Field modulation technique

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. 

A thorough discussion Is given of the field modulation technique, a new stationary relaxation method based on electric field perturbation of Ionic equilibria. Concomitantly the theory of electric field effect In Ionic systems Is reviewed especially stressing their Importance for conductance phenomena In low polar solutions. 

Conductance relaxation Is also shown to be critically dependent upon aggregation equilibria affecting non-conducting (ion-pairs) as well as Ionic species. The relaxation behavior In the presence of quadrupoles (ion-pair dimers) and triple Ions Is thoroughly analyzed. The experimental results show the potential of the field modulation techniques as a method for the Investigation of ionization processes, independent of conductance measurements. 

Before proceeding to a complete calculation relating the conductance Increase to the amplitude of the field dissociation effect and Its temporal behavior we will give a short description of the actual circuit used for the field modulation technique (Figure 1) with reference to the previous discussion. 

From the general discussion of the use of nonlinear properties to investigate dynamic phenomena it is clear that the field modulation techniques is but one example of a broader class of methods for the study of fast processes. A drawback, particular to electric field modulation, is the prohibitive heat dissipation in conducting systems. However, any forcing parameter imposing a conductance modulation could be used in principle as, for example, in the study of the dynamics of photoconductive phenomena. 

The field modulation technique has been successfully applied to a broad variety of systems showing a field dependent conductance. Since a large amount of conductance data Is available for low polar solutions of tetraalkylammonium salts these systems were studied most thoroughly. In the subsequent discussion we will rely mainly on the results obtained for these systems but the general conclusions apply directly to other systems, e.g. fluorenyl-salte In ethereal solvents, metal complexes In benzene, etc. 

Figure 1. Schematic circuit for the field modulation technique.

The dHvA measurements were carried out using a field modulation technique at liquid helium temperatures and in magnetic fields up to 6 T. Second harmonic frequency signals of the pick-up coil were detected and analyzed by fast-Fourier analysis. 

Solid-state electron spin resonance (ESR) spectra at 9.25 GHz were also measured, using microwave power levels <1 mW and conventional field-modulation techniques. 

Upon applying a field modulation technique, it was possible to record directly field-induced changes in the AT/T spectra. Therefore, the kinetic traces in Fig. 2.2 reflect the time dependence of the field-induced differential transmission (AT/T)fm. which is the difference between AT/T recorded in the presence and absence of the electric field (AT/T)fm = (AT/T)f-(AT/T)f o. 

From the pressure dependence of the dissociation process of the tetrabutylammoniurapicrate (TBAP) ion-pair in mixtures of benzene-chlorobenzene (2.27 D 3.87) the volume change of the dissociation (AV) and the activation volumina for ionic dissociation, respectively ionic recombination (AV, resp. AV ) were obtained. The volume change of ionic dissociation was evaluated from conductance data at different pressures while the activation volumina were obtained from the pressure-dependence of the relaxation time of the dissociation equilibrium. Relaxation measurements were performed with the electric field modulation technique. 

Changes in cell-constant at high pressure are irrelevant for the kinetic measurements performed by electric field modulation techniques (8) the results of which are independent of cell geometry. The density of the benzene-chlorobenzene mixtures was calculated, assuming ideality, from the compressibility of the pure solvents. Measurements of the dielectric constant in the sample cell at different pressures yielded the change of dielectric constant with pressure (9D/9P), which is the important parameter for the description of electrostriction. The viscosity at different pressures was extrapolated from the pressure dependence for the pure solvents and the viscosity, at 1 atm, of the mixtures. 

The activation volumina for the solutions at the lower polarity are lacking because in this region the relaxation time is predominantly determined by triple-ion formation even at the lowest TBAP concentration measurable with the field modulation technique. If the ionisation equilibrium is treated as the association-dissociation of hard, charged, spheres subjected to Brownian motion in a continuous medium of dielectric constant D and viscosity n it is possible to give theoretical expressions for and k ... 

Figure 3.28 shows the magnetoresistance of a-(ET)2KHg(SCN)4 at 0.1 K for H b axis (whereby the fc -axis is the normal to the ac-plane) [527]. One can see that magnetoresistance oscillations are observed for FI> cal T. Figure 3.29 shows the corresponding SdH signal at 0.05 K for H Zi -axis, obtained by a standard low-ffequency field modulation technique [527]. The Fourier transform spectrum is presented in the inset of Figure 3.29. This spectrum...

Figure 3.29. SdH signal of a-(ET)2KHg(SCN)4 measured at 0.05 K by the field modulation technique. (Reproduced by permission of Pergamon Press from ref. 527.)...

By modulating the electric field and using phase-sensitive detection methods, Uehara et al. 8 ) were able to increase the sensitivity considerably and they could even detect Stark splittings of less than the doppler width of the components. Fig. 3 shows the Stark spectrum of HDCO for different electric field strengths. Because of the Stark modulation technique the absorption lines appear differentiated the zero points represent the center of each line. 

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

A subset of electron-hole radical pairs exhibits features of Spin Correlated Radical Pair (CRRP) electron spin polarization mechanism [101] which can be observed at somewhat longer times via light/field modulated (LFM) EPR measurements. This technique is only sensitive to the light dependent part of the EPR spectrum on the time scale of the light modulation frequency (millisecond regime, insert Fig. 1.15). Using LFM EPR it was observed that both the transitions of the holes localized on the surface modifier and electrons localized on the Ti02... 

The use of magnetic field modulation together with phase-sensitive detection using a lock-in amplifier renders CW-NMRI an extremely narrow-bandwidth detection technique, and thus the penalty inherent in all other techniques of a reduced SNR with increasing gradient strength (due to the increased frequency bandwidth) is removed. 

Although such a 2-MHz ESR apparatus was very sophisticated, its time resolution was not enough for measurement of CIDEP. In 1973, Fessenden [5] found that the direct ESR measurement without field modulation improved the time resolution, observing CIDEP signals in solution with pulse radiolysis. This method was applied for laser-photolysis measurements in solids [6] and in solution [7]. A spin-echo ESR technique was also found to be useful for CIDEP [8]. Since then, CIDEP experiments with cw-ESR and pulsed-ESR spectrometers without field modulation have become much more popular than before. Through such transient ESR measurements, CIDEP due to not only the radical pair mechanism but also several other mechanisms have been observed in many chemical reactions including biologically important ones such as photosynthesis reactions. In this chapter, we will show several mechanisms for CIDEP with several typical examples. 

It was found that the surface of the photosensitive film was modulated by the intensity distribution of light induced by the fine structures of the specimen. The cilia of a paramecium were clearly observed. One could recognize that the end of each cilium branched into two cilia. Because the spatial resolution of the observed result is smaller than 100 nm, the evanescent wave distribution generated by the fine structures of the specimens were imaged with the near-field recording technique. 

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