Photon Factory Activity Report 2017 #35 (2018)

BL-20A/2016G521

Ultra-low-energy total cross sections for electron scattering from O2
Masashi Kitajima1,*, Takuma Okumura1, Naomasa Kobayashi1, Hirofumi Akasaka1, Kouichi Hosaka1,
Takeshi Odagiri2, and Masamitsu Hoshino2
1
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan2
Department of Material and Life Sciences, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
1 Introduction
Accurate absolute cross section data for electron
scattering from atoms and molecules provide important
information not only for the fundamental physics ofelectron collisions but also for many fields such as
electron-driven processes in the Earth and planetaryphenomena, gaseous discharges, radiation chemistry andplasmas physics. Consequences of several interesting
scattering phenomena such as Ramseur - Townsend effect,
shape resonances, vibrational Feshbach resonances, andthreshold structure due to a virtual state, appear in thescattering cross section curves especially at very-low
collision energies, where the de Broglie wavelength of anelectron become much longer than the typical size oftarget particles. The low energy behaviors of the electron
scattering cross sections are also related to the scattering
length which gives zero-energy scattering cross section.
Present group have developed a unique experimental
technique in order to measure absolute total cross sections
for electron scattering from atoms and molecules, whichmakes use of photoelectrons produced by thephotoionization of atoms using Synchrotron Radiation(SR) instead of using the conventional hot-filamentelectron sources [1-4]. The technique enables one tomeasure total electron scattering cross sections down tovery-low energies with extremely high energy resolution
in the single collision condition.
In the present project, total cross sections for electron
scattering from oxygen molecule were measured in awide energy range including the very-low energy region,
where rich structures due to shape resonances appear.
2 Experiment
In the present project, total cross sections for electron
scattering from oxygen molecule were measured in awide energy range including the very-low energy region
below 100 meV with a very high energy resolution. The
experiment has been carried out at the beamline 20A of
the Photon Factory, KEK. Present experiment employs
the threshold photoelectron source utilizing the SR which
utilizes the penetrating field technique together with thethreshold photoionization of noble gas atoms by the SR.
The threshold photoelectrons produced by the thresholdphotoionization of Ar are extracted by a weakelectrostatic field formed by the penetrating fieldtechnique and formed into a beam. The intensity of theelectron beam passing through the collision cell without
any collision with the target was detected by the channel
electron multiplier. The counting rates of the detectedelectrons were measured as a function of the number

density of the target gas filled in the gas cell in order toobtain the total cross section for electron scattering
according to the attenuation law. The continuous injection
operation of the PF ring is one of the key features for the
reliable absolute value measurements of the total crosssections.
3 Results and Discussion
Total cross sections for electron scattering from O2 atelectron energies from 10 eV down to about 20 meV
obtained in the present work are shown in Fig. 1 together
with previously reported experimental cross sections.
Present results showed that recommended cross sections
reported in 2009 [5], which have been obtained fromseveral previous experimental results, have overestimatedthe cross section values at energies below 1 eV. The
striking vibrational features observed in the present total
cross section curve below 1.5 eV represent the strong
contribution of the 2Πg shape resonance. The 2Πg shaperesonance occurs due to a temporary electron capture by
O2 to form vibrationally excited states of the negative ionof oxygen molecule, i.e., O2(X 2Πg). By analyzing thepresent cross sections, we also determined the resonanceenergies and the resonance widths of the 2Πg resonance.

Fig. 1 Total cross sections for electron scatteringfrom O2.
References[1] M. Kurokawa et al., Phys. Rev. A 82, 062707 (2010).
[2] M. Kurokawa et al., Phys. Rev. A 84, 062717 (2011).
[3] K. Shigemura et al., Phys. Rev. A 89, 022709 (2014).
[4] M. Kitajima et al., Eur. Phys. J. D 71, 139 (2017)[5] Y. Itikawa, J.Phys. Chem. Ref. Data, 38 1 (2009).
* mkitajim@chem.titech.ac.jp