Novel method for state selective determination of electron-impact-excitation cross sections from 0° to 180°

Marvin Weyland,Xueguang Ren,Alexander Dorn,Woon Yong Baek,Klaus Bartschat,Dmitry V Fursa,Thomas Pflüger,Igor Bray,Hans Rabus,Oleg Zatsarinny

doi:10.1140/epjti/s40485-014-0006-2

Abstract

We use an improved target recoil momentum spectroscopy setup to determine differential cross sections for excited metastable state production in atoms and molecules by electron impact and show its capabilities for an atomic helium target. A crossed beam setup with a supersonic helium jet and a pulsed electron beam at energies close to the excitation threshold of 19.82 eV was used. Measuring the recoil momentum vector of the target instead of the momentum of the scattered electron removes common restrictions to the accessible scattering angles while the microchannel plate detector ensures a high counting efficiency. Using a photoemission electron source we reach an energy resolution of about 200 meV at 1 µA peak current. Results are compared with simulations using theoretical convergent-close-coupling (CCC), R-matrix with pseudo-states (RMPS) and B-spline R-matrix (BSR) calculations and show good agreement.

Highlights

In traditional electron impact experiments, the scattered electron is measured using a movable detector
Our improved setup allows for state selective measurements close to the threshold energy by using a time- and positionsensitive microchannel plate detector, where events from all electron scattering angles can be registered simultaneously and by using a photo-emission electron source
The metastable state production starts at the excitation threshold of the lowest 23S state and shows a characteristic shape due to negative ion resonances and additional excited states whose energies are marked in the diagram

Summary

Introduction

In traditional electron impact experiments, the scattered electron is measured using a movable detector. The scattering angles are scanned by changing the position of the detector [1] This technique allows measurement within a limited angular range, as the electron detector would not be able to measure the scattered electrons in the backward and forward directions due to the interference of the spectrometer with the incoming or outgoing projectile beam. Detecting the recoil momentum of the excited target instead of the scattered electron has the advantage that the projectile beam has no influence on the measurement. This principle has been shown by Murray and Hammond [2,3] for electron impact excitation at intermediate projectile energies. Our improved setup allows for state selective measurements close to the threshold energy by using a time- and positionsensitive microchannel plate detector, where events from all electron scattering angles can be registered simultaneously and by using a photo-emission electron source

Methods

Results

Conclusion