利用焦距为300 mm和1500 mm的透镜对纳秒激光束进行聚焦, 产生了在空间发散度上具有较大差别的两激光束, 采用飞行时间质谱计分别测量了两光束诱导C60分子电离和碎裂时的离子产物分布随激光通量的变化特征。实验结果表明, 激光束的空间发散度越大, 轻碎片离子C+n(n＜25)的总产额、C60分子的碎裂程度以及延迟电离产额随激光通量的变化趋势就越快, 延迟电离成分占总电离成分的百分比就越高。该实验结果可用多光子吸收的内能分布理论给予合理解释。

Two laser beams with different divergences are obtained using 300 mm and 1500 mm lens. The ionic fragments distributions of the two laser beams are measured by a time-of-flight mass spectrometer. The results show that with the increasing divergence of laser beam, the fragmentation degree of C60, the yield of delayed ionization of C60 and the total yield of the ionic fragment C+n(n＜25) have steeper change depending on fluences. In addition, the yield of delayed ionization has a larger proportion than direct ionization. The experimental results are in reasonable agreement with the latest prediction by multiple photon absorption theory.