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95Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2017
IONIZATION OF MOLECULES AT THE FLUID-FLUID PHASE TRANSITION IN WARM
DENSE HYDROGEN
Genri E. Norman, Ilnur M. Saitov National Research University Higher School of Economics, Russia, E-mai: genri.norman@gmail.com
An idea is introduced that ionization of H2 molecules takes place at the fluid-fluid phase transition in warm dense hydrogen/deuterium with formation of molecular ions H2+ and H3+. Density functional theory with the VASP plane-wave code is used. Proton-proton pair correlation functions (PCF) g(r), conductivity and pressure are calculated. Three PCF treatment procedures are applied to reveal a nature and character of the phase transition. (1) PCF varies slowly with the density in the range of distances larger than 2Â. However, the situation changes drastically for distances smaller than 2Â. The values of the PCFs first local maxima g(rmax1) and first local minima g(rmin1) are changed dramatically in the narrow density range. To emphasize the character of g(r) changes, plots of g(rmax1)/ g(rmin1) are obtained. Strongly pronounced jumps for T = 700, 1000, 1500 and 2500 K are clear indications of the phase transition since they take place at the same densities where small density jumps are observed. The dependence of discontinuity of ratio g(rmax1)/ g(rmin1) on temperature gives us the estimation of critical temperature Tc~4000K which agrees with recent data of Mochalov et al.
(2) The value of rmax1 is equal to the interatomic distance 0.74Â, in the H2 molecule. The value of rmin1 is close to the interatomic distances 1.06Â, and 0.92Â, in the molecular ions H2+ and H3+. Let g1(r) and g2(r) are PCF's which are the closest to the phase transition before and after it. Ag(r)= g2(r)-g1(r) is close to zero for r>2Â. The function Ag(r) has a deep minimum at r=0.74Â and a strongly pronounced maximum in the range of r from 0.92Â to 1.06Â. It means that the number of H2 decreases and ions H2+ and H3+ appears at the phase transition.
(3) The ratio of the second maxima and minima g(rmax2)/ g(rmin2) varies smoothly with the density. It turns out that the PCF's obtained can be modeled for r > 2Â, by the soft sphere PCF's at number densities which are equal to the total density of H2, H2+ and H3+. The latter value remains close to the constant one at the phase transition. The repulsion "diameters" is close to the theoretical estimate. A two-step mechanism is suggested. The first stage is related to the partial ionization of H2 molecules at the phase transition with formation of the molecular ions H2+. The second stage is a reaction of H2 molecules and H2+ ions to form H3+ ions. The nature of the phase transition combines the ionization and the structure transformation. Strong ionization can be related to the Norman-Starostin plasma phase transition prediction. However, it differs from it by inherent structural changes. The transition in WDH is an exceptional case.
The study has been funded by the Russian Academic Excellence Project '5-100'.
