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A crossed-beam experiment has been performed to measure differential vibrationally inelastic cross sections for electron scattering from CO2 in the energy range from 1.5 to 30 eV over the scattering angles of 10-130°. Near the 2Πu resonance at 3.8 eV, the angular and energy dependences of the differential cross sections (DCS) have been determined in detail. Absolute values are obtained by normalizing the inelastic cross sections to the elastic ones. To determine the intensity of the individual vibrational modes from the energy-loss spectra, a deconvolution procedure was employed. The DCS for the pure fundamental modes (010), (100), (001) and the overtone of (020) are reported in detail for the first time over wide ranges of impact energy and scattering angle.

Elastic scattering of electrons from a molecule is studied theoretically. Differential, as well as integral and momentum-transfer, cross sections are calculated at the energies 3 - 60 eV. The calculation is based on an ab initio electrostatic potential taken with the approximate effects of electron exchange and target polarization. The fixed-nuclei approximation is made and the coupled channel equations are solved for partial waves. The resulting cross sections are satisfactory in the energy range 10 - 60 eV, when compared with experimental data. The resonance structure at around 4 eV is not well reproduced by the present calculation. Detailed comparisons are also made with previous theoretical works.

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...