Time-dependent view of an isotope effect in electron-nuclear nonequilibrium dynamics with applications to N2
Time-dependent view of an isotope effect in electron-nuclear nonequilibrium dynamics with applications to N2
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DOI：
10.1073/pnas.1804455115
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发表时间：
2018-05
期刊：
影响因子：
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通讯作者：
J. Ajay;K. Komarova;F. Remacle;F. Remacle;R. Levine;R. Levine
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作者：
J. Ajay;K. Komarova;F. Remacle;F. Remacle;R. Levine;R. Levine
在地外来源中发现的大范围同位素异常表明，我们寻求对远紫外线辐射诱导的光化学过程的更好的机制见解。为了能够跟踪这一过程，我们模拟了在真空紫外（VUV）中被超快脉冲激发的N2分子的时间进展。如此短的脉冲必然会引发我们在计算机中追踪的分子的非稳态状态。在VUV中，N2被泵浦到价态激发态和里德伯态。超短脉冲产生了这些电子态的相干组合，定位在frank - condon区域，导致动态同位素效应。N2光解过程中的同位素分馏可以解释银河系不同区域中14N/15N比值的巨大变化。我们之前提出，这种同位素效应是由于光激发束缚价和Rydberg电子态在有强态混合的频率范围内的耦合。我们在这里通过一个时间相关的量子力学模拟来确定质量在动力学中的作用的特征。N2的光激发是由一个超短脉冲引起的，所以这个过程在时间上有一个明确的起源，这样我们就可以及时地监测孤立分子的动力学。一个超快脉冲的频率必须很宽，并且跨越几个激发态。因此，每个激发态分子不是处于给定的电子态，而是处于叠加态。在激发后的短时间内，当同一分子中两个不同电子态上的波包重叠时，会出现相对剧烈的依赖质量的大种群转移。在强耦合区域中，不同电子状态上的波包的相干重叠允许非常依赖质量的人口的有效转移。转移的程度取决于两种不同电子态的居群乘积及其相对相位。这就好像两个分子发生碰撞，但这个过程发生在一个分子内，而这个分子同时处于两种状态。一个解析玩具模型恢复了（强烈的）质量和能量依赖。
Significance Wide-range isotopic anomalies found in extraterrestrial sources suggest that we seek better mechanistic insights on photochemical processes induced by far UV radiation. To be able to follow the process, we simulate the progress in time of an N2 molecule excited by an ultrafast pulse in the vacuum UV (VUV). Such a short pulse necessarily initiates a nonstationary state of the molecule that we follow in silico. In the VUV, N2 is pumped to a valence excited and Rydberg states. The ultrashort pulse creates a coherent combination of these electronic states, localized in the Franck–Condon region, leading to a dynamical isotope effect. 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.