Photon Factory Activity Report 2004 #22 Part B (2005)
Atomic and Molecular Science

3B,16B/2004G010

Observation of interatomic Coulombic decay in Ne dimer above and below the Ne
2s-1 threshold
T. Aoto,*1 K. Ito,1 Y. Hikosaka,2 F. Penent,3 and P. Lablanquie,31KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
2UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3LCP-MR, Université Pierre et Marie Curie, 11 rue P et M Curie, 75231 Paris, FranceIntroduction
It is well now that, in rare gas atoms, inner-shell coreholes usually decay by emitting a secondary Augerelectron. This pathway is forbidden in case of innervalence holes, for their binding energy lies below thelowest double ionization potential. Consequently, they
can only experience a radiative decay. However, when
the rare gas atom is a part of a weakly bound van der
Waals cluster, an alternative interesting decay path isopened whereby the initially ionized atom transfers its
energy to a neighboring atom. We have demonstrated that
this process occur in Ar, Kr and Xe dimers [1]. The Ne
dimer case is even more spectacular as the transferredenergy is sufficient to ionize the neighboring atom. Sucha decay process is called Inter-atomic Coulombic Decay(ICD) and was for the first time theoretically predicted byCederbaum and co-workers [2,3] to be much faster thanthe competing intra-atomic radiative decay.

Results and Discussion
Ne dimer is the most suitable species in order toinvestigate the ICD mechanism. The process can be
schematically depicted in the following way: an innervalence 2s electron is photoionized from a Ne atom inside
the Ne dimer ([Ne---Ne]), the hole then decays bytransferring its energy to the neighboring Ne atom andejecting an outer-valence 2p electron, that is:[Ne---Ne] + hv
photoionization+-1[Ne (2s )---Ne] + ephotoelectron
ICD+-1+-1[Ne (2p ) --- Ne (2p )] + eICD.
The final state is thus a doubly charged Ne dimer that
dissociates through charge separation. Recently, ICD wasobserved experimentally in 2s ionized Ne dimer by+
detecting the two Ne fragments and the ICD electron incoincidence at an excitation energy 10eV above the 2sionization threshold [4].

In the present study, we used a very simple andsensitive experimental approach to investigate ICD
process in Ne dimer, namely coincidence between+
the two receding energetic Ne ions. Experiment was

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performed at undulator beamline BL16B. Ne dimers
are produced in a supersonic expansion with a proper
stagnation pressure in order not to produce large size
clusters. The two fragment ions were detected incoincidence by two apparatus: a simple ion filter that
rejects low energy or thermal ions and a hemisphericalelectrostatic analyzer [5] which allows the measurement+of the kinetic energy distribution of energetic Ne ions.
+We confirmed the ICD occurrence by detecting two Ne
ions simultaneously with the two detectors. The ICD
cross section were measured as a function of the photon
energy, which is strongly affected by resonances due to4
double excitations, [Ne*(2p) (3s)(np)---Ne], located just+-1
above the ionisation threshold of the [Ne (2s )---Ne] state.
+The Ne kinetic energy observed in the present study is ingood agreement with the previous theoretical prediction[3] and experimental observation [4] and remains
constant within a few eV above threshold. Surprisingly,
we observed that ICD process is also present below the 2s
ionization threshold. Work is in progress to understandthis phenomenon which we attribute to a spectator
process upon excitation of a 2s electron to a Rydberg
orbital.

References[1] R. Thissen, P. Lablanquie, R. I. Hall, M. Ukai, and K.
Ito, Eur. Phys. J. D 4, 335 (1998)[2] L. S. Cederbaum, J. Zobeley, and F. Tarantelli, Phys.
Rev. Lett. 79, 4778 (1997)[3] S. Scheit et al, J. Chem. Phys. 121,8393 (2004)[4] T. Jahnke et al, Phys. Rev. Lett. 93, 163401 (2004)[5] Y. Hikosaka, F. Penent, P. Lablanquie, R. Hall and K.
Ito, Meas. Sci. Technol. 11, 1697 (2000)*tomohiro.aoto@kek.jp