Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Resonance-enhanced multiphoton ionization in the x-ray regime   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.

2
The categories of this paper are: math
23
The categories of this paper are: solv-int
22
The categories of this paper are: chao-dyn
30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
36
The categories of this paper are: cmp-lg
18
The categories of this paper are: econ
10
The categories of this paper are: q-bio
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat
1
The categories of this paper are: cs
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys