Vibronic conduction

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-... 

The photoconductivity and absorption spectra of the multilayer polydiacetylene are shown in Fig. 22 [150]. The continuous and dotted line relate to the blue and red polymer forms respectively. Interpretation is given in terms of a valence to conduction band transition which is buried under the vibronic sidebands of the dominant exciton transition. The associated absorption coefficient follows a law which indicates either an indirect transition or a direct transition between non-parabolic bands. The gap energies are 2.5 eV and 2.6 eV for the two different forms. The transition is three dimensional indicating finite valence and conduction band dispersion in the direction perpendicular to the polymer chain. 

In conclusion, at weak molecule-to-lead (tip, substrate) coupling the usual vibronic side-band peaks in the differential conductance are observed at stronger coupling to the leads (broadening) these peaks are transformed into step-like features. A vibronic-induced decreasing of the conductance with voltage is observed in high-conductance junctions. The usual IETS feature (in-... 

Fig. 33 Vibronic emission in the symmetric multilevel model voltage-current curve, differential conductance, and the number of excited vibrons in the off-resonant (triangles) and resonant (crosses) cases (details see in the text).

Since the interfacial ET process is faster than the vibrational relaxation, one needs the transition rate from a single molecular vibronic level to the conduction band (or local states coupled to the adsorbed molecule) of the... 

Different approaches have also been proposed. For instance, Painelli et al. [23] have expressed the frequency-dependent conductivity of dimerized and trimerized organic conductors on the base of vibronic adiabatic Mulliken theory. They have shown that the calculated spectrum is virtually identical to the one obtained from linear response theory. 

All dynamic mechanical measurements employed a Rheo-vibron direct reading viscoelastometer> model DDV-II (Vi-bron, manufactured by the Toyo Measuring Instrument Co.j Ltd., Tokyo, Japan.) The temperature range employed was from -100°C to +100 C, with a heating rate of about 1 C° per minute. To better correlate with sound damping experiments conducted in our laboratory, a frequency of 110 Hz was employed. As per requirements of the instrLunent, the sample dimensions were of the order of 10 cm x 10 cm x 2 cm. 

The real part ai of a divided by o is the fraction of energy absorbed per second. All above mentioned relations are valid for the free carrier part as well as for the vibronic part of the dielectric function. In cases where the absorption process dominates the interaction, the latter may be described by the imaginary part of the dielectric function or by the real part of the optical conductivity in the form... 

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