Hole Profiles and Electron-Phonon Interactions

The absorption spectrum of atoms and molecules in low temperature solids is composed of a sharp zero-phonon line and a phonon side band (Table 2.12 ). The phonon side band corresponds to light absorption accompanied by phonon absorption or emission. The absorption shown in bold in Table 2.12 is a zero-phonon line. The sum of the absorption, drawn in a finer line, yields the phonon side band. The phonon side band appears on the higher energy side of the zero-phonon line at low temperatures. A measure of the interaction between guest molecule and host matrix is given by the Debye-Waller factor, DW(T), defined as a function of temperature, T, in Eq. (2.3), using the areas of the zero-phonon line, S0(T), and the phonon side band, SP(T). 

A shift of the potential curve of the excited state (e state) in the configurational coordinate and an increase in temperature are reflected by a decrease in DW(T), indicating changes in the fraction of zero-phonon lines. 

An experimental PHB hole profile generally consists of three parts a sharp zero-phonon hole, a small, broad hole of the phonon side band on the higher energy side, and a broad hole, called pseudo-phonon side hole, on the lower energy side of the laser frequency (Table 2.12 ). The pseudo-phonon side hole results from the overlap of the zero-phonon holes which exhibit phonon side bands at the laser frequency. 

Among organic dye/polymer systems, hole formation has been reported to take place at 80-120 K for TPP in a phenoxy resin, for a water-soluble TPP in poly(vinyl alcohol), and for TPP in an epoxy resin.25 A typical example of hole formation up to 80 K is shown in Table 2.12 .26 The Arrhenius plot of the quantum efficiency of hole formation, 4 , for TPP in several polymers (Table 2.13 ) shows that 0 gradually decreases at 4-20 K, irrespectively of the nature of the polymer. Above a certain temperature, Tr, 0 decreases markedly, as a result of spectral diffusion and hole filling, attributed to local structural relaxation of the polymer chains. 

The temperature dependence of hole formation and hole profile is affected by four factors decrease in the Debye-Waller factor, broadening of the hole width, spectral diffusion, and laser-induced hole filling. The first two effects are reversible phenomena and recover at low temperatures. The latter two are irreversible and their influence cannot be eliminated by cooling the sample again. The temperature dependence of the Debye-Waller factor (DiV(T) — S0(T)/S 4)) for TPP/PMMA and TPP/phenoxy resin systems, shown in Table 2.13 by a dotted line, agrees well with the slope of 0 at 4-20 K. The temperature dependence of the Debye-Waller factor is smaller in poly(vinyl alcohol), which shows a higher Es value (23 cm4). Thus, hole formation efficiency is controlled by the temperature dependence of Debye-Waller factor for temperatures below T and, for temperatures above T it is affected mainly by the simultaneous occurrence of spectral diffusion and laser-induced hole filling due to structural relaxation. 

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