Absorbance-potential curve

In Fig. 9.5, curve (a) is the cyclic voltammogram (CV) for the oxidation of tri-p-anisylamine (TAA) in 0.1 M Et4NCl04-AN, and curves (b) and (c) are the absorbance-potential curve and its derivative curve, respectively. They were obtained using an OTE with a vapor-deposited platinum film [8]. The excellent agreement of curves (a) and (c) shows that the reaction in the CV is purely the oxidation and re-reduction of the TAA. However, the two curves are different in that the peak current for curve (a) is proportional to the square root of the voltage sweep rate, while the peak height of curve (c) is inversely proportional to the square root of the voltage sweep rate. 

Figure B3.4.12. A schematic ID vibrational pre-dissociation potential curve (wide flill line) with a superimposed plot of the two bound fimctions and the resonance fimction. Note that the resonance wavefiinction is associated with a complex wavevector and is slowly increasing at very large values of R. In practice this increase is avoided by iismg absorbing potentials, complex scaling, or stabilization.

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

Figure 3.16A shows spectra of o-tolidine in an optically transparent thin-layer electrode (OTTLE) for a series of applied potentials. Curve a was recorded after application of +0.800 V, which caused complete oxidation of o-tolidine ([0]/[R] > 1000). Curve g was recorded after application of +0.400 V, causing complete reduction ([0]/[R] < 0.001). The intermediate spectra correspond to intermediate values of Eapplied. Since the absorbance at 438 nm reflects the amount of o-tolidine in the oxidized form via Beer s law, the ratio [0]/[RJ that corresponds to each value of Eapplied can be calculated from the spectra by Equation 3.18. 

Quite similar remarks pertain to the passage (Figure 1) from chemisorbed to absorbed (dissolved) states in the bulk of some solid phases accessible to certain atoms of small radius (H, C, N, O), as exemplified by the solution of hydrogen in palladium or tungsten trioxide. Spillover therefore depends upon the crossing point of the physi- and chemi-sorption potential curves for the two solids, upon the number and kind of the chemisorbed states, and upon the possible interstate transitions. The approach of AB to homolytic or heterolytic, dissociative adsorption along the reaction co-ordinate may be represented as follows—... 

The electromagnetic radiation field is taken into account by adding the energy of the photons to the various molecular potential curves, Vi(R). If the photon number is initially N, when n photons are absorbed, the remaining number of photons is N — n. The resultant field-dressed diabatic state has potential energy... 

Fig. 10.6 — A comparison of absorbance-time curves for a potential step followed by open circuit relaxation for the ece and disp 1 mechanisms. Reproduced with permission from A. Bewick, J. M. Mellor, B. S. Pons, Hlectrochem. Acta, 23, (1978), 77.

As a further practical tip, for an OTE based on a thin conductive optically transparent layer, the resistance of the working electrode can be reduced if an additional thicker metal layer is coated on the optically transparent film in the region not exposed to the light beam under the insulation layer. If the insulation is prepared via lamination, a thin metal foil can be simply inserted instead of the deposited metal layer. This new cell design (Fig. II.6.5) enables cyclic voltam-mograms as well as current time curves in the case of a potential step experiment (chronoamperograms) simultaneously with a series of time-resolved spectra to be recorded. This experimental approach allows absorbance-potential or absorbance-time curves to be recorded to analyse the reaction kinetics of the generation of intermediates and the final product up to a time scale of approximately 0.1 s life-time. 

Another approach to laser isotope separation is offered by predissociation of laser-excited molecular isotopes into stable fragments. If the potential curve of the excited state of AB is intersected by a repulsive potential (Fig.14.3), the molecule may dissociate without absorbing a second photon. 

Sketch a potential energy diagram which might represent an endothermic reaction. (Label parts of curve representing activated complex, activation energy, net energy absorbed.)... 

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