Two-colour experiments in the gas phase
Two-colour experiments in the gas phase
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DOI：
10.1088/0953-4075/43/19/194006
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发表时间：
2010-10
期刊：
影响因子：
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通讯作者：
Michael Meyer;John T Costello;S. Düsterer;W. B. Li;P. Radcliffe
中科院分区：
文献类型：
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作者：
Michael Meyer;John T Costello;S. Düsterer;W. B. Li;P. Radcliffe
首先，利用汉堡自由电子激光(闪光)与单独的近红外(NIR)飞秒激光相结合的前所未有的特性，进行了原子光电离和分子解离的实验。在一系列双色实验中，系统地研究了稀有气体在强近红外修饰场中的光致电离及其与偏振的关系。对低修饰场下双色双光子电离过程的部分截面进行了详细的分析。较高的修饰场产生了多光子过程，在没有不良干扰的情况下进行了观察和分析，这是闪光辐射单色性的有利结果。实验结果与用二阶微扰理论和“软光子”近似得到的双色阈值以上电离的理论描述进行了比较。此外，还获得了Ne的双光子顺序双电离的补充信息，这是由短而强的闪光脉冲实现的。作为未来分子解离时间分辨研究的起点，对氢双原子体系进行了原理验证实验。在典型的泵浦-探测装置中，通过延时光学激光诱导的单光子和多光子电离，可以识别在闪光光解离过程中形成的受激发的中性碎片。
First experiments on atomic photoionization and molecular dissociation have been performed by taking advantage of the unprecedented characteristics of the free electron laser in Hamburg (FLASH) combined with a separate near-infrared (NIR) femtosecond laser. In a series of two-colour experiments, the photoionization of rare gases in the presence of a strong NIR dressing field as well as the polarization dependence of this process were investigated systematically. A detailed analysis of the partial cross sections for the two-colour two-photon ionization process was carried out for low dressing fields. Higher dressing fields gave rise to multi-photon processes, which were observed and analysed without undesirable interferences, a beneficial consequence of the monochromaticity of the FLASH radiation. The experimental results were compared with theoretical descriptions for two-colour above-threshold ionization obtained by employing second-order perturbation theory and the ‘soft-photon’ approximation. In addition, complementary information was obtained on the sequential two-photon double ionization of Ne, which was made possible by the short and intense FLASH pulses. As a starting point for future time-resolved studies of molecular dissociation, a proof-of-principle experiment on the hydrogen diatomic system was carried out. In a typical pump–probe arrangement, excited neutral fragments, which were formed during photo-induced dissociation by the FLASH radiation, were identified via single- and multi-photon ionization induced by the time-delayed optical laser.