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首页> 外文会议>NATO Advanced Research Workshop on Super-Intense Laser-Atom Physics Sep 24-30, 2000 Han-sur-Lesse, Belgium >TUNNELLING IONISATION AND THE FRANCK-CONDON PRINCIPLE
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TUNNELLING IONISATION AND THE FRANCK-CONDON PRINCIPLE

机译：隧道电离和弗兰克-康登原理

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摘要

According to the Franck-Condon principle, transition probabilities between two molecular states are determined by the electronic transition probability, weighted by the overlap of the two vibrational states involved. In this paper we discuss a situation where this approximation is invalid, namely the tunnelling ionisation of H_2. The fragmentation of H2 in intense laser fields of visible and infrared wavelengths has been studied experimentally for more than a decade. It is usually assumed that the complete fragmentation occurs in three, more or less independent, steps. First there is the ionisation of H_2. Due to the small photon energy, the high ionisation potential and the absence of appropriate resonances, the ionisation takes place in the tunnelling or even the over-the-barrier regime. This type of ionisation occurs at the peaks and troughs of the coherent electromagnetic wave, i.e. in short periods of less than 1 fs during which the vibrational motion is essentially frozen. Quite naturally this leads to the idea of vertical transitions centered on the equilibrium distance of H_2, see e.g. Walsh et al. (1998). The population distribution amongst the vibrational levels of H_2~+ is then given by the standard Franck-Condon factors, which are shown in figure 1. However, it would be more accurate to take the large deformation of the H_2~+ electronic ground state in the strong laser field into account. This would be expected to shift the population towards higher vibrational levels , but we have strong reasons to believe that the population distribution is actually shifted towards lower levels.
机译：根据Franck-Condon原理，两个分子态之间的跃迁概率由电子跃迁概率确定，并由所涉及的两个振动态的重叠加权。在本文中，我们讨论了这种近似无效的情况，即H_2的隧穿电离。在可见光和红外波长的强激光场中，H 2的碎片化已经进行了十多年的实验研究。通常假设完整的碎片发生在三个或多或少独立的步骤中。首先是H_2的电离。由于光子能量小，电离势高以及没有适当的共振，电离发生在隧穿甚至是跨势垒状态。这种类型的电离发生在相干电磁波的波峰和波谷处，即在小于1 fs的短时间内发生，在此期间振动运动基本上被冻结。很自然地，这导致了以H_2的平衡距离为中心的垂直过渡的想法，例如参见Walsh等。 （1998）。然后，通过标准的Franck-Condon因子给出H_2〜+振动能级之间的总体分布，如图1所示。但是，将H_2〜+电子基态的大变形考虑在内会更准确。考虑到强激光场。可以预期这将使人口向更高的振动水平转移，但是我们有充分的理由相信人口分布实际上已经向较低的水平转移。

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