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L11_Molec_spectr_I

L11_Molec_spectr_I - Molecular Spectroscopy ­part I ...

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Unformatted text preview: Molecular Spectroscopy ­part I Interrogate molecules with electromagne2c radia2on When a molecule undergoes a transi8on from one quantum state to another, energy is conserved by emi>ng or absorbing a photon: If the energy of the transi8on is in the radio ­frequency, it is between nuclear spin states. If it is in the microwave region, it can be between electron spin states for an unpaired electron or between rota2onal states. If it is in the infrared, it is between vibra2onal states (including or not rota8onal transi8ons). If it is in the visible and UV, it is between electronic states (including vibr. and rota8onal). If it is in the far UV and X ­ray region, dissocia2on and ioniza2on can occur. Within the BO approxima8on: Degrees of freedom: A molecule with N atoms can have 3N mo8ons. It has 3N degrees of freedom. The molecule as a whole can translate and the transla8on mo8on can be decomposed into 3 direc8ons (x,y,z). It has 3 transla8onal degrees of freedom. The molecule as a whole can rotate around the three Cartesian axes. If the molecule is linear, two of those rota8ons are indis8nguishable. If it is linear, it has 2 rota8onal degrees of freedom. If it is nonlinear, it has 3 rota8onal degrees of freedom. The vibra8onal degrees of freedom are the remaining ones: for a linear molecule: 3N  ­ 5 for a nonlinear molecule: 3N  ­ 6 Within the BO approxima8on, we will assume that the types of energy are separable. Molecules as harmonic oscillators and rigid rotors E rot 2 h2 = J ( J + 1) = J ( J + 1) = ( Be hc ) J ( J + 1) 2I 8πI € Molecules as harmonic oscillators and rigid rotors Interac8on between rota8ons and vibra8ons Spectroscopic constants Calcula8ons of spectroscopic parameters Anharmonicity Electronic spectra ...
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