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Lecture 8

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

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  1. Lecture 8 Detectors Spectrophotometer design Background correction Width of the spectral peak Ch. 20 Harris

  2. Monochromator (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 Several electrons for each incident electron Several electrons for each photon Quarz envelope Light, h Photoemissive cathode Anode

  5. Photoelectromultiplier dynode dynode electron photon

  6. Single beam Double beam

  7. Double beam with the beams separated in time

  8. Single beam design with 210-element area detector

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

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

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

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

  13. Original Second 4th First

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

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

  16. 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

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

  18. A primitive rule: Molecular spectra - widebands Atomic spectra - narrowlines

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