14b - Electronic (optical) Spectroscopy Spectroscopy of the...

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1 Electronic (optical) Spectroscopy Spectroscopy of the Hydrogen atom anything 1 0, 1 1s 2s is a forbidden transition ( 0 0 0). 1s 2p is an allowed transition ( 1 0 0). 1s 3d is a forbidden transition ( 2 0 2). 2p 3s is a Selection rules : Example n s: fi nn n ll l m l l l ∆= − = ∆= − =± ∆=± ∆=−= →∆ = = = = allowed transition ( 0 ) . 11 l () * ,, , s i s f if n nlmm lmm d er µµ τ ψ µ = =− G r v e Ze
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2 * 2 The Frank-Condon factor: i f v v d ψ τ Diatomic molecules The Frank-Condon Principle Electrons can rearrange much faster than the nuclei. Hence, the nuclear configuration does not change during electronic transitions. Hence, the most probable vibronic transitions will be between vibrational states with considerable overlap (that is, transitions that do not involve significant changes in nuclear configurations). Note: (1) The selection rule v=±1 does not apply for electronic spectra. (2) Homonuclear diatomic molecules are optically-active (although they are IR-inactive and microwave-inactive. Unshifted electronic potentials Shifted electronic potentials
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3 Pathways of energy flow in photoexcited molecules (usually a singlet state) 13 10 s nonradiative vibrational relaxation 3 10 s Radiationless
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14b - Electronic (optical) Spectroscopy Spectroscopy of the...

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