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Journal of Physics B Atomic and Molecular Physics

Ionisation in atom-Rydberg atom collisions: ejected electron energy spectra and reaction rate coefficients

A. A. Mihajlov, R.K. Janev

Published May 28, 1981• 58 citations
Publisher

Abstract

Using the resonant internal energy conversion mechanism for inelastic atom-Rydberg atom slow collisions, the energy spectra of ejected electrons and the rate coefficients for the ionisation process are calculated analytically. Reaction rates for H*(n)+H, Cs*(n)+Cs, Rb*(n)+Rb and K*(n)+Rb colliding systems are calculated and compared with the available experimental data.

DOI
10.1088/0022-3700/14/10/013

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Journal of Physics B Atomic and Molecular Physics•Sep 14, 1987•7 citations
Using the resonant internal energy conversion mechanism for inelastic atom-Rydberg atom slow collisions, the energy spectra of ejected electrons and the rate coefficients for the ionisation process are calculated analytically. Reaction rates for H*(n)+H, Cs*(n)+Cs, Rb*(n)+Rb and K*(n)+Rb colliding systems are calculated and compared with the available experimental data.
The process of ionization in slow-atom-Rydberg-atom collisions is investigated. A resonant energy exchange mechanism is considered responsible for the electron transition into the continuum. Both symmetrical and asymmetrical systems are considered. The ionization probabilities and cross sections for a number of Rydberg states in the colliding systems H∗(n)+H{\mathrm{H}}^{*}(n)+\mathrm{H}H∗(n)+H, Li∗(n)+Li{\mathrm{Li}}^{*}(n)+\mathrm{Li}Li∗(n)+Li, H∗(n)+Li{\mathrm{H}}^{*}(n)+\mathrm{Li}H∗(n)+Li are calculated.
Absolute rate constants for ionization of Rb(ns,nd) atoms in collisions with SF6{\mathrm{SF}}_{6}SF6​ have been measured for values of n in the range 38--106. In addition, the kinematics of such reactions have been investigated using Xe(nf) atoms. The data confirm that collisions of Rydberg atoms with targets that attach free low-energy electrons are well described by the ``essentially-free''-electron model. Analysis of the data shows that the cross section \ensuremath{\sigma}(v) for electron attachment to SF6{\mathrm{SF}}_{6}SF6​ at very low electron energies (\ensuremath{\lesssim}10 meV) is inversely proportional to the electron velocity v and, if v is expressed in cm ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$, is given by \ensuremath{\sigma}(v)=(4.2\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$/v cm2{\mathrm{cm}}^{2}cm2.
In the present work very-high-n Rydberg-atoms (nsim100−nsim400)(n \\sim 100 - n \\sim 400)(nsim100−nsim400) are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low rm(sim85mueV−sim1.4meV),\\rm({\\sim}85\\mu eV - {\\sim}1.4meV),rm(sim85mueV−sim1.4meV), analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\\sim111\\sp\\circ$K, which are far below those accessible using any alternate approach.\nRate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions rmK(itnp/)+XYtoKsp++(XY)sp−∗ortoKsp++Xsp−+Yeqno(1)\\rm K({\\it np\\/}) + XY \\to K\\sp+ + (XY)\\sp{-*}\\ or\\ \\to K\\sp+ + X\\sp- + Y\\eqno(1)rmK(itnp/)+XYtoKsp++(XY)sp−∗ortoKsp++Xsp−+Yeqno(1)and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization rmK(itnp/)+XY(itJ/)toKsp++itesp−+XY(itJ/−1)eqno(2)\\rm K({\\it np\\/}) + XY({\\it J\\/}) \\to K\\sp+ + {\\it e}\\sp- + XY({\\it J\\/}-1)\\eqno(2)rmK(itnp/)+XY(itJ/)toKsp++itesp−+XY(itJ/−1)eqno(2)\nThe n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process.\nThe essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\\eqalignno{&e\\sp- + \\rm XY \\to (XY)\\sp{-*}\\ or\\ \\to X\\sp- + Y&(1\\sp\\prime)\\cr &e\\sp- + \\rm XY({\\it J\\/}) \\to {\\it e}\\sp- + XY({\\it J\\/}-1)&(2\\sp\\prime)\\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.
The excitation and deexcitation processes in slow collisions of highly excited (Rydberg) atoms with ground-state parent atoms are considered. A mechanism, based on the quasiresonant energy transfer within the electronic part of the colliding system, is proposed for these processes. Both direct and exchange reaction channels are included into the treatment. Expressions for the $n\ensuremath{\rightarrow}{n}^{\ensuremath{'}}$ transition probabilities are obtained in closed form. Probability and cross section calculations for some transitions and level decays in the H∗(n){\mathrm{H}}^{*}(n)H∗(n)-H system are presented.
Rate constants for state changing in collisions of Rb(ns) Rydberg atoms (40\ensuremath{\le}n\ensuremath{\le}48) with HF molecules have been measured and have been calculated using a semiclassical approach. Agreement between the total rate constants measured experimentally and derived theoretically is very good. The results point to the near-resonant nature of n-changing collisions, but indicate that the rate constants for such reactions are less dependent on energy imbalance than suggested by calculations based on the impulse approximation.
The ionisation of highly excited Rydberg atoms in collisions with ground-state atoms is considered. The stationary diffusion-like mechanism of the ionisation is presented. The investigation is based on the Fokker-Planck equation. Analytical expressions for the probabilities of stationary diffusive ionisation are obtained and applied for the explanation of experimental results on the ionisation of highly excited Rydberg states of caesium in collisions with ground-state Cs atoms. It is concluded that the diffusive mechanism is the main mechanism for collisional ionisation of Rydberg atoms with n>or approximately=25-30.