Quadratic Stark spectrum

Figure 7 shows the quadratic Stark spectrum of a poly(methyl metacrylate) film doped with a azobenzene-linked amphiphile, 4-octadecyloxy-4 -nitroazobenzene. Using eq. (5) and the most characteristic spectral point on the AT/T curves, where dD/dX = 0 and d2D/dXa = 0, the value of Ap was evaluated to be 5.4 debye. Further, the p value of the azobenzene-linked amphiphile was calculated to be 24 x 10 30 esu at a fundamental wavelength of 1064 nm. The p values of azobenzene-linked amphiphiles employed in this study were evaluated by the procedure mentioned here. The values are listed in Table 2 in the section 1.1.1. 

Fig.7. Quadratic Stark effect spectrum of a poly(methylmetacrylate) film doped with an azobenzene-linked amphiphile C180AZ0C00H (solid line). Dotted line, broken line, and dash and dotted line show an absorption spectrum of the film, its first derivative, and second derivative, respectively.

The reason that I am elaborating in such detail on these c.t.s. s is that they are practically the only states in region 3, and also I believe that if we can only get a clear understanding of these states, the question of optical transition will sort itself out automatically. There are also many other effects in molecular complexesjyhere the c.t.s. s enter. I have already mentioned the cases of the quadratic Stark effect and of tfie asymmetric crystal field, where the c.t.s. s must be allowed to play an equally important and indeed analogous role. A further effect relates to the width of the charge transfer bands. The main cause of the breadth is essentially the same as that for the width of the crystal-field spectrum, except that it is much... 

Electro-absorption (EA) spectroscopy, where optical absorption is observed under the application of an electric field to the sample, is another method that can distinguish between localised and inter-band excitations. The electric field produces a Stark shift of allowed optical absorptions and renders forbidden transitions allowed by mixing the wavefunctions of the excited states. Excitons show a quadratic Stark (Kerr) effect with a spectral profile that is the first derivative of the absorption spectrum for localised (Frenkel) excitons and the second derivative for charge transfer excitons, i.e. 

One of the most notable examples of the application of EA spectroscopy to organic semiconductors is polydiacetylene, in which EA spectroscopy was able to separate absorption bands of quasi-ID excitons from that of the continuum band [78]. The confined excitons were shown to exhibit a quadratic Stark effect, where the EA signal scales with and the EA spectrum is proportional to the derivative of the absorption with respect to the photon energy. In contrast, the EA of the continuum band scales with and showed Frank-Keldish (FK) type oscillation in energy. The separation of the EA contribution of excitons and continuum band was then used to obtain the exciton binding energy in polydiacetylene, which was found to be -0.5 eV [78]. 

Ultimately, the absorption spectrum is affected by both the linear and quadratic field-induced Stark effects. The electric field induces a change in the transition energy A (T), expressed as a combination of both terms from Equation 19.32 and Equation 19.34 [87,100] ... 

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