| Summary: | 碩士 === 國立彰化師範大學 === 物理學系 === 92 === We study the low-lying two-photon excited states in two-photon absorption in poly(p-phenylene vinylene)（PPV） and its derivatives. We find that , within the Pariser-Parr-Pople（PPP） model , the state in PPV is the lowest unbound state of electron-hole pair. This hence indicates that the binding energy of the singlet exciton （ ）is ∼0.2-0.4 eV in a single PPV chain limit. We show that the state in PPV is exactly the lowest two-photon excited state（i.e. the state）. Due to the breaking of charge-conjugation symmetry , the in substituted PPVs , originating from the , is weakly two-photon allowed , while the with has its origin from the , hence giving a strong peak in two-photon absorption spectra.
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