Symmetric Probe Research Articles

Density of states, electron-transport mechanisms, and chemical potentials in the presence of inelastic processes.

We present a formulation for the estimation of the density of states for quantum structures, such as the quantum wire and the single- and multiple-barrier structures. Our formulation is based on the quantum kinetic approach as opposed to the single-particle approach; therefore, the model inherently accounts for the exclusion principle and incorporates phase randomizing and/or inelastic collisions. Also in this paper, we propose a physically meaningful decomposition of the density of states for left- and right-moving electrons. The total and the decomposed densities of states are then used to define chemical potentials that may be measured by the symmetric and asymmetric voltage probes. The present approach eliminates some unphysical features reported in earlier work and enables us to propose an alternative, but equivalent, expression to the well-known Tsu-Esaki equation for the tunneling current. In our transport equations, the usual transmission coefficients are not needed; instead, we introduce the concept of a state-current density that may be used to describe the current through tunneling barrier structures. Moreover, we use this concept of the state-current density to comment on the coherent and the sequential tunneling mechanisms for double-barrier structures.

Modeling the behavior of the magnetic force microscope

Results of some numerical simulations are presented that model the behavior of the magnetic force microscope under various probe/sample configurations. Analysis is carried out under the assumption of uniform magnetization of an axially symmetric probe of various types including: sphere, cylinder, conical tip, and rounded magnetically coated conical tip. In these cases the analysis can be made by studying the effective magnetic surface charge on the probe. We also consider the case of a spherical probe with magnetization that can rotate in response to the local field. The sample configurations studied are also of the axially symmetric type, which can be used to model the fields generated by arrays of cylindrical samples, e.g., alumite.

Detection of the bunyavirus Germiston in VERO and Aedes albopictus C6/36 cells by in situ hybridization using cDNA and asymmetric RNA probes

Using Germiston virus infected vertebrate (VERO) and invertebrate ( Aedes albopictus C6/36) cells, paraformaldehyde-glutaraldehyde fixative allowed the best preservation of cellular morphology and the highest hybridization signals with cDNA and asymmetric RNA probes against the viral S segment. Asymmetric RNA probes always gave higher sensitivity and better specificity of in situ hybridization than the nick-translated symmetric DNA probe in both vertebrate and invertebrate cells. The study of Aedes albopictus C6/36 cells persistently infected with Germiston virus showed that only a small number of cells contained the S segment, and that the replication and transcription of the S segment took place in the cytoplasm of acutely and persistently infected cells.

Measurement of velocity vectors with orthogonal and non-orthogonal triple-sensor probes

A new method of interpreting the signals from triple-sensor thermal anemometer probes has been developed based on fast solution for all the roots of the non-linear Jorgensen (1971) equations describing the directional response of each cylindrical sensor. The sensors can be oriented at arbitrary angles to each other, but always within a range of probe geometries that keep prong interference and thermal wake interference below acceptable levels. The properties of a class of non-orthogonal symmetric tetrahedral probe geometries are studied in relation to the range of flow vector angles that can be measured, the sensitivity of the probe with respect to changes in flow angle, and the sensitivity of the computed velocity components due to angular errors associated with the construction of the probe. The solutions of Jorgensen's equations are inherently multiple-valued, but if the velocity vector is restricted to be within a cone of angles, they are unique. It is shown that measurements with non-orthogonal triple sensor signals are sensitive to angular deviations of a few degrees of the sensor angles from the nominally orthogonal probe geometry, indicating the need of a non-orthogonal algorithm. The mean, rms, Reynolds stress, and power spectrum of the velocity in fully developed turbulent pipe flow were measured using a specially designed triple sensor probe and the proposed algorithm.

On the significance of the fourth-rank orientational order parameter of fluorophores in membranes

Using information theory, the orientational distribution function of cylindrically symmetric probe molecules in uniaxial systems is constructed from orientational order parameters. If only the second-rank order parameter 〈 P 4 > is known, a distribution of the gaussian type results. If also the fourth-rank order parameter 〈 P 4 > is known, it is possible to distinguish between several hypothetical models. A reanalysis of time-resolved fluorescence anisotropy data indicates that the so-called 〈 P 4 >-distribution (introduced by Zannoni and described below) is a good model for diphenylhexatriene in non-oriented membranes. The behaviour of the distribution function is related to the fluctuation in P 2. This fluctuation is proportional to 〈 P 4 >, at fixed 〈 P 2 >.

Detection of mRNAs in sea urchin embryos by in situ hybridization using asymmetric RNA probes

Asymmetric RNA probes, which contain only the mRNA coding strand, provide a large increase in hybridization efficiency in situ over that observed with either symmetric (both strands represented) RNA or DNA probes. Asymmetric RNA probes are synthesized in vitro by transcription from recombinants formed between sequences encoding sea urchin mRNAs and the transcription vector R7Δ7. Using a probe representing early variant histone mRNA sequences we have characterized hybridization to sections of sea urchin embryos with respect to thermal stability of the hybrids formed, optimum temperature, effect of sequence divergence on hybrid thermal stability, and dependence of the hybridization signals on probe concentration and hybridization time. Estimates from the observed signals indicate that a large fraction of target RNAs is both retained in sections and hybridized with probe at saturation. Coupled with measurements of nonspecific background binding of heterologous probes, these data indicate that the method has sufficient sensitivity to detect many moderately abundant mRNAs (20–75 molecules per cell in the 1500-cell pluteus). In situ hybridizations to embryos at different developmental stages show that while histone mRNAs are uniformly distributed in cleaving embryos, different cell lineages of older embryos show large differences in accumulation of these mRNAs.

Theory of axially symmetric probes in a collisionless magnetoplasma: Aligned spheroids, finite cylinders, and disks

A theory is presented for current collection by electrostatic probes in a collisionless, Maxwellian plasma containing a uniform magnetic field B, where the probes are spheroids or finite cylinders whose axis of symmetry is aligned with B, or disks perpendicular to B. The theory yields upper-bound and adiabatic-limit currents for the attracted particle species. For the repelled species, it yields upper and lower bounds. This work is an extension of existing theory for spherical probes by Rubinstein and Laframboise.

Langmuir probes of general shape and electrode current distribution in a flowing plasma

The physical basis of ion collection by a negatively charged Langmuir probe in a quasineutral plasma is elucidated and arguing from the general solution of the stationary collisionless Boltzmann equation the dichotomy of current/voltage relationships below and above saturation voltage is explained. Without restriction of shape it is shown how the 3/4 law of plane probe theory may be extended in an averaging sense. It is argued that the removal of electrons to form a positive sheath adjacent to the probe proceeds isoenergetically along the equipotentials of Laplace's equation so that these remain the equipotentials of the Poisson equation for the positive sheath region which develops although the potential and field will satisfy the latter rather than the former equation. This idea is pursued for the case of 3D probe shapes matching orthogonal coordinate systems and for cylindrically symmetric probe shapes of arbitrary simply-connected cross-section and differential equations for potentials in the space-charge regions are derived. For the case of a probe in a plasma with flow velocity much above thermal maximal current densities on the collector may occur at points distinct from electrostatic field maxima loci. This is handled by treating the current density as a statistical distribution and expressing the standard deviation as a function of voltage. That which is new in this paper may be summarized as follows: (1) The generalization of shapes among Langmuir probes far exceeding anything previously attained. (2) The isoenergetic argument for the retention of equipotential surface shapes in passing from Laplace's to Poisson's equation. (3) An explanation why the current concentration lines do not follow electrostatic field lines above saturation by balancing the statistical and deterministic arguments.

Measurement of volt/meter vertical electric fields in the middle atmosphere

A test flight for a series of middle atmosphere electrodynamics rockets was launched from Wallops Island, Virginia, at 7:18 EST on July 31, 1980. The mother‐daughter configuration contained a three axis symmetric double probe electric field instrument and a blunt probe on the daughter payload, and a Gerdien condenser and a single axis (vertical) asymmetric double probe electric field instrument on the mother payload. The payloads reached an apogee of 111 km, and data were gathered from all instruments on the downleg. A downward vertical electric field with a maximum amplitude of about 4 V/m was observed in a layer between about 57 and 67 km. The integrated potential across this layer was approximately 20 kV. Conductivity measurements indicated that free electrons were absent from the region of large electric fields; however, the decrease in conductivity was insufficient to maintain vertical current continuity through the layer. These results establish the existence of large mesospheric electric fields, supporting previous results from single axis measurements.

Symmetric theory of probe-plasma interactions

The theory of interactions between a probe and the surrounding plasma at rest is developed in a spherically and in a cylindrically symmetric model (probe theory). The theory is based on the Vlasov-Poisson system; a general numerical program was developed to solve this system by means of an iterative procedure. Various ambient plasma and charged particle emission properties are described by the complete set of boundary conditions for the distribution functions in the phase space. By use of this numerical method, potential and space charge density in the whole surroundings of the probe as well as the current densities of all plasma constituents are calculated self-consistently. Furthermore, the regions of the phase space with particle trajectories of the same kind can be approximated depending on the plasma properties. Then, the current densities can be estimated analytically. This approach to the problem yields self-consistent approximations and is the only stringent derivation of the thick sheath and of the thin sheath approximation of the classical Langmuir theory. These approximations are generalized with respect to the charged particle emission from the surface. The symmetric probe theory is applied to the following problems of spacecraft environment and spacecraft charging: (i) a spacecraft in the ionosphere with very negative surface potential, (ii) a spacecraft in the solar wind with strong photoelectron emission, and (iii) a spacecraft in the transition region of comet Halley with very strong secondary plasma emission.

A theory of fluorescence depolarization in membranes

A theory of time dependent fluorescence depolarization for a probe in a membrane vesicle or in a poly-liquid-crystalline system is presented. General, model independent expressions for the polarization ratio r(t) as a function of time are obtained in terms of orientational correlation functions ϕmn(t). It is shown that only certain combinations of ϕmn(t) corresponding to relative orientations correlation functions contribute to r(t). Short time, r(0) and long time, r(∞) values are worked out in detail for biaxial and cylindrically symmetric probes in a locally uniaxial mesophase. The dynamical information available in the experiment is discussed in detail for uniaxial probes using the diffusional and strong collision reorientation model. It is shown that the two models predict different initial slopes r(0). Steady state expressions are obtained and the differences with previous theories discussed. Finally the effect of overall vesicle reorientation is briefly examined with a view to the application of the...

First measurements of electron temperature in theDregion with a symmetric double probe

Measurement of the altitude profile of electron temperature in the ionospheric D region with the aid of a symmetric double probe flown on a Nike-Cajun payload launched on Oct. 13, 1971. The procedure for determining the electron temperature from the parameters of the double probe's current-voltage characteristic under conditions of nonnegligible ion-atom collision frequencies is described. It is shown that in its first lower ionospheric application the technique of the symmetric double probe has yielded the lowest values of electron temperature yet measured and has provided the very first direct measurement of electron temperature in the D region.

Transition Region Response of the Symmetric Double Probe and Its Application in the Lower Ionosphere

The technique of the symmetric double probe lends itself to the in situ measurement of plasma temperature in the ionospheric D region because it can lead to meaningful results under relatively high collision frequencies where the Langmuir probe has been observed to fail. For a completely thermalized plasma and over parametric ranges 0.1≤λia / Rp ≤100 and 0.004≤Rp/λD≤100, where λia is the ion-atom mean free path, Rp is the probe radius, and λD is the Debye length, it is shown that the modification to the original collision-free double-probe theory of Johnson and Malter for the determination of electron temperature is never greater than 12%, with a value of (8 ± 2)% nominally applicable in the case of D-region diagnostics. This technique has been successfully operated on a Nike-Cajun payload flown at midday from White Sands, N. M. to an apogee of 78.5 km. The associated electronics and deployed double-probe configuration are presented, and a current-voltage characteristic collected in the ascent stage at 73.7 km is briefly discussed. The values of electron temperature indicated by the sampled data are [inverted lazy s] 30% higher than those predicted by theory for the anticipated state of thermal equilibrium with the ambient neutrals. The importance, however, is that these values, of the order of 300 K, are the lowest ever reported by any in situ method operative in the D region and consequently point out the strong potential of the double-probe technique in future applications to regions of planetary ionospheres which cannot be considered as collision-free.

A continuum theory of electrostatic probes in a slightly ionized gas

A systematic continuum theory based on the method of matched asymptotic expansions is developed to deal with electrostatic probes in a slightly ionized gas. To the first approximation of thin space charge sheath, the sheath solution is shown to become common to the geometries considered, i.e. plane, cylindrically and spherically symmetric probe surfaces: the main interest lies in the last geometry. The second-order approximation strongly affects the last two geometries. Detailed numerical calculations are presented to illustrate the effects of a finite ratio of the Debye length to the characteristic body dimension; including a contraction of the sheath region and enhancement of the electric field from the asymptotic (first-order approximation) values.

Probe measurements in an electrodeless discharge.

Radial distributions of number density, electron temperature and plasma potential measured in electrodeless discharge, using symmetric and asymmetric double probes