Core-excitation-induced dissociation in CD
4
after participator Auger decay
J. Rius i Riu,
1,
* E. Melero Garcı
´
a,
1
J. A
´
lvarez Ruiz,
1
P. Erman,
1
P. Hatherly,
2
E. Rachlew,
1
and M. Stankiewicz
3
1
Section of Atomic and Molecular Physics, The Royal Institute of Technology, SCFAB, SE-10691Stockholm, Sweden
2
J.J. Thomson Physical Laboratory, The University of Reading, Whiteknights, P.O. Box 220, Reading, RG66AF, United Kingdom
3
Instytut Fizyki im. Mariana Smoluchowskiego, Uniwersytet Jagiellon ´ski, ul. Reymonta 4, 30-059 Krako ´w, Poland
Received 17 December 2002; revised manuscript received 22 May 2003; published 29 August 2003
The fragmentation of the CD
4
molecule after selective ionization of the 1 t
2
and 2 a
1
electrons with photons
from 70 to 290 eV has been studied with the energy-resolved electron-ion coincidence technique. The mass
spectra acquired in coincidence with 1 t
2
electrons reveal CD
4
+
, CD
3
+
, and CD
2
+
fragments, depending on the
excitation energy used. The production of CD
3
+
is strongly enhanced after C 1 s excitation to different core
excited states, with respect to the production observed after direct ionization of the 1 t
2
orbital. This enhance-
ment is correlated with the changes of the molecular geometry when it relaxes from the core-excited state.
DOI: 10.1103/PhysRevA.68.022715 PACS numbers: 33.80.Eh, 82.80.Rt, 33.80.Gj
I. INTRODUCTION
The constant developments in synchrotron radiation light
sources and particle detecting techniques have allowed the
performance of coincidence experiments between energy-
resolved electrons and mass-resolved ions, with count rates
convenient enough to make such otherwise time consuming
experiments feasible 1. Selecting the electrons by their ki-
netic energy allows monitoring the fragmentation from a spe-
cific doorway state, while the remaining reactions are dis-
criminated. The main advantage of this approach over
traditional mass spectroscopy lies in the selective monitoring
of processes occurring from initially ‘‘prepared’’ ionic states,
which allows for a detailed analysis of the properties of the
different electronic states. Early experiments of this kind
were performed more than 20 years ago 2.
From the work by Simm and co-workers, the technique
evolved into more sophisticated and advanced approaches
using different experimental setups. Thus, the first triple co-
incidence measurements by Frasinski et al. 3 allowed vi-
sual insight into the dissociative photoionization processes
studied, although without electronic state resolution. The
fragmentation associated with a particular final valence-hole
state after Auger decay was first studied by Eberhardt et al.
4 Hanson and co-workers 5 united both aforementioned
techniques, implementing energy-resolved, Auger-electron,
multiple-ion coincidence experiments. Ueda et al. measured
resonant Auger-electron–ion coincidences first 6 and then
further developed their experimental setup, combining
energy-selected electron and mass- and energy-selected ions
with angle-resolved photoion spectroscopy 7. Since then,
other interesting work, such as the study of the role of inter-
nal energy 8, bending 9, and nuclear motion 10 in dis-
sociation of site-selected core-excited molecules, triple coin-
cidence measurements with normal Auger electrons 11,12,
and the study of the electronic structure of shape resonances
13, have been reported.
There are two types of Auger processes following mo-
lecular resonant excitation of a core electron to an unoccu-
pied orbital. In spectator Auger processes, the excited elec-
tron does not participate in the decay process. Thus, the final
electronic state reached has two holes in valence orbitals and
one electron in the excited orbital. However, in participator
Auger decay processes, the excited electron participates in
the decay, leaving the molecule in a final state with a hole in
a valence orbital only. This final state is identical to the final
state of direct valence photoionization. For this reason, par-
ticipator Auger processes are especially suitable to examine
the effect of the nuclear motion in the core-excited state on
the molecular dissociation.
Particularly relevant for this paper is the work presented
by Ueda et al. about the correlation between nuclear motion
in the core-excited CF
4
molecule and the molecular dissocia-
tion after resonant Auger decay 10. There, experimental
evidence of core-excitation-induced dissociation in mol-
ecules was observed. The experimental data were compared
with the results of theoretical calculations based on the vi-
bronic model. Finally, it was concluded that the dissociation
to CF
2
+
from the CF
4
+
2 t
2
-1
C state is enhanced by core
excitation. This was explained in terms of the nuclear motion
energy transferred via the participator Auger decay to the
CF
4
+
2 t
2
-1
C state during the core-excited state lifetime.
In our experiments, the deuterated methane molecule
(CD
4
) was chosen as a convenient system to further inves-
tigate the nuclear relaxation toward the equilibrium position
after the creation of a core hole in the molecule and the
dissociation after resonant Auger decay 14,15. The isotopic
substitution of hydrogen is used to enhance the ion mass
resolution of our experiment. However, methane molecular
data are used for the interpretation of the results obtained.
The neutral ground state of CH
4
(CD
4
) is of tetrahedral T
d
symmetry 16,17, with an electron configuration 1
1 a
1
2
2 a
1
2
1 t
2
6
. While the 1 a
1
molecular orbital closely re-
sembles the atomic carbon 1 s orbital, the 2 a
1
and 1 t
2
orbit-
als are molecular valence orbitals. During the experiments,
we measured dissociations from the 1 t
2
-1
state of CD
4
+
formed either by direct ionization of valence electrons, or by
transitions through neutral core-excited states. In the latter
*Corresponding author. Present address: Department of Physical
Sciences, University of Oulu, P.O. Box 3000, FIN-90014, Oulu,
Finland. Email address: jaume.rius.i.riu@oulu.fi.
PHYSICAL REVIEW A 68, 022715 2003
1050-2947/2003/682/0227156/$20.00 ©2003 The American Physical Society 68 022715-1