1. Lecture 8 Detectors Spectrophotometer design Background correction
Width of the spectral peak
Ch Harris

2. Spectrometer Monochromator Detector (filter, wavelength selector)
Light Source
Detector
Sample
Spectrometer
Data Processing

3. A phototube
K/Cs/Sb
GaAs - +
Ag/O/Cs
90 V
Wavelength, nm

4. Photomultiplier Anode Several electrons for each incident electron
for each photon
Quarz
envelope
Light, h
Photoemissive
cathode
Anode

5. Photoelectromultiplier
dynode
dynode
photon
electron

10. Single beam
Double beam

11. Double beam with the beams separated in time

13. Single beam design with
210-element area detector

14. Flat background Ipeak Ipeak – I0 I0 I Intensities at two different
wavelengthes
Peak I0
Baseline
wavelength

15. Flat background Ipeak I0 I Peak First derivative eliminates
constant
Peak
First derivative eliminates
flat background I0
Baseline
Wavelength (l)

16. Linear Background Ipeak Ipeak – (I1+I2) / 2 I1 I2 I Background  
Wavelength (l)

17. Linear Background Ibackground = Kl+b I Background
First:
Background
Second:
Second derivative eliminates
linear background
Wavelength (l)

18. Original
Second
4th
First

19. Well defined peaks and valleys
High noise
Some peaks visible
Almost featureless:
little information
Low noise

20. Only photons of one specified wavelength are
absorbed
Excited state
Ground state

21. 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 nm in visible area
Multiple energy levels

22. Energy levels
of excited state
Energy sublevels
Energy levels
of ground state

23. Molecular spectra - wide bands
A primitive rule:
Molecular spectra - wide bands
Atomic spectra - narrow lines