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Isotope effect in the photodissociation process of nitrous oxide

一氧化二氮光解过程中的同位素效应

基本信息

批准号：
11640501
负责人：
UMEMOTO Hironobu
金额：
$ 2.3万
依托单位：
Japan Advanced Institute of Science and Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640501/
关键词：
nitrous oxide photodissociation isotope effect atmospheric chemistry

项目摘要

The stratospheric N_2O is enriched in heavy isotopes. In order to explain this isotope anomaly, Yung and Miller have demonstrated, ou the basis of their calculations, that isotopically heavy N_2O, such as ^<14>N^<15>N^<16>O and ^<14>N_2^<18>O, has smaller probability to be photolyzed at wavelengths longer than 200 nm. The peak of the absorption of N_2O is around 180 nm. However, sun light around 180 nm is absorbed by O_2 and less important for the photolysis. Photolysis around 205 nm is more important. Since the absorption coefficient of N_2O changes with the wavelength steeply in this region, a small isotope shift in the wavelength causes a large difference in the absorption coefficient.In the present work, the photodissociation products of N_2O, N_2(X^1Σ^+_g) and O(^1D_2), were monitored to examine the ^<14>N/^<15>N isotope effect. ^<14>N_2^<16>O, ^<14>N^<15>N^<16>O, and ^<15>N^<14>N^<16>O were photolyzed between 202 and 206 nm. O(^1D_2) and N_2(X ^1Σ^+_g) were monitored by a resonance-enhanced multiphoton ionization technique combined with a time-of-flight mass spectrometric technique. The O(^1D_2) signal was weaker when heavy isotopomers were photolyzed at 205.47 nm. The isotope effect was as large as 10 % for ^4N^<15>N^<16>O, while it was 3 % for ^<15>N^<14>N^<16>O.The absorption coefficients must be smaller for heavy isotopomers. An inverse isotope effect was observed when N_2(X ^1Σ^+_g) was monitored. ^<14>N_2 signal was less than that for ^<14>N^<15>N.There must be an isotope effect in the detection efficiency of rotationally highly excited N_2(X ^1Σ^+_g). The rotational state distributions of N_2(X ^1Σ^+_g) were also measured at two photolysis wavelengths, 205 and 210 nm. It was found that N_2(X ^1Σ^+_g) is highly rotationally excited and that 60 % of the available energy is partitioned into the rotational mode in both cases. No large isotope effect was found in the internal state distributions of N_2(X ^1Σ^+_g).

平流层N_2O富集重同位素。为了解释这种同位素异常，Yung和Miller在他们的计算基础上证明，同位素重的N_2O，如^<14>N^<15>N^<16>O和^<14>N_2^<18>O，在波长大于200 nm的波长下被光解的概率较小。N_2O的吸收峰在180nm左右。然而，180 nm左右的太阳光被O_2吸收，对光解作用不太重要。205 nm左右的光解更为重要。由于该区域N_2O的吸收系数随波长变化非常大，因此波长的微小同位素位移会导致吸收系数的较大差异。本文对N_2O的光解产物N_2（X^1Σ^+_g）和O（^1D_2）进行了监测，考察了^<14>N/^<15>N同位素效应。^<14> n2 ^<16>O, ^<14>N^<15>N^<16>O和^<15>N^<14>N^<16>O在202 ~ 206 nm之间被光解。O（^ 1d2）和N_2（X ^1Σ^+_g）采用共振增强多光子电离技术结合飞行时间质谱技术进行监测。在205.47 nm处，重同位素体的O（^1D_2）信号较弱。^4N^<15>N^<16>O的同位素效应高达10%，而^<15>N^<14>N^<16>O的同位素效应为3%。重同位素的吸收系数必须更小。当监测N_2（X ^1Σ^+_g）时，观察到逆同位素效应。^<14>N_2信号小于^<14>N^<15>N信号。旋转高激发N_2（X ^1Σ^+_g）的探测效率一定存在同位素效应。测定了N_2（X ^1Σ^+_g）在205和210 nm光解波长下的旋转态分布。结果表明，N_2（X ^1Σ^+_g）是高度旋转激发的，在这两种情况下，60%的可用能量被分配到旋转模式。在N_2（X ^1Σ^+_g）的内态分布中没有发现明显的同位素效应。