Light Scattering and Spectroscopy Lecture Notes PDF

Topics to be covered  Molecular level interactions  Absorption, spontaneous emission, stimulated emission  Fate of excited molecules  Fluorescence  Light scattering  Rayleigh, Mie, Raman, spectroscopy 1  Introduction...

Topics to be covered  Molecular level interactions  Absorption, spontaneous emission, stimulated emission  Fate of excited molecules  Fluorescence  Light scattering  Rayleigh, Mie, Raman, spectroscopy 1  Introduction  Coloured objects - scattering  Mie theory (or solution): scattering off single homogeneous isotropic spheres  Rayleigh theory: particle smaller than 1/10 of the wavelength  Mie theory: particle size ~ wavelength  Direction changed, energy unchanged  Elastic scattering https://www.lavision.de/en/techniques/mie-rayleigh-raman/ 2  Electric field scattering  X-polarized electric field: particle oscillates along the x-axis  Emits EM radiation: light re-emitted  Mostly in the yz plane and x-polarized.  No radiation: +x and –x axis  Classical interpretation 3  Quantum mechanics’ view  Photon energy ≠ quantized changes in energy levels  Virtual energy state (the dotted line)  Short lived – 10-14 seconds  Return to original state: same energy photon  Any wavelength: non-resonant effect From Fundamentals of Photonics 4  Rayleigh Scattering 5  Raman Scattering Stokes Scattering anti-Stokes Scattering From Fundamentals of Photonics  Higher or lower energy photon results  Stokes scattering: hfs =hf-hfv  Anti-Stokes scattering: hfa=hf+hfv  Inelastic scattering 6  Raman Scattering - 2  Energy imparted to or extracted from vibrational and/or rotational states  Fluorescence: finite resonance lifetime (ns region)  Raman scattering: almost instantaneous (ps region)  Non-resonant effect 7  Summary of light scattering processes Type of Scattering at Relative change Scattering of photon energy ΔE/E Elastic: Rayleigh Particles λ 0 Inelastic: Raman Vibrations 10-3 – 10-2 8  What is spectroscopy?  Interaction of radiation (EM waves) with matter  Radiation split into constituent components  Radiation intensity vs. wavelength (or frequency) Emission spectrum Absorption spectrum http://www.physics.uc.edu 9  Absorption spectroscopy  UV or visible range studies  Two optical paths P. Atkins and J. de Paula, Physical Chemistry 8th ed., Oxford University Press, 2006  Measure relative absorption by the sample at different wavelengths From Wikipedia Absorption spectrum of sun light shows some frequencies are missing 10  Absorbance and Transmittance  No units, but AU (absorbance units) used  Optical Density (O.D) What proportion of the input light passes through a sample if its O.D. is 3? 11  Molar attenuation coefficient  Responsible for absorption: chromophore.  Fluorophore if it fluoresces  Density of chromophores -> absorption coefficient.  Concentration c (mol/L)  : molar attenuation coefficient (L/mol.cm) 12  http://www.oceanoptics.com 13  Emission spectroscopy  Specific wavelength source  Emitted and/or scattered P. Atkins and J. de Paula, Physical Chemistry radiation recorded 8th ed., Oxford University Press, 2006  The Neon emission spectrum is shown here From Wikipedia 14  Raman spectroscopy  Curved reflecting Image of a sectioned tooth, surface highlighting the enamel (green),  dentine (blue) and areas of highMonochromator or fluorescence (red) interferometer for analysis by the detector Renishaw  Very low light intensity http://www.renishaw.com/en/raman-image-gallery--8087 P. Atkins and J. de Paula, Physical Chemistry 8th ed., Oxford University Press, 2006 15  Summary  Light scattering is a non-resonant event and as such can occur at any frequency  Mie theory describes the scattering of planar EM waves off spherical homogeneous objects  For small objects, Mie theory reduces in complexity and is better known as Rayleigh scattering  Rayleigh (and Mie) is elastic while Raman is inelastic scattering  Spectroscopy is an important tool in the study of electron energy levels in atoms and bond lengths and strengths in molecules  Electronic and vibrational transitions can be observed through fluorescence, IR, Raman spectroscopy techniques 16