Electroabsorption spectra

A further clear establishment of the absoiption due to singlet excitons and the phonons coupled to them is the electroabsorption experiment reported in Ref. [18]. The main results are shown in Figure 9-14 the top panel shows the absorption spectrum of m-LPPP at 20 K. It becomes clear that the peaks at 2.7, 2.9, and 3.1 eV, representing A0, A i, and A2 (see Fig. 9-10) are not the only vibronic replicas. There are additional peaks between these dominant ones if the experiment is conducted at low temperature. The bottom panel in Figure 9-14 shows a so-called electroabsorption spectrum which is obtained as the modulation (or change) of the absorption under the application of an electric field. Below 3.2 eV the electroab-... 

Figures 6 and 7 show absorption and electroabsorption spectra of [ (NH3)5Ru 2(/A-pyz)]5+ and [ (NH3)5Ru 2(M,4 -bpy)]5+, respectively. The change in AA as a function of x is uniform for the bands, which indicates that the molecular properties that give rise to AA are identically oriented with respect to the transition dipole moment. The electroabsorption spectra in the near-IR region (MMCT bands) give the greatest differences between complexes when analyzed with Eq. (31) and these are shown in Fig. 8. For the Creutz-Taube ion (Fig. 8A), the spectrum does not satisfactorily reduce to a sum of derivatives but nevertheless shows that AA(p) line shape to be modeled primarily by a negative zeroth derivative (Ax) term, especially at energies below 6500 cm-1. The fit in this case yields a value for Ap. = 0.7 0.1 D, which when compared with the maximum permanent electric dipole moment ( A/u max = 32.7 D, assuming a metal-to-metal distance) is strong evidence for a delocalized ground state. Contrast this result with the analysis of the electroabsorption spectrum of [ (NH3)5Ru 2(ja-4,4 -bpy)]5+ shown in Fig. 8B.

Figure 15 The absorption spectra, the first derivative of the absorbance with respect to photon energy, and the electroabsorption spectrum of AZO-FO.

It should be noted that the above classification of the electroabsorption spectrum is valid only approximately, because first of all eqn (11.11) is correct only in the case of weak absorption and, second, the Frenkel and CT exciton states usually mix. We finally mention that the change of the refractive index Sn is of the same order as 5k new experimental techniques are required to measure this change, however. Good candidates for such methods have been proposed by War-man and coworkers (18). The success of such measurements could be the basis of electrorefraction spectroscopy, complementary to the existing electroabsorption spectroscopy. 

Figure 10.2 illustrates the electroabsorption spectrum of phenyl-substituted traras-polyacetylene thin film (Liess et al. 1997). The feature at 2.0 eV is the red-shifted l Bu exciton. The feature at 2.5 eV is attributed to a dipole-forbidden state, namely the m Ag state. Unlike polydiacetylene crystals, disordered trans-polyacetylene thin film does not exhibit Pranz-Keldysh oscillations (described in Chapter 8) and therefore a definite assignment of a conduction band edge cannot made. However, because disordered polydiacetylene also does not exhibit Pranz-Keldysh oscillations, but a smeared-out feature similar to the one exhibited at 2.5 eV in Fig. 10.2 it is sometimes assumed that this feature does mark the band edge. Another interpretation is that this feature represents the n = 2 Mott-Hubbard exciton, described in Chapter 6, with the particle-hole continuum lying close in energy (possibly at 2.7 eV, which is three times the THG feature at... 

Fig. 10.2. The electroabsorption spectrum of phenyl-substituted trans-polyactelyene thin film. Reprinted with permission from M. Liess, S. Jeglinski, Z. V. Vardeny, M. Ozaki, K. Yoshino, Y. Ding, and T. Barton, Phys. Rev. B 56, 15712, 1997. Copyright 1997 by the American Physical Society.

Finally, we remark on the m Ag state shown in the electroabsorption spectrum of Fig. 10.2. While it is possible that 2.5 eV is the vertical transition energy of the 2 A+ state, the THG experiments of Fann et al. (1989) indicate that the vertical transition energies of the 2 A+ and states are virtually degener-... 

A phase transition in PTS at low temperatures (15) causes splitting of the Tf-ir transitions. Similar splitting occurs for the defect electroabsorption peak as shown in Fig. 4. At higher temperature down to 150 K the electroabsorption spectrum shows a broad, slightly asymmetric peak which shifts to higher energy as the temperature decrea-... 

FIGURE 4 Dependence on temperature of the absorption (dashed curve) and of the electroabsorption spectrum in the region of defect absorption. 

From the experimental observations we conjecture that the electroabsorption spectrum arises from a transition of an electron local -... 

The electroabsorption spectrum in a static electric field F is conventionally recorded as 7(cu,F) - 7(w) and is formally Im -w w,0,0). Stark shifts are familiar in atomic, molecular, and extended systems. F mixes and shifts electronic states. We neglect mixing of vibrational states, whose transition dipoles are much smaller. The spectrum 7(w,F) contains both allowed and field-induced transitions. 

Fig. 8. (A) Fit (—) to electroabsorption data (O) for l (NH3)5Ru 2(/ -pyz)]5 at x = 90° in the near-IR region of the spectrum. Zeroth ( ), first ( ), and second < ) components of the fits are also shown. (B) Fit (—1 to electroabsorption data (O) for [ (NH3)5Ru 2-(/u.-4,4 -bpy)]5 at x = 90° in the near-IR region of the spectrum. Zeroth ( ), first ( ), and second (A) components of the fits are also shown. [Reproduced with permission from Ref. 118). Copyright (1991) American Chemical Society.]...

The formation of the impurity exciton leads to an appearance of photo-induced lattice vibrations. Sokolov has offered an experimental electroabsorption (EA) method, which has a very high sensitivity for detecting such vibrations. However, the analysis of experimental data supposes a theoretical study of the vibrational spectrum of the defective crystal. 

The existence of even parity states lying above the have been identified by two-photon absorption (TPA), two-photon fluorescence excitation [31], and electroabsorption (EA) spectroscopy. We measured the TPA spectrum of DOO-PPV by Z-scan [32] showing both the real and imaginary components of Figure 7-13 compares the linear absorption spectrum of DOO-PPV to the spectra of the real and imaginary molecular second hyperpolarizability. The imaginary component, y", is proportional to the TPA coefficient a2, while the real component, y, corresponds to the non-linear dispersion, 2- The y" spectrum shows a clear peak at % 3.2,5 eV, 0.5 eV above the peak of the l.B absorption band, and a shoulder at se3.5 eV, which may correspond to a second TPA band. The y spectrum clearly indicates dispersion near the two y" peaks, as expected from Kra-mers-Kronig analysis, and permits us to identify two distinct TPA bands. 

An alternative means for the experimental evaluation of //da is by means of electroabsorption spectroscopy. " The electric field dependence of the absorption spectrum can be used to obtain the difference in the ground state and excited state dipole moments, A/UdaI- This may be combined with the transition dipole moment (in Debyes), l gel = 16[Cda/(108 x 10 da)]. to obtain (neglecting vibronic and nonresonance overlap contributions), ... 

Another method to assess delocalization is based on the measurement of the electric dipole change associated with the intervaience transition. This is done by electroabsorption spectroscopy which measures the effect of an electric field on the spectrum of the sample in a glassy matrix. " In a weakly coupled system, the dipole change should be large as the electron is... 

Both defect absorption and electroabsorption grow during the thermal polymerization process though differently. The absorption band saturates after about 6 h at 80°C. The electroabsorption peak increases rapidly when the autocatalytic range is reached, together with a red shift by about 70 meV similar as the excitons (Fig. 2, PTS-7). As shown in Fig. 3 the growth of Aa follows closely the polymerization curve (14). When most of the material is polymerized, however, Aa decreases again to about half its peak value quite in contrast to the absorption spectrum. 

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