摘要:

Two state‐of‐the‐art two‐dimensional photon detectors, a high quantum efficiency charge‐coupled device, and a microchannel plate photomultiplier with a position sensitive resistive anode (Mepsicron) have been used to observe spontaneous Raman scattering from multiorder phonon modes of Si in the backscattering geometry. These two detectors and an intensified linear photodiode array are compared using the multiorder spontaneous Raman signal from Si as a weak optical source. An attempt is made to assign the observed Raman scattering peaks in the first‐ through fourth‐order Raman shift regions to known peaks in the density of states of the appropriate Si phonon branches.

ISSN:0021-8979    

DOI:10.1063/1.341701

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Time‐of‐flight mass spectrometry was used to measure the cluster size and distribution in photo‐ and electron‐impact ionized pure vapor expansions. In addition, fluorescence excitation spectroscopy measurements were made on the expansion of tellurium to determine the degree of cooling and identify the dominant cooling mode. Mass spectra were taken as a function of nozzle diameter, ranging from 0.2 to 13 mm, and source vapor pressure up to 400 Torr. Cluster size measurements were made on the following materials of moderately low and high melting point: Te, Zn, Se, Mg, Al, Ge, and Ag. Modeling of the dye laser excitation spectra of the Te dimer indicates that the vapor is rotationally cooled down to 150 K for a source pressure of 140 Torr, corresponding to a source temperature of 1073 K. Vibrational temperatures are significantly higher than rotational temperatures for all source conditions. All spectra obtained show only small clusters (<10 atoms per cluster) which are present in the source vapor before expansion. We conclude that a pure vapor source under the expansion conditions investigated does not produce measureable quantities of large clusters.

ISSN:0021-8979    

DOI:10.1063/1.341702

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

KDP, KD*P, and LiNbO3, three nonlinear optical materials that have been difficult to coat, are treated with polymeric surface layers. These layers hermetically seal the hygroscopic crystal surfaces. Their optical properties, thermal compatibility, high‐power laser damage behavior, abrasive resistance, and suitability for overcoating with traditional, dielectric antireflection multilayers are reported.

ISSN:0021-8979    

DOI:10.1063/1.341703

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In order to obtain dry‐etched lasers with high spatial uniformity, the relation between the etching profile and the laser structure is investigated in detail for two types of GaAs/AlGaAs multiquantum‐well lasers, with and without selective Zn diffusion, with reactive‐ion‐beam‐etched facets. It is found that a laser structure with an abrupt step on the surface locally roughens the facets during etching through the generation of etching residues on the etched bottom. Such an abrupt step on a wafer can be typically introduced by the conventional selective Zn‐diffusion process including surface chemical treatment. The etched lasers fabricated with the selective Zn‐diffusion process show a wide distribution in the threshold current. In contrast, the lasers without the selective diffusion process realize a narrow distribution in the theshold current.

ISSN:0021-8979    

DOI:10.1063/1.341682

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A fast‐beam deflection technique is used to investigate the transient behavior of a laser‐induced surface plasma generated by CO2‐laser pulses of 2‐kW average power near an aluminum surface in various protective gas atmospheres, with a typical pulse energy of 3.3×10−2J, a pulse repetition rate of 60 kHz, and an average peak power per pulse of 30 kW, corresponding to an intensiy of 6×1012W/m2. As a laterally probing beam, a cw 10‐W CO2laser is used, whose beam is focused into a small volume in front of the surface. The electrons of the laser‐induced plasma passing this volume diffract the cw CO2laser beam, and the resulting beam deflection is determined by means of a partially absorbing CaF2wedge in connection with a fast infrared detector. The evaluation of beam deflection 0.15 cm above the surface renders a maximum electro density of 2×1023m−3for a surrounding argon atmosphere and 0.7×1023m−3for pure helium. Additives of O2reduce the attainable electron density even more. No plasma can be detected below a threshold of, say, 10 mJ energy per laser pulse. It is verified that high electron density as in the case of argon shields the surface against the incoming CO2laser pulses so that material processing, such as welding, is rather poor. Better welding results are achieved in helium‐oxygen mixtures.

ISSN:0021-8979    

DOI:10.1063/1.341657

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Simultaneous measure of Thomson and Rayleigh light scattering were performed in order to determine electron density, electron temperature, and neutral density distributions in a hollow‐cathode arc. These measurements were performed in argon, nitrogen, helium, and hydrogen at approximately 1 Pa. The hollow‐cathode arc has also been studied in nitrogen and argon at high pressures ranging from 1–550 Pa. In this paper we present only high‐pressure data for nitrogen, although comparison with some results obtained in argon is provided. A plasma scrape‐off effect leads to a reduction in electron density in front of the anode and consequently to an enhancement of the anode fall. This enhancement was detected through an increase in electron temperature.

ISSN:0021-8979    

DOI:10.1063/1.341658

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An experiment has been performed to study the loss of charge from a high velocity probe as it passes through a high‐pressure, low‐density plasma generated by a flame. Charged projectiles are fired through the flame and the loss of charge is measured as a function of the flame density and the projectile speed. Expressions are derived for the charge loss based on the thin sheath, thick sheath, and the field‐limited/sheath convection models normally used to describe the behavior of Langmuir probes. For a low‐density flame plasma, the sheath conduction turns out to be field limited and the exponential function describing the charge loss has been verified experimentally. The charge loss from the ballistic probe is used to compute the electron density profile of the flame. At the sound velocity in air, the residual charge after passing through the flame suddenly drops to near zero, suggesting that the sheath conduction mechanism changes radically as the velocity passes through the transonic region. This result may have far‐reaching implications for the use of Langmuir probes at transonic and supersonic speeds.

ISSN:0021-8979    

DOI:10.1063/1.341659

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An x‐ray standing wave investigation of the structure of a high‐quality mosaic crystal of Cd0.725Mn0.275Te reveals that the Cd and Mn cations occupy the same sublattice in this zinc‐blende material, as expected. This conclusion is made by inspection of the data obtained from a simple measurement made at the Cornell high energy synchrotron source. Furthermore, these data clearly indicate that the two cations have slightly different positions within the cation sublattice. Fitting these data to a simple model using x‐ray dynamical diffraction theory shows that these distinct cation positions are consistent with the bimodal CdTe and MnTe bond lengths previously observed with extended x‐ray absorption fine structure spectroscopy [A. Balzarotti, M. Czyzyk, A. Kisiel, N. Motta, M. Podgorny, and M. Zimnal‐Starnawska, Phys. Rev. B30, 2295 (1984)], but contradict the assertion that the cation sublattice is undistorted. This application of the standing wave technique to a mosaic crystal demonstrates that dynamical diffraction methods can produce important structural information from a much larger class of crystals than previously considered.

ISSN:0021-8979    

DOI:10.1063/1.341660

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The distribution of the orientation of thebandcaxes of crystallites in the piezoelectric polymer, poly(vinylidene fluoride), is inferred from the distribution (with varying sample orientation) of diffracted x‐ray intensity of the (111) and (001) reflections. Measurements of drawn and poled samples are reported, and results from these measurements as well as those for rolled material are analyzed. Thecaxes of the crystallites are narrowly distributed about the drawing (or rolling) direction. Thebaxes are broadly disturbed in the plane perpendicular to the drawing (rolling) direction. The effects of drawing, rolling, poling, and thermal aging upon the distribution of crystallite orientations are discussed. The distribution of dipole orientations and the resulting net polarizations are calculated, using fast Fourier transforms to deconvolve the diffraction data. A model is proposed, for the distribution of thebaxes, and is found to fit the data well in most cases. The correlation between the alignment of crystallographic axes and the piezoelectric constants is discussed. Possible improvement in the net polarization of the polymer of a factor of 2 is suggested.

ISSN:0021-8979    

DOI:10.1063/1.341661

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In continuation of our earlier investigations on the microstructures of cold‐worked Cu‐(Sn,Ni,Mn)‐Zn ternary alloys by x‐ray diffraction line profile analysis [J. Appl. Phys.54, 6652 (1983);ibid.48, 3560 (1977);ibid.56, 1213 (1984)] the present work with Cu‐Mn‐Si alloy system in the fcc phase has been undertaken to elucidate the effects of Mn and Si, when present together, on the cold‐worked substructure of this system. From detailed analyses of peak shift, peak asymmetry, and Fourier line shapes, quantitative estimates of the related microstructural parameters, namely, intrinsic (&agr;’), extrinsic (&agr;‘), and twin fault (&bgr;) probabilities, domain sizes, rms strain, dislocation density, and stacking fault energy (for pure Cu), have been made. The results indicate, as in the earlier cases, increased presence of intrinsic stacking faults (&agr;’)—primarily responsible for the observed peak shift and domain size broadening with relatively small presence of extrinsic faults (&agr;‘) and complete absence or very small presence of deformation twins (&bgr;) responsible for the observed asymmetry in the profiles. Unlike earlier observations, the presence of transitional solute Mn along with nontransitional Si as another solute appears to have a pronounced effect on the stacking fault probability of the present Cu‐base alloys in the fcc phase.

ISSN:0021-8979    

DOI:10.1063/1.341662

出版商:AIP    

年代:1988

数据来源: AIP