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Final Report

Theoretical studies of Rydberg atom collisions

R.E. Olson

Published Nov 28, 1984• 0 citations
OpenAlex

Abstract

During the last two years, theoretical studies were performed on a variety of topics related to collisions involving Rydberg atoms. Progress was made towards the understanding of ion-Rydberg atom, Rydberg atom-Rydberg atom, and ground state atom-Rydberg atom collisions. Cross sections were calculated for electron capture, ionization, and excitation processes and parametrized in terms of atomic parameters. A strong dc electric field was also incoporated in calculations of electron capture and ionization cross sections for ion-Rydberg atom collisions. The results of this latter investigation indicate the design of a far ir photon detector which is based on the field ionization of Rydberg atoms may be limited by the collision processes. (Author)

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Highly excited (Rydberg) atoms are produced and can exist for significant periods of time in various plasma environments. It is necessary to know cross sections for the various collision processes involving these atoms if a complete understanding of the physics of these environments is to be attained.\nThe experimental investigation of collisional phenomena between high Rydberg sodium atoms and electrons is described in this thesis. A fraction of the atoms in a ground state sodium beam is optically excited (using lasers) to well-defined, high Rydberg states (principal quantum numbers chosen around 40). Subsequent to their production the Rydberg atoms are allowed to interact with 25 eV electrons for a specified time interval following which the collision products are detected. The electrons are produced in a beam which is crossed at right angles with the sodium beam. Important collision processes are (1) electron impact ionization of the Rydberg atom, and (2) electron-Rydberg atom interactions in which the quantum state of the atom is changed (state changing collisions).\nResults are presented for an observed state changing process which is identified as angular momentum state changing only (no change in the value of the principal quantum number). Cross sections for this process are measured to be 10('-10) to 10('-9) cm('2). Problems encountered during the work which prevented the measurement of electron impact ionization cross sections are discussed. In the course of the work two other topics required study: (1) the process of electric field ionization of Rydberg atoms (used to detect the atoms), and (2) the effects of background (300 K) radiation (from the chamber walls, for example) on Rydberg atoms. Results of these studies are also presented.

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Resonant ionization in slow-atom-Rydberg-atom collisions

R.K. Janev, A. A. Mihajlov

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 ${\mathrm{H}}^{*}(n)+\mathrm{H}$, ${\mathrm{Li}}^{*}(n)+\mathrm{Li}$, ${\mathrm{H}}^{*}(n)+\mathrm{Li}$ are calculated.

Physical Review A•Mar 1, 1980•64 citations
During the last two years, theoretical studies were performed on a variety of topics related to collisions involving Rydberg atoms. Progress was made towards the understanding of ion-Rydberg atom, Rydberg atom-Rydberg atom, and ground state atom-Rydberg atom collisions. Cross sections were calculated for electron capture, ionization, and excitation processes and parametrized in terms of atomic parameters. A strong dc electric field was also incoporated in calculations of electron capture and ionization cross sections for ion-Rydberg atom collisions. The results of this latter investigation indicate the design of a far ir photon detector which is based on the field ionization of Rydberg atoms may be limited by the collision processes. (Author)
Highly excited (Rydberg) atoms are produced and can exist for significant periods of time in various plasma environments. It is necessary to know cross sections for the various collision processes involving these atoms if a complete understanding of the physics of these environments is to be attained.\nThe experimental investigation of collisional phenomena between high Rydberg sodium atoms and electrons is described in this thesis. A fraction of the atoms in a ground state sodium beam is optically excited (using lasers) to well-defined, high Rydberg states (principal quantum numbers chosen around 40). Subsequent to their production the Rydberg atoms are allowed to interact with 25 eV electrons for a specified time interval following which the collision products are detected. The electrons are produced in a beam which is crossed at right angles with the sodium beam. Important collision processes are (1) electron impact ionization of the Rydberg atom, and (2) electron-Rydberg atom interactions in which the quantum state of the atom is changed (state changing collisions).\nResults are presented for an observed state changing process which is identified as angular momentum state changing only (no change in the value of the principal quantum number). Cross sections for this process are measured to be 10('-10) to 10('-9) cm('2). Problems encountered during the work which prevented the measurement of electron impact ionization cross sections are discussed. In the course of the work two other topics required study: (1) the process of electric field ionization of Rydberg atoms (used to detect the atoms), and (2) the effects of background (300 K) radiation (from the chamber walls, for example) on Rydberg atoms. Results of these studies are also presented.
Rydberg states of atoms have intrinsic properties that should allow their use as highly sensitive and tunable detectors at far infrared wavelengths. The intrinsic limit of such atomic detectors are collisional processes. This report describes the results of a research program to better understand an important class of collisions--namely, the resonant collision of two Rydberg atoms. We have extensively measured the linewidths and the collision cross sections and have determined thier dependence on parameters such as the principal quantum number and the Rydberg state angular momentum. The resonant collisions are shown to occur for a wide variety of initial states with cross sections of the order of 10 to the 9th power square-Angstroms and scaling as (n*O to the 4th order).
Atoms in highly excited Rydberg states possess physical characteristics quite unlike those associated with atoms in the ground or low-lying excited states. In particular, they are physically very large and are only very weakly bound. In consequence, collisions can lead to a wide variety of reaction processes many of which are unique to Rydberg species and have very large collision cross sections. In collisions with neutral targets, Rydberg atoms behave not as an atom but rather as a pair of well-separated independent scatterers, namely the core ion and the excited Rydberg electron. In the present article we discuss many of the different reactions that can occur when Rydberg atoms collide with neutral targets, focusing principally on reactions that are dominated by (binary) Rydberg electron-target interactions and include collisions with molecules that attach free low-energy electrons and with polar targets. In certain situations, however, interactions involving the Rydberg core ion are important. This is illustrated using as an example the destruction of ultralong-range Rydberg molecules excited in a cold dense gas.
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.
New experimental results have been obtained in collisions of singly charged ions with Na Rydberg atoms. (a) The distribution of final n states populated by electron capture shows non-monotonic variations of width with incident energy over the range 60-2100 eV. (b) The total capture cross section for alkali ions on Rydberg targets shows peaks and dips over the same energy range, correlated with particular reduced velocities v and the polarization of the exciting laser. (c) {ell}-Changing collision of Na{sup +} on Na(nd) targets, n=26 and 28, at low v preferentially transfer multiple units {h_bar} of orbital angular momentum under single-collision conditions, despite very large impact parameters. (d) Inelastic impacts of Na{sup f+} on Na 36s and 30p states, perturbed by electric fields, populate a strongly field-dependent mixture of nearby Stark levels. A synopsis of recent progress will be given.
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.