Photon Factory Activity Report 2022 #40 (2023)

BL-20A/2022G529

Influence of the forward scattered electrons in the attenuation transmission method inelectron-molecule scattering experiments
Masashi KITAJIMA1,*, Hidenori DAIKI1 Taisei NISHIYAMA1, Minori AKASAKA1, Kenta
MURAKAMI1, Ryuta MIZUMOTO1, Hiroaki MURAKAMI1 and Hibiki Azuhata1
1

Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan

1 Introduction
Present group has developed 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 [1]. The cross-section data concerning
electron-atom or -molecule scattering are important not
only for the understanding of fundamental physics of theelectron collisions but also for applications where electron
driven processes take place, which include huge fields.
Among the measurable quantities describing the electron
scattering, the grand total cross section, which is the sumof the integral cross sections for all scattering channels, isthe most experimentally reliable since it can be obtained
without any normalization procedures. Therefore, accurate
grand total cross sections for electron scattering fromvarious atoms and molecules have been regarded as thestandard values for the normalization of cross-section datafor individual scattering processes and also the bench markof theoretical models [2].
The standard experimental technique for themeasurements of the grand total cross sections is theelectron transmission method based on the Lambert-Beer’s
attenuation law. The method has been widely employed fornumerous experimental works determining the electron
scattering total cross sections. The electron transmission
method, however, has a problem of omission of theforward scattered electrons due to incomplete
discrimination against the electrons scattered at small
angles with forward direction due to the finite angularresolution [1]. This problem becomes important for the
cross-section measurements of electron scattering frompolar molecules, since electron scattering is dominated bysmall scattering angles due to the long-range potential
between electron and polar molecule. Therefore, almost all
the experimental total cross sections for electrons
scattering from polar molecules in the literature are
criticized by theorists [3]. On the other hand, the demandfor cross sections of electron collisions with polar
molecules such as H2O, NH3 and so on, has been increasing.
The experimental technique of present group which
utilizes SR has an advantage of high-stability of electron
beam against the existence of the target gas compared tothe conventional hot-filament electron sources [1]. This
makes it possible to resolve the forward scattered electron

problem accurately. Therefore, the influence of the forward
scattered electrons in the transmission experiment utilizing
the present threshold photoelectron source were studied in
the present project.
2 Experiment
In the present project, in order to determine the influenceof the forward scattered electrons in the transmission
experiment utilizing the threshold photoelectron source,
intensities of transmitted electrons under various
experimental conditions have been measured.
The electron transmission measurements were carriedout at the beamline 20A of the Photon Factory, employing
the threshold photoelectron source utilizing thesynchrotron radiation [1]. The electron beam produced by
the threshold photoelectron source transmitted through thecollision cell with several different conditions of electron
lens system were analyzed by its intensities.
Simulations for electron trajectories of transmitted
electrons have also been carried out. The trajectory of theelectrons passing through the collision cell together withthe electrons scattered at several positions inside thecollision cell were calculated with the Charged Particle
Optics computer program [4].
3 Results and Discussion
Comparison of the experimental and simulated electron
intensities with several experimental conditions showedthat strong position dependence for the detection efficiencyof the electron detector exist. This position dependent
detection efficiency results in a very strong angle
discrimination of the forward scattered electron which maylead to a very small influence of the forward scatteredelectron, although the influence is not negligible. Further
precise analysis on the influence of the forward scatteredelectron is continuing.
References[1] M. Kurokawa et al., Phys. Rev. A 82, 062707 (2010).
[2] E. Gargioni and B. Grosswendt, Rev. Mod. Phys. 80 451(2008).
[3] I. I. Fabrikant, J. Phys. B 49 222005 (2016).
[4] Charged Particle Optics programs [http://www.
electronoptics.com].
* mkitajim@chem.titech.ac.jp