Mode Dependence in Vibrational Excitation of aCO2Molecule by Electron and Positron Impacts

Abstract

We have found theoretically, for the first time, that vibrational excitations of a CO 2 molecule by electron ͑e 2 ͒ and positron ͑e 1 ͒ impacts are strongly dependent on the charge of the projectile at impact energy below 6 eV. For the symmetric-stretching mode, the excitation cross section of e 2 impact is larger by 2 to 3 orders of magnitude than that of e 1 impact, while for bending and asymmetric-stretching modes the magnitude of both cross sections for e 2 and e 1 impacts is nearly comparable. These results are qualitatively confirmed experimentally, and are interpreted as the difference of interactions and incident e 2 or e 1 wave functions. [S0031-9007(98)05939-0] PACS numbers: 34.80. Gs, A comparative study of electron ͑e 2 ͒ and positron ͑e 1 ͒ scattering from atoms, molecules, and condensed matter is known to provide a fundamental knowledge of underlying physics for electronic structure and scattering dynamics, and hence, is important for atomic physics, condensed matter physics, and nuclear physics [1]. In addition, this knowledge helps in understanding physics for other collision processes which involve different types of exotic particles. Total cross sections by e 1 impact on atoms and molecules are known to give smaller values than those for e 2 impact below about 100 eV to a few eV region, and this feature has been interpreted as a result of cancellation of static and polarization potentials for e 1 impact while these potentials are added up, along with the additional exchange interaction for e 2 impact, thus causing the stronger interaction. For specific inelastic processes, unfortunately, very few systematic studies by e 1 impact have been performed for gaseous targets, and a detailed comparative study between e 2 and e 1 impact is virtually nonexistent except for some preliminary investigation of experimental results for raregas atoms and simple molecules [2]. Only recently, Gianturco and colleagues [3(a)] have carried out a careful study on e 1 scattering from CO 2 molecule for elastic as well as vibrational excitation processes for the symmetric stretching mode, and have compared it with their previous study [3(b)] for e 2 scattering in order to shed some light on the coupling mechanism.Earlier in our study [4], we have observed that the total cross section for CO 2 for e 2 impact is indeed larger by 40% than that for e 1 impact below 100 eV or so, and continues to be so until the impact energy reaches down to 3 eV. Below 3 eV, however, the magnitude of the cross section is found to reverse, i.e., the total cross section for e 1 impact becomes larger by 25%, although it reverses again at much lower energy around 0.5 eV. At the time, we speculated a possible cause for this phenomena as being due to anomalously large cross sections for either rotational or vibrational excitation, or both for e 1 scattering. An exploratory study for rotational excitations by both projectiles was reported some time ago by Takayanagi and Inokuti [5] based on the Born approximation, who showed that for a molecule wi...

Threshold efficiency of vibrational excitation of methane by electron/positron impact: A comparison

“…1) The comparison of experimental inelastic processes at higher energies for both leptonic projectiles led to similar conclusions for linear targets like CO 2 [3] and OCS [2].…”

“…-The wealth of experimental data on the low-energy behavior of collisions between polyatomic molecular gases and leptonic projectiles like e − and e + has certainly increased in recent years [1] and has been therefore providing a greater variety of comparative behavior that requires detailed theoretical understanding. This is particularly true when it comes to the analysis of the vibrational excitation processes, where the two leptons can provide in principle very different outcomes although the scattering events are being driven by essentially the same set of Coulomb forces between each projectile and the electronuclear aggregates which constitute the vibrating, bound molecular states [2,3]. The theoretical analysis of the vibrational mechanisms at the microscopic level, however, has been somewhat lagging behind in the sense of being limited to linear molecules only [2,3] or only to the elastic processes of the polyatomic gases [4,5], with only very recent attempts at analysing a polyatomic vibrating target being collisionally excited by e + [6] or e − [7] in the low-energy region.…”

Positron Scattering from Atoms and Molecules at Low Energies

“…Study of positron-matter interactions is an active area of research [1][2][3][4][5][6][7]. The simplest form of such interactions are two-body collisions with atoms and molecules.…”

“…Except for measurements of total cross sections [2,3], virtually all studies of positron-atom and positron-molecule scattering have concentrated on energies greater than a few electron volts. The lowest energy positron differential cross-section ds͞dV (DCS) measurement was for argon at 2.2 eV [8], and the only other total vibrational crosssection measurement we are aware of was for CO 2 at energies above 3 eV [7]. Current scientific questions of interest involving low-energy positrons include the possibility of bound states of positrons with atoms and molecules and the role of vibrational excitation in the formation of longlived positron-molecule resonances and fragmentation following positron annihilation [5].…”

“…While the electron cross section peaks above the y 3 threshold (0.0157 eV), the positron data is qualitatively different, raising potentially interesting theoretical questions. Although there is theoretical work on the excitation of vibrational modes in molecules by positrons [7,17] scattering from CF 4 . The only other measurements we are aware of for CF 4 are the total cross sections measured above 1 eV by Sueoka et al [3].…”