Speaker
Description
The electron impact ionization of the metastable state 1s2s3S of helium is particularly important in the modelling of plasmas, and, as a simple system, is also a benchmark for theories. Indeed, there are many theoretical studies devoted to the calculation of the ionization cross section [2, 3], ranging from Born calculations to sophisticated close-coupling calculations. In contrast, only a few absolute measurements exist, a fact explained by the difficulty to produce metastable helium in a controlled manner. The latest and most comprehensive measurement, by Dixon et al. [1], lies about a factor of two higher than the most up-to-date calculations and therefore another absolute measurement is highly demanded by the community.
We designed a novel source for the production of a fast, intense beam of metastable helium atoms in the 1s2s3S state. A fast beam of He+ ions, produced in a duoplasmatron source, is sent into a sodium vapour cell where double charge exchange converts He+ into He− with an efficiency of 1%. A continuous wave CO2 laser is then used to photodetach a large fraction of He− ions, leaving the remaining neutral helium atoms in the 1s2s 3S state exclusively. The beam of neutral, metastable atoms is injected downstream in a setup designed to measure cross sections for electron-ion collisions [4] using the animated-crossed-beam method.
The present results lie significantly lower than the previous experiments. On the other hand, they are in very good agreement with the theoretical results of Fursa and Bray [2], who performed a set of calculations including the effect of doubly excited states. The reference values reported more recently by Ralchenko et al. do not include doubly excited states [3], and the agreement with our data is poorer at higher electron energies. This highlights the influence of such autoionizing states in the ionization process. We also measured for the first time, to our knowledge, the double ionization of He− and He(1s2s3S) by electron impact. The analysis of the data is currently under way.
References
[1] A. J. Dixon, M. F. A. Harrison and A. C. H. Smith, J. Phys. B 9, 2617 (1976).
[2] D. V. Fursa and I. Bray, J. Phys. B 36, 1663 (2003).
[3] Y. Ralchenko, R. K. Janev , T. Kato, D.V. Fursa, I. Bray and F.J. de Heer, At. Data Nucl. Data Tables 94, 603 (2008).
[4] J. Lecointre, S. Cherkani-Hassani, D.S. Belic, J.J. Jureta, K. Becker, H. Deutsch, T.D. Märk, M. Probst, R.K. Janev and P. Defrance, J. Phys. B 40, 2201 (2007).
