LaForge Aaron C, Son Sang-Kil, Mishra Debadarshini, Ilchen Markus, Duncanson Stephen, Eronen Eemeli, Kukk Edwin, Wirok-Stoletow Stanislaw, Kolbasova Daria, Walter Peter, Boll Rebecca, De Fanis Alberto, Meyer Michael, Ovcharenko Yevheniy, Rivas Daniel E, Schmidt Philipp, Usenko Sergey, Santra Robin, Berrah Nora
Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA.
Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany.
Phys Rev Lett. 2021 Nov 19;127(21):213202. doi: 10.1103/PhysRevLett.127.213202.
Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a 1s electron, a different mechanism is required to obtain ionization to Ar^{17+}. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a 1s→2p transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar^{16+}, but also through lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile.
在此，我们报告了利用欧洲X射线自由电子激光装置产生的超强X射线，在短波长区域对氩原子进行非线性电离的情况。经过连续多光子电离后，可获得高电荷态。对于不足以直接电离一个1s电子的光子能量，需要一种不同的机制来实现对Ar^{17 +}的电离。我们提出这是通过双色过程发生的，其中自由电子激光脉冲的二次谐波通过1s→2p跃迁共振激发系统，随后由自由电子激光脉冲的基波进行电离，这是一种X射线共振增强多光子电离（REMPI）。这种共振现象不仅发生在Ar^{16 +}上，也发生在较低电荷态上，在这些电荷态中，多次电离与衰变寿命相互竞争，使得X射线REMPI有别于传统的REMPI。借助最先进的理论计算，我们解释了X射线REMPI对相关离子产率和光谱轮廓的影响。
