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Physical Review A

Ionization of low-Rydberg-state He atoms by dipole-forbidden rotational deexcitation of NH3

Toshizo Shirai, Hiroki Nakamura

Published Dec 1, 1989• 0 citations
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Abstract

A theoretical study of the ionization of He(n=14 $^{1}P$) atoms in thermal collisions with ${\mathrm{NH}}_{3}$ was carried out within the semiquantal approximation. The charge-dipole Glauber differential cross section was employed for the electron-molecule rotational deexcitation process. Calculated thermally averaged cross sections show quite a strong n dependence, especially for n\ensuremath{\le}17. A comparison is made between the present results and recent experiments.

DOI
10.1103/physreva.40.7377

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A theoretical study of the ionization of He(n=14 1P^{1}P1P) atoms in thermal collisions with NH3{\mathrm{NH}}_{3}NH3​ was carried out within the semiquantal approximation. The charge-dipole Glauber differential cross section was employed for the electron-molecule rotational deexcitation process. Calculated thermally averaged cross sections show quite a strong n dependence, especially for n\ensuremath{\le}17. A comparison is made between the present results and recent experiments.
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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.
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The cross-section formula of Flannery [Phys. Rev. A 22, 2408 (1980)] in the semiquantal approximation for the processes referred to in the title is rewritten so as to make it more useful in practice. The formula is shown to be further simplified by taking an average over the azimuthal quantum number of a highly excited hydrogenic atom to be ionized. Numerical applications with use of the Glauber amplitude for the electron-atom inelastic scattering are made to the ionization collisions between two excited hydrogen atoms with simultaneous excitation and deexcitation of one of the atoms. The results are compared with those obtained by using the Born amplitude, and are analyzed in terms of the Glauber generalized oscillator strengths.
Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.