Effect of dipole interactions on the properties of an expanding ultracold plasma: A study using quantum mechanical scattering theory

Abstract

While generating ultracold plasma (UCP) by photoinization of laser-cooled atoms, only a small fraction of atoms are ionized, and the remaining neutrals interact with the electrons present therein. These interactions, in addition to the Coulomb interactions between ions and electrons, cause phenomena such as ionization of Rydberg atoms and three body recombination, all of which affect the overall behaviour of the ultracold plasma. We had earlier developed a quantum treatment to analyze these interactions and investigated the ionization of Rydberg atoms in Cesium, which showed good agreement with measured results. We now extend it to other atomic species to investigate Rydberg ionization, and other effects such as three-body recombination and a resulting additional `quantum pressure' which causes a faster expansion of the UCP. Our results successfully explain experimental observations which were hitherto deemed as `anomalies'.

Figures (4)

• Figure 1: Total electron impact cross-section for the alkali atoms( Li, Na, K, Rb, Cs) atoms at lower energy range$(\sim 1eV)$ plotted against electron momentum $ka_o$. The cross-section values are calculated for different energy levels of n=20 (squares), 40 (circles) and 60(triangles). For comparison, the cross-section values have been plotted together(bottom-right) for n=60.
• Figure 2: Unscreened total electron-atom cross-section calculation for Cesium (1–200 eV), shown against Brode (diamonds), Jaduszliwer $\&$ Chan (horizontal bar), Kauppila $\&$ Stein (cross), CCC method(star), and R-matrix method (lower triangle). Our results using the unscreened polarisation potential is shown by Vertical barsprakashmacaskillbraybray2.
• Figure 3: Plasma parameters calculated using the ionisation/recombination rate present in the UCP medium as a function of electron momentum $ka_o$. The graph (a) shows the competing processes of ionisation/recombination in phase space. Graph (b) shows the coupling parameter. The graphs (c) shows Debye wavelength and electron pressure, and (d) shows plasma frequency in the phase space.
• Figure 4: (a)The upper graph shows total number of Rydberg atoms in the plasma medium as a function of electron momentum. The number has been plotted for different electron densities viz. $10^5, 2 \times 10^5, 5 \times 10^5$( in $cm^{-3})$ for a constant plasma temperature of 100K.g (b)The lower graph shows the total number of Rydberg atoms at different temperatures viz. 100K, 278 k, and 625 K. Density has been kept constant at $5 \times 10^5 cm^{-3}$