Stark effect, electrochemically

In EMIRS and SNIFTIRS measurements the "inactive" s-polarlsed radiation is prevented from reaching the detector and the relative intensities of the vibrational bands observed in the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that vibrational coupling to adsorbate/adsorbent charge transfer (11) and also w electrochemically activated Stark effect (7,12,13) can lead to apparent violations of the surface selection rule which can invalidate simple deductions of orientation. 

Because of the short lifetime of ions in gaseous atmospheres, even at low pressure, gas-phase IR measurements are limited to adsorption of neutral molecules. Electrochemical applications of the IR method offer the interesting possibility of providing data on the adsorption properties of charged particles (Secs. 8 and 9). In the electrochemical environment the applied potential allows ionic adsorbates to be studied under energetically controllable conditions. Otherwise the electrochemical double layer offers exceptional conditions to investigate the Stark effect on vibrational transitions by setting tunable electric fields of the order of 10 V cm at the interface. This phenomenon will be discussed in Sec. 10. 

In electrochemical systems the energetics of the interface can be controlled very elegantly by changing the electrode potential. In the case of CO adsorption, mechanisms such as backbonding should be very sensitive to potential change, i.e., to the electronic density of the surface. Furthermore, the strong electric field in the inner double layer can produce a band shift known as the Stark effect, as discussed in Sec. 10 in more in detail. 

The Stark effect requires the application of electric fields of the order of 10 Vcm or higher. The electrochemical interface, where molecules and ions are subjected to fields in the order of 10 Vcm S seems to be the ideal place to study this phenomenon. 

In the next sections we discuss the available data on the electrochemical Stark effect on the vibrational spectrum of adsorbed carbon monoxide and adsorbed sulfate ions at platinum. 

In contrast to the Stark effect on the vibrational frequency, the changes of the transition probability with the electric field have been investigated less. For electrochemical systems an expression for the integrated absorption coefficient, B, was proposed by Korzeniewski et al. [164] as follows ... 

The electrochemical double layer offers the exceptional possibility of investigating the Stark effect at very high electric fields. Some important progress has been made in the theoretical treatment of the problem. Experimental data of potential effects upon the frequency and/or the intensity of vibrational modes must discriminate between the pure electric field effect and the secondary effect of potential on the coverage and, consequently, on the lateral interactions. 

The variations of vcol versus Es on the five nm-PtRu/GC electrodes are shown in Figure 16 [48]. The Stark shift for COl on all nm-PtRu/GC alloy film electrodes was measured to be around 34 cm V In contrast to the nm-Ru/GC electrode, where two Stark shift rates (a small value in the low potential region and a large value in the high potential region) were obtained, only one straight line can be drawn through the experimental data points. The results clearly demonstrate that the properties of an nm-PtRu/GC electrode are not a simple combination of the properties of nm-Pt/GC and those of nm-Ru/GC. The fact that the band center, the FWHM, and the Stark effect of the COl band all lie in between the values of nm-Pt/ GC and those of nm-Ru/GC confirmed that the alloy of PtRu thin film was formed by electrochemical co-deposition under cyclic voltammetric conditions. 

Lambert, D.K. (1996) Vibrational Stark effect of adsorbates at electrochemical interfaces. Electrochimica Acta, 41, 623-630. 

Hence Korzeniewski and Pons [75] chose to study pyrene as a possible example of this "electrochemical Stark effect [76] since it was known to adsorb flat on an electrode and was highly polarizable and it did, indeed, exhibit a band forbidden by the surface selection rule. 

Tunneling is a ubiquitous phenomenon. It is observed in biological systems (1), and in electrochemical cells (2). Alpha particle disintegration (3), the Stark effect (4), superconductivity in thin films (5), field-electron emission (6), and field-ionization (7) are tunneling phenomena. Even the disappearance of a black hole (or the fate of a multi-dimensional universe) may depend on tunneling, but on a cosmological scale (S-9). 

The vibrational frequency of an adsorbate on a surface varies with applied electrostatic field [29,30]. This effect is referred to as the Vibrational Stark Effect (VSE), and is observed both under UHV and under electrochemical conditions (i.e. conditions in which the metallic surface constitutes the working electrode in a three-electrode cell). This naming of the phenomenon comes from the fact that the observed frequency shifts may be properly accounted for as consequences of a physical Stark effect interaction of the (field-free) adsorbate dipole moment with the field, as must rigorously be the case in... 

Electrochemical Stark effect As described already in Section 2.10.2, the Stark effect is based on the interaction of the interfacial static electric field with the transition electric dipole or molecular polarizability. The Stark effect may give rise to the first (or sometimes second) derivative of the absorption spectmm, depending on the type of interaction with the electric field. It is important to note that the ER signal due to the Stark effect should have the same frequency dependence as the ac change of the static electric field insofar as orientation change does not take place simultaneously, because the Stark effect is a field effect. In fact, this has been experimentally confirmed by frequency domain analysis [82]. 

Before moving on, one important exception to the SSR is worth noting, which arose from the work of Bewick, Pons and coworkers [92, 93] on the adsorption of acrylonitrile, and the later work of Ko-rzeniewsld and Pons [94-96]. In essence, the work showed that the vibrations of an adsorbed molecule that are parallel to the electrode surface may become activated as a result of the electric field, and this was termed the electrochemical Stark effect [94-96] as would be expected, this effect depends very strongly on the nature of the adsorbed molecule. Thus, for example, Pons and coworkers [96] observed a bipolar band centered near 1600 cm in the in situ infrared spectrum of anthracene on reducing it to its radical anion the band was attributed to the Ag C—C symmetric stretch of the anthracene shifting to lower V on reduction. As the molecule adsorbs... 

Electrochemical Stark Effect and the Influence of a Solvent Surface vibrational spectra of adsorbed species on an electrode are often characterized by... 

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