摘要:

Microwave transmission measurements of the electrical properties of shock‐ionized air in the range 8≤Ms≤12 and initial pressures of 1, 2, 5, 10, and 20 Torr are presented. The test arm of the microwave transmission bridge isXband waveguide, which passes through the shock tube and has an affixed nozzle to section out a portion of the shocked gas. This gas is constrained to a rectangular cross section and passes through the waveguide where it interacts with the microwave signal. An 0.5‐in. and a 1.5‐in. nozzle are used, and it is shown that nozzle length has a negligible effect on both the ionization rate and the level. The data are presented in terms of dielectric constant, electron density, collision frequency, conductivity, attenuation, and reflection coefficient. The data are compared with results obtained from other experimental methods and discrepancies are noted. Values ofNeand &ngr; are calculated initially from the Lorentz relation with &ngr; independent of velocity; these data are then recalculated by use of Hochstim's correction factors. Ionization profiles obtained from the microwave measurements at 1 Torr agree belowMs=11 with theoretical values obtained from the rate program of Garr and Marrone. Evaluation of ionization profiles in terms of rate constants is complicated at all pressures by the effects of accelerating shocks.

ISSN:0021-8979    

DOI:10.1063/1.1709758

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Several workers have reported the somewhat curious phenomenon that in ball‐indentation fracture experiments on glass plates, the average fracture load is directly proportional to the indenter radius for a wide range of small indenters. As a part of the present investigation, similar experiments have been performed and confirm these observations. That such a definite relation (known as Auerbach's Law) should exist serves as one of the more puzzling barriers to nearly universal acceptance of statistical theories of glass fracture. This paper discusses such behavior from a probabilistic view in the content of the Hertz solution of classical elasticity. In particular a statistical solution is demonstrated, which, based on the hypothesis that there is a distribution of flaws with a density which varies inversely as the cube of the flaw severity, not only yields the linear range for average fracture loads, but also gives good agreement with other detailed characteristics of experimental fracture load‐frequency data.

ISSN:0021-8979    

DOI:10.1063/1.1709759

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

In the application of lasers in such fields as nonlinear optics, one is often faced with the problem of limitation in the highest available laser power. A potential solution to this situation is to achieve higher power by adding separate lasers in parallel. However, whether such schemes would have an inherent limitation imposed by the lack of coherence of separate lasers is not so obvious. We have found that separate lasers can indeed be used as uncorrelated pump sources for parametric interactions. For optical parametric oscillations, each additional pump for amplification (with Stokes radiation) would enhance the exponential gain and reduce the oscillation threshold. Conversely, each additional pump for up‐conversion (with anti‐Stokes radiation) would reduce the exponential gain and raise the oscillation threshold. The contribution of each elementary scattering process can be evaluated individually allowing the net effect to be summed together in simple expressions.

ISSN:0021-8979    

DOI:10.1063/1.1709760

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A variational study of the periodic Bloch wall is carried out. It is found that the periodic Bloch wall is energetically favorable only if the anisotropy constant of the material is smaller than a critical value; otherwise, the ordinary Bloch wall is energetically favored. Computations of the appropriate lengths are carried out, and the results agree fairly well with those obtained by Shtrikman and Treves for small values of the anisotropy constant.

ISSN:0021-8979    

DOI:10.1063/1.1709761

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The diffusion coefficients of carbon in single and polycrystalline ZrC have been measured in the temperature range 1350°–2150°C, using radioactive tracer techniques. Volume and short‐circuit enhanced tracer diffusion coefficients are represented by the expressionsD*=1.32×102exp (−113 200/RT) cm2/sec andD*=1.6 exp (−90 000/RT) cm2/sec, respectively. The results are compared with apparent diffusion coefficients determined from existing carburization and high‐temperature creep data.

ISSN:0021-8979    

DOI:10.1063/1.1709762

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

An optical lever system was used to observe the impact shock wave arrival and free‐surface motion at the rear surface of a 2.54‐cm‐thick 2024‐T4 aluminum target plate. The projectile was a 0.636‐cm‐diam. steel ball and struck the target at a velocity of 5.28±0.11 mm/&mgr;sec. The shock‐wave data, in the range 30–1 kbar, extend data by Fowles to lower pressures. The measured elastic shock velocity of 6.23±3% agrees with ultrasonic values and with elastic shock values by other investigators.Elastic shock amplitudes do not maintain a constant value as in one‐dimensional experiments, but decay at a rate faster than predicted for spherical elastic waves.

ISSN:0021-8979    

DOI:10.1063/1.1709763

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The reflectivity of epitaxial PbS films has been compared with the reflectivity of bulk PbS in the energy range 2.1 to 6.2 eV and found to agree in both magnitude and structure within a few percent at energies where interference effects can be neglected. The transmission of several films with thicknesses varying from 335 to 550 Å has been measured and the structure and shape of the spectra shown to be the same for the films with different thicknesses. The reflectivity of bulk PbS between 2.1 and 6.2 eV has been combined with measurements made by other workers outside of this range to compute optical constants from the Kramers Kronig relation. The optical constants obtained in this manner appear to agree quite well (within about 10% in most cases) with those obtained from the reflectivity and transmission of the films, considering the experimental difficulties experienced in measuring film thickness and the absolute magnitude of the reflectivity.

ISSN:0021-8979    

DOI:10.1063/1.1709764

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Radiation‐field‐dependent frequency shifts arising in atomic‐beam spectroscopy are treated theoretically and experimentally. Shifts due to fundamental and unavoidable interactions between the radiation field and the atoms comprising the beam are distinguished from those due to various ``apparatus effects.'' Precise measurements of frequency shifts are made for a cesium beam experiencing Ramsey‐type excitation. For the magnetic‐field‐sensitive transitions (F, MF) = (4, ±1) ↔ (3, ±1), the magnitude of the shifts is about 1 part in 1010of the resonance frequency value, per milliwatt variation of input power to the radiation field. The shifts vary with input intensity in a nonmonotonic fashion and are shown to result from nonuniformity in the static magneticc‐field of the apparatus. Much smaller shifts of about 5 parts in 1013per milliwatt are observed for the magnetic‐field‐insensitive transition (F, MF) = (4, 0) ↔ (3, 0). The major features of these shifts are shown to arise from spectral impurities in the radiation exciting the transition and a small phase difference between the pair of separated radiation fields. The bearing these results have on evaluating the accuracy of an atomic beam frequency standard is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1709765

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The interaction between longitudinal optical phonons in an insulator and electrons tunneling through the insulator is investigated on the basis of a simple one‐dimensional model. This interaction is found to alter the mechanism of metal‐insulator tunneling drastically, the tunneling current being constant at low temperatures and highly temperature‐dependent at room temperature. The low‐field (≲106V/cm) current‐voltage relationship has an algebraic form similar to that for thermionic emission but differs from it in magnitude and temperature dependence. It is concluded that the high‐ and low‐temperature regions of charge transfer from metal to insulator are continuous, representing the evolution of the electron‐phonon interaction with temperature.

ISSN:0021-8979    

DOI:10.1063/1.1709766

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Square‐wave phase modulation of the microwave signal which induces transitions allows one to measure velocity‐dependent frequency shifts in Ramsey resonance experiments. The output signal from the molecular beam apparatus is demodulated by a synchronous detector operating in phase with the modulation. This output is proportional to the derivative of the transition probability and provides the usual resonance signal which is used in a servo loop to keep a harmonic of a voltage tunable crystal oscillator on the central peak of the resonance. A quadrature synchronous detector operating 90° out of phase with the modulating signal provides a measure of any velocity‐dependent frequency shifts. Such shifts arise when there is a phase difference between the oscillating fields or when an electric field applied to a magnetic system produces av×&egr;shift. We have theoretically and experimentally demonstrated that this quadrature signal is proportional to the phase difference between the microwave cavities of a Ramsey apparatus with the in‐phase signal constrained at zero. We have also shown that the sensitivity of the quadrature system depends on the width of the available velocity distribution.

ISSN:0021-8979    

DOI:10.1063/1.1709767

出版商:AIP    

年代:1967

数据来源: AIP