Resonance-enhanced multiphoton ionization in the x-ray regime

TL;DR

This paper demonstrates resonance-enhanced multiphoton ionization in argon atoms using ultraintense x-ray pulses, revealing a two-color resonant process that produces high charge states and differs from conventional REMPI.

Contribution

It introduces the first observation of x-ray resonance-enhanced multiphoton ionization (REMPI) in the x-ray regime, supported by theoretical calculations.

Findings

High charge states achieved via multiphoton ionization.

Identification of a two-color resonant ionization mechanism.

Theoretical explanation of ion yield and spectral features.

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 $1s$ 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 $1s \rightarrow 2p$ 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 through multiple 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.