Conservation of the nuclear spin modification of CH4 in the cooling process by supersonic jet expansion
Conservation of the nuclear spin modification of CH4 in the cooling process by supersonic jet expansion
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超音速射流膨胀冷却过程中CH4核自旋修饰守恒
DOI：
10.1006/jmsp.1994.1074
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发表时间：
1994
影响因子：
1.4
通讯作者：
K.M.T. Yamada
中科院分区：
文献类型：
--
作者：
M. Hepp;G. Winnewisser;K.M.T. Yamada
图2.假设热平衡，针对各种温度绘制了归一化为5+ 9+ 2= 16的A、E和R状态的相对布居。总体表示与所示对称性相关的所有水平的总体之和; A表示A，+ A，F表示F，+ F。高温近似适用于温度T> 100 K。如果我们假设这些选择规则在碰撞能量转移过程中保持不变，那么处于A、E和F态的分子的布居数将不会因超音速膨胀而改变;这三个核自旋态保持它们的相对布居数值，该值由膨胀前的温度决定，在我们的实验中，该温度对应于大约300 K。在由于超音速膨胀引起的强烈冷却之后，该初始玻尔兹曼分布将对应于极端非热平衡分布。另一方面，如果允许核自旋状态在碰撞过程中发生变化，则应观察到在约15-20 K的低温下热平衡的分子分布。图2说明了在假设热平衡（即玻尔兹曼分布）的情况下，在各种温度下计算的A、E和F对称态的CH 4分子的相对布居。在我们以前的论文（1）中，室温分布（-300 K）的理论预期比值被错误地计算为5.0888：9。1596年：1.75 16由于意外遗漏了一个水平。然而，如图2所示，高温近似对室温下的甲烷是合适的，其比值应该正好是5：9：2。在本研究中，我们扩展了早期的测量，有两个目的：（i）提高布居测定的精度;（ii）证实前一篇文章的结论，即核自旋态守恒。用半导体激光光谱仪测量了超声速射流中各个振转跃迁的谱线强度。实验步骤的详细信息在我们早期的论文（I-3）中给出。与前面的工作（1）一样，通过流量控制器将样品气体CH 2与氩气混合，并通过喷嘴注入真空室。测量了在总压力为2巴的氩气中接种的浓度为25%的甲烷的V1带的红外跃迁的线强度。
FIG, 2. The relative populations for the A, E, and R states, normalized to 5+ 9+ 2= l 6, are plotted for various temperatures assuming thermal equilibrium. The populations represent the sum of the populations for all levels pertaining to the symmetry indicated; A for A,+ A, and F for F,+ F. The high-temperature approximation is appropriate for temperatures T> 100 K. łf we assume these selection rules to hold during the collisional energy transfer process, the populations of the molecules in the A, E, and F states will not be changed by the supersonic expansion; these three nuclear spin states keep their relative population values set by the temperature just before the expansion, which in our experiments corresponds to about 300 K. After the strong cooling due to the supersonic expansion this initial Boltzmann distribution will correspond to an extremely non-thermal-equilibrium distribution. On the other hand, if the nuclear spin states are allowed to change during the collision process, a molecular distribution of thermal equilibrium at cryogenic temperatures of about 15–20 K should be observed. Figure 2 illustrates the relative populations of the CH4 molecules for the A, E, and F symmetry states calculated for various temperatures assuming the thermal equilibrium, ie, Boltzmann distributions. In our previous paper (1) the theoretically expected ratio for the room temperature distribution (-300 K) was erroneously computed to be 5.0888: 9. 1596: 1.75 16 by accidental omission of one level. However, as shown in Fig. 2, the high-temperature approximation is appropriate for methane at room temperatures and the ratio should be exactly 5: 9: 2.In the present study we have extended the earlier measurements with a twofold purpose:(i) to improve the precision in the population determination and (ii) to confirm the conclusion of the previous paper, namely the conservation of the nuclear spin states. A diode laser spectrometer was used to measure the line intensity of the individual rovibrational transitions in the supersonic jet. The details of experimental procedure are given in our earlier papers (I–3). As in the previous work (1), the sample gas, CH,, was mixed with the Argas by flow rate controllers and injected through a nozzle into the vacuum chamber. Line intensities of the infrared transitions of the v, band were measured for methane with 25% concentration seeded in Argon with a total pressure of 2 bar.