Photon Factory Activity Report 2019 #37 (2020)

BL-20A/2018G516

High-resolution measurements of cross section for low-energy and very-low-energy
electron scattering from NH3
Masashi KITAJIMA1,*, Tomokazu EJIRI1, Takuma OKUMURA1, Daichi ITO1, Ryutaro OOWADA1,Kouichi HOSAKA1, Takeshi ODAGIRI2, and Masamitsu HOSHINO2
12Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
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 the virtual state, appear in the
scattering cross section curves. Investigations of scatteringcross sections are especially interesting at very-low
collision energies, where the de Broglie wavelength of anelectron become much longer than the typical size of target
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.
Beam experiments with hot-filament electron sources
have provided accurate cross-sections in a wide energy
region. However, experiments in the collision energy
region below a hundred meV have been limited, due to thedifficulty for producing an electron beam at this lowenergy region.
Ammonia (NH3) is one of the simplest polyatomic
molecules and its accurate electron scattering cross section
has been required in many fields. Therefore, several
experimental work on measurements of cross section forelectron scattering from NH3 have been reported as shownin the recent compilation [1]. For the total cross section,
however, experimentally reported cross sections differ
from each other below 10 eV. Thus, accurate
measurements of total cross section at low energy region
are highly demanded.
Recently, we have measured total cross section forelectron scattering from diatomic molecules such as N2 andO2, in a wide energy range including the very-low energy
region below 100 meV [2,3]. A unique experimental
technique to measure absolute total cross sections forelectron scattering from atoms and molecules at very-low
energies, which makes use of photoelectrons produced by
the photoionization of atoms using Synchrotron Radiation(SR) instead of using the conventional hot-filamentelectron sources [4] were employed. The technique enables
one to measure total electron scattering cross sections

down to very-low energies with extremely high energy
resolution in the single collision condition.
In the present project, cross sections for electron
scattering from NH3 were measured in the energy range
below 10 eV, especially for the energy region, whereaccurate cross sections were not known.
2 Experiment
The experiment has been carried out at the beamline 20Aof 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 weak electrostatic
field formed by the penetrating field technique and formed
into a beam. The intensity of the electron beam passing
through the collision cell without any collision with thetarget was detected by the channel electron multiplier. The
counting rates of the detected electrons were measured asa function of the number density of the target gas filled in
the gas cell in order to obtain the total cross section forelectron scattering according to the attenuation law. The
continuous injection operation of the PF ring is one of thekey features for the reliable absolute value measurementsof the total cross sections.
3 Results and Discussion
Present measurement shows that total cross section of
NH3 are much larger than previously reported values
measured using the conventional technique. Cross section
below 3 eV, the obtained total cross section curve showed
increasing feature with decreasing the electron energy
toward the lowest energy and reaches to 1000 ×1020 m2
which is a huge value compared to the size of the molecule.
References[1] Y. Itikawa, J. Phys. Chem. Ref. Data, 46 043103 (2017).
[2] M. Kitajima et al.,Eur. Phys. J. D 71, 139 (2017).
[3] Okumura et al., J. Phys. B 52, 035201 (2019).
[4] M. Kurokawa et al., Phys. Rev. A 82, 062707 (2010).
* mkitajim@chem.titech.ac.jp