Resonance-enhanced multiphoton ionization in the x-ray regime*

Multiphoton ionization is one of the fundamental nonlinear processes when matter interacts with intense laser fields. In particular, Resonance-enhanced multiphoton ionization (REMPI) has been a widely-used spectroscopic technique but has yet been extended to the x-ray regime since it requires entirely different physical processes and interpretation. Conventional REMPI relies on the resonant excitation of a valence electron where the only relaxation pathway is radiative decay. On the other hand, a core-excited state after x-ray resonant excitation is subject to Auger-Meitner decay, which is orders of magnitude faster than radiative decay. Thus, the complex interplay between ultrafast decay processes and REMPI renders this process challenging to fully resolve in the x-ray regime. We present a first observation of REMPI in the x-ray regime. We observe nonlinear ionization to create Ar¹⁷⁺, where photon energies are insufficient to directly ionize a 1s electron. With the aid of state-of-the-art theoretical modeling, we attribute the ionization to a two-color REMPI-like process where the second harmonic creates a 1s → 2p transition and the fundamental pulse subsequently ionizes the system. The measured resonance profile of x-ray REMPI shows a broad, asymmetric, red-shifted distribution, which is a clear distinction from the conventional REMPI case. Moreover, theoretical results demonstrate a strong pulse length dependence of the resonance profile. Our analysis shows that the REMPI process occurs not only for Ar¹⁶⁺ but also for lower charge states, where multiple ionization competes with Auger lifetimes. We find the observed broadband nature and pulse-length dependence of the resonance profile to be due to overlapping resonances with lower Ar charge states.