wavelength
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Flat background
Ipeak
I
constant
Peak
First derivative eliminates flat background
I0
Baseline
Wavelength (l)
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Linear Background
Ipeak
I
Ipeak (I1I2) / 2
I1
Background
I2
??
??
Wavelength (l)
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Linear Background
I
Ibackground Klb
First
Background
Second
Second derivative eliminates linear background
Wavelength (l)
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Original
Second
4th
First
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Well defined peaks and valleys
High noise
Some peaks visible
Almost featureless little information
Low noise
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Only photons of one specified wavelength
are absorbed
Excited state
Ground state
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Width
Velocity of a particle gt1000 m/s
Doppler
Speed of light 300,000,000 m/s
collisions
Approximately as Doppler
Both effects increase with the temperature and
decrease with the size of the molecule/atom
Width is around 0.01-0.001 nm in visible area
Multiple energy levels
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Energy levels of excited state
Energy sublevels
Energy levels of ground state
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A primitive rule
Molecular spectra - wide bands
Atomic spectra - narrow lines