Two-photon-absorption spectrum of poly(di-n-hexylsilane) films

Two-photon-absorption (TPA) spectra of poly(di-n-hexylsilane) (PDHS) films are obtained from 605 to 410 nm at 295 and 11 K, where the intensity is an order of magnitude higher. A strong TPA band is found above 5 eV and interpreted in terms of interacting σ electrons in a Pariser-Parr-Pople (PPP) model. PPP models for (Si${)}_{\mathit{n}}$ chains relate the excitonic (one-photon) absorption at ${\mathit{E}}_{\mathit{g}}$=3.4 in PDHS to the 4.2-eV TPA at the alternation gap and the high-energy TPA derived from two-electron excitations at ${\mathit{E}}_{\mathit{g}}$. The smaller alternation gap in π-conjugated polymers and their intense TPA above ${\mathit{E}}_{\mathit{g}}$ also indicate correlated states and differ qualitatively from single-particle descriptions.