We studied photoelectron angular distributions (PADs) and energy spectra of sodium atoms photo-ionized with intense short laser pulses in the near-infrared region. Energy spectra of photoelectrons due to multistep ionization of sodium atoms through intermediate levels 5p, 6p, 7p, and 4f were observed in above threshold ionization (ATI) at laser intensities of 2 10¹³Wcm⁻² and 1 10¹³Wcm⁻². The PADs from the 5p(4f) state for zero, first, second and third order ATI peaks were measured. We observed that for the two selected laser intensities the angular distribution of photoelectrons showed only slight variations. The PADs exhibit main lobes within ∼60 degrees from the laser polarization and additional lobes that become more pronounced with the increasing ATI peak number. Besides a general analytical representation of the observed angular dependences, the experimental PADs for each ATI peak were fitted with Legendre polynomials accounting for different ionization channels. The good agreement between the experimental data and fitting functions allowed to evaluate the contributions of the ionization channels to the observed ATI orders. The calculations of the photoelectron spectra by numerically solving the time-dependent Schrdinger equation in a single active electron approximation produced agreement with the experiment in the main features of the spectra, while differences in more subtle details were observed.

我们研究了钠原子的光电子角分布 (PAD) 和能谱，在近红外区域用强短激光脉冲进行光电离。在高于阈值电离 (ATI) 中，在 2 10 ¹³ Wcm ^-2和 1 10 ¹³ Wcm ^-激光强度下观察到由于钠原子通过中间能级 5 p、6 p、7 p和4f 的多步电离引起的光电子能谱² . 来自 5 p ( 4f) 测量零、一阶、二阶和三阶 ATI 峰值的状态。我们观察到，对于两种选定的激光强度，光电子的角分布仅显示出轻微的变化。PAD 显示出距激光偏振约 60 度以内的主瓣和随着 ATI 峰值数增加而变得更加明显的附加瓣。除了观察到的角度依赖性的一般分析表示外，每个 ATI 峰的实验 PAD 都拟合了勒让德多项式，说明了不同的电离通道。实验数据和拟合函数之间的良好一致性允许评估电离通道对观察到的 ATI 阶次的贡献。