摘要:

Positron backscattering was observed at interfaces of polymer/metal using positron annihilation lifetime spectroscopy in the Teflon and Mylar films and poly (MMA‐co‐nBMA) acrylic coatings on tungsten (W) and molybdenum (Mo) substrates. Aluminum (Al) used as a comparison substrate did not have significant backscattering. Major changes were observed in the third,I3, and fourth,I4, intensity components of thin layers of Teflon and Mylar films on W and Mo substrates compared to Al. The average value of positron backscattering was estimated at 39% and 32% for W and Mo, respectively, with a22Na source. A model was presented for calculation of the backscattering coefficient (B) in the polymer/metal systems and compared with experimental results. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363170

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Nanosecond laser pulses, with 2.33 eV photon energy and ∼0.6 MW/cm2radiation flux, have been used to initiate a transient increase of tunneling current between a W tip and a Si sample surface in an ultrahigh vacuum scanning tunneling microscope (STM) apparatus. As the laser power is increased to ∼2.5 MW/cm2, single atom transfer from the tip to a silicon surface occurs. For both polarities, the laser induced tunneling current is linear with laser pulse energy up to ∼0.6 MW/cm2. A transient tunneling current up to 15 &mgr;A has been observed. The similarity of the laser induced transient tunneling for both polarities, and hence its independence on material, suggest that the same mechanism is operative in both directions of tunneling. Both ballistic electron tunneling and band bending effects have been considered in the analysis of the electron transfer. It is proposed, however, that pulse laser heating of the tip causes this transient increase of the tunneling current due to a transient thermal expansion, reducing the tip‐sample tunneling distance. The increase in tunneling current may lead to additional Nottingham heating of the tip apex. At a laser flux of 2.5 MW/cm2, single atom transfer between the W tip and the silicon surface occurs. The number of atoms transferred can be controlled by the laser flux, and the transfer process is virtually independent of the tip‐sample bias polarity. Since a maximum tip temperature of 650 K is estimated during the pulse, W atom transfer must occur under the influence of strong W–Si chemical interaction. The speed of the pulse laser atom transfer (8 ns) exceeds by orders of magnitude the transfer speed that could be achieved by pulsing the STM piezodrive. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363171

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A novel approach for enhancing the selectivity of the desired isotope in the molecular laser isotope separation (MLIS) process is presented. The scheme consists of simultaneously applying two laser beams with frequencies corresponding to those between the ground and the first energy excitation level and the ground and the second energy excitation level, respectively. Practical relations on the properties of the spherical‐top molecules are derived and a semiclassical analysis of the electromagnetic interaction within the limits of the experimental conditions applied in actual MLIS experiments shows that the selectivity, defined as the ratio of the absorption cross sections of the two isotopes, increases by a factor of 10–20 times in the case of the uranium isotopes. In addition, it is demonstrated that during the multiphoton absorption process energy‐level splittings due to induced magnetic dipoles and induced electric quadrupoles are by no means negligible. They become significant during multiphoton processes where two or more photons are lost during the interaction process. At high pumping powers they become dominant and inhibit selectivity. They cancel out during interaction processes where there is no change in the total number of photons, such as scattering. These effects can be avoided by applying the laser beams to the molecular gas in arrangements which in principle are equivalent to a Mach–Zehnder interferometer with the molecules substituted for the reuniting beam splitter. Moreover, the induced electric quadrupoles (E2) are fully exploited. The application of the results and the concepts described herein can render the MLIS process the most economic and practical method for the commercial separation of the uranium isotopes. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363172

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A cholesteric liquid crystal with negative dielectric anisotropy and with a visible wavelength selective reflection presents high memory effects when encapsulated in a polymer film. Three states at least are accessible within the same film: reflecting state, memory state, and refreshed state. The last two states are stable over months. When the sample is annealed, the sample goes through an irreversible transformation, after which the memory state is no longer accessible. Through the data collected as the samples were progressively annealed it was guessed how annealing changes the properties of the film, and which phenomena are responsible for such changes. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363173

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Pulse narrowing characteristics in traveling‐wave optical parametric generation in KTiOPO4crystals are investigated. By utilizing the pulse narrowing effect, output pulses with pulse duration of 250 fs can be obtained from 550 fs pump pulses in the experiment. It is also demonstrated that when the conversion efficiency becomes large by increasing the pump intensity or by changing the phase‐matching angle, the pulse narrowing effect becomes small. These tendencies are confirmed by numerical calculations of the parametric propagation process, which include the effects of temporal pulse shape, group‐velocity difference, transverse beam profile, walk‐off, two‐photon absorption, and depletion. The theoretical results indicate that when the conversion efficiency becomes greater than 1%, the back‐conversion process becomes large at the high‐intensity part of the signal and idler pulses. This causes the pulse width to increase and reduces the pulse narrowing effect. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363174

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A pseudo‐Rayleigh wave traveling along the edge of a thin plate is excited by a pulsed laser and detected by optical interferometry. The field confinement feature at the top of the plate edge is investigated by means of acoustic dilatation measurement. A nearly dispersionless behavior and little diffraction loss are observed during the guided mode propagation which agree with theoretical prediction. The possibility of using this wave for viscosity sensing is also discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363175

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Employing impedance analysis of quartz resonators, we have monitored the complex viscoelastic compliance of a 1.67‐&mgr;m‐thick film of poly(isobutylene) during swelling in toluene vapor. The optical thickness was simultaneously measured by ellipsometry. The acoustic model used for analysis is discussed. We can quantitatively derive the shear modulus. The results show a decrease of the glass transition temperature of about 60 °C. We suggest that the method is of high value for the investigation of sorption and drying kinetics in the regime of low solvent content. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363176

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Optical emission spectroscopy is used to investigate trends with changes in processing parameters for Ar/Cu plasmas in an electron‐cyclotron‐resonance (ECR) plasma deposition system. The primary motivation for this work is to monitor trends in ionization fractions for copper deposition plasmas using a noninterfering diagnostic tool. The system, which consists of a solid copper sputter target coupled to a permanent magnet ECR microwave plasma system, is operated in the range of 1–6 mTorr argon with net microwave input power of 500–1500 W. Emission from the following excited states is monitored: Ar neutrals (696.5 nm); Ar ions (488 nm); Cu neutrals (521.8 and 216.5 nm); and Cu ions (213.6 nm). Cu ion emission and Cu neutral emission monotonically increase with net microwave input power but at slightly different rates for different pressures, while argon‐ion emission as a function of pressure shows a broad peak around 4 mTorr. The ratio of Cu ion emission to Cu neutral emission is used as an indicator of the relative ionization efficiency for Cu and peaks near 5 mTorr. Spectroscopic estimates of electron temperature differences between pure Ar and Ar/Cu plasmas are also presented.

ISSN:0021-8979    

DOI:10.1063/1.363177

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A key need for the development and testing of models suitable for chemically active, low pressure plasmas is detailed comparisons between model predictions and experimental measurements. In this paper, a two‐dimensional, axisymmetric fluid model of an inductively coupled plasma is described, and model predictions are compared to the experimental measurements of P. A. Miller, G. A. Hebner, K. E. Greenberg, P. D. Pochan, and B. P. Aragon [J. Res. Natl. Inst. Stand. Technol.100, 427 (1995)] of electron density, electron temperature, and plasma potential. Comparisons between model predictions and experimental measurements were made in argon and chlorine discharges. Qualitative to semiquantitative agreement between the model predictions and experimental diagnostics was observed, suggesting that assumptions made in the model are reasonably accurate. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363169

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Spatially resolved, line integrated, excited state densities, and neutral and ion temperatures have been measured in inductively coupled argon plasmas. Absorption spectroscopy was used to measure the line integrated density and temperature of the argon 1s5, 1s4, 1s3, and 1s2energy levels. Laser‐induced fluorescence was used to confirm the neutral temperatures and to measure argon metastable ion temperatures. For rf powers between 50 and 300 W and pressures of 4–50 mTorr, the line integrated density of the 1s5energy level varied between 1×1016and 2×1016m−2. The densities of the 1s4, 1s3, and 1s2levels were approximately 4–10 times smaller. In the center of the plasma, the ion and neutral temperatures were identical, between 550 and 1000 K for plasma powers between 30 and 240 W and pressures between 4 and 50 mTorr. The neutral temperature had a maximum in the center of the discharge and decreased towards the edge of the discharge. However, the ion temperature increased to between 3000 and 4000 K at the edge of the discharge. Ion drift velocity in the radial direction was between 1×105and 2×105cm/s at the edge of the plasma. No significant changes in the spatial density distribution or temperature were observed when either a rf bias was applied to the lower electrode or when the stainless‐steel lower electrode was covered with a bare silicon wafer. The addition of nitrogen to the argon discharge resulted in the density of the 1s5state decreasing by a factor of 2 and the density of the 1s4state decreasing by a factor of 10. Implications of these measurements on the radial electric fields, radiation trapping, and the energy transport in the plasma are discussed.

ISSN:0021-8979    

DOI:10.1063/1.363178

出版商:AIP    

年代:1996

数据来源: AIP