摘要:

Ion motion in a Penning trap and the electrical signals it can produce have been analyzed for the purpose of identifying the important causes of uncertainty in high‐accuracy mass measurements of heavy ions. The role of the azimuthal quadrupole electric field in signal pickup, and its effects on ion motion at the sum frequency of the cyclotron and magnetron motions, have been identified. A useful scheme for calculating the signal strength and strength of the interaction between an applied field and the ion motion has been developed. The important sources of uncertainty in using the sum frequency of the cyclotron and magnetron motions for determining the ion mass are discussed. Particular application is made to the case of cyclotron resonance detection by observation of the time of flight of ejected ions.

ISSN:0021-8979    

DOI:10.1063/1.346185

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Time‐dependent production of higher‐silane gases anda‐Si:H film are measured relative to decomposed silane in rf and dc, hot and cold cathode, static‐gas discharges. From the absence of higher‐silane production in very low silane partial‐pressure discharges, it is inferred that most higher silanes are produced by gas‐phase SiH2‐initiated reactions. The higher silanes are thus tracers of SiH2, while the film production traces the fraction of H, SiH, and SiH3in the initial decomposition. From the measured stable product yields, we deduce that SiH4→SiH2+2H is the dominant electron‐collisional dissociation channel.

ISSN:0021-8979    

DOI:10.1063/1.346186

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Penetration of an applied quasistatic electric field inside an open, conducting, spherical shield is analyzed. The shield is either empty or can be loaded with a concentric spherical conductor, and its opening is assumed to be circular. The electric potential and field multipole expansions are determined and, in the case of a hollow cavity, closed analytical expressions are provided. When the cavity is loaded, convergence of the multipole expansions is improved by a preconditioning technique which isolates the field singularity at the sharp edge of the aperture, and makes its analytical treatment possible. Numerical results for the electric field complement the theoretical analysis.

ISSN:0021-8979    

DOI:10.1063/1.346187

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper, the Cˇerenkov radiation in an unbounded homogeneous isotropic chiral medium is studied and analyzed classically. Starting from the Maxwell equations and the proposed constitutive relations for isotropic chiral media, we formulate the problem for the electric and magnetic fields emitted from a charged particle moving with a constant speed in a chiral medium, and find a formal solution for the electromagnetic field components and energy spectral density of radiation. Notable features, such as double cone of propagation, and important characteristics of the Cˇerenkov radiation in such media in terms of the relative velocity of the particle with respect to the two characteristic phase velocities in the medium are discussed.

ISSN:0021-8979    

DOI:10.1063/1.346188

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The scale dependence of transmission of microwave radiation atK‐band frequencies (18–26.5 GHz) through a dense distribution of conducting spheres is studied for densities where localization is expected to occur. Scatterer packing fractions ranged from 0.16 to 0.22. Values of the elastic and transport mean free paths, and effective absorption lengths were obtained by fitting the transmission data to classical diffusion models. At the lowest densities the elastic mean free paths are within ∼15% of the value calculated with the single scattering formula. Atf=0.22 the difference is about 30%; this result together with an absorption length of ∼6 cm could account for the failure to observe strong coherent effects and associated nonclassical transport.

ISSN:0021-8979    

DOI:10.1063/1.346189

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Anisotropic gels were produced by photopolymerization of an oriented liquid‐crystalline (LC) mixture containing LC diacrylates and conventional LC molecules. Gels produced in this way were clear and did not give rise to excess scattering of light. Upon application of an electric field across the gel, the system turned translucent. Light scattered by the gel was found to be highly dependent on the direction of polarization and wavelength of the incident light.

ISSN:0021-8979    

DOI:10.1063/1.346190

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The optimal quasi‐static‐electric bias for the diffraction efficiency of degenerate four‐wave mixing process in a homeotropically aligned nematic liquid‐crystal film is studied. The nonlinearity of this process is characterized by the behavior of field‐induced molecular reorientation. In our theoretical calculation, it is pointed out that the determination of optimal field is crucially influenced by the twist deformation in the reorientational grating and is also subjected to the competing influences of splay and of bend elastic torques on the molecular reorientation. In experiments, the careful measurement of diffraction efficiency is carried out automatically by using a microcomputer to investigate the optimal bias for this nonlinear effect in nematicN‐(4‐methoxybenzylidene)‐4‐butylaniline films. It is unambiguously shown both by theoretical and experimental results that the optimal bias is decreasing with the increasing of grating period and optical intensity.

ISSN:0021-8979    

DOI:10.1063/1.346191

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have successfully attained low‐noise and high‐power operation in GaAlAs lasers by applying high reflectivity facet coating approach to nonabsorbing mirror lasers. High reflectivity coating is used to reduce feedback induced noise coming from hopping between the diode cavity modes and between the external cavity modes. A nonabsorbing mirror structure is used to obtain high‐power operation by suppressing mirror degradation and catastrophic optical damage effectively even with the use of high reflectivity coating. Nonabsorbing mirror lasers with the front facet reflectivity 50% showed the relative intensity noise level less than −130 dB/Hz at 3 mW under 0%–10% optical feedback and showed the cw output power as high as 50 mW in fundamental spatial mode operation. Under 30‐mW cw operation at 50 °C, on the other hand, obvious degradation has not been observed over 4000 h in the nonabsorbing mirror lasers with 50% front facet reflectivity. Thus the present laser is useful as a low‐noise source at a low‐power level and also as a high‐power source with high reliability.

ISSN:0021-8979    

DOI:10.1063/1.346192

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Three methods of evaluating the second‐order contributions to the mechanical velocity shift and reflection coefficient for isotropic groove gratings are presented. The second‐order terms contain the effect of evanescent and bulk modes generated through the interaction of a surface acoustic wave with a periodic overlay. All three methods will be shown to be numerically equivalent when calculated self‐consistently, verifying the validity of all three within their respective limitations. The extension of these methods to arbitrary anisotropic substrates and overlays is discussed.

ISSN:0021-8979    

DOI:10.1063/1.346193

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Studies of nonlinear acoustic interactions in superfluid helium at temperatures below 0.2 K have culminated in the construction of an all‐acoustic parametric amplifier at gigahertz frequencies. This device represents the shortest wavelength parametric amplifier ever made, with signal wavelengths shorter than 1000 A˚ and pump wavelengths shorter than 600 A˚. In the experiment, plane waves are mixed at a predefined angle in the superfluid helium. Two gain regimes are observed. The first regime is a noncollinear phase‐matched process, in which the signal phonon stimulates decay of the pump phonon to create gain at the signal frequency. The second regime is a four‐phonon collinear process, in which gain on the signal is created by the second harmonic of the pump. This four‐phonon process is unusual and is shown to be a combination of three‐phonon processes wherein the lack of conservation of energy and momentum in one process is mostly compensated by the other.

ISSN:0021-8979    

DOI:10.1063/1.346194

出版商:AIP    

年代:1990

数据来源: AIP