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分子强场近似（SFA）理论虽然在描述强激光场中分子的超快动力学方面取得了巨大的成功，但是理论本身存在关键的矛盾。一方面SFA基本思想要求初态为无场下的系统本征态，另一方面物理过程的空间平移不变性要求系统初态应当为激光场缀饰态，这两个相互矛盾的要求分别对应非缀饰态和缀饰态两种形式的分子SFA理论，两种理论的有效性和适用条件存在广泛的争议。本文中，我们对（椭）圆偏振激光场中N$_{2}$和Ne$_{2}$分子的电离过程进行了研究，期望能对上述争议给出解答。椭圆偏振光能有效抑制再散射过程及各种干涉效应的影响，使得电离过程更加干净，因此可以有效甄别缀饰态和非缀饰态的适用条件。我们采用强场近似方法（SFA）及库仑修正强场近似方法（CCSFA）计算了缀饰态和非缀饰态下不同分子轨道对应的光电子动量分布，并与已有的实验结果进行了对比。我们发现，对于Ne$_{2}$这样核间距较大的分子，必须采用缀饰态才能准确地描述其电离特征；而对于N$_{2}$这样核间距较小的分子，缀饰态描述则不适用。本文的结论为准确描述激光诱导分子超快过程及相应理论的进一步发展提供了参考。Although the molecular strong-field approximation (SFA) theory has achieved significant achievements in characterising the ultrafast dynamics of molecules in strong laser fields, there are fundamental contradictions in the system itself. On the one hand, the basic principle of SFA requires that the initial state be an eigenstate of the system in the absence of the field, and on the other hand, the spatial translation invariance of the physical process requires that the initial state of the system should be a laser-field-dressed state, and these two contradictory requirements correspond to the two forms of molecular SFA theories, namely, the undressed and the dressed states, respectively, and the two theoretical validity and applicability conditions of these are widely disputed. In this paper we investigate the ionization processes of N2 and Ne2 molecules in (elliptically) circularly polarized laser fields, with the expectation of providing an answer to the above controversies. Elliptically polarized laser can efficiently suppress the re-scattering process and the influence of various interference effects, which makes the ionization process cleaner, and thus can effectively screen the applicable conditions for the dressed and undressed states. We have calculated the photoelectron momentum distributions corresponding to different molecular orbitals in the dressed and undressed states by using the strong-field approximation (SFA) and the Coulomb-corrected strong-field approximation (CCSFA) and compared them with the previous experimental results. For molecules with large nuclear spacing such as Ne$_{2}$, we find that the dressed state is necessary to accurately characterise their ionization, whereas for molecules with small nuclear spacing such as N$_{2}$, the dressed state description is not applicable. The conclusions of this paper provide a reference for the accurate description of laser-induced molecular ultrafast processes and the further development of the corresponding theories, and provides a reference for the further development of molecular ultrafast imaging schemes.
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Keywords:
- Elliptically polarized laser field /
- Molecule ionization /
- Dressing /
- PMD
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