221 resultados para 185-1149
Resumo:
Absorption spectrum from 400 to 2000 run and upconversion fluorescence spectra under 940 nm pumping of YAG single crystal codoped with 5 at.% Yb3+ and 4 at.% Tm3+ were studied at room temperature. The blue upconversion emission centered at 483 nm corresponds to the transition (1)G(4) -> H-3(6), the emission band around 646 nm corresponds to the transition (1)G(4) -> F-3(4) of Tm3+. Energy transfer from Yb3+ to Tm3+ is mainly nonradiative and the transfer efficiency was experimentally assessed. The line strengths, transition probabilities and radiative lifetimes of (1)G(4) level were calculated by using Judd-Ofelt theory. Gain coefficient calculated from spectra shows that the upconversion corresponding with transitions (1)G(4) -> H-3(6) in YAG doped with Yb3+ and Tm3+ is potentially useful for blue light Output. (c) 2006 Elsevier B.V. All rights reserved.
Resumo:
Based on scalar diffraction theory, we investigated far-field intensity distribution (FFID) of beam generated by Gaussian mirror resonator. We found usable analytical expressions of diffracted field with respect to variation of diffraction parameters. Particular attention was paid to the parameters such as mirror spot size and radius of the Gaussian mirror, which determine the FFID. All analyses were limited to TEM00 fundamental mode. (c) 2004 Elsevier B.V. All rights reserved.
Resumo:
项目针对热带亚热带森林生态系统服务功能及所属区域社会发展的需求，结合全球变化（CO2浓度上升导致的全球变暖、氮沉降、降水格局演变）所面临的关键科学问题，系统开展森林生态系统碳、氮、水等过程演变规律的研究。经过十多年的努力，取得如下创新性理论：⑴成熟森林土壤可持续积累有机碳。发现亚热带成熟森林表土层（0-20cm）有机碳以0.61t/hm2•a的速度增加，为确认成熟森林作为新的碳汇奠定基础。⑵成熟森林趋于氮饱和。发现热带亚热带成熟森林生态系统趋于氮饱和，氮沉降增加将导致系统养分平衡的破坏。⑶退化生态系统恢复限制因子理论。水热季节分配不均限制了退化生态系统的恢复。⑷森林生态系统恢复/演替过程中其结构与功能、地上和地下不同步理论。创新方法：计算土壤C贮量长期变化的新方法；建立森林地下NPP关联估算模型；基于C/N确定森林土壤硝化与反硝化作用速率；推出降水动能及其受林冠分配调控的理论计算方法；提出任意时空尺度的生态系统水热状况量度指标及计算公式。发表Science等SCI论文52篇，核心期刊论文185篇，专著3部，被SCI论文引用312篇次，核心期刊引用2277篇次，核心内容之一被评为“2006年度中国基础研究十大新闻”。该成果催生生态系统非平衡理论框架的建立，引起国际同行的极大关注并获得高度评价，达到了世界领先水平。
