摘要:

With the model of equivalent charge distribution, we calculated the exact electrostatic force acting on the real (conical) tip of an atomic force microscope. This model applies to a conductive tip in front of a conductive plane. We compared the equivalent charge model with several analytic models used to date to approximate the electrostatic forces and discussed their degree of validity. We estimated the contribution of the cantilever to the total force and showed, on the basis of theoretical calculations and experimental results, that the contribution of cantilever may constitute the essential part of the electrostatic force in the range of distances used in electrostatic force microscopy in the air. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363884

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Phosphor screens which convert x-ray images to visible light images are key components in two-dimensional charge coupled device (CCD) based detector systems used for x-ray diffraction. Some experimental and theoretical aspects of phosphor screen performance are described in this article. The efficiencies of x-ray-to-light conversion were measured using a CCD camera for transmission phosphor screens fabricated from two different phosphor powders, Y2O2S:Eu (P22R) and Gd2O2S:Tb (P43), for screen mass thicknesses of 3–50 mg/cm2and for x-ray energies of 7–34 keV. A model was developed and evaluated for the dependence of the emitted light brightness on screen thickness and x-ray energy. Inputs to the model are x-ray absorption coefficients available from published compilations, and light attenuation versus thickness data, which were determined experimentally for the phosphors and found to be dominated by scattering rather than absorption. The angular distribution of emitted light, measured for one of the phosphor screens, was found to be nearly Lambertian. Broadening of image features in the x-ray-to-visible-light conversion by phosphors for 19.6 keV x-rays was found to increase approximately linearly with phosphor screen thicknesses in the range of 30–160 &mgr;m, but with a minimum width of 110 &mgr;m for P22R phosphor and 70 &mgr;m for P43 phosphor. In the range of 7–15 keV, maximum brightness was obtained for P43 phosphor screens of about 10 mg/cm2mass thickness (60 &mgr;m). For P22R screens, the thickness for maximum brightness increased from about 8 mg/cm2(50 &mgr;m) for 7 keV to more than 46 mg/cm2(210 &mgr;m) for 15 keV. For 7 keV the maximum brightnesses for P22R and P43 phosphors were about the same. For 10 keV the maximum brightness for P43 phosphor was about 60&percent; greater than the maximum brightness for P22R phosphor samples tested. For 15 keV the maximum brightness for P43 phosphor was again about 60&percent; greater than that for the P22R samples tested. In the range of 20–34 keV, maximum brightness would occur for thicknesses greater than 46 mg/cm2(210 &mgr;m) for P22R phosphor and greater than 40 mg/cm2(160 &mgr;m) for P43 phosphor. Comparing the brightness for 90 &mgr;m thickness for the two phosphors, P43 was about 30&percent; brighter for 20 keV, 20&percent; brighter for 24 keV, and 10&percent; brighter for both 29 and 34 keV. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364439

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The energy states of oxygen-deficient type defects in the vacuum ultraviolet region are discussed based on the experimental results of vacuum ultraviolet absorption, temperature dependence of the photoluminescence (PL) intensities at 4.3 and 3.1 eV, and lifetimes of the PLs. It was found that the oxygen-deficient type glass has a large absorption tail above 6 eV in addition to an absorption band around 5 eV and that the 3.1 eV PL intensity scarcely depends on temperature when excited above 6 eV. It was also found that the lifetime of the 4.3 eV PL is 9 ns and that of the 3.1 eV PL is 113 &mgr;s irrespective of the excitation photon energy. The obtained results are explainable by assuming that electrons excited into the conduction band by photons above 6 eV contribute to the PLs. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363885

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Energy levels of the4f2electronic configuration of Pr3+in SrAl12O19(SAM) have been determined from polarized absorption and fluorescence spectra using crystals, charge-compensated with Mg2+, at temperatures between about 20 and 300 K. Energy level assignments were made initially by comparing the crystal spectra with energy levels calculated by using crystal-field parameters,Bkm, compatible with those previously reported for Nd3+in SAM. TheBkmwere varied to obtain a best fit between experimental and theoretical energies, and the final values:B20=−94,B40=1047,B60=−1397, andB66=−1245cm−1give an rms fit of 14.5 cm−1. TheBkmfor Pr3+in SAM were also used along with a result relating theBkmto crystal-field components,Anm, derived in a lattice sum calculation to obtainBkmvalues for all the triply ionized rare-earth ions in SAM. Odd-foldAkmare also given that are required in the intensity and lifetime calculations and yield results in good agreement with the measurements. In addition, the results of our energy level fitting were useful in estimating absorption and emission cross sections, as well as lifetimes and branching ratios, for the variousJmultiplets. The evidence given here that the room temperature fluorescence quantum efficiency of3P0in lightly doped Pr in SAM is near unity strengthens the case that this is a potentially useful laser material. The theoretical model and method of analysis used is useful for the calculation of energy levels, wave functions, transition probabilities, etc., for any rare-earth RE3+ion in any host of known crystal structure. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363886

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Three wave mixing using a fiber ring resonator scheme is discussed, and experimental results are presented. In this scheme, an optical nonlinear medium is placed into the fiber ring laser cavity, and a semiconductor optical amplifier and a tunable optical band pass filter (TOBPF) are used as the gain medium and lasing wavelength selector. The phase-matching condition, which is necessary for efficient wave mixing, can easily be satisfied by tuning the TOBPF. Second-harmonic generation (SHG) and sum-frequency generation (SFG) were demonstrated using a LiNbO3waveguide with a periodically domain inverted structure. Enhancements of SHG and SFG were achieved. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363887

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A new type bistable reflective cholesteric liquid crystal display is reported. The texture and the relaxation process are very different from other types of the reflective cholesteric displays reported previously. The contrast ratio of the display is about 2.5 times higher than regular cholesteric displays. The response time of the display is 10 ms, which is much faster than the 230 ms response of regular cholesteric displays made with the same liquid crystal materials. A model to explain the unique textures of the display is presented. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363867

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A topic of great present interest is the investigation of heat transfer in materials with memory (HTMM). The correct description of different experimental heat transfer phenomena (in liquid helium, in dielectrics at helium temperature, in organic liquids, etc.) is conducted on the framework of HTMM theory. Also, new physical effects have been predicted in HTMM theory. The most important problem for HTMM theory and practical applications is analyzed—the propagation of harmonic thermal waves in an infinite plate from material with thermal memory. It is shown that in such materials there are appreciable effects produced by the interference of thermal waves, similar to multibeam interference of electromagnetic waves in optics. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363849

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A transient increase is observed in both the spectral energy decay rate and the degree of chaotic complexity at the interface of a shock wave and a turbulent ionized gas. Even though the gas is apparently brought to rest by the shock wave, no evidence is found either of prompt relaminarization or of any systematic influence of end-wall material thermal conductivities on the turbulence parameters. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364441

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Transient phenomena which accompany the breakdown of gas in a semiconductor-gas discharge (SGD) system with 100 &mgr;m interelectrode distance have been studied experimentally and with numerical calculations. This system serves as the basis for an image converter operating in the infrared (IR) spectral region. The experiments are done for a cryogenic discharge in helium at a temperature close to that of liquid nitrogen. Depending on specific experimental conditions, oscillatory or aperiodic modes approaching to the steady-state current are observed after breakdown has been induced by a step-like voltage pulse. Numerical calculations of transient phenomena were performed for experimental conditions within the framework of the Townsend model, taking into account direct ionization, Penning ionization and secondary emission of electrons from the cathode. The main features of discharge kinetics obtained are in accordance with experimental data. At the same time, with the steady-state discharge current density varying fromj= 1×10−4A/cm2to7×10−2A/cm2,inconsistencies between experimental and theoretical results are observed. A possible reason for this discrepancy is related to the complex process of secondary ionization at the cathode under the conditions of a cryogenic discharge and to the dependence of the secondary electron emission factor on current density, that has been neglected in constructing the model. The results obtained are considered from the viewpoint of the response time of gas-discharge IR image converters. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363850

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The radiative efficiency of the 147 nm resonance radiation of Xe excited in a low pressure, high-frequency surface wave sustained plasma has been investigated. The radiative UV power has been obtained from optical absorption spectroscopic measurements of the Xe resonance level population and from Monte Carlo calculations of the effective decay rate of this level. Precise measurements of the rf power absorbed by the plasma enable the determination of the absolute vacuum ultraviolet discharge efficiency for the Xe surface wave discharge. Results show efficiencies up to more than 80(±11)&percent; . ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363851

出版商:AIP    

年代:1997

数据来源: AIP