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Physical Review A

Ionization of Rydberg atoms in thermal collisions with polar molecules

Toshizo Shirai, Hiroki Nakamura

Published Nov 1, 1987• 5 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.36.4290

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