CHEM3440Lec9F06 - CHEM*3440 Chemical Instrumentation Topic...

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Unformatted text preview: CHEM*3440 Chemical Instrumentation Topic 9 Photoluminescence Types of Luminescence Fluorescence Happens quickly after the absorption of the initial photon (μs to ps lifetime). Phosphorescence Happens slowly after the absorption of the initial photon (min to ms lifetime). Chemiluminescence Excitation arises from a chemical reaction instead of photoabsorption. Electron Spin Spin is property inherent to all subatomic particles (just like mass or charge). Electrons have only two values of spin, usually abbreviated as “up” and “down” or sometimes as +1/2 or –1/2. Spin on charged particles produces a magnetic ¡eld. These ¡elds can interact with other neighbouring particles, leading to states of different energy. Molecules with all electrons paired (each “up” electron has a corresponding “down” electron with which to partner) are diamagnetic. Unpaired electron systems are paramagnetic. Spin Multiplicity States Electronic states can be categorized according to the pairing behaviour of their electrons. All paired electrons gives a singlet state. One unpaired electron gives a doublet state. Two unpaired electrons give a triplet state. Almost all molecules have a singlet ground state. Radicals have doublet ground states. Excited states of a singlet ground state molecule can be either singlets or triplets; both occur in every molecule. The ¡rst excited triplet state is usually lower in energy than the ¡rst excited singlet. General Energy Level Diagram First Excited Triplet State, T 1 Energy More states at still higher energy. Various rovibrational levels with S Ground Electronic State, S First Excited Electronic State, S 1 Second Excited Electronic State, S 2 Luminescence Starts With Excitation Photoabsorption occurs from the ground vibronic state to many vibrational levels of an upper electronic state. The absorption of a photon occurs within a few femtoseconds. T 1 Energy S 1 S 2 S Absorption from a singlet to a triplet state is spin forbidden and is very weak compared to singlet-singlet processes. At room temperature, virtually all molecules are in the ground electronic state, S , and most of them are in the ground vibrational state. They are distributed over many closely spaced rotational levels. Rovibrational Relaxation Energy stored in rotational and vibrational degrees of freedom are quickly dissipated through collisions with solvent molecules. This heats the solvent (ever so slightly) and brings the molecules to their ground rovibrational state within the excited electronic state. This is complete within picoseconds of the photoabsorption event. T 1 Energy S 1 S 2 S Internal Conversion An excited molecule is able to transfer energy from one excited state to another of the same symmetry . Usually a high electronic/low vibrational state suddenly becomes a lower electronic/high vibrational state....
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CHEM3440Lec9F06 - CHEM*3440 Chemical Instrumentation Topic...

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