EGAS41 - Swansea University

41 st EGAS PL 2 Gdańsk 2009
Quantitative experiments on electron-molecule collisions
Michael Allan
Department of Chemistry, University of Fribourg, chemin du Musée 9, 1700 Fribourg,
Switzerland
E-mail: michael.allan@unifr.ch,
The talk will start with a brief overview of areas of application of electron induced chemistry
and point out which demands and challenges do these applications place on the
fundamental research of electron-molecule collisions. A few examples of how these challenges
have been met will be presented:
Combined theoretical and experimental effort has recently improved our understanding
and capacity to deliver cross sections for electronic excitation of atoms [1], important for
lightning and plasma propulsion.
The emerging Focused Electron Beam Induced Processing (FEBIP) technology inspired
the research of electron collisions by directing our attention to new types of compounds
– the metalorganic compounds used as precursors there [2]. An example of such
a precursor is Pt(PF 3 ) 4 [3] and electron scattering results on this compound will be presented.
Chemical change in polyatomic molecules is very important in the applied systems,
but quantitative calculations are nearly always limited to diatomic molecules and absolute
measurements are still quite rare. An important recent progress towards improving this
situation is the ab-initio calculation [4] and absolute measurement [5] of the DEA cross
section for the prototype polyatomic molecule acetylene and its di-deutero isotopomer.
The assignment of the resonant bands and our understanding of the dissociative electron
attachment mechanism for polyatomic molecules at higher energies – the domain of
Feshbach resonances, with two excited electrons – is still very limited. Light has recently
been shed into this problem, primarily from experiment, by studying the trends in a large
number of compounds [6]. A number of rules and selectivities have been identified and in
certain cases qualitatively rationalized by calculations of the potential surfaces of the parent
Rydberg states of the Feshbach resonances. These processes still present an unsolved
challenge for the theory.
An important conclusion is that electron scattering in the past has often been motivated
by the desire to discover new resonances and cross sections were often measured
only in relative units, for only one process, and over limited energy and angular ranges to
permit the observation of a given resonance. The primary challenge posed by the applications
is the need to measure (and calculate) cross sections in absolute units, over wide
energy and angular ranges, and for all the accessible processes.
References
[1] M. Allan, K. Franz, H. Hotop, O. Zatsarinny, K. Bartschat, J. Phys. B: At. Mol. Opt.
Phys. 42, 044009 (2009)
[2] I. Utke, P. Hoffmann, J. Melngailis, J. Vac. Sci. Technol. B 26, 1197 (2008)
[3] M.H. Ervin et al., J. Vac. Sci. Technol. B 25, 2250 (2007)
[4] S.T. Chourou, A.E. Orel, Phys. Rev. A 77, 042709 (2008)
[5] O. May, J. Fedor, B.C. Ibanescu, M. Allan, Phys. Rev. A, 77, 040701(R) (2008)
[6] B.C. Ibanescu, M. Allan, Phys. Chem. Chem. Phys. (2009) DOI:10.1039/B904945B
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