中国物理B

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AN APPROACH OF ESTIMATING EXCITATION CROSS SECTIONS IN MULTIPHOTON PROCESSES

冉琴, 张立敏, 戴静华, 马兴孝

1997 (2): 81-91. doi: 10.1088/1004-423X/6/2/001

摘要 ( 1230 )

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We suggest a simple approach to roughly estimate the excitation cross sections of various steps in a multiphoton ionization (or dissociation) process by applying the measured dependence of product signal on laser intensity. The validity of this method is demonstrated theoretically by means of the rate equations based upon a two-step kinetic model and can, in principle, be extended to the multiphoton process with steps more than two. Some results of numerical test and experiment are quoted.

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LASING WITHOUT INVERSION AND ORIGIN OF GAIN IN A Λ SYSTEM

张登玉

1997 (2): 92-96. doi: 10.1088/1004-423X/6/2/002

摘要 ( 1040 )

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A three-level Λ-type atomic system is pumped by an incoherent radiation field. The system lases without population inversion in the steady state. We analyze the inversionless gain and origin of the gain.

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COHERENT POPULATION TRAPPING IN THE SYSTEM OF A Λ-CONFIGURATION MULTILEVEL ATOM DRIVEN BY A TWO-MODE QUANTIZED FIELD

李高翔, 彭金生

1997 (2): 97-109. doi: 10.1088/1004-423X/6/2/003

摘要 ( 1212 )

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We have studied the atomic coherent population trapping in the system of a two-mode quantized field interacting with a Λ-configuration multilevel atom with one lower discrete state mad two sets of quasicontinuum states, or a set of quasicontinuum states and a set of true continuum states. The influence of the atom's initial preparation and the properties of the atomic upper levels on whether or not atomic coherent population trapping occurs, and the states of the field which trap the atom are analyzed.

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ION FLUX BEHAVIOR IN HIGH DENSITY PLASMAS

邵福球, 王龙, 吴汉明, 姚鑫兹

1997 (2): 110-124. doi: 10.1088/1004-423X/6/2/004

摘要 ( 1165 )

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The available electron cyclotron resonance plasma sources have been simulated in two-dimensional configuration space (Z, R) and three-dimensional velocity space (V_z, V_r, V_θ). Using a hybrid model, we treat ions as particles and electrons as fluid. The simulation is specified for plasma parameters of the available apparatus and is focused on the ion flux distributions. For a set of given operating conditions, the spatial distributions of electron temperature, plasma potential, plasma density, ionization rate and the contours map of distributions of particles in different volume cells in velocity space are obtained; statistics on ions that hit the bottom surface are also reported. The distributions of ion flux vectors on different cross-section planes in the plasma are given, and the phenomena of large radial diffusione of ions across magnetic force lines are also demonstrated. Finally, the influences of microwave power and neutral gas pressure on the distributions of ion flux vectors on ZR-plane, ion flux radial distributions and ion currents to all walls have been studied.

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INVESTIGATION OF GROWTH CHARACTERISTICS OF MONOCRYSTALLINE C₆₀ FILMS IN HELIUM GAS ENVIRONMENT

马国斌, 杨映虎, 陈光华

1997 (2): 125-129. doi: 10.1088/1004-423X/6/2/005

摘要 ( 1315 )

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The growth characteristics of C₆₀ films were investigated in helium gas environment at pressures of 0, 2.667×10^-2 and 6.667×10^-2 Pa, respectively. X-ray diffraction results showed that perfectly (111) oriented monocrystalline C₆₀ films with large grains were successfully deposited on synthetic 1M phlogopite mica-fluorophlogopite (0Ol) plane at a substrate temperature of 160℃ when the helium gas pressure is 2.667×10^-2 Pa. The results indicated that under appropriate helium gas pressure, the energy change of C₆₀ molecules through collisions with helium atoms, and the decrease of the supersaturation of the vapor pressure in the vicinity of the substrate resulted in a quasi-thermodynamic equilibrium deposition system. As a result, well-structured monocrystalline C₆₀ films were deposited on (001) plane of the fluorophlogopite substrate.

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YBaCuO BICRYSTAL JUNCTIONS AND DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

王世光, 戴远东, 曾祥辉, 郑培辉, 陈名玲, 王守证, 熊光成, 连贵君, 甘子钊

1997 (2): 130-139. doi: 10.1088/1004-423X/6/2/006

摘要 ( 1311 )

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We have prepared yttria-stabilized-zirconia bicrystal substrates using a simple hot-pressing method. The grain-boundary junctions have been fabricated with YBa₂Cu₃O₇ thin films grown epitaxially on the bicrystals. The patterns are defined by conventional photolithography, The dc and microwave characteristics of the junctiorts and the dc superconducting quantum interference devices (SQUIDs) have been intensively studied. The current-voltage curves are bridge-typed with noise rounding near the critical current. Resistive tail has been observed in the resistance versus temperature curves. The results are compared with the theoretical prediction for classical Josephson junctions. It is found that the behavior of bicrystal junctions can be described in the frame of classical theory. The deviations are attributed to the nonuniformity of the junctions. The small loop dc SQUIDs demonstrate diffraction and interference effects with regard to the applied magnetic field. A large square-washer with a new configuration has been designed to enhance the effective area of dc SQUID as a magnetometer. It has achieved a magnetic field resolution down to 1 pT/(Hz)^1/2(at 10Hz) at 77K.

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PHOTOCHROMISM AND ITS MECHANISM IN POLYDIACETYLENE CRYSTALS

彭景翠

1997 (2): 140-150. doi: 10.1088/1004-423X/6/2/007

摘要 ( 1234 )

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Based on summarizing and analyzing experimental facts of photochromism in polydiacetylene crystals, a theoretical model describing the photochromic process was presented. It is considered that the photochromism is the result of phase structural conversion during photoexcitation excitonic relaxation process, and its dependence on the energy and density of incident photons can be explained correctly. The micro-mechanism of photochromism is discussed in detail.

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