Probe Photon Energy Research Articles

PHOTO INDUCED INFRARED ACTIVE PHONONS IN TRANS POLYACETYLENE

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

Method of virtual sources in the underwater-acoustical description of high-frequency sound fields in a wedge : Kuznetsov, V.K. 18 (October–December 1972) 223

The results of transient transmission studies utilizing femtosecond laser pulses on hydrogenated amorphous silicon alloys are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different alloys are similar, but the technique is able to clearly distinguish between them based on their alloy composition and bandgap. It is shown that the results can be modeled as a free-carrier absorption spike followed by a residual plateau caused by both recombination and intraband heating. This model is adapted to work in the band tail region.