2000 Fiscal Year Final Research Report Summary
Isotope effect in the photodissociation process of nitrous oxide
| Project/Area Number |
11640501
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Physical chemistry
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| Research Institution | Japan Advanced Institute of Science and Technology |
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Principal Investigator |
UMEMOTO Hironobu Japan Advanced Institute of Science and Technology School of Materials Science Associate Professor, 材料科学研究科, 助教授 (80167288)
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| Project Period (FY) |
1999 – 2000
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| Keywords | nitrous oxide / photodissociation / isotope effect / atmospheric chemistry |
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| Research Abstract |
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).
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