Abstract
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 electron, a different mechanism is required to obtain ionization to . We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a 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 , 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.
- Received 15 February 2021
- Accepted 14 October 2021
DOI:https://doi.org/10.1103/PhysRevLett.127.213202
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