We study the multiphoton ionization of potassium atoms in 800 nm and 400 nm femtosecond laser fields. In the 800 nm laser field, the potassium atom absorbs three photons and emits one electron via one photon resonance with the 4p intermediate state with the help of the ac-Stark shift. The resonance feature is clearly shown as an Autler–Townes (AT) splitting and is mapped out in the electron kinetic energy spectrum. In a 400 nm laser field, although one photon resonance is possible with the 5p state, no splitting is observed. The different transition amplitudes between 4s–4p and 4s–5p explain the observed results. Due to the AT effect, an unexpected peak in the photoelectron energy spectrum that violates the dipole transition rule is observed. A preliminary explanation involving the spin-orbit interaction in the p state is given to account for this component. The observed AT-splitting in the electron kinetic energy distribution can be used as an effective method to calibrate the intensity of a laser field.

我们研究了 800 nm 和 400 nm 飞秒激光场中钾原子的多光子电离。在 800 nm 激光场中，钾原子在 ac-Stark 位移的帮助下吸收三个光子并通过与 4 p中间态的一个光子共振发射一个电子。共振特征清楚地显示为 Autler-Townes (AT) 分裂，并在电子动能谱中绘制出来。在 400 nm 激光场中，虽然 5 p态可能发生一个光子共振，但没有观察到分裂。4 s –4 p和 4 s –5 p之间的不同跃迁幅度解释观察到的结果。由于 AT 效应，在光电子能谱中观察到违反偶极跃迁规则的意外峰。给出了涉及p状态中的自旋轨道相互作用的初步解释以解释该分量。观察到的电子动能分布中的 AT 分裂可用作校准激光场强度的有效方法。