氮气与甲烷混合气生成CN自由基研究

Investigation of formation of CN radical from nitrogen and methane mixture

CN自由基在生命形成过程中起着重要作用。在早期行星大气中,CN的生成与氮气(N2)和甲烷(CH4)有密切关系。以土卫六大气成分为参考,利用CH4和N2的混合气体模拟早期大气环境,并对模拟大气进行辉光放电产生CN自由基,通过测量CN自由基的吸收光谱研究了CH4、N2比例和土卫六大气中的痕量气体对CN生成的影响。在N2和CH4混合气体的低压辉光放电中,当CH4气压占总气压的20%左右时,CN自由基的生成浓度最大。保持N2和CH4气压配比不变而改变总气压时,起初CN自由基浓度随总气压增加而增加;当总气压超过60 Pa时,CN自由基浓度随着总气压的增加趋缓;而当总气压大于90 Pa时,CN浓度随着总气压的增加缓慢减少。在给定气压和CH4-N2浓度配比条件下,CN自由基的浓度都随放电电流增加而增大。土卫六大气中存在痕量水汽(H2O)、二氧化碳(CO2)和一氧化碳(CO),在N2和CH4混合气体放电过程中加入少量这些气体都会抑制CN自由基的生成。

CN radical plays an important role in the process of life formation.The generation of CN is closely related to nitrogen(N2) and methane(CH4) in the early planetary atmosphere.Considering the atmospheric compositions of Titan,N2 and CH4 are mixed to simulate the early atmospheric environment,then the simulated atmosphere is discharged to generate CN.The influence of CH4 to N2 ratio and the trace gases in Titan atmosphere on the generation of CN are studied by measuring the absorption spectrum of CN radicals.It is shown that,under the low-pressure glow discharge of N2 and CH4 mixture,the concentration of CN radicals is the largest when the CH4 gas pressure accounts for about 20% of the total gas pressure.When the pressure ratio of N2 and CH4 is kept constant,the concentration of CN radicals increases with the increase of total pressure at first.When the total pressure reaches 60 Pa,the concentration of CN radicals will slow down with the increase of total pressure,and when the total pressure is more than 90 Pa,the concentration of CN radicals starts to decrease slowly with the increase of total pressure.Given the gas pressure and the CH4 to N2 concentration ratio,the concentration of CN radicals increases with the increase of discharge current.It is well known that there are trace amounts of water vapor(H2 O),carbon dioxide(CO2) and carbon monoxide(CO) in Titan atmosphere,and it is found that the addition of a small amount of these gases in the discharge of N2 and CH4 mixture will suppress the formation of CN radicals.