International Journal of Mass Spectrometry 228 (2003) 507–516
Internal energy effects in the reactivity of CO
2
2+
doubly charged molecular ions with CO
2
and CO
Pietro Franceschi
a
, Roland Thissen
a,∗
, Ján Žabka
a
,
Jana Roithová
b
, Zdenek Herman
b
, Odile Dutuit
a
a
Laboratoire de Chimie Physique, Université Paris-Sud, CNRS UMR 8000, bˆ at 350, 91405 Orsay, France
b
V.
ˇ
Cermák Laboratory, J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejškova 3, CZ-182 23 Prague 8, Czech Republic
Received 29 November 2002; accepted 19 March 2003
Dedicated to Professor Helmut Schwarz on the occasion of his 60th birthday in appreciation
of his manifold contributions to mass spectrometry and ion chemistry.
Abstract
Reactivity of the dication CO
2
2+
in dependence on its internal energy was studied in collision with CO
2
and CO. The
dication was obtained by photoionization of the CO
2
molecule by synchrotron radiation and the guided beam apparatus
CERISES was used in these studies. Spontaneous dissociation of CO
2
2+
to CO
+
and O
+
was observed without any target
gas in the reaction cell. With CO
2
or CO as a target gas, CO
+
and O
+
fragment ions were formed as products of dissociative
charge transfer, while the contribution of collision-induced dissociation of the reactant CO
2
2+
to their formation was found
negligible. The CO
+
yield showed a considerable dependence on the internal energy of the reactant dication CO
2
2+
: the
branching ratio CO
+
/O
+
increased gradually with photon energy between 38.7 and 39.4eV. We interpret this observation as
being due to the increase in the instantaneous distances at the extrema of vibrational motion of the dication with increasing
internal energy. This modifies the Franck-Condon factors in the charge transfer process and leads to a preferential population
of the vibrationally excited states of the product cation CO
2
+
(C) known to lead exclusively to the dissociation products
CO
+
+ O. An indirect mechanism that requires the coupling of the first excited (a) state of the dication with the higher
vibrational states of the CO
2
2+
(X) state is proposed to explain the threshold behaviour observed in our results.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Dication; Ion–molecule reactions; Photoionization; CO
2
2+
; Vacuum ultraviolet
1. Introduction
The last two decades registered a growing inter-
est for small molecular multiply charged ions. At first
∗
Corresponding author. Tel.: +33-1-6915-7573;
fax: +33-1-6915-3053.
E-mail address: roland.thissen@lcp.u-psud.fr (R. Thissen).
glance, the presence of two positive charges on a
molecular assembly questions its stability [1]. In fact,
in many cases, molecular dications have rather long
lifetimes due to the characteristics of their potential en-
ergy surface [2–5] (e.g., 4 s for the X state of CO
2
2+
).
The existence of doubly charged molecular ions as
metastable chemical species allows the manifestation
of their particular and new chemical behaviour. In most
1387-3806/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-3806(03)00157-X