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Tel Aviv University
Absorption and Laser Induced Fluorescence Spectra of Molecular IodinePhysical Chemistry Laboratory Cheskis S, Urbakh M
"Iodine is the heaviest common halogen (atomic number=53, atomic mass=127) and exists as a solid at room temperature in sublimation equilibrium with its vapor. Like the other halogens, this vapor consists of a weakly bound diatomic molecule, I2. The homonuclear diatomic has no dipole moment, so the vibrational frequency of this molecule cannot be determined via conventional IR absorption. The vapor has the appearance of a violet gas, indicating a visible absorption. This absorption corresponds to a spin-forbidden transition from the lowest vibrational levels of the singlet electronic ground state to high vibrational levels of a triplet excited state. This transition is stronger and 'redder' for I2 than for the lighter halogen dimers.
"Electronic spectroscopy of small molecules provides chemists with the fundamental understanding of the nature of the chemical bond in quantum mechanical detail. We all know that a chemical bond is not infinitely strong, i.e. it has a well defined bond energy. We also know that because the bond is stretchy, the frequency of vibration of the nuclei attached by a chemical bond will be determined by how stretchy or stiff the bond is. The bond has a length, which is characterized by the vibrationally averaged position of the atoms at the end of the bond. To a good approximation, these and all other properties of the bonding between atoms is derivable from the potential energy curve. "
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