EGAS41 - Swansea University

41 st EGAS CP 66 Gdańsk 2009
Electron collisions with cyclopropane
M. Allan
Department of Chemistry, University of Fribourg, chemin du Musée 9, 1700 Fribourg,
Switzerland
E-mail: michael.allan@unifr.ch,
Electron interactions with alkyl groups are important for a number of applications. The
cross sections of the prototype compounds, the alkanes, do not show pronounced resonant
structure, however, only extremely broad humps covering the 2-15 eV region [1]. This is
somewhat surprising, alkanes have many empty orbitals, which could give rise to shape
resonances.
In cyclopropane, however, the high symmetry has the consequence that one virtual
orbital couples only to f-wave and not to lower l waves, thus retarding the ‘leaking out’
of the electron through the centrifugal barrier in the corresponding a ′ 2 shape resonance
at 5.5 eV [1]. As a consequence, this resonance is exceptionally narrow for a shape
resonance in a saturated hydrocarbon (Fig. 1), an effect which could be called ‘symmetryinduced
narrowing’. The present work characterizes this resonance and generally electron
collisions with cyclopropane by measuring the absolute differential cross sections for elastic
scattering and for vibrational excitation. Since the resonance is, in chemical terms, due to
a temporary capture of the incoming electron in a C-C antibonding σ ∗ orbital, it excites
primarily the symmetric ring breathing vibration ν 3 . A representative cross section, drawn
as a function of energy and of scattering angle, is shown in Fig. 1. The calculation of
Čurík and Gianturco [2] are in good agreement with the present measurement.
Cross Section (pm /sr)
2
400
300
200
100
0
5.5 eV
= 180°
2 3
0 5 10 15 20
Electron Energy (eV)
400
300
200
100
0
E i = 5.5 eV
f-wave
2 3
÷2
0 30 60 90 120 150 180
Scattering Angle (deg)
Figure 1: Cross section for exciting the first overtone of the ν 3 symmetric ring breathing vibration,
shown as a function of the incident electron energy (left) and of scattering angle at the
5.5 eV resonance (right).
References
[1] M. Allan, L. Andrić, J. Chem. Phys. 105, 3559 (1996)
[2] R. Čurík, F.A. Gianturco, J. Phys. B: At. Mol. Opt. Phys. 35, 717 and 1235 (2002)
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