EGAS41 - Swansea University

41 st EGAS CP 68 Gdańsk 2009
Isotope effect in dissociative electron attachment cross sections
in acetylene
O. May ∗ , J. Fedor, M. Allan
Department of Chemistry, University of Fribourg, chemin du Musée 9, 1700 Fribourg,
Switzerland
∗ Corresponding author: olivier.may@unifr.ch,
We present absolute partial cross sections for dissociative electron attachment to C 2 H 2
and C 2 D 2 which have been measured using a new time-of flight ion spectrometer combined
with a trochoidal electron monochromator. For C 2 H 2 the new data is in good agreement
with the total cross sections measured previously in our laboratory [1] and elsewhere [2]
using the total ion collection principle.
The present measurement on deuterated acetylene reveals a pronounced isotope effect
at the first dissociative electron attachment band at 3 eV, with the cross section for
C 2 D − /C 2 D 2 being 14.7 times smaller than that for C 2 H − /C 2 H 2 . The dramatic magnitude
of the effect is attributed to slower motion of the D atom as compared to H atom, combined
with unfavorable competition of the dissociation with autodetachment. The recent
calculations by Chourou and Orel are in good agreement with our experimental values
for C 2 H − /C 2 H 2 but underestimate values for C 2 H 2 /C 2 D 2 , their ratio σ(C 2 H − )/σ(C 2 D − )
being 28.9 [2].
A second band is observed at around 8 eV corresponding to the light fragments H − or
D − which dominate above the C − 2 fragment, in contrary to previous assignment. These
bands exibit much weaker isotope effects, in agreement with their assignment to Feshbach
resonances with narrow autodetachment widths.
References
[1] O. May, J. Fedor, B.C. Ibanescu, M. Allan, Phys. Rev. A 77, 040701(R) (2008)
[2] R. Azria, F. Fiquet-Fayard, J. Physique 33 663 (1972)
[3] S.T. Chourou, A.E. Orel, Phys. Rev. A 77, 042709 (2008)
[4] M. Allan, L. Andrić, J. Chem. Phys. 105, 3559 (1996)
[5] R. Čurík, F.A. Gianturco, J. Phys. B 35, 717 and 1235 (2002)
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