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CHEM3440Lec7F06 - Spectrum of Electromagnetic Radiation...

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CHEM*3440 Chemical Instrumentation Topic 7 Spectrometry Spectrum of Electromagnetic Radiation Electromagnetic radiation is light. Different energy light interacts with different motions in molecules. Radiofrequency MHz Nuclear Spin NMR Microwave GHz Electron Spin Molecular Rotation ESR (EPR) Microwave Spec. Infrared THz Molecular Vibrations I.R. Spectroscopy UV-Visible PHz Valence Electron Transitions UV/Vis Spectroscopy Far UV - X-ray PHz Core Electron Transitions XUV - X-ray Spectroscopy Gamma Ray EHz Nuclear Transitions Mössbauer Spectroscopy Photon Energy Units Different units for light quanta energy are used for different experiments. E = h ! = h c " = h c ! 1 eV = 8065.6 cm -1 1 eV = 1.602 x 10 -19 J 1 eV = 241.8 THz 1 eV = 1239 nm MHz NMR GHz ESR cm -1 IR eV, nm UV-Visible eV, Å Far UV/X-ray Units used in different spectroscopic experiments or regions. Correspondence between different units used in spectroscopy. Spectroscopic Experimental Concept Spectroscopic experiments involve probing a system with some particles so as to disturb its equilibrium and then monitor its response by observing the emission of other particles. Photons Photons Electrons Heat Chemical Reaction Photons Absorption Experiment Fluorescence or Scattering Experiment Sample
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Spectral Bandwidth Radiation emission is distributed over a certain wavelength range in a close- to-gaussian manner. Two defining parameters: Peak position Full Width at Half-Maximum (FWHM) 0 0.25 0.5 0.75 1 540 550 560 570 580 Peak at 559 nm for both curves. 9 nm 13 nm The Spectrometer Spectroscopy infers the analysis of the distribution of radiant power by wavelength. Spectrometers separate the various spectral components into the spectrum. Generally we can categorize different spectrometers based upon the way they deal with the spectrum. Broadband radiation from which one selects a monochromatic band Dispersive spectrometers (diffraction gratings, prisms) Non-Dispersive spectrometers (filters, Fourier transform) Narrow emission lines Coherent sources (lasers, synchrotrons) Incoherent sources (atomic lamps) Dispersive Spectrometers Measure one wavelength at a time. A serial detection scheme. Monochromators. Light source Lens Detector Grating Slits Slits Array Detectors Measure a large range of wavelengths simultaneously. A parallel detection scheme. Light source Lens Detector Array Slits Grating Non-dispersive Spectrometers Interferometer. Encodes spectral data into time domain. Many advantages. Moving mirror Fixed mirror Encoded beam Detector Light source Lens Beam splitter Good to detect a single band of wavelengths. Filter Detector Light source Lens Filter-selecting spectrometer
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Spectral Resolution When are two different colours really “different? 0 0.25 0.5 0.75 1 540 560 580 600 620 90% valley !" The resolution if any spectral feature can be quantified by the peak separation and the valley depth.
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