Lowering of the ionization energy of ions in plasmas and solids

Atomic physics in plasmas is crucial in astrophysics, cosmology, planetary science and inertial confinement fusion research. It is also fundamentally important to understand the interplay of a quantum atom with charged particles at finite temperature. In a low-density environment, where atoms and ions are essentially free, calculations of the atomic structure, population kinetics and radiative properties (emission, absorption, scattering) have been very successfully applied in many different scientific and technical disciplines [1-3].

            As density increases, the free atom model breaks down because the environment impacts on the atomic structure and the transition rates [1]. Among a large variety of dense plasma effects, the Ionization Potential Depression IPD is of fundamental interest. Consequently IPD has attracted the atomic and plasma physics community since generations [4-17] and stimulated sophisticated high-resolution X-ray spectroscopic methods to measure line and continuum shifts induced by dense plasmas. However, goaded by lacks of suitable experiments and codes implementing general models with spectroscopic precision, IPD is under controversial discussion up to present days. In addition the fluctuating plasma electric microfield results in a perturbations of cross sections and transition rates that are directly related to the radiative properties of the matter.

            With the emergence of the X-ray Free Electron lasers new measurements of the ionization potential depression have been attempted and the data seemed to question our present understanding [18]. It was claimed that the performed measurements support the early model of Ecker & Kröll EK [5] whereas the well-accepted model of Stewart & Pyatt SP [6,7] is in disagreement with the data. On the other hand, subsequently performed ionization potential depression measurements at high-energy density laser experiments [19,20] confirmed the validity of the SP model and demonstrated a worse agreement with the EK model.

            In the framework of the recently developed Atomic-Solid-Plasma ASP model [21] it has been shown, that both, the EK and SP models fail to interpret the data: they fail in fact with respect to both, absolute values and Z-scaling relations. The analysis showed [21,3] that previous statements [18] are based on a misconception and accidental coincidence in scaling relations.

            The plenary talk provides an introductory overview of the long lasting controversial discussion of the lowering of the ionization energy, illuminates why current models fail in some experiments while describing well other ones and why the new Atomic-Solid-Plasma model ASP provides a solution and pathway to understand the current findings.

            Finally, recent developments of general parameter free models with spectroscopic precision are outlined that invoke the dual characteristics of bound and continuum states.

References

[1] V.S. Lisitsa, Atoms in Plasmas, Springer 1994
[2] H.R. Griem, Principles of Plasma Spectroscopy, Cambridge University Press (1997).
[3] F.B. Rosmej, V.A. Astapenko, V.S. Listisa, Plasma Atomic Physics, Springer (2021).
[4] R.D. Inglis, E. Teller, Astr. Phys. J. 90, 439 (1939).
[5] G. Ecker, W. Kröll, Physics Fluids 6, 62 (1963).
[6] J. Stewart, K. Pyatt, Astrophys. J. 144, 1203 (1996).
[7] R.M. More, LLNL technical report, no. ucrl-84991 (1981).
[8] D.G. Hummer, D. Mihalas, Astr. J. 331, 794 (1988).
[9] F.B. Rosmej, K. Bennadji, V.S. Lisitsa, Physical Review A 84, 032512 (2011).
[10] P. Beiersdorfer et al., G.B. Brown, A.McKelvey et al., R. Shepherd, D.J. Hoarty, C.R.D. Brown, M.P. Hill, L.M.R. Hobbs, S.F. James, J. Morton, L. Wilson, Phys. Rev. A 100, 012511 (2019).
[11] X. Li, F.B. Rosmej, V.A. Astapenko, V.S. Lisitsa, Phys. Plasmas 26, 03301 (2019).
[12] X. Li, F.B. Rosmej, Physics Letters A 384, 126478 (2020).
[13] J. J. Bekx, S.-K. Son, B. Ziaja, R. Santra, Phys. Rev. Res. 2, 033061 (2020).
[14] D. Dawra, M. Dimri, A.K. Singh, R.K. Pandey, R. Sharma, M. Mohan, Phys. Plasmas 28, 112706 (2021).
[15] G. Röpke, D. Blaschke, T. Döppner, C. Lin, W.-D. Kraeft, R. Redmer, H. Reinholz, Phys. Rev. E 99, 033201 (2019).
[16] G. Massacrier, JQSRT 51, 221 (1994).
[17] J.-C. Pain, D. Benredjem, HEDP 38, 100923 (2021).
[18] O. Ciricosta et al., Phys. Rev. Lett 109, 065002 (2012).
[19] L.B. Fletcher et al., Phys. Rev. Lett. 112, 145004 (2014).
[20] D.J. Hoarty et al., Phys. Rev. Lett. 110, 265003 (2013).
[21] F.B. Rosmej, Letter J. Phys. B. 51, 09LT01 (2018).