Temperature Dependence of the Picosecond Photocurrent in Stretched Trans-Polyacetylene Films with Below Gap Excitation
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TEMPERATURE DEPENDENCE OF THE PICOSECOND PHOTOCURRENT IN STRETCHED TRANS-POLYACETYLENE FILMS WITH BELOW GAP EXCITATION A. D. WALSER*, R. DORSINVILLE*, R. TUBINO** and R. R. ALFANO*
*Institute for Ultrafast Spectroscopy and Lasers Electrical Engineering Department The City College of New York, 138th St. and Convent Ave. N.Y., N.Y. 10031 **Istituto di Fisica dellUniversita', Via Vienna 2, Sassari and Istituto di Chimica delle Macromolecole, CNR, Italy ABSTRACT
The temperature dependence of the picosecond photocurrent with below gap excitation (1.06 pm) has been measured for a highly oriented form of trans-polyacetylene. The 1-d picosecond photocurrent is independent of temperature. The 3-d picosecond photocurrent is temperature dependent with an activation energy of 63 meV. These results demonstrate the photoproduction of nonlinear charged carriers ( solitons and polarons) at energies that are below the principal interband absorption edge. INTRODUCTION Trans-polyacetylene (trans-(PA)) is a linear polymer consisting of weakly coupled chains of CH units. Trans-(PA) has a twofold degenerate ground state which is believed to lead to mobile topological chain distortions or kinks called solitons which are responsible for many of the interesting properties of the polymer 1-4. According to the Su, Schrieffer and Heeger (SSH) model, solitons can be photogenerated in transpolyacetylene when an incident photon excites an e-h pair, and the lattice then distorts around the photogenerated charged carriers leading to a soliton-antisoliton (S+S-) pair. This model stimulated a host of experiments to challenge or prove the existence of solitons 5 -16 . The results of photoinduced experiments and simultaneous electrical conductivity and ESR measurements confirmed the predictions of Su and Schrieffer and also established the reversed spincharge relation of the soliton model 5. 6. Theoretical calculations using the SSH or
Takayama et al. (TLM) models have shown that the energy to create a soliton at rest (E s ) was less than one-half the single particle gap (A). Therefore, direct photoproduction of soliton pairs at energies well below the principle interband absorption edge is possible as shown experimentally by Blanchet et al. 8. The role played by nonlinear charge carriers such as solitons and polarons in the photoconductive response of trans-(PA) has been studied by several groups by comparing the excitation profile of the photoinduced absorption to that of the photoconductive response of the polymer 9-14 . The similarity between the two profile is generally considered as evidence that charged solitons play a crucial role in the photocurrent of trans-(PA). However, most of the past photoconductive experiments were performed with above gap (hco>2A) excitation where the charged soliton pairs are generated indirectly via electronhole pairs. The photoconductive response of trans-(PA) with below gap excitation has received relatively little attention. In addition, there is strong evidence both from photoinduced absorption measurements a
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