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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