摘要:

The spherical and chromatic aberrations of a converging electron mirror are of opposite sign from those of electron lenses. This important property makes it possible in principle to compensate the aberrations of electron lenses by means of an electron mirror and to design electron microscopes based on a corrected optics system incorporating an electron mirror. In this paper the properties of the hyperbolic electron mirror are calculated, and the conditions for simultaneous correction of spherical and chromatic aberrations are worked out for several types of electron microscopes. The hyperbolic mirror field is a rotationally symmetric potential field between two electrodes. The electrodes are shaped as equipotential surfaces of the hyperbolic field, except for an aperture on the axis of the positive electrode for entrance and exit of electrons. The effect of the aperture is to create a thin diverging aperture lens at the termination of the hyperbolic field. The properties of the mirror are calculated analytically. The problem of separating the electron beam incident on the mirror from the beam returning from the mirror without impairing the image quality is solved by means of magnetic deflecting fields located at image planes. The mirror corrections can be applied to either magnetic or electrostatic lenses. The parameters for correction of aberrations are calculated for systems using electrostatic lenses. With appropriate polarity of the accelerating voltage and the lens and mirror voltages the calculations apply to ion imaging systems as well.

ISSN:0021-8979    

DOI:10.1063/1.345212

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A plasma‐mixing device in which a helium plasma beam is crossed with a molecular nitrogen beam was used to measure the branching ratio of the excitation of individual vibrational levels of theB2&Sgr; electronic state of the molecular nitrogen ion in charge‐transfer collisions between He+2ions and ground‐state N2molecules. The branching ratio of the excitation of individual vibrational levels of theC3&Pgr; electronic state of the neutral nitrogen molecule due to the resonant energy‐transfer collisions between argon metastables and ground‐state nitrogen molecules was also measured using a plasma beam of a mixture of argon and helium. These results allow comparison of our measurements with results obtained using other techniques.

ISSN:0021-8979    

DOI:10.1063/1.345213

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A model for ion stopping in semiconductors, which considers separate stopping contributions from valence and core electrons, and explicitly includes the effect of the gap, has been used to calculate the electronic stopping power of energetic B, P, and As in Si, Ge, GaAs, and CdTe for projectile energies 10 keV–100 MeV. Account was taken of the partially stripped incident ions by means of the effective charge. There is good agreement at low ion velocity with Lindhard and Scharff’s [J. Lindhard and M. Scharff, Phys. Rev.124, 128 (1961)] values which for heavy ions do not depend on effective charge theory, as well as with the semiempirical curves at energiesE≥0.2 MeV/nucleon where they can be compared.

ISSN:0021-8979    

DOI:10.1063/1.345214

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have developed a simple analytical model for the electric and ponderomotive (trapping) potentials in linear ion traps. We have used this model to calculate the required voltage drive to our mercury trap, and the result compares well with our experiments. The model gives a detailed picture of the geometric shape of the trapping potential and allows an accurate calculation of the well depth. The simplicity of the model allowed us to investigate related, more exotic trap designs which may have advantages in light‐collection efficiency.

ISSN:0021-8979    

DOI:10.1063/1.345215

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The theory of free‐electron‐laser instability for a tenuous relativistic electron beam propagating parallel to a longitudinal wiggler field has been extended to include the influence of finite radial geometry on its stability properties. The growth rate of the instability shows a sensitive dependence on (&ngr;/&ggr;)1/3, where &ngr; is the Budker parameter and &ggr; is the beam energy. The growth rate increases with increasing beam intensity. However, the instability bandwidth with respect to the beam location inside the waveguide decreases for beams with somewhat higher &ngr;/&ggr; values. The instability is broadband for several harmonics excited.

ISSN:0021-8979    

DOI:10.1063/1.345216

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The ‘‘resonance problem’’ is that at certain values of the wave numberk(the resonantk’s), the second‐kind integral equation for solving scattering problems can become extremely ill‐conditioned. This adversely affects both the accuracy and speed of numerical solutions. We consider transverse‐magnetic scattering from a conductor (Dirichlet problem). The integral equation (derived using double‐layer potentials) is discretized using approximately fourth‐order convergent quadrature formulas. At resonantk’s for circular and elliptical scatterers, we find very large condition numbers for the discrete matrices [up toO(107) ], generally leading to poor solutions. We apply two approaches to alleviate the resonance problem. The first is to use a different integral equation, based on both single‐ and double‐layer potentials. This leads to low condition numbers and good solutions at resonantk. The second method is to use the original second‐kind integral equation, introduce a small imaginary part ink, and extrapolate back to the real axis. Solutions obtained by the two methods are in excellent agreement. The extrapolation technique will be particularly useful in the case of the exterior Neumann problem, when the application of the first technique will be numerically more difficult. By solving the resonance problem, we ensure that fast and accurate solutions are obtainable at any arbitrary wave number.

ISSN:0021-8979    

DOI:10.1063/1.345217

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report the optical bistability and the self‐oscillation of a nonlinear Fabry–Perot etalon filled with Norland optical adhesive 81. A normal and an inverted bistable loop were observed for a short (80msec) and a long (1.2sec) input optical pulse duration, respectively. The self‐oscillation properties depended sensitively on the substrate and the spacer thicknesses. The measured self‐oscillation period became longer with an increase of the spacer thickness and was in the range o 0.2 to 3 seconds. The inversion of the bistable loop, and the self‐oscillation in this experiment can be explained by the competition between the fast thermal refractive index change and the slow linear thermal expansion of the etalon.

ISSN:0021-8979    

DOI:10.1063/1.345218

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Evanescent wave absorption in an aqueous dye solution has been performed using multimode fused silica fiber which was unclad at the sensing region. Evanescent absorbance values for methylene blue in a concentration range 3×10−8to 5×10−6M are reported. In order to produce modes close to cutoff in the sensing region, tunneling modes were launched into the clad fiber. Spatial filtering was used to restrict the light launched to those modes which have substantial power in their evanescent field in the unclad region. The measured evanescent absorbance of the dye solution was found to vary linearly with the exposed core length and to exhibit a square root dependence on concentration. The former effect is predicted from standard theory while the latter is attributed to adsorption on the core surface which obeys a Debye–Huckel‐type concentration dependence. In addition, a concentration enhancement of two orders of magnitude was observed due to this surface adsorption. While this effect limits the use of the technique for a reversible sensor it may be exploited in disposable probes.

ISSN:0021-8979    

DOI:10.1063/1.346085

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Infrared upconversion offers advantages over direct detection of thermal imagery, but has been of limited use because of low upconversion efficiencies and because of the complexity of the upconversion systems. Photon‐limited imaging techniques can overcome the disadvantages of upconversion. The low‐power levels required by photon‐counting detectors make large upconversion efficiencies unnecessary. At the same time, photon‐limited imaging offers advantages in speed of operation and ease of implementation. We investigate the application of photon‐limited imaging to sum‐frequency upconverters, infrared quantum counters in alkali‐metal vapor, and a recently reported infrared phosphor. Upconversion efficiencies and noise effects associated with the different upconversion methods are derived. Two figures of merit are used to compare the upconversion methods. One figure of merit is the conventional noise equivalent differential temperature (NE&Dgr;T). The other is a criterion based on the statistics of photon‐limited images applied to scene matching. We find that the sum‐frequency upconverter and the phosphor require fewer detected photons than the infrared quantum counter to achieve the same values for the figures of merit. We also find that reliable scene matching can be performed with fewer detected photons than would be expected from the NE&Dgr;T figure of merit.

ISSN:0021-8979    

DOI:10.1063/1.345167

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A mathematical model is derived for the steady‐state temperature of an evaporating water droplet with a monolayer coating. The model uses the flux‐matching arguments of Fuchs to describe the concentration and temperature fields outside the evaporating droplet. Assuming the mean molecular speed, the gas‐phase diffusion coefficient, and the gas thermal conductivity to be temperature independent, a closed‐form expression is derived for the steady‐state temperature of the droplet as a function of the monolayer accommodation coefficient. Comparison between model predictions and recent fluorescence thermometry experiments shows good agreement.

ISSN:0021-8979    

DOI:10.1063/1.345168

出版商:AIP    

年代:1990

数据来源: AIP