Polyacetylene photo-induced absorption

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

In summary, semiconductor polymers such as polyacetylene and polythiophene have experimentally demonstrated nonlinear optical processes (photo-induced absorption, photo-induced bleaching and photo-luminescence) with characteristic time scales in the picosecond range or faster. These phenomena are intrinsic and originate from the instability of these conjugated polymers toward structural distortion. 

Figure 10.6 also provides a possible explanation of the high energy feature observed at 1.35 eV in the photo-induced absorption spectrum of trans-polyacetylene, which is attributed to neutral states. A vertical excitation from... 

According to theoretical considerations and experimental evidence obtained from electron energy-loss spectroscopy by Fink and Leising [79], from photo-induced absorption measurements by Vardeny et al. [80] and from electron-nuclear double resonance (ENDOR) experiments by Grupp et al. [81], the value of U seems to be between 7 and 9 eV in polyacetylene. This case where neither A nor U dominates is very difficult to treat theoretically. 

CW experiments as well as time-resolved measurements down to picosecond resolution have been carried out for both PA and PC. Photo-induced absorption measurements yield information on the number of photo-excited particles (charged and neutral), whilst the photocurrent is due to the product of number of carriers and their mobility, and is sensitive only to charged excitations. Therefore comparison of the results of both types of experiment gives maximum information on photoexcitations in polyacetylene. 

Figure 1.60. Photo-induced absorption spectrum of trans-polyacetylene after illumination with 2.4 eV (at 10 K). (Reprinted with permission from ref 80)...

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