CONDENSED MATTER THIRD SERIES, VOLUME 32, NUMBER 6 15 SEPTEMBER 1985 Infrared-photoinduced-absorption studies in soluble trans-polyacetylene R. Dorsinville, R. Tubino, S. Krimchansky, R. R. Alfano, and Joseph L. Birman Institute for Ultrafast Spectroscopy and Lasers, Physics Department and Electrical Engineering Department, The City College of New York, New York, New York 10031 A. Bolognesi, S. Destri, M. Castellani, and W. Porzio Istituto di Chimica delle Macromolecole, Consiglio Xazionale delle Ricerche, Milano, Italy (Received 5 April 1985) We report on the observation of photoinduced excitations in trans-polyacetylene in its liquid form in the frequency range from 2000 to 6000 cm '. These measurements strongly suggest that trans- polyacetylene is capable of supporting charged solitons even in solution. INTRODUCTION Most recently, absorption, fluorescence, Raman scatter- ing, and time-resolved emission spectroscopy were mea- sured' in a new liquid form of soluble polyacetylene (PA) consisting of polyene chains grown on activated sites of polybutadiene wtuch acts as a soluble carrier. These measurements have shown that the liquid form has simi- lar absorption and emission properties to that of the solid form. The blue shift of the absorption peak and the po- larization properties of the emission spectrum of the solu- ble form indicate a reduction of the interchain interac- tions and of the conjugation length compared to the solid form. ' The question of whether or not trans PA is able to- support "solitons" in its liquid-form was left o' pen. In this paper we report on the observation of photoinduced exci- tations in trans-PA in its liquid form. These measure- ments imply that the soluble trans-PA should be able to support free solitons even in solution. According to Su and Schrieffer a photoinduced electron-hole pair evolves to a soliton-antisoliton pair in about 100 fsec. The pair is coupled to nonbonding local- ized states at the center of the gap and an important test of the existence of solitons in the soluble trans PA should- be the observation of photoinduced absorption below the interband threshold. Recently, infrared optical absorption due to photogenerated carriers has been reported in solid, insoluble trans-PA. We were able to observe the photoinduced-absorption spectrum of the soluble trans- PA in the energy range from 2000 to 6000 cm METHODS Two different samples were used in the experiments: a solution of trans PA in toluene (conce-ntration 1 mg/cm ) in a 1-mm-thick quartz cell and a solid sample prepared from the solution by evaporating it on a sapphire plate. These samples were mounted on the cold finger of a nitro- gen Dewar. The photoexcitation was produced by a 100- mw cw argon-ion. laser (h v=2. 4 eV) modulated at 90 Hz. The ir emission of a glowing silicon resistor was focused into the sample and the transmission analyzed with an ir spectrometer coupled to a InSb detector. The photoinduced-absorption changes were recorded by lock- in detection and displayed on a recorder. RESULTS AND DISCUSSION In Fig. 1 the photoinduced absorption of the evaporated soluble trans-PA at 77 K is plotted versus photon energy. This spectrum is similar to the photoinduced spectra of nonsoluble solid trans PA The m-ain . feature is a broad asymmetric band peaking at about 0. 49 eV with full width at half maximum of 0. 3 eV. The ir photoinduced- absorption spectrum of a toluene solution of trans-PA at 180 K just above the freezing temperature of toluene is also displayed in Fig. 1. Below this temperature crystalli- zation of the solvent occurs which increases the scattering and causes the signal to disappear. Although the detected signal from the solution at 180 K was about an order of magnitude weaker than the signal from the evaporated film, we were able to measure the photoinduced spectrum and found it similar to the spectrum of the film or of standard solid trans-PA. %'e have also measured the magnitude of the photoin- duced absorption as a function of temperature for both samples. In Fig. 2, the induced absorption at 0. 49 eV is plotted versus temperature. These curves are similar to the one measured for the nonsoluble polyacetylene. The amplitude of the photoinduced change in absorption de- creases with increasing temperature, becoming unobserv- 32 3377 1985 The American Physical Society