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Physical Review Letters

Electron Scattering Resonances in Fast D(High n )- N2 Collisions

Peter M. Koch

Published Aug 6, 1979• 30 citations
Publisher

Abstract

The results of the first measurements of the velocity dependence of the total ionization ${\ensuremath{\sigma}}_{I}$ and the total destruction ${\ensuremath{\sigma}}_{d}$ (ionization+excitation+deexcitation) cross sections for fast Rydberg-atom-neutral-molecule collisions are presented. The observed resonant structure in and the absolute magnitudes of ${\ensuremath{\sigma}}_{I}$ and ${\ensuremath{\sigma}}_{d}$ unambiguously confirm the prediction that the Rydberg electron bound to a fast atom scatters quasifreely. The data also strongly hint that the ionic core does the same.

DOI
10.1103/physrevlett.43.432

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Electron Impact Ionization of Sodium Rydberg Atoms below 2 eV

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C. Ronge, A. Pesnelle, M. Perdrix +1 more

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Physical Review A•Nov 1, 1988•8 citations
The results of the first measurements of the velocity dependence of the total ionization \ensuremathσI{\ensuremath{\sigma}}_{I}\ensuremathσI​ and the total destruction \ensuremathσd{\ensuremath{\sigma}}_{d}\ensuremathσd​ (ionization+excitation+deexcitation) cross sections for fast Rydberg-atom-neutral-molecule collisions are presented. The observed resonant structure in and the absolute magnitudes of \ensuremathσI{\ensuremath{\sigma}}_{I}\ensuremathσI​ and \ensuremathσd{\ensuremath{\sigma}}_{d}\ensuremathσd​ unambiguously confirm the prediction that the Rydberg electron bound to a fast atom scatters quasifreely. The data also strongly hint that the ionic core does the same.
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The ionization of He atoms, prepared in a well-defined (n=14) 1^{1}1P Rydberg state, in collisions with electron-attaching molecules like SF6{\mathrm{SF}}_{6}SF6​ and NO2{\mathrm{NO}}_{2}NO2​, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for SF6{\mathrm{SF}}_{6}SF6​, reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ cm2{\mathrm{cm}}^{2}cm2, and to be smaller for NO2{\mathrm{NO}}_{2}NO2​, close to ${10}^{\mathrm{\ensuremath{-}}15}$ cm2{\mathrm{cm}}^{2}cm2. The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for NO2{\mathrm{NO}}_{2}NO2​ indicates that, for this molecule, the reaction product is an ion pair, as for SF6{\mathrm{SF}}_{6}SF6​, and that ionization is not due to rotational energy transfer even though NO2{\mathrm{NO}}_{2}NO2​ has a dipole moment. This dependence cannot be explained by the ``free-electron model'' usually used in Rydberg-atom--molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.
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.
The title process with simultaneous rotational deexcitation of molecules was investigated theoretically by using the semiquantal approximation. The dipole Born and dipole Glauber differential cross sections were employed for the electron-molecule rotational deexcitation process. Numerical applications were made to the following systems: Xe+CO, HCl, HF, and LiF. A comparative study of the two approximations was carried out concerning the dependences of the cross sections on the following various quantities: n (principal quantum number), J (initial rotational quantum number), \ensuremath{\Delta}J (rotational quantum number change), D (dipole moment), V (relative collision velocity). As the molecular dipole moment becomes larger, there appears a prominent difference in the n dependence of the cross section. The Glauber approximation brings about a decrease in the cross section and yields a sawlike n dependence in contrast to a steplike n dependence in the case of the Born approximation. In the collisions involving lowly excited Rydberg atoms, ionization with simultaneous dipole-forbidden transition of a molecule becomes dominant compared to the ionization with simultaneous dipole-allowed transition. The cross section was found to be inversely proportional to the relative collision velocity; thus the ionization rate becomes independent of temperature. A comparison was also made between the experiment for the ionization rate at 300 K for Xe(27f)+HF and Xe(31f)+HCl.
We have observed electron impact ionization of highly excited sodium Rydberg atoms in nsnsns and ndndnd states, n=35−−51n=35--51n=35−−51, below E=2  eVE=2\text{ }\text{ }\mathrm{e}\mathrm{V}E=2  eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in nnn. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.
The ionization of low-Rydberg-state He atoms in the n 1P^{1}P1P state (n=14) in collisions with the polar molecules NH3{\mathrm{NH}}_{3}NH3​, SO2{\mathrm{SO}}_{2}SO2​, and acetone (C3{\mathrm{C}}_{3}C3​H6{\mathrm{H}}_{6}H6​O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\ensuremath{\sigma}}_{i}^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/vr2{v}_{r}^{2}vr2​ in a large part of the studied velocity range, while a 1/vr{v}_{r}vr​ velocity dependence is predicted by the free-electron model for the higher Rydberg levels.