Isotopic fractionation in the photodissociation of N2 could explain the considerable variation in the 14N/15N ratio in different regions of our galaxy. We previously proposed that such an isotope effect is due to coupling of photoexcited bound valence and Rydberg electronic states in the frequency range where there is strong state mixing. We here identify features of the role of the mass in the dynamics through a time-dependent quantum-mechanical simulation. The photoexcitation of N2 is by an ultrashort pulse so that the process has a sharply defined origin in time and so that we can monitor the isolated molecule dynamics in time. An ultrafast pulse is necessarily broad in frequency and spans several excited electronic states. Each excited molecule is therefore not in a given electronic state but in a superposition state. A short time after excitation, there is a fairly sharp onset of a mass-dependent large population transfer when wave packets on two different electronic states in the same molecule overlap. This coherent overlap of the wave packets on different electronic states in the region of strong coupling allows an effective transfer of population that is very mass dependent. The extent of the transfer depends on the product of the populations on the two different electronic states and on their relative phase. It is as if two molecules collide but the process occurs within one molecule, a molecule that is simultaneously in both states. An analytical toy model recovers the (strong) mass and energy dependence.
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机译:N2的光解离中的同位素分级可以解释我们银河系不同区域 14 sup> N / 15 sup> N比值的显着变化。我们以前提出,这种同位素效应是由于光激发的结合价和里德堡电子态在强态混合的频率范围内的耦合引起的。我们在这里通过时间相关的量子力学模拟确定质量在动力学中的作用特征。 N2的光激发是通过超短脉冲进行的,因此该过程在时间上具有明确定义的起源,因此我们可以及时监控分离的分子动力学。超快脉冲的频率必然很宽,并且跨越了几个激发的电子状态。因此,每个受激分子不是处于给定的电子状态而是处于叠加状态。激发后的短时间内,当同一分子中两个不同电子态的波包重叠时,就会出现大量依赖质量的大量迁移。在强耦合区域中,波包在不同电子态上的这种相干重叠允许非常有效地依赖于质量的人口的有效转移。转移的程度取决于两种不同电子状态及其相对相位的总体乘积。好像两个分子发生碰撞,但过程发生在一个分子内,一个分子同时处于两种状态。分析玩具模型可以恢复(强)质量和能量依赖性。
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