Isotope effect in the photodissociation process of nitrous oxide

平流层N_2O中富含重同位素。为了解释这种同位素异常现象，Yung和米勒在计算的基础上证明了重同位素N_2O，如^<14>N^<15>N^<16>O和^<14>N_2^<18>O，在波长大于200 nm时光解的几率较小。N_2O的吸收峰在180 nm附近。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附近的光解作用更为重要。由于N_2O的吸收系数在此区域随波长的变化很陡，所以波长的小的同位素位移会引起吸收系数的很大差异，本工作通过监测N_2O的光解产物N_2（X^1 &lt;$^1 +_g）和O（^1D_2）来研究^<14>N/^<15>N同位素效应。在202 ~ 206 nm波长范围内，N ^<14>N_2^<16>O、<14>N^<15>N^<16>O和<15>N^<14>N^<16>O发生光解。用共振增强多光子电离技术结合飞行时间质谱技术对O（^1D_2）和N_2（X^1 &lt;$^+_g）进行了监测。当重同位素在205.47 nm处光解时，O（^1D_2）信号较弱。对于^4N^ N^ O，同位素效应高达10%<15><16>，而对于^<15>N^<14>N^<16>O，同位素效应仅为3%。对于重同位素，吸收系数必须较小。当监测N_2（X^1 &lt;$^+_g）时，观察到了反同位素效应。在<14><14><15>高旋转激发态N_2（X ^1 &lt;$^+_g）的探测效率中，一定存在同位素效应。在205和210 nm两个光解波长下测量了N_2（X ^1 &lt;$^+_g）的转动态分布。发现N_2（X^1 &lt;$^+_g）是高转动激发态，在两种情况下，都有60%的可用能量被分配到转动模中.在N_2（X^1 &lt;$^+_g）的内态分布中没有发现大的同位素效应。

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).