Photoelectron spectroscopy method to reveal ionization potential lowering in nanoplasmas

Here we propose a scheme for probing the outer-shell atomic energy levels within a laser-created nanoplasma, using photoelectron spectroscopy data obtained from the irradiation of the nanoplasma with an ultraintense 'probing' pulse from a soft x-ray free-electron laser. The proposed method can then detect shifts of outer-shell energy levels of an atom or an ion within a plasma, due to the effect of charged plasma environment on atomic potentials, known as the 'plasma screening effect'. Various theoretical models exist that estimate the magnitude of the screening effect. However, the first experimental data that can verify theoretical models have become available only recently (Vinko et al 2012 Nature 482 59). They were obtained with a hard x-ray-based experimental method which uses the information encoded in fluorescence spectra and is, therefore, restricted to deep atomic shells. Below, we show that our photoelectron spectroscopy method of probing a nanoplasma with a destructive, high-intensity soft x-ray pulse that brings the irradiated system to the regime of 'massively parallel' ionization (Gnodtke et al 2012 Phys. Rev. Lett. 108 175003) enables access to the information on the energy level (and ultimately energy level shift) of the valence orbital. In particular, the result of such a photoelectron spectroscopy experiment could help to clarify the discrepancy between the ion abundances in nanoplasmas observed during the recent high-harmonic-generation and free-electron-laser experiments with intense soft x-ray pulses. (paper)