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Effect of Thermal Defocusing on Backward Stimulated Raman Scattering in CH4
Xiao-qing Hua,Jing Leng,He-ping Yang,Guo-he Sha*,Cun-hao Zhang
Author NameAffiliationE-mail
Xiao-qing Hua State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China  
Jing Leng State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China  
He-ping Yang State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China  
Guo-he Sha* State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ghsha@dicp.ac.cn,Tel:0411-84379311 
Cun-hao Zhang State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China  
Abstract:
The conversion efficiency of stimulated Raman scattering (SRS) in CH4 is studied by using a single longitudinal mode second-harmonic Nd:YAG laser (532 nm, linewidth 0.003 cm-1, pulse-width (FWHM) 6.5 ns).Due to the heat release from vibrationally excited particles, SRS processes often suffer from the thermal defocusing effect (TDE). In view of 6.5 ns laser pulse width is much shorter than the vibrational relaxation time of CH4 molecules, TDE can only affect the SRS processes afterwards. In the cases of low laser repetition, TDE will be not serious, because it will be removed by the thermal diffusion in Raman medium before the next pulse arrives. At the laser repetition rate 2 Hz, CH4 pressure 1.1 MPa and pump laser energy 95 m J, the quantum conversion efficiency of backward first-Stokes (BS1) has attained 73%. This represents the highest first-stokes conversion efficiency in CH4. Furthermore, due to the relaxation oscillation, the BS1pulses are narrowed to about 1.2 ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Its beam quality is also much better and is only slightly affected by TDE. This reason is that BS1 represents a wave-front-reversed replica of the pump beam, which can compensate the thermal distortions in Raman amplify process. Under the same conditions, but pump laser repetition rate as 10 Hz, the conversion efficiency of BS1 goes down to 36% due to TDE. From this study, we expect that a well-behaved 630 nm Raman laser may be designed by using a closed CH4/He circulating-cooling system, which may have some important applications.
Key words:  Nd:YAG laser, Stimulated Raman scattering, Thermal defocusing e?ect, CH4 gas
FundProject:
热散焦效应对甲烷后向受激喇曼散射的影响
花晓清,冷静,杨何平,沙国河*,张存浩
摘要:
利用自制的C12A7-Mg催化剂,研究了催化水蒸汽重整生物油、石脑油和CH4制备氢气的性能,以及催化剂寿命,并用X射线光电子能谱对催化剂进行了表征.温度测试范围为250?850 ℃.对于催化水蒸汽重整生物油反应,在750 ℃时,氢气产率最大达到80%,碳的转化率接近95%.在相同的反应温度下,催化水蒸汽重整石脑油和CH4的氢气产率和碳的转化率要低于重整生物油反应.催化剂的失活主要是由于重整过程中的积碳.
关键词:  
DOI:10.1360/cjcp2006.19(3).193.4
分类号: