摘要:

Optical transitions between the5D4excited state of the Tb3+(4f)8configuration and the crystal‐field split components of the (4f)7(5d) configuration are observed using high‐resolution laser excitation of Tb‐doped powder samples at room temperature. Excited‐state absorption spectra of Tb3+in YAG, YAGaG, Y2SiO5, and LaOBr are presented. Superimposed on the broadband excitation spectrum of these transitions we find the relatively narrow 4f→4flines. We present a theoretical model for interpreting the 4f→5dtransitions, which predicts a strict proportionality between the (4f)8 5D4→(4f)7(5d) and the (4f)8 7F6→(4f)7(5d) transitions. This model is used to estimate the optical‐absorption cross section for the former transitions. Although the terminating 4flevels lie at the same energy as the 5dbands, the 4f→4ftransitions can be interpreted on the basis of the Judd‐Ofelt theory. We derived a method for calibration of the Tb3+4f→5dfluorescence‐excitation spectra from powder samples. For YAG:Tb, the resulting optical cross sections were confirmed by direct‐absorption measurements on a Tb3+‐doped epitaxial YAG layer. To our knowledge, this is the first time that the probabilities for excited‐state absorption to highly excited 5dand 4fstates in Tb3+have been obtained theoretically and experimentally from Tb‐doped powder materials.

ISSN:0021-8979    

DOI:10.1063/1.346916

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A physical model based on resonant electric dipole interaction between excited activators is presented that may be used for gaining insight into the second‐order energy‐loss process in cathode‐ray‐irradiated Tb phosphors. It is shown that resonant up‐conversion of the5DJexcitons (J=3,4) to highly excited 4fand 5dstates may be of equal importance. The derivation of the rate constants for this second‐order energy‐loss process is given and experimental values for the5D4‐5D4and5D3‐5D4interaction are presented for Tb‐doped YAG, YAGaG, LaOBr, and Y2SiO5. The values depend on previously determined optical cross sections for excited‐state absorption. Based on this model we explain well‐known differences in saturation behavior between these Tb‐doped phosphors under cathode‐ray excitation.

ISSN:0021-8979    

DOI:10.1063/1.346892

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An analytical investigation of the application of a large magnetostatic field on the second‐order optical susceptibility &khgr;(2)in III‐V semiconductors like GaAs and InSb is reported. The model is based upon the parity indefiniteness of the electron wave functions in crystals lacking inversion symmetry. The generalized force and the electronic dipole moment operators are accordingly taken to be 2×2 matrices with all elements being finite. We have followed perturbation technique for the density matrix. The magnetostatic field renormalizes the transition frequency and sharpens the density of states. Both the resonant as well as nonresonant three‐wave parametric interactions have been discussed. Numerical estimates made for InSb crystal under resonant regime reveal considerable enhancement in the real part of &khgr;(2)even at moderately low magnetic field while the imaginary part exhibits a near saturation behavior. Under nonresonant laser excitation, both &khgr;(2)rand &khgr;(2)irise very sharply with magnetic field for effective cyclotron frequency &ohgr;c≤&ohgr;0−&ohgr;g, &ohgr;0and &ohgr;gbeing the pump and crystal band‐gap frequencies, respectively. Quite interestingly, &khgr;(2)rand &khgr;(2)idecay almost exponentially to very low values at &ohgr;c>2(&ohgr;0−&ohgr;g).

ISSN:0021-8979    

DOI:10.1063/1.346893

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In nondestructive evaluation the inspection of bond quality is a very important problem. In this paper we suggest an experimental method based on dispersion curve measurements for evaluating the quality of the bond between a layer and a substrate bonded by the inertia‐friction welding process. The experimental results are in good agreement with theoretical calculations which show that the behavior of the lowest velocity modes is very sensitive to the interface conditions and to the quality of the bond.

ISSN:0021-8979    

DOI:10.1063/1.346894

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Noncontacting laser‐ultrasonic measurements of the sintering of ceramics in real‐time are described. Lasers are used for both generation and detection of ultrasonic waves propagating through the material. A pulsed laser generates the ultrasonic wave from thermoelastic absorption at the material surface. Detection is accomplished by a confocal Fabry–Perot interferometer, which is insensitive to the speckle nature of the scattered light from the sample surface and can obtain measurements from samples with rough or diffusely reflecting surfaces. The densification of the ceramic sample is determined by changes in the bulk longitudinal wave velocity and the sample shrinkage.

ISSN:0021-8979    

DOI:10.1063/1.346895

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An acoustic array driven with a designed set of localized‐wave (LW) solutions of the scalar‐wave equation generates a robust, well‐behaved, transient pencil beam of ultrasound in water. The performance of the LW‐pulse‐driven array theoretically and experimentally exceeds a tenfold improvement over related continuous‐wave excitations of the same array.

ISSN:0021-8979    

DOI:10.1063/1.346896

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Time‐resolved infrared radiometry (TRIR) has been shown to be a valuable thermal characterization technique for the nondestructive evaluation of layered coatings. In this method an external step heating pulse is applied to the coating system and the resulting change in surface temperature is monitored as a function of time. Characteristic times and amplitudes can be ascribed to internal thermal reflections that return heat to the coating surface. A full description of the time dependence of the coating surface temperature can be obtained from these quantities. This paper describes a method for determining these characteristic times and amplitudes and shows their relationship to internal heat‐flow patterns. Uses and limitations of the method are discussed, and comparison of the theory with experimental TRIR results is presented.

ISSN:0021-8979    

DOI:10.1063/1.346897

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Condensed matter, when subjected to intense disrupting forces through impact or radiation deposition, will break up into a randomly distributed array of fragments. An earlier analysis of random fragmentation is extended to account for fragmentation in bodies which are finite in extent and for bodies within which the minimum fragment size is bounded. The statistical fragment size relations are compared with molecular dynamic simulations of dynamic fragmentation, with fragmentation caused by the high‐energy collision of nuclear particles, and with the distribution of galaxies in the universe which are assumed to be fragment debris from the primordial Big Bang.

ISSN:0021-8979    

DOI:10.1063/1.347188

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An exact solution to the problem of collisionless, space‐charge‐limited flow of cold ions across a one‐dimensional (planar) dc plasma sheath of negligible electron density is derived for general values of the presheath ion velocityv0and electric fieldE0. For a given ion current densityJand sheath thicknessd, the exact solution reduces to the classical Child–Langmuir model in the case thatv0=0 andE0=0. When eitherv0orE0is sufficiently large, however, the exact solution may differ appreciably from the Child–Langmuir law. The existence of a closed‐form expression for the spatial variation of the sheath potential is shown to be contingent upon the satisfaction of a simple inequality relatingv0andE0toJ. Whenv0obeys the Bohm criterion and the magnitude ofE0suggests that the Bohm energy is acquired over a distance not less than one Debye length, this inequality is indeed satisfied.

ISSN:0021-8979    

DOI:10.1063/1.346898

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An electron‐beam probe has been used to make time‐ and space‐resolved measurements of the electric field in a parallel plate rf discharge. Measurements were taken at a time resolution of 5 ns and a field resolution ±1 V/cm at increments of 0.32 cm from the powered electrode. The detection method provides the weighted average of the electric field along the beam trajectory. Measurements of fields less than 20 V/cm were made throughout the entire discharge. Data were obtained for 13.6‐MHz argon discharges at 2.3 and 20 mTorr, and at rf voltage amplitudes of 100 and 600 V at each pressure. The data for the 2.3‐mTorr, 600‐V case show (1) that a propagating double layer forms during the collapse of the sheath, (2) that the field near the electrode points in toward the plasma at the extreme collapse of the sheath, (3) that the collapse and expansion of the sheath proceeds asymmetrically for measurable field values, and (4) that the trajectory averaged electric field in the plasma has an anomalous phase shift and magnitude. Similar phenomena are observed for the remaining three discharge cases.

ISSN:0021-8979    

DOI:10.1063/1.346899

出版商:AIP    

年代:1990

数据来源: AIP