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Lecture 8

Detectors

Spectrophotometer design

Background correction

Width of the spectral peak

Ch. 20 Harris

(filter,

wavelength selector)

Light Source

Detector

Sample

Spectrometer

Data Processing

Several electrons

for each incident electron

Several electrons

for each photon

Quarz

envelope

Light, h

Photoemissive

cathode

Anode

Double beam

210-element area detector

Ipeak

I

Ipeak – I0

Intensities at two different

wavelengthes

Peak

I0

Baseline

wavelength

Ipeak

I

constant

Peak

First derivative eliminates

flat background

I0

Baseline

Wavelength (l)

I

Ibackground = Kl+b

First:

Background

Second:

Second derivative eliminates

linear background

Wavelength (l)

Well defined peaks and valleys

High noise

Some peaks visible

Almost featureless:

little information

Low noise

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

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