PHYSICAL REVIEW B VOLUME 52, NUMBER 10 1 SEPTEMBER 1995-II Near-infrared dynamics of photoexcitations in a substituted polyacetylene Satoshi Takeuchi Department of Physics University of Tokyo, 7 3 1-, H-ongo, Bunkyo ku-, Tokyo 113, Japan Toshio Masuda Department of Polymer Chemistry, University of Kyoto, Yoshida Hon-machi, Sakyo ku, -Kyoto 606, Japan Takayoshi Kobayashi Department of Physics, University of Tokyo, 7 3 1, H-o-ngo, Bunkyo ku-, Tokyo 113, Japan (Received 15 March 1995) Near-infrared {0.52-1.37 eV) transient spectra of a substituted polyacetylene, poly[[(o- trimethylsilyl)phenyl]acetylene], were measured with a time-resolution better than 300 fs. Whole spectra of hot self-trapped excitons and relaxed, overall neutral pairs of oppositely charged, spatially confined soliton-antisoliton were measured deep in the band gap. The confinement parameter y =0.23+0. 07 and on-site Coulomb repulsion Uo =2. 8+0.4 eV were determined from the results. I. INTRODUCTION Nonlinear excitations with geometrical relaxation have been the main subjects of research for quasi-one- dimensional conjugated polymers in relation to their large optical nonlinearities and potential use for applica- tions. ' A number of spectroscopic works on topological soliton excitations in a degenerate system such as irans- polyacetylene have been extensively made so far by means of charge-transfer doping and/or photoexcitation, and successfully explained basically within the frame developed by Su, Schrieffer, and Heeger. The formation process of the soliton, however, has not yet been investi- gated experimentally because of the difhculties in directly probing the associated in-gap states appearing in the near-infrared (NIR) region with a sufficiently high time resolution. Though the near- to midinfrared (2. 5 — 5. 0 pm) photoinduced absorption (PA) of a thin film of trans-polyacetylene was previously reported by Rothberg et al. , it was measured only at several wavelengths, and did not allow one to discuss the time-dependent spectral shape to disclose the formation process of the soliton. In this paper we report the NIR (0.52 — 1.37 eV) transient spectra of a substituted polyacetylene, poly[ [(o- trimethylsilyl)phenyl]acetylene] (PMSPA). A broad- band difference-frequency generation as well as a NIR portion of a femtosecond continuum is exploited to cover the spectral region. The temporal variation of the spec- tral shape observed at early delay times allows us to definitely determine the spectra of both hot self-trapped excitons (STE's) just after photoexcitation and spatially confined, intrachain soliton-antisoliton pairs formed fol- lowing the charge separation. A trans-rich PMSPA film is preferentially polymerized using a WC16 catalyst. The chemical structure of the sample is shown in Fig. 4, and it is written as ( — A= B — )„, where A=CHz and B=CH-C6H4-Si(CH&)3. In the limit that the chain is infinitely long (x ~ oo ), two configurations with opposite bond alternations, — A =B — A=B — A= and =A — B=A —B= A—, are totally equivalent in energy, because their sterical conformations are identical to each other and only the phase of the bond alternation is different be- tween them. The PMSPA, therefore, has a doubly degen- erate ground state to support the formation of the topo- logical soliton excitations as in the case of trans- polyacetylene ( — CH = CH — ), polyyne ( — C = C — )„, and halogen-bridged mixed-valence complexes. s However, a finite chain length of the real sample may lift the ground-state degeneracy by a small amount, and this effectively acts as an attractive force between the two photoexcited charges. An introduced sidegroup also causes a local distortion of the chain. It limits the delo- calization of the conjugated ~ electrons along the chain, which is inferred from a systematic blue shift of the m —+a* interband transition energy observed in several substituted polyacetylenes as compared to that of trans- polyacetylene. The attractive force as well as the shor- tening of the conjugation length confines a photoexcited soliton-antisoliton pair within a segment. The confined soliton-antisoliton pair has been confirmed in this study by a doubly peaked spectrum of the associated in-gap states. The degree of confinement of the solitons and electronic correlation of one-dimensional m electrons, the essential factors in understanding the dynamic behavior of excited species, are to be discussed. EXPERIMENT The schematic of the experimental apparatus for pump-probe measurements is shown in Fig. 1. Pulses with a 110 fs duration at 0.8 pm are generated from a Kerr-lens mode-locked, modified Ti sapphire oscillator (Spectra Physics, Model 3900 S) pumped by a 7 W, all- line argon-ion laser (Coherent, INNOVA 310). They seed a Ti:sapphire regenerative amplifier with the opti- mized cavity design after being stretched temporally up 0163-1829/95/52(10)/7166(5)/$06. 00 O~1995 The American Physical Society