We demonstrate a deep-learning neural network (DNN) method for the measurement of molecular alignment by using the molecular-alignment-based cross-correlation polarization-gating frequency resolved optical gating (M-XFROG) technique. Our network has the capacity for direct measurement of molecular alignment from the FROG traces. In a proof-of-principle experiment, we have demonstrated our method in O2 molecules. With our method, the molecular alignment factor ⟨cos2θ⟩(t) of O2, impulsively excited by a pump pulse, was directly reconstructed. The accuracy and validity of the reconstruction have been verified by comparison with the simulations based on experimental parameters.

The dephasing of molecular alignment can lead to the deformation of the alignment signal during its periodic revivals. Most studies are concentrated on the first few rotational revival periods of the molecular alignment and neglect the dephasing effect. However, study of the alignment dephasing is still of great significance for both the long-term dynamics of the molecular alignment and the dephasing itself. In this work, we theoretically demonstrate that the dephasing effect is correlated with both the rotational temperature and the rotational revival period of the molecules. The results present that the dephasing is especially significant for those molecules with long rotational revival period at high rotational temperatures. The physics behind it is explored by taking advantage of the coherence of the rotational quantum state population. This work deepens our understanding of rotational dynamics and rotational spectroscopy in molecular alignment.

弱场偏振探测技术是分子取向探测技术的一种。该探测技术不会破坏无外场取向分子的量子状态从而便于取向后的分子进行各种实际应用，因此这种技术在分子取向的研究中运用较为广泛。详细介绍了弱场偏振探测技术和其改进技术——平衡弱场偏振探测技术的探测原理和实验系统组成，以泰勒级数为基础采用近似方法分析、比较了两种探测技术的灵敏度，探讨了二者的噪声来源和影响。研究发现，在分子取向度较小时，后者的灵敏度比前者较强；探测激光能量涨落较大时，后者的噪声明显低于前者；探测激光能量涨落可以忽略时，二者的噪声大小比较接近。