Experimentally obtained anisotropy parameters β2 and β4, and the extracted values of W and Δ

Table 1. Experimentally obtained anisotropy parameters β2 and β4, and the extracted values of W and Δ. Abstract The two-photon ionization of helium atoms by ultrashort extreme-ultraviolet free-electron laser pulses, produced by the SPring-8 Compact SASE Source test accelerator, was investigated at photon energies of 20.3, 21.3, 23.0 and 24.3 eV. The angular distribution of photoelectrons generated by two-photon ionization is obtained using a velocity map imaging spectrometer. The phase-shift differences and amplitude ratios of the outgoing s and d continuum wave packets are extracted from the photoelectron angular distributions. The obtained values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to a resonance with an excited level or Rydberg manifold. The difference stems from the co-presence of resonant and non-resonant path contributions in the two-photon ionization by femtosecond pulses. Since the relative contribution of both paths can be controlled in principle by the pulse shape, these results illustrate a new way to tailor the continuum wave packet.表1 实验测得的各向异性参数β₂和β₄，以及提取得到的W与Δ值。 摘要 本研究针对由SPring-8紧凑型自放大自发辐射（SASE）测试加速器产生的超短极紫外自由电子激光脉冲，在20.3 eV、21.3 eV、23.0 eV及24.3 eV的光子能量下，开展了氦原子双光子电离的实验探究。本研究利用速度映射成像光谱仪，获取了双光子电离产生的光电子角分布。从光电子角分布中提取出出射s波与d波连续态波包的相移差及振幅比。当光子能量调谐至与激发能级或里德伯流形（Rydberg manifold）发生共振时，所测得的相移差与散射相移差存在显著差异。该差异源于飞秒脉冲双光子电离过程中共振路径与非共振路径贡献的共同存在。由于原则上可通过脉冲波形调控两条路径的相对占比，本研究结果为连续态波包的定制调控提供了全新途径。