Stark tuning

Stamenkovic V, Chou KC, Somoijai GA, Ross PN, Markovic NM. 2005. Vibrational properties of CO at the Pt(l 1 l)-solution interface The anomalous Stark-Tuning slope. J Phys Chem B 109 678-680. 

Corrigan and Weaver, 1998 Kunimatsu et al., 1985a, b Korzeniewski et al., 1986 Tian et al., 1997]. At the onset of chemisorbed CO oxidation, the CO frequency evidences the anomalous Stark tuning behavior noted by Ross and Markovic and coworkers (Fig. 12.8), who associated it with compression of CO islands by adsorbed anions [Stamenkovic et al., 2005]. 

Figure 12.8 CO-saturated electrolyte in the thin cell of Fig. 12.2. (a) CO oxidation the first and second scans are shown, (b) Comparison with the CO stretch frequency shift. (Filled circles denote linear Stark tuning behavior while open circles correspond to deviations from linear behavior during oxidation.)...

Figure 12.16 Potential dependent SFG spectra (a) and the Stark tuning plot (b) from chemisorbed CO on Pt nanoparticles in a CO-saturated 0.1 M H2SO4 electrolyte. Each spectrum was acquired for 10 s (forward scan data only are shown). The potential was scanned from — 0.20 to 0.70 V (vs. Ag/AgCl) at 1 mV/s. Pt nanoparticles were of approximately 7 nm size, and were immobilized on an Au disk.

BB-SFG, we have investigated CO adsorption on smooth polycrystaHine and singlecrystal electrodes that could be considered model surfaces to those apphed in fuel cell research and development. Representative data are shown in Fig. 12.16 the Pt nanoparticles were about 7 nm of Pt black, and were immobilized on a smooth Au disk. The electrolyte was CO-saturated 0.1 M H2SO4, and the potential was scanned from 0.19 V up to 0.64 V at 1 mV/s. The BB-SFG spectra (Fig. 12.16a) at about 2085 cm at 0.19 V correspond to atop CO [Arenz et al., 2005], with a Stark tuning slope of about 24 cm / V (Fig. 12.16b). Note that the Stark slope is lower than that obtained with Pt(l 11) (Fig. 12.9), for reasons to be further investigated. The shoulder near 2120 cm is associated with CO adsorbed on the Au sites [Bhzanac et al., 2004], and the broad background (seen clearly at 0.64 V) is from nomesonant SFG. The data shown in Figs. 12.4, 12.1 la, and 12.16 represent a hnk between smooth and nanostructure catalyst surfaces, and will be of use in our further studies of fuel cell catalysts in the BB-SFG IR perspective. 

The dependence on dosing order is immediately evident upon examination of the plots of versus 0K and vCo, the CO stretching frequency, versus . Dosing K and CO prior to D20 yields larger Stark tuning slopes and larger work function... 

H The Stark effect is the result of altering the charge of the metal surface, which can strengthen or weaken the adsorption of the surface species. The effect can be quantified by the change in vibrational peak against the surface potential (i.e., Stark tuning slopes). 

Figure 6. Infrared COl stretching frequency and Stark tuning rate as a function of particle size, determined from SNFTIR spectra of CO species resulting from the adsorption of methanol from 0.5 M CH3OH in 0.1 M HCIO4 solution. ...

Potential-dependent frequencies in spectra of adsorbates in electrochemical interfaces are commonly observed. Thus so-called Stark tuning rates, Qv/QE, of 30cm have been reported for adsorbed CO on platinum [65, 111] (Fig.59) and adsorbed CN on silver [109, 111, 162]. Even higher values were found for sulfate species adsorbed on polycrystalline platinum (100 cm V [36, 38]) or on single crystal Pt(lll) [141, 143] (120cm V ). In some cases, as for adsorbed tetra-cyanoethylene [163] and anthracene [164], vibrational features which are forbidden by the surface selection rule become active under the influence of the electric field at the interface. 

To a good approximation the Stark tuning rate is given by... 

Applying the model to adsorbed CO, Lambert has evaluated the coefficients a from experimental data. Thus 020 was taken from the unperturbed frequency (vq), 03 0 from gas-phase values of a Morse potential, o, j from the IR cross-section of adsorbed CO and 021 from the first overtone observed by EELS. Equation (18) has been applied successfully to CO/Ni(lll) and CO/Pt, both in the gas phase and at electrodes, reproducing the observed Stark tuning rate with a good accuracy. 

It is very interesting that the Stark tuning rate is coverage-dependent for adsorbed CO, in electrochemical media [55-57] as well as in vacuum [172]. It is found that at lower degrees of coverage the Stark tuning rate increases significantly. This depen-... 

The BaO A "1" state is perturbed by the b3II and A1 states. The effect of perturbations on the Stark tuning coefficients of a 1E+ state is more complicated than the situation described by Eq. (6.5.21) (because parity remains an almost good quantum number in A = 0 states at f-fields that are sufficiently large to cause complete parity mixing in A > 0 states). There will be Stark tuning... 

We now turn our attention to the field-dependent C-0 stretching frequency. Stark tuning slopes were calculated from the slope of the v-F curve at F = 0, for both CO and NO (nitric oxide) on the four different transition-metal surfaces Rh, Ir, Pd, and Pt. A comparison with the DFT-computed Stark tuning slopes with the experimental behavior is shown in Figure 18. The experimental Stark tuning slopes dv/dE, expressed in wavenumbers per Volt, were converted into field units, i. e. wavenumbers per Volt/Angstrom, by using the relation... 

A. The dvoprldF values are seen to be in rough accordance with the dVaxp IdF estimates, although the variations of the former are smaller. These discrepancies are probably due to the role of co-adsorbed solvent in affecting the local electrostatic field. Nevertheless, the larger Stark tuning slopes computed for NO vs. CO are in accordance with experimental observations. 

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