PHOTO INDUCED INFRARED ACTIVE PHONONS IN TRANS POLYACETYLENE

Abstract

We measured photogenerated ir active phonons in ~-(CH) by the photoinduced absorption technique. A correlation was found to exi~t with a photoinduced absorption band in which electronic transitions are involved. Our results show photoproduction of free charged defects with long lifetimes. In the photoinduced optical absorption technique 1 (PA), the signal arises from the change ~~ in the optical absorption spectrum of the semicond~ctor due to the presence of excited carriers. Recently, Orenstein and Baker have reported measurements of the PA spectrum in (CH) at a delay time of 10~sec following excitation, in the energy range 0.5eV t~ 1.6eV. In trans (CH) they observed two new subgap FA bands at 1 .36eV and at O.5eV. These bands app~ar to be uncorrelated to each o~her due to their different decay time and different temperature dependence. Very recentlY4the resultant interband bleaching at 2ev 3 and the PA spectrum from 1 .2eV to 1.geV were 4 measured with picosecond pulse excitation. It was shown that the gap_f~ates and their resultant interband bleaching are produced in less ~hen 10 sec and that the photogenerated carriers are localized but highly mobile . In this work we report the steady state PA spectrum in trans (CH) ig the energy range 0.09 to 1.1 eV, which includes also the phonons energy ran~e . The photoinduced changes in the sample optical absorption spectrum were measured with an incandescent light source dispersed by a monochromator. The light transmitted through the sample(T) and its changes (~T) were measured with a solid ~tnte detector having a wide spectral range. The laser beam used for excitation was a cw Ar+ with photon energy of 2.4eV which is larger than the gap 0~5trans-(CH)x. Phase detection techniques improved the sensitivity of 6T/T to 10 for most parts of the spectrum. The sample was in the form of thin film of (CH)xgrown on NaCl substrate, initially polymerised as cis-(CH) and subsequently isomerized to trans(CH) . x x In Fig. 1 the i~~uced absorption (-~T/T) at 10K obtained with a laser absorbed Power of 20mWcm is plotted versus the probe photon energy. The PA spectrum consists of an assymetric band peaked at 0.43 eV with a F~M of O.35eV and a m~?h narrower doubly peaked feature wi~? maxima at 170meV(1360cm ) and 157meV(1256cm ) with FWHM of less than 4meV (32cm ). At higher temperatures these bands decrease in intensity and shift up in energy (172meV and O.48eV at 200K). We ident~f7 the narrow PA 5 bands as induced ir active phonons, due to their energy location ' and sharpness. The broader assymetric PA band is due to electron transitions from photoinduced localized states in the gap (with a delta function energy distribution) C3-326 JOURNAL DE PHYSIQUE