Hole profiles

Recently Jain et al. [41] have examined the validity of the approximation Pt p used in deriving Eq. (3.42). They calculated the hole profile in a polymer sample at two different applied voltages. The profiles are shown in Fig. 3.8. Fig. 3.8 shows that near... 

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. 

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. 

The thermal stability of holes burned at low temperatures has been investigated by temperature cycling experiments. As an example, we present (Table 2.13 ) the changes in hole profiles of the TPP/phenoxy resin system during successive increase in excursion temperature and subsequent cooling of the hole burned initially at 4 K.27) The detection of a hole for TPP at an excursion temperature is limited to 50 K in PMMA and most conventional polymers, but it can reach up to 80 K in a phenoxy resin and in poly(vinyl alcohol). In this temperature domain, hydrogen bonding of TPP to these polymer matrices, is believed to help suppress structural relaxations. 

Methacrylate Esters. Figure 1 shows typical hole profiles of TPP/PMMA during the temperature cycle experiment and a schematic diagram of the temperature cycling. At first a hole is burned and measured on the profile at 20 K. Then the temperature d die sample is elevated to a certain excursion temperature. The sample is kept at the excursion temperature for 5 min. Then we cool the sample again to 20 K and measure die hole profile. We repeat the temperature cycle with increasing elevated temperature. 

Figure 1. Upper half Typical hole profiles of TPP/PMMA during temperature cycling. Hie values in ( ) correspond to the initial hole width before temperature cycling. Lower half Schematic diagram of temperature cycle experiment.

We measured the irreversible changes in hole profiles and the low-energy excitation modesfor several porphyrin-doped polymer systems. The cause of the hole broadening... 

Elongational behavior is induced in the entrance of the spinning hole and in the transition region from backhole to actual capillary. In practice hardly any permanent orientation is built up in this way, however, because molecular relaxation is rapid. Spinning hole profiles are smoothened only to prevent the formation of vortices which would lead to extrudate distortion. Promoting orientation already in the spinning holes is not common for melt spinning. It could be beneficial for the orientation of melt-spun liquid-crystalline polymers, however, for example in the production of carbon fiber from pitch. 

Figure 10.2 Broadening and pressure shift of a spectral hole (H2PC in an argon matrix). Trace (1) Original hole profile Trace (2) Spectral hole after a pressure increase of 88 kPa.

It is instructive to have a closer look at hole-profiles, and as simple example, we wiU consider the hydrogen molecule H2 with only two electrons or one electron pair. In O Fig. 4-3, hole densities for H2 are shown the two nuclei Ha and Hp are separated by 72 pm, and the reference electron is placed 15 pm to the left of nucleus Hp. 

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