Collision-energy-resolved angular distribution of Penning electrons for N2–He(23S)

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Abstract

The collision-energy-resolved angular distributions of Penning electrons for individual ionic state of N2–He(23S) were measured. The angular distributions showed increasing intensity in the backward (rebounding) directions with respect to initial He(23S) beam vector because Penning ionization occurs with a collision against repulsive interaction wall followed by the electron emission from 2s orbital of He. We also analyzed internal angular distribution by means of fitting parameters using classical trajectory calculations for N2–He(23S) on the modified interaction potential. These internal angular distributions suggested the electron emission from 2s orbital of He and they depended on collision energy and electron kinetic energy.

Graphical abstract

(Left) Electron emission direction θ(= η + γ) with angles η between M–He vector and He beam, and γ between M–He vector and electron emission direction.

(Right) The angular distributions of Penning electrons by experiment and calculation for the N2–He system at the collision energy of (a) 100 meV, (b) 200 meV, and (c) 300 meV.

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Introduction

The ionization reaction of a molecule (M) with an excited atom (A) as a following equation (1) is called Penning ionization [1]:A+MA+M++e-.A kinetic energy analysis of the electrons ejected via the ionization process (1), which is called Penning ionization electron spectroscopy (PIES), was first performed by Čermák [2]. The PIES technique is in many respects similar to ultra-violet photoelectron spectroscopy (UPS). Measurement of the angular distributions of the ejected electrons in Penning ionization is expected to clarify the mechanism of this process. Ebding and Niehaus have measured the angular distributions of electrons emitted in Penning ionization for He(23S) with Ar, Kr, Xe, CO, N2, and Hg [3] as a function of the angle θ with respect to the initial He(23S) beam vector. They found the angular distributions to be strongly anisotropic and asymmetric in the case of practically no interaction potential well. Other works [4], [5] have investigated the ionization mechanism by angle-resolved measurements of Penning electrons. Takami and Ohno found that an excimer-like state involving C 2s hole characters induces Auger-like transitions for the system of the hydrocarbon molecule having more than two adjacent carbon atoms [4]. Lescop et al. showed that vibrational states of CO+ (X2Σ+) beyond v = 2, which are forbidden by the Franck–Condon principle, were due to an autoionization process, because those angular distributions of ejected electrons are isotropic in spite of the anisotropic angular distribution for v = 0 [5]. Collision-energy-resolved angular distributions of Penning electrons also have been measured by a few studies. Niehaus measured the variation of the angular distribution for Ar–He(23S) system with changing collision energy from 80 meV to 350 meV as dividing the thermal distribution into some parts for similar intensity using the time-of-flight (TOF) technique [6]. They found that the behavior of the angular distribution curves is insensitive to collision energy in the range 80–350 meV, probably because of a weak attractive interaction between Ar and He(23S). Mitsuke et al. changed the collision energy with controlling nozzle temperature and measured the angular distribution for He(23S)–H2S at two collision energies [7]. They found that the angular distribution curves vary with collision energy in this system, because of the deflection of trajectories by a long-range attractive potential for He(23S)–H2S.

In this letter, we present the collision-energy-resolved angular distributions of Penning electrons for each ionic state of N2–He(23S) system. In order to resolve collision energy, we used the two-dimensional (collision energy Ec and electron-energy Ee) Penning ionization electron spectroscopic technique (2D-PIES), in which the produced electron intensity is observed as a function of both collision energy and electron kinetic energy [8]. We also present the calculated angular distributions and internal angular distributions with trajectory analysis in order to discuss the difference of both distributions corresponding to ionic states of N2.

Section snippets

Experimental

Fig. 1 shows a scheme of the experimental apparatus for collision-energy-resolved angular distribution of Penning electrons with electron spectroscopy. A metastable beam of He(21S, 23S) was produced by a nozzle discharge excited beam source, and pulsed by a mechanical chopper to perform collision-energy-resolved experiment. The He(21S) component was quenched by a water-cooled helium discharge lamp and the charged particles were removed by a deflector. Then the He(23S) beam was introduced

Calculation

To reproduce and analyze the angular distribution of Penning electrons, classical trajectory calculations were performed on the modified potential energy surface [11] with interaction parameters from Li model calculation which uses a Li atom in place of a He(23S) based on the similarity of the outermost electron. This method improves the deference in interaction between He–M and Li–M by overlap expansion as following equations:VOE(R,ω)=V0(R,ω)-iCi|ϕi|χ|2,χ=ζ3πexp[-ζr],where V0 is the Li

Result and discussion

Fig. 3 shows the laboratory angular distribution of ejected electron in Penning ionization of N2–He(23S) at collision energies (Ec) of 100 meV, 200 meV, and 300 meV. Vibrational states of each ionic state are not resolved. The obtained intensity is multiplied by sin θ in order to correct the ionization volume and transformed to a relative value with respect to the intensity at θ = 90°. Symbols are experimental data and curves are obtained by optimization of parameters. All ionic states (X2Σg+, A2Πu,

Acknowledgements

The present work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. M.Y. is supported by the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.

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