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Lecture 8. Detectors Spectrophotometer design Background correction Width of the spectral peak. Ch. 20 Harris. Monochromator (filter, wavelength selector). Light Source. Detector. Sample. Spectrometer. Data Processing. A phototube. K/Cs/Sb. GaAs. -. +. Ag/O/Cs. 90 V.

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Lecture 8

Detectors

Spectrophotometer design

Background correction

Width of the spectral peak

Ch. 20 Harris


Monochromator

(filter,

wavelength selector)

Light Source

Detector

Sample

Spectrometer

Data Processing


A phototube

K/Cs/Sb

GaAs

-

+

Ag/O/Cs

90 V

Wavelength, nm


Photomultiplier

Several electrons

for each incident electron

Several electrons

for each photon

Quarz

envelope

Light, h

Photoemissive

cathode

Anode


Photoelectromultiplier

dynode

dynode

electron

photon


Single beam

Double beam



Single beam design with

210-element area detector


Flat background

Ipeak

I

Ipeak – I0

Intensities at two different

wavelengthes

Peak

I0

Baseline

wavelength


Flat background

Ipeak

I

constant

Peak

First derivative eliminates

flat background

I0

Baseline

Wavelength (l)


Linear Background

Ipeak

I

Ipeak – (I1+I2) / 2

I1

Background

I2





Wavelength (l)


Linear Background

I

Ibackground = Kl+b

First:

Background

Second:

Second derivative eliminates

linear background

Wavelength (l)


Original

Second

4th

First


Well defined peaks and valleys

High noise

Some peaks visible

Almost featureless:

little information

Low noise


Only photons of one specified wavelength are

absorbed

Excited state

Ground state


Width

Velocity of a particle >1000 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


Energy levels

of excited state

Energy sublevels

Energy levels

of ground state


A primitive rule:

Molecular spectra - widebands

Atomic spectra - narrowlines


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