Luminescence - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Luminescence - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Luminescence – light emission accompanying a transition from higher to lower energy levels 1) Phosphorescence – example of photoluminescence (excited state generated by photons) often exhibited by solids like glow in the dark key chains, television screens (CRTs) & “fluorescent” lights 2) Fluorescence – also photoluminescence which is usually observed in solution like quinine that is added to tonic water many analytical applications docsity.com 3) Bioluminescence – excited state induced biologically (enzymatic process) exhibited by fireflies, some algae & fish 4) Chemiluminescence – excited state induced chemically by bond breakage, often oxidation & used for light sticks & some analytical applications 5) Triboluminescence – excited state induced mechanically
bite down on hard candy, also Curad bandaids 6) Electroluminescence – electrical excitation found in some polymers & electric pickle docsity.com When absorption of a photon occurs (process #1), several things can happen: Vibrational Relaxation (VR) - is a non- radiative process (#2) by which the upper vibrational levels lose energy & go to the lowest vibrational level in a given electronic energy state (very fast ~ 10-12 s) Internal Conversion (IC) - non-radiative process (#4) where excited state couples to upper vibrational level of lower electronic energy level followed by VR docsity.com Because of Vibrational Relaxation (VR) all absorbed photons result in the promoted electron ending up in the lowest vibrational level of the first excited singlet state This level can then do one of three things: 1) Fluoresce
make transition to one of the vibrational levels of the ground state giving up energy as a photon (process #3) docsity.com 2) Radiationless Deactivation (#4) – return to ground state giving up energy as heat, by internal conversion or some sort of collisional deactivation 3) Intersystem Crossing (#5) – lowest vibrational level of first excited singlet couples to upper vibrational level of triplet state followed by vibrational relaxation Once formed the triplet state can go to ground state radiationlessly or by emitting a photon = phosphorescence (#6) docsity.com Moderately interesting website showing an animated Jablonski Diagram for absorption, VR, fluorescence, phosphorescence, etc. http://micro.magnet.fsu.edu/primer/java/jablo nski/lightandcolor/ docsity.com Spectra 1) Excitation Spectrum – fluorescence or phosphorescence intensity (at fixed λ) as a function of excitation λ or absorption λ 2) Fluorescence Emission Spectrum - fluorescence emission intensity vs. λ for a fixed excitation λ (= absorption λ, max.) 3) Phosphorescence Emission Spectrum – phosphorescence emission intensity vs. λ for a fixed excitation λ (= absorption λ, max.) docsity.com Phenanthrene Spectra Relative intensity Fluorescence Ll Excitation 200 300 400 500 600 Wavelength, nm ® docsity.com