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A study of ultra-low-energy electron-molecule collisions using very-high-n Rydberg atoms

Xiaohui Ling

Published Jan 1, 1993• 0 citations
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Abstract

In the present work very-high-n Rydberg-atoms $(n \\sim 100 - n \\sim 400)$ 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 $\\rm({\\sim}85\\mu eV - {\\sim}1.4meV),$ 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 $\\sim$1$\\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 $$\\rm K({\\it np\\/}) + XY \\to K\\sp+ + (XY)\\sp{-*}\\ or\\ \\to K\\sp+ + X\\sp- + Y\\eqno(1)$$and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $$\\rm K({\\it np\\/}) + XY({\\it J\\/}) \\to K\\sp+ + {\\it e}\\sp- + XY({\\it J\\/}-1)\\eqno(2)$$\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.

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Studies of ultra-low energy electron-molecule collisions using very-high-n Rydberg atoms

M. T. Frey, Xiaohui Ling, K. A. Smith +1 more

Potassium atoms in selected high-lying np Rydberg states (80 {<=} n {<=} 400) are being used as a tool to examine electron-polar molecule collisions at ultra-low energies ({approximately}80 {mu}eV - 1 meV). This energy regime has not been investigated previously and is of interest because the corresponding electron de Broglie wavelength is large, {approximately}1300 - 330 {Angstrom}, and because the collision time is comparable to the period of molecular rotation. A variety of target molecules has been studied, including simple rotors such as HF, and more complex species such as NH{sub 3}, CH{sub 3}I, CH{sub 2}Br{sub 2} and C{sub 6}H{sub 5}NO{sub 2}. These targets encompass a range of dipole moments and rotational behavior, but analysis of the data shows that in each case the cross section {sigma}({epsilon}) for rotationally-inelastic electron-polar molecule scattering varies approximately as 1/{epsilon} where {epsilon} is the electron energy. Such threshold behavior is not predicted by the Born approximation and possible reasons for this discrepancy will be discussed. In the case of NH{sub 3}, the data show that near-resonant energy transfer associated with transitions between different inversion levels is an important process leading to collisional state changing.

Bulletin of the American Physical Society•May 1, 1993•0 citations

Thermal energy collisions of potassium(nd) Rydberg atoms with electron-attaching molecules at intermediate n

Z. Zheng

Thermal energy collisions of K(nd) Rydberg atoms with electron attaching molecules which result in reactions of the following types K(nd) + SF$\\sb6$ $\\to$ K$\\sp+$ + SF$\\sb6\\sp-$ K(nd) + CF$\\sb3$I $\\to$ K$\\sp+$ + I$\\sp-$ + CF$\\sb3$ have been investigated at intermediate n. Using the field ionization technique, both rate constants for Rydberg atom destruction and for formation of free negative ions are directly measured for n as low as 12.\nThe data show that, at intermediate n, the rate constants for Rydberg atom destruction start to decrease with decreasing n and to deviate from those predicted on the basis of the simple free electron model. A semi-classical model is presented to explain this decrease. It reveals explicitly the relationship between the rate constant for Rydberg atom destruction and the rate constant for free electron attachment.\nThe data also show that, at intermediate n, only a fraction of the Rydberg electron attachment events leads to formation of free negative ions. Post-attachment interaction between the positive and negative ions formed is investigated. The kinetic energy distribution of the ion pairs in their center-of-mass frame and their separation probability are calculated. The angular dependence of the kinetic energy distribution and the separation probability are also studied. The present experiments, in conjunction with the theoretical calculations, advance understanding of the collisions substantially.\nInvestigation of Zeeman-effect quantum beats observed in the selective field ionization spectra of D state potassium Rydberg atoms in also presented.

•Jan 1, 1990•0 citations
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 sim\\simsim1spcirc\\sp\\circspcircK, 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.
Potassium atoms in selected high-lying Rydberg states (n <<< 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CClsb4\\sb4sb4 are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few mu\\mumueV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.
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.
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.
Rate constants for Rydberg-atom destruction through electron transfer in collisions between K(np) Rydberg atoms and SF6{\mathrm{SF}}_{6}SF6​ and CCl4{\mathrm{CCl}}_{4}CCl4​ have been measured for values of principal quantum number n up to n\ensuremath{\sim}400 using an apparatus specifically developed for very-high-n collision studies. Analysis of the data using the free-electron model provides cross sections for electron attachment to SF6{\mathrm{SF}}_{6}SF6​ and CCl4{\mathrm{CCl}}_{4}CCl4​ at electron energies down to \ensuremath{\sim}10 \ensuremath{\mu}eV, which are far below those accessible using any alternate technique. The measured cross sections are inversely proportional to electron velocity and are consistent with the Wigner threshold law for s-wave attachment.
Potassium atoms in selected high-lying np Rydberg states (80 {<=} n {<=} 400) are being used as a tool to examine electron-polar molecule collisions at ultra-low energies ({approximately}80 {mu}eV - 1 meV). This energy regime has not been investigated previously and is of interest because the corresponding electron de Broglie wavelength is large, {approximately}1300 - 330 {Angstrom}, and because the collision time is comparable to the period of molecular rotation. A variety of target molecules has been studied, including simple rotors such as HF, and more complex species such as NH{sub 3}, CH{sub 3}I, CH{sub 2}Br{sub 2} and C{sub 6}H{sub 5}NO{sub 2}. These targets encompass a range of dipole moments and rotational behavior, but analysis of the data shows that in each case the cross section {sigma}({epsilon}) for rotationally-inelastic electron-polar molecule scattering varies approximately as 1/{epsilon} where {epsilon} is the electron energy. Such threshold behavior is not predicted by the Born approximation and possible reasons for this discrepancy will be discussed. In the case of NH{sub 3}, the data show that near-resonant energy transfer associated with transitions between different inversion levels is an important process leading to collisional state changing.
Thermal energy collisions of K(nd) Rydberg atoms with electron attaching molecules which result in reactions of the following types K(nd) + SFsb6\\sb6sb6 to\\toto Ksp+\\sp+sp+ + SFsb6sp−\\sb6\\sp-sb6sp− K(nd) + CFsb3\\sb3sb3I to\\toto Ksp+\\sp+sp+ + Isp−\\sp-sp− + CFsb3\\sb3sb3 have been investigated at intermediate n. Using the field ionization technique, both rate constants for Rydberg atom destruction and for formation of free negative ions are directly measured for n as low as 12.\nThe data show that, at intermediate n, the rate constants for Rydberg atom destruction start to decrease with decreasing n and to deviate from those predicted on the basis of the simple free electron model. A semi-classical model is presented to explain this decrease. It reveals explicitly the relationship between the rate constant for Rydberg atom destruction and the rate constant for free electron attachment.\nThe data also show that, at intermediate n, only a fraction of the Rydberg electron attachment events leads to formation of free negative ions. Post-attachment interaction between the positive and negative ions formed is investigated. The kinetic energy distribution of the ion pairs in their center-of-mass frame and their separation probability are calculated. The angular dependence of the kinetic energy distribution and the separation probability are also studied. The present experiments, in conjunction with the theoretical calculations, advance understanding of the collisions substantially.\nInvestigation of Zeeman-effect quantum beats observed in the selective field ionization spectra of D state potassium Rydberg atoms in also presented.