Electromodulation measurements

The abrupt increase in slope at 3.37 eV signals the presence of a higher-energy transition, in accord with the increased absorption found by electromodulation measurement (15). The decrease in slope at 3.5 eV corresponds to a saturation of this contribution to the total absorption and is not understood. Nevertheless, several other materials give evidence of similar behavior. 

The fact that both the COad bands and the CO2 band appeared in the same direction on an nm-Ru/GC electrode illustrates one of the main characteristics of the AIREs. It may be worth noting that the COl band center measured in Figure 11(a) is close to that reported by Lin et al. but is about 15cm lower than the value for CO adsorbed on a massive Ru electrode in CO saturated 0.5 M HCIO4 solution studied by using electromodulated infrared spectroscopy (EMIRS) [50]. It is evident that the appearance of the COb band in the spectrum of the nm-Ru/GC electrode is due to significant enhancement of IR absorption. 

The reflectance of an electrode is modulated when an alternating potential is applied to an electrode. This technique has been applied to the study of the anodic layer on Pt by Conway and Gottesfeld. These ac electromodulation reflectance measurements yield results analogous to ac admittance measurements whereas the change of reflectance and ellipsometric parameters with and without the film present are analogous to the charge vs. potential curves. The ac electroreflectance measurements confirm the reversible adsorption at low coverage ( h < 0.9 V) and the irreversible character of film formation at Eu > 1.0 V. 

From these relations they estimated the electromodulation effect on the ellipsometry measurements, the magnitude of which turned out to be of the same order of magnitude as the effect of adsorption layer. 

Under the conditions used for the electro-optical measurements on the various device structures that are reported in sections S and 6, we will expect to see a substantial electric field applied across at least a fraction of the polyacetylene layer, and we need to characterise the modulation of the %-n absorption edge with electric field. This electromodulation response is the Franz-Keldysh effect, and arises through modulation of the electron states near the band edge in the applied field. It is found to be very large in Shirakawa polyacetylene [54,55] and it has been pointed out that this is due to the strong non-linear electronic response that characterises the conjugated polymers [55]. In the low field limit, we expect to see a response that varies quadratically with the applied electric field, and that is proportional to the second differential of the absorption coefficient, a, 92a/aE2 [54]. 

Figure 9. Electromodulation of the optical transmission for an unoriented film of Durham polyacetylene, of thickness 480 run, measured with an i lied field of 3 x 10 V/cm.

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