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High Temperature

The influence of interaction between Rydberg atoms and slow electrons on distribution function in the ultracold plasma

Boris B. Zelener, B. V. Zelener, Edward A. Manykin, Danila Khikhlukha

Published Apr 1, 2012• 0 citations
Publisher

Abstract

Expressions for the cross sections and rate constants of transitions during collisions of Rydberg atoms with electrons whose energy E is lower than the transition energy E if are proposed. The populations of Rydberg states were calculated using the numerical solution of the set of kinetic balance equations as functions of time in an ultracold plasma based on the obtained transition rate constants. The calculation results are in good agreement with the experiment.

DOI
10.1134/s0018151x12020216

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Journal of Physics B Atomic Molecular and Optical Physics•Aug 9, 2012•18 citations
Expressions for the cross sections and rate constants of transitions during collisions of Rydberg atoms with electrons whose energy E is lower than the transition energy E if are proposed. The populations of Rydberg states were calculated using the numerical solution of the set of kinetic balance equations as functions of time in an ultracold plasma based on the obtained transition rate constants. The calculation results are in good agreement with the experiment.
The distribution functions and diffusion coefficient of electrons in the Rydberg energy space in an ultracold plasma are obtained as functions of temperature by numerically solving a system of kinetic balance equations. As the initial conditions, the results obtained in previous papers by the molecular dynamics method are used. From calculation of Rydberg electron fluxes, the temperature dependence of the recombination coefficient is obtained, in good agreement with the recombination coefficient calculated previously by the molecular dynamics method.
We present extensive Monte Carlo calculations of electron-impact-induced transitions between highly excited Rydberg states and provide accurate rate coefficients. For moderate energy changes, our calculations confirm the widely applied expressions in P. Mansbach and J. Keck [Phys. Rev. 181, 275 (1969)] but reveal strong deviations at small energy transfer. Simulations of ultracold plasmas demonstrate that these corrections significantly impact the short-time dynamics of three-body Rydberg atom formation. The improved rate coefficients yield quantitative agreement with recent ultracold plasma experiments.
Inelastic n-changing collisions play an important role in the evolution of Rydberg atoms into ultracold plasmas. However, for the initially intermediate n (n ∼ 40) Rydberg states, these collisions can hardly be observed due to the low electron temperature in ultracold plasmas. In this work, we designed an experimental scheme to facilitate collisions between free electrons at 1.5 eV and intermediate n Rydberg atoms. Using the field ionization technique, we measured the state distributions resulting from the evolution of initially cold rubidium atoms in the 45P3/2 Rydberg state. The experimentally obtained probability of inelastic collisions excitation agrees well with the Monte Carlo simulation results. In addition, our experimental results indicate that the n-changing population induced by hot electrons is significant for lower nP Rydberg states. Our work plays a significant role in calculating the rates of electron-ion three-body recombination in ultracold plasmas.
When ultracold plasma is generated using the photoionization of laser-cooled atoms, some atoms reach only up to Rydberg states. These, in turn, interact with the free electrons of the plasma and get ionized further, thus enhancing the plasma. Theoretical models until date have used a semiclassical approach that does not explain all the properties of Rydberg ionization. We present an analytical solution for the Rydberg–electron ionization cross section using the Quantum-mechanical scattering process, which shows a good agreement with experimental data. In particular, the dependency of ionization on the Rydberg state is clearly shown in the form of relation with orbital size and the scattering length. This calculation is valid for all alkali atoms and, hence, is useful for estimating the electron density of ultracold plasma.
A kinetic model is developed to describe ultracold nonideal Rydberg plasmas, which allows all stages of the generation and decay of such a plasma to be sequentially traced. The plasma kinetics is considered on the basis of available experimental data corresponding to a nonideality parameter of γ ∼ 1. The results of theoretical analysis are in good agreement with experiment. Calculations show evidence of a significantly decreased recombination rate and, hence, of the possible formation of a metastable structure in the plasma under consideration. The distribution of the number of excited atoms is determined for the plasma with N e = N i = 7 × 10 5 and E e = 9 K. The observed behavior of the number and density of particles as functions of the time and principal quantum number is explained. It is suggested that the distribution of excited atoms for the given parameters has a maximum for the state with k = 25.
We present a kinetic model based on coupled rate-equation simulations that accounts for state-detailed rate processes in the evolution of a molecular Rydberg gas to plasma. Calculations support a steady-state picture of Rydberg predissociation and its contribution to plasma dissipation. Inelastic collisions of electrons with Rydberg states efficiently transfer energy from and to the thermal bath of free electrons, giving rise to quasi equilibria of relatively high temperature. Dissipative effects that remove low- n population change the time course of this heating. Including statistically sampled channels of Rydberg predissociation with n - and ℓ-dependent lifetimes slows the relaxation to quasi equilibrium and retards the initial electron temperature increase. At later times, the dissociative loss of low- n population displaces this quasi equilibrium leading to continuous electron heating. Dissociation products appear at a rate that is controlled initially by the rate of Rydberg relaxation to quasi equilibrium. Thereafter, the overall rate at which the total dissociation consumes neutral molecules determines the decay rate for all levels. Simulations predict a total predissociation rate that is smaller than the initial rate of direct electron–ion dissociative recombination. At later times, residual predissociation displays an apparent first-order decay with a rate constant that fits well with experimental observations.