X-ray techniques have evolved over decades to become highly refined tools for a broad range of investigations. Importantly, these approaches rely on X-ray measurements that depend linearly on the number of incident X-ray photons. The advent of X-ray free electron lasers (XFELs) is opening the ability to reach extremely high photon numbers within ultrashort X-ray pulse durations and is leading to a paradigm shift in our ability to explore nonlinear X-ray signals. However, the enormous increase in X-ray peak power is a double-edged sword with new and exciting methods being developed but at the same time well-established techniques proving unreliable. Consequently, accurate knowledge about the threshold for nonlinear X-ray signals is essential. Herein we report an X-ray spectroscopic study that reveals important details on the thresholds for nonlinear X-ray interactions. By varying both the incident X-ray intensity and photon energy, we establish the regimes at which the simplest nonlinear process, two-photon X-ray absorption (TPA), can be observed. From these measurements we can extract the probability of this process as a function of photon energy and confirm both the nature and sub-femtosecond lifetime of the virtual intermediate electronic state.
A strong enhancement of electron-ion recombination over expected rates at sub-eV relative energies is revealed for highly charged ions. Measured Ne' + and Ar' + recombination rates were found to be enhanced at low (&1 rneV) energies by factors of 4 and 10 over theoretical predictions of radiative recombination rates, respectively.It is shown that the majority of the enhancement in Ar' is caused by very low lying An = 0 dielectronic recombination resonances; nevertheless, the nonresonant contributions to the measured recombination rates for both systems were found to be significantly higher than predicted by presently available models.PACS numbers: 34.80.Kw, 32.80.HdAmong the fundamental electron-ion interactions which occur in plasmas [1,2], recombination at low energies has turned out to be surprisingly unpredictable.Measurements performed under the most precise conditions yet achieved have consistently yielded experimental rates far above theoretical expectations [3 -6]. No convincing explanation of these enhanced rates exists, nor have detailed quantitative spectra in the sub-meV region been presented.In models describing recombination in electron coolers one considers three basic processes: radiative recombination (RR), where energy and momentum conservation is balanced by the emission of a photon, e + a '-A" "'+ hv, three-body recombination (TBR), where the excess energy is imparted to a neighboring free electron, e +e +A+ A~'~+ e and, for nonbare ions, dielectronic recombination (DR), where a free electron is captured resonantly with the excitation of a bound electron in the ion. This process can be viewed as the time reverse of Auger emission.
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