External electric excitation

A current carrying electrode is used to trigger (excite, pace) a nerve. The electrode may be on the skin (transcutaneous excitation), as a needle inserted through the skin, as a catheter electrode, as a part of an active implant, or as a microelectrode into an exposed nerve or axon. 

The choice of the best parameter for defining a trigger level is difficult The level may be expressed as current [mA], current density [mA/mm ], voltage [V], electric field [V/m], 

Weiss (1901) and the Lapicque (1907) presented slightly different models of current-duration relationships for rectangular pulses (Reilly, 1998). Weiss used the equation 

The empirical current-duration relationship is in somewhat better accordance with the hyperbolic model than the exponential, but the exponential model is directly derived from the electric circuit model with a current source supplying an ideal resistor and capacitor in parallel. The empirical current-duration relationship is different for myelinated and naked axons. Also, it must be remembered that the excitation process is nonlinear and not easily modeled with ideal electronic components. 

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Semiconductor control modules gate the thyristors, which switch cm rent to the motor field at the optimum motor speed and precise phase angle. This assures synchronizing with minimum system disturbance. On pull-out, the discharge resistor is reapplied and excitation is removed k> provide protection to the rotor winding, shaft, and external electrical system. The control resynchronizes the motor after the cause of pull-out i.n removed, if sufficient torque is available. The field is automatically applied if the motor synchronizes on reluctance torque. The control is calibrated at the factory and no field adjustment is required. The opti-... 

The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

Another important property of the outer membranes of nerve and muscle cells is their susceptibility to excitation under the effect of electric action. Excitation can be brought about, for example, by an external electric current pulse. Pulses can be applied to the membrane with the aid of two microelectrodes, one residing in the extracellular fluid and the other introduced through the membrane into the cytoplasm. 

W.Liptay, Dipole Moments of Molecules in Excited States and the Eifect of External Electric Fields on the Optical Absorption of Molecules in Solution, in Modem Quantum Chemistiy, ed by O.Subabiglu, Academic Press, New York, 1965. 

An optical microcavity produced by the latter process has been applied to tune the emission from erbium-doped PS [Zh6], Erbium compounds like Er203 are known to exhibit a narrow emission band at 1.54 pm, which is useful for optical telecommunications. Several methods have been used to incorporate erbium in PS. A simple and economical way is cathodic electrochemical doping. External quantum efficiencies of up to 0.01% have been shown from erbium-doped PS films under electrical excitation [Lo2]. The emission band, however, is much broader than observed for Er203. This drawback can be circumvented by the use of an optical cavity formed by PS multilayers. In this case the band is narrowed and the intensity is increased because emission is only allowed into optical cavity modes [Lo3]. 

The cantilever is excited into resonance by electrically exciting the piezoelectric cantilever mount. The frequency of the excitation wave is scanned in a given frequency range, and the frequency of maximum cantilever amplitude is taken as the resonance frequency. The frequency spectrum of the cantilever response shows the fundamental frequency as well as the harmonics of cantilever vibration. The cantilevers, however, also resonate in response to ambient conditions such as room temperature or acoustic noise without requiring any external power. 

The effect of an external electric field is to produce an acceleration of the electrons in the direction of the field, and this causes a shift of the Fermi surface. It is a necessary condition for the movement of electrons in the fc-space that there are allowed empty states at the Fermi surface hence electrical conductivity is dependent on partially filled bands. An insulating crystal is one in which the electron bands are either completely full or completely empty. If the energy gap between a completely filled band and an empty band is small, it is possible that thermal excitation of electrons from the filled to the empty band will result in a conducting crystal. Such substances are usually referred to as intrinsic semiconductors. A much larger class of semiconductors arises from impurities... 

Finally we should mention the electrochromic shifts which are the changes in the energy of electronic transitions when an external electric field is applied to the sample. These effects are quite small but have proved useful for the measurements of the dipole moments and polarizabilities of excited molecules. 

Interest in spectral hole burning comes from its potential application as high density computer memory. An external electric field can be used to shift the spectral holes so as to obtain an even larger number of detectable holes than with the excitation wavelength alone. 

An external electric field leads to three alterations in the electron structure of an atom. Firstly, the energy levels of the atom are shifted and split (the Stark effect). The theory of this effect is well-known [8], Secondly, the highly excited states of the atom disappear. The potential for the outer electron of the highly excited atom, is equal to... 

Thus - beside the electrical pretreatment - photo-excited charge carriers play an important role during the determination of the optical hysteresis. Consequently it is a decisive experimental requirement that illumination is performed when a steady state of Ps is reached after the application of an external electric field Ea, and in addition, that the scattered light is detected in the steady state of the light scattering process. Both requirements are met in our experiments. 

It was revealed, that the photocurrent increases in MF B=0.4 T have a maxima in external electrical fields (EF) at 4.2xl04, 3.1xl04 and 2xl04 V/m at excitation by light with photon energies 2.64, 3.07, 3.87 eV accordingly (Fig. lc). 

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