Stark electrodes

Figure 9.23 Laser Stark spectroscopy with the sample inside the cavity. G, grating S, Stark electrodes W, window M, mirror D, detector...

The(2/ + l)-fold degeneracy ofan energy level with rotational quantum number J can be partially lifted by placing the molecule in a static electric field. This so-called Stark effect can be utilized to measure the magiutudes of the dipole moment components, fit, Mo in die principal inertial axis system of the molecule. For this purpose, Stark electrodes can be mounted outside the microwave cavity to generate a... 

Fig. III. 12. Zeeman multiplets of the 2i2—221 rotational transition of ethyleneoxide measured with AM = 0 (upper trace) and AM = 1 (lower trace) selection rule. The zero field transition frequency is marked by a dagger. The Stark lobes are pushed out of the frequency range shown in the figure by application of a sufficiently high square wave voltage to the Stark electrode...

To achieve large electric fields, the separation of the Stark electrodes is made as small as possible (typically about 1 mm). This generally excludes an intracavity arrangement because the diffraction by this narrow aperture would introduce intolerably large losses. The Stark cell is therefore placed outside the resonator, and for enhanced sensitivity the electric field is modulated while the dc field is tuned. This modulation technique is also common in microwave spectroscopy. The accuracy of 10 " for the Stark field measurements allows a precise determination of the absolute value for the electric dipole moment. 

FIGURE 2 Basic elements of a Stark-modulated microwave spectrometer. The mounting of the Stark electrode in the absorption cell is shown in the inset. 

Figure 12.10 Analysis of SFG spectra from atop CO on Pt(lll) using a CO-saturated 0.1 M H2SO4 electrolyte and a scan rate of 5 mV/s. The (2 x 2) —3CO (- /l9 x - /l9)R23.4°— 13CO phase transition resulted in a jump in atop amplitude (a). Stark tuning (b) and peak width versus electrode potential data (c) are also shown. (Filled circles denote hnear Stark tuning behavior while open circles correspond to deviations from linear behavior during oxidation.)...

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. 

In many problems for which no direct solution can be obtained, there is a wave equation which differs but slightly from one that can be solved analytically. As an example, consider die hydrogen atom, a problem that was resolved in Section 6.6. Suppose now that an electric field is applied to the atom. The energy levels of the atom are affected by the field, an example of the Stark effect. If die field (due to the potential difference between two electrodes, for example) is gradually reduced, the system approaches that of the unperturbed hydrogen atom. 

The samples for the linear Stark effect measurement were prepared as follows. First, 9 monolayers of cadmium arachidate were deposited on fused quartz plates with semitransparent A1 electrodes. Next, test layers which contained 30 layers of compound C180AZ0SN were deposited. Then, further 10 layers of cadmium arachidate were deposited. Finally, the semitransparent top A1 electrodes were vacuum-deposited. 

The measurement of the Stark effect were carried out with the electric-field modulation technique at room temp, in vacuo (about 10 3 torr). A sinusoidal ac voltage (500 Hz) was applied between the A1 electrodes. Then, the change in transmittance induced by the applied electric field were measured with a phase-sensitive detector (NF Electronic Instruments LI-575A) at the fundamental frequency. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Strongly potential dependent spectral features observed in the optical linear elec-troreflectance spectroscopy of various single crystal noble metal electrodes have been attributed to Stark shifts in surface states [105, 106, 140]. For the Ag(l 1 l)/electrolyte interface, an energy state is presumed to shift from about 5 eV at -0.20 V bias potential to about 4eV at -0.80 V. An analogous explanation is suggested to account for the behavior of Ag(l 10), which is reported to have two surface states [105,140] in... 

The real situation is less stark than that indicated. For one thing, the material on the plates may not be uniform and that which electrochemically converts to another substance at first may do so more easily than the material at deeper layers on the electrodes. One of the properties by which the value of a battery is judged is the length of the almost flat plateau, i.e., a good battery discharges for many hours with only a... 

A value of 36 has been calculated using equation (2). The variation of the center of the COb band (vb) versus E for massive Pd electrode and Pd Ec-NaA/GC is plotted in Figure 3a and 3b. Two straight lines can be observed in the case of Pd°Ec-NaA/GC. One is for E below -0.6 V, which yields a Stark shift rate (dva / dE) of 52 cm v. The second linear part is observed in the potential range between -0.6 V to -0.2 V, from which a Stark shift rate of 16 cm v has been evaluated. In comparison with the value of Stark shift rate of 47 cm v on massive Pd electrode, the small values of Stark shift rate in Figure 3 a may be attributed to the structure of Pd nanoparticles and geology in NaA zeolite. 

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