Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy
We have studied fluorescence spectra of small Xem and Xem(NF3)k clusters (m, k ≤ 10²) embedded in large host Ne₇₅₀₀ clusters, excited with SR excitation in the VUV spectral region above the Xe⁺(²P₁/₂) atomic limit. The IR emission bands due to ionic chromophores Xe⁺p (p = 2, 3, 4) as well as UV (...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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| Цитувати: | Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy / A. Kanaev, L. Museur, T. Laarmann, T. Muller // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1426–1433. — Бібліогр.: 28 назв. — англ. |
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Kanaev, A. Museur, L. Laarmann, T. Muller, T. 2017-06-13T09:12:36Z 2017-06-13T09:12:36Z 2006 Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy / A. Kanaev, L. Museur, T. Laarmann, T. Muller // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1426–1433. — Бібліогр.: 28 назв. — англ. 0132-6414 PACS: 36.40.–c, 36.40.Vz, 33.50.Dq https://nasplib.isofts.kiev.ua/handle/123456789/120892 We have studied fluorescence spectra of small Xem and Xem(NF3)k clusters (m, k ≤ 10²) embedded in large host Ne₇₅₀₀ clusters, excited with SR excitation in the VUV spectral region above the Xe⁺(²P₁/₂) atomic limit. The IR emission bands due to ionic chromophores Xe⁺p (p = 2, 3, 4) as well as UV (B–X) and visible (C–A) bands due to desorbed Xe⁺F⁻– excimer molecules have been observed. The triatomic Xe₂F excimer emission has only been observed under low-energy excitation close to the lowest xenon exciton absorption band. A competition between ionic and charge-transfer reaction channels is followed as a function of the cluster sizes m and k. This work was supported by the IHP-Contract HPRI-CT-1999-00040 of the European Commission. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Clusters Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| title |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy |
| spellingShingle |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy Kanaev, A. Museur, L. Laarmann, T. Muller, T. Clusters |
| title_short |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy |
| title_full |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy |
| title_fullStr |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy |
| title_full_unstemmed |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy |
| title_sort |
ionic chromophores xe⁺p (p≤ 4) in multishell rare-gas clusters xem(nf₃)kne₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy |
| author |
Kanaev, A. Museur, L. Laarmann, T. Muller, T. |
| author_facet |
Kanaev, A. Museur, L. Laarmann, T. Muller, T. |
| topic |
Clusters |
| topic_facet |
Clusters |
| publishDate |
2006 |
| language |
English |
| container_title |
Физика низких температур |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
We have studied fluorescence spectra of small Xem and Xem(NF3)k clusters (m, k ≤ 10²) embedded
in large host Ne₇₅₀₀ clusters, excited with SR excitation in the VUV spectral region above the
Xe⁺(²P₁/₂) atomic limit. The IR emission bands due to ionic chromophores Xe⁺p (p = 2, 3, 4) as
well as UV (B–X) and visible (C–A) bands due to desorbed Xe⁺F⁻– excimer molecules have been
observed. The triatomic Xe₂F excimer emission has only been observed under low-energy excitation
close to the lowest xenon exciton absorption band. A competition between ionic and
charge-transfer reaction channels is followed as a function of the cluster sizes m and k.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/120892 |
| citation_txt |
Ionic chromophores Xe⁺p (p≤ 4) in multishell rare-gas clusters Xem(NF₃)kNe₇₅₀₀ (m, k ≤ 100) studied with fluorescence spectroscopy / A. Kanaev, L. Museur, T. Laarmann, T. Muller // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1426–1433. — Бібліогр.: 28 назв. — англ. |
| work_keys_str_mv |
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| first_indexed |
2025-11-26T02:05:56Z |
| last_indexed |
2025-11-26T02:05:56Z |
| _version_ |
1850607864100421632 |
| fulltext |
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11, p. 1426–1433
Ionic chromophores Xep
� (p � 4) in multishell rare-gas
clusters Xem(NF3)kNe7500 (m, k � 100) studied with
fluorescence spectroscopy
A. Kanaev1, L. Museur2, T. Laarmann3, and T. M�ller4
1 Laboratoire d’Ing�nierie des Mat�riaux et des Hautes Pressions, C.N.R.S.,
Institut Galil�e, Univerit� Paris-Nord, Villetaneuse 93430, France
E-mail: kanaev@limhp.univ-paris13.fr
2 Laboratoire Physique des Lasers, C.N.R.S., Institut Galil�e, Univerit� Paris-Nord,
Villetaneuse 93430, France
3 Max-Born Institute, Max-Born Str. 2a, Berlin 12489, Germany
4 Institut f�r Atomare Physik, Technische Universit�t Berlin, Hardenbergstr. 36, Berlin 10623, Germany
Received July 7, 2006
We have studied fluorescence spectra of small Xem and Xem(NF3)k clusters (m, k � 102) embed-
ded in large host Ne7500 clusters, excited with SR excitation in the VUV spectral region above the
Xe+(2P1/2) atomic limit. The IR emission bands due to ionic chromophores Xep
� (p = 2, 3, 4) as
well as UV (B–X) and visible (C–A) bands due to desorbed Xe+F– excimer molecules have been
observed. The triatomic Xe2F excimer emission has only been observed under low-energy excita-
tion close to the lowest xenon exciton absorption band. A competition between ionic and
charge-transfer reaction channels is followed as a function of the cluster sizes m and k.
PACS: 36.40.–c, 36.40.Vz, 33.50.Dq
Keywords: ionic chromophores, excited state distributions, charge transfer reaction, doped
rare-gas clusters, excimer molecule emission, fluorescense.
Introduction
A fundamental difference between the neutral and
ionized rare gas clusters is well known. The neutral
clusters are bound by the weak van-der-Waals pair-
wise interaction while polarized forces induced by the
ionized core strongly increase the cluster solidity. This
has a consequence on the atomic desorption process,
accompanied by the cluster cooling. Another point of
difference is that while in the neutral cluster the rep-
resenting building unit is the rare gas dimer, a larger
unit can be produced in the ionic cluster due to
a charge delocalization process. For example, Xe p
�
(p = 2–4) units have been experimentally observed
from analysis of absorption and fluorescence spectra
[1–3] and theoretically predicted in free xenon clus-
ters [4–6]. IR fluorescence of self-trapped hole in
rare-gas solids has been recently studies in Refs. 7–9.
A steady interest to the rare gas ionic clusters is ex-
plained by their role as model systems in understand-
ing of the charge transfer and solvation processes in
bulk media. The use of clusters offers a possibility to
study its evolution from the atomic limit towards the
bulk material as a function of cluster size, which is an
important issue of the fundamental physics. More-
over, because of the large percentage of surface atoms,
small clusters are extremely interesting objects for in-
terface studies.
Recently, the so called «pick-up» technique al-
lowed the growth of small guest rare-gas clusters in-
side large host rare-gas clusters and the investigation
of embedded clusters with a shell-like geometric struc-
ture [10,11]. This method has the advantage of con-
trolling the temperature, the surface or bulk localiza-
© A. Kanaev, L. Museur, T. Laarmann, and T. M�ller, 2006
tion and the size of the guest cluster, as well as the size
of the host cluster. Neon is a good solvent system for
such experiments for several reasons: NeN > 1000 clus-
ters are soft enough and easily pick up atoms or mole-
cules. They efficiently thermalize the dopant mo-
lecules at the characteristic cluster temperature of
� 10 K [12]. Moreover, the surounding neon cluster
atoms do almost not perturb the energy levels of em-
bedded molecules. Since neon clusters are transparent
within the VUV spectral range, they are well-suited
to study electronic properties of molecules and heavier
rare-gas clusters made of Ar, Kr and Xe atoms.
Recently, investigation of ArmNeN [11] and KrmNeN
[13] and XemRgN clusters (Rg = He, Ne) [14]
(m � 100 << N � 7500) gave insight into the tightly
bound bulk interface excitons in small rare-gas clus-
ters. It has been shown that a large penetration depth
of the interface exciton into the bulk of the embedded
cluster, induced by the surrounding neon matrix, pro-
hibits a formation of the electronic structure inherent
to the corresponding bulk solids.
However, the solvation effect on ionic cations Xe p
�
(p = 2–4) inside multishell clusters has never been a
subject of experimental studies. Influence of the sur-
rounding medium on the ionization, charge delo-
calization and charge transfer processes could be in-
spected in this way.
In the present paper we report on the spectroscopy
of small ionized clusters (m � 100) embedded
inside large 7500 Ne clusters. Using the multiple
cross-beam technique, we have also prepared multi-
shell Xem(NF3)kNeN clusters (m, k � 100), where a
competition between the charge delocalization onto
the Xe p
� (p = 2–4) units and charge transfer reaction
leading to the rare-gas halide excimer formation is ex-
pected. The experiments were carried out using the
energy resolved fluorescence technique. As in our pre-
vious studies, we make use of the IR ionic chromo-
phores fluorescence [2,3] on the transitions down to
the lowest bound and first excited unbound states
(Fig. 1). We have followed the evolution of the IR
band shape and relative IR and UV band intensities as
a function of the number of picked-up Xe atoms and
NF3 molecules. From a detailed analysis of the experi-
mental data we gain new information on the charge
delocalization in the solvated xenon clusters.
Experiment
The measurements have been performed at the
CLULU experimental station [15] at synchrotron
DESY. Neon clusters were prepared in a supersonic
expansion of the gas neon at 200 mbar pressure thourgh
a conical nozzle (200 �m, 2� = 4�) cooled down until
30 K. The average cluster size 7500 � N was de-
termined using well-known scaling laws according
to the formula [16,17]: N /� 33 1000 235( * ) .� , with
�* . .� K pd /Tch eq
0 85 22875, Kch Ne( ) � 185, and p in mbar,
T in K and d in �m are used. The width of the cluster
size distribution (FWHM) is N � N. Using a succes-
sive «pick-up» technique these large neon clusters
were doped firstly with NF3 molecules and secondly
with Xe atoms from two cross jets. The embedded
clusters are cold: their temperature of 10 K is defined
by that of the neon host cluster [18]. The Poisson sta-
tistic determines the average number of picked-up
molecules (k) and atoms (m). The mean sizes of the
embedded Xem and (NF3)k clusters have been esti-
mated by measuring the VUV-fluorescence excitation
spectra of the Arm clusters inside large neon clusters in
the range of 12.4 eV, prepared by expanding the argon
gas correspondingly through xenon (m) and nitrogen
trifluorine (k) nozzles, as it is explained in Ref. 13.
Laarmann et al. [11] have shown that the absorption
lineshape of tightly bound excitons in Arm clusters
changes with their size m according to the Frenkel
exciton model. By comparing the experimental spectra
lineshapes with those from Ref. 11, one obtains a rela-
tion between the cross-jet pressure and the average
number of embedded atoms. Since the probability for
a Ne cluster to pick-up atoms is mainly depending on
the Ne cluster size and the average cross-jet particle
density along the beam axes, the calibration is also
valid in the case of Xe and NF3 doping.
Monochromatized synchrotron radiation (
=
= 0.25 nm bandpass) in the spectral range of
100–140 nm (Al-grating) or 40–100 nm (Pt-grating)
was focused on the doped multishell XemNe7500 and
Xem(NF3)kNe7500 cluster beam 10 mm downstream
from the nozzle. Generally, we used to excite cluster
beam in the lowest excitonic band of the host neon
clusters at 17.7 eV (70 nm). After the excitation the
energy is efficiently transferred onto impurity center,
that is a small embedded xenon cluster. Fluorescence
excitation spectra in the VUV-UV (
� 300 nm) and
in the UV-visible-IR (200 nm �
� 900 nm) were
recorded by two photomultipliers with CsI and
GaAs(Cs) photocathodes, respectively. The backgro-
und pressure was kept below 10–3 mbar during the ex-
periments.
Before presenting experimental results, a remark
on the cluster composition can be made. The pick-up
atoms and molecules by large Ne7500 clusters results
in a release of energy. Neon atoms are weakly bound
and evaporate from the XemNeN cluster by heating. In
fact, the binding energies per atom of rare-gas neon
and xenon solids are 26.5 meV and 172.3 meV [19].
Therefore, doping may decrease the initial cluster size
by � 6.5 Ne-atoms per adsorbed Xe atom if the kinetic
Ionic chromophores Xe p
� (p � 4) in multishell rare-gas clusters
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1427
energy of the free atoms is neglected. Therefore, dop-
ing by m � 102 xenon atoms do not change consider-
ably the neon cluster size. It can be shown that the
same conclusion is valid for the doping by a small
number of NF3 molecules. This conclusion about sta-
bility of the host neon clusters with N = 7500 in the
experimental doping conditions (m, k � 100) has been
confirmed by measurements of the luminescence exci-
tation spectra in the spectral range of the neon absorp-
tion of h � 17.7 eV.
Results and discussion
Fluorescence spectra of xenon clusters excited
above the atomic ionisation limit originate from mo-
lecular states related to the 5p5 2P1/2 � 5p5 2P3/2
transitions in ionic chromophores Xe p
� (p = 2–4)
[2,3]. The relevant potentials of the two-atomic ions
are schematized in Fig. 1. Because of a low barrier be-
tween the trimer and tetramer ionic cores, the corre-
sponding transitions are not resolved in relatively hot
(T = (79 ± 8) K [18]) free xenon clusters. Experimen-
tal results on fluorescence of Xe N
� (N � 2·104) have
been reported in Refs. 2, 3. However, Laarmann et al.
[3] have remarked that because of two main radiation
channels associated with 2(1/2)g � 1(1/2)u and
2(1/2)u � 1(1/2)g transitions and three possible
ion cores (Xe2
� , Xe3
� , and Xe 4
�), six fluorescence
bands can be resolved in fluorescence spectra at suffi-
ciently low temperatures. These new spectral bands
have been demonstrated in small Xe40 clusters embed-
ded in large neon clusters using the pick-up method
[3]: the temperature of such cluster is defined by the
host neon temperature, which is about 10 K [18].
Size-dependence of the IR fluorescence spectra of
these embedded Xem clusters (m � 102) is studied in
the present work.
XemNe7500 clusters
The IR fluorescence spectra of XemNe7500 clusters
excited at
exc = 70 nm are presented in Fig. 2. The
multi-peaks gaussian fit has been used to resolve the
contributions of six bands labelled by a, b, c, d, e1,
and e2. They have been assigned by Laarmann et al.
[3] to radiative transitions in the ionic dimer
(subbands e1, e2), trimer (b, d) and tetramer (a, c)
cores inside xenon clusters.
According to the schema in Fig. 1, the bands e1, b,
a originate from the higher positioned (more shallow)
electronic g-state, whereas the bands e2, c, d originate
from the stronger bound u-state: the corresponding
dimer ion states are 2(1/2)g and 2(1/2)u [20]. As
one can see from this figure, the bands d and c domi-
nate the fluorescence of very small xenon clusters
m � 5. The bands b and a appear at m � 30 and became
stronger in clusters larger than m � 50. With further
increase of the size m � 70, the spectral shape of the IR
fluorescence remains almost unchanged. Their inte-
grated relative intensities issued from the multi-peaks
1428 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
A. Kanaev, L. Museur, T. Laarmann, and T. M�ller
3.0 4.0 5.0 6.0
–1.0
–0.5
0
0.5
1.0
Xe2
+
Internuclear distance,�
E
n
e
rg
y,
e
V
3.0
2.5
2.0
1.5
Xe ( P ) + Xe( S )+ 2 1
1/2 0
Xe ( P ) + Xe( S )+ 2 1
3/2 0
2(1/2)g
2(1/2)u
1(1/2)u
1(3/2)g
Fig. 1. Schematic potential curves of the relevant states
related to 5p5 2P1/2 and 5p5 2P3/2 atomic xenon ion li-
mits.
m = 20
m = 30
m = 40
e2
e1
d
c b
a
m = 50
In
te
n
si
ty
,a
rb
.u
n
its
hv, eV
1.2 1.4 1.6 1.8 2.0 2.2 2.4
Fig. 2. fluorescence spectra of XemNe7500 excited at
17.7 eV.
gaussian fit are shown in Fig. 3. Because of a low
contribution, the dimer bands are not shown in this
figure.
Different state origin of the two groups of the
bands d/c and b/a can be evidenced from fluores-
cence decay measurements, presented in Fig. 4. All
these curves can be well fitted by two-exponents re-
sulting in excited-state lifetimes of t1 = 1.5 �s and t2 =
= 150 ns. On the other hand, the corresponding decay
amplitudes A1 and A2 vary with the cluster size m:
only the long-lived decay component is present at
m � 20, while the short-lived decay component inten-
sifies in larger xenon clusters (see Fig. 4,a). Its rela-
tive intensity as a function of the size m is shown in
Fig. 4,b. A correlation between relative intensity of
the bands b/a and shortlived fluorescence decay al-
lows their straightforward identification. We assign
the longer lifetime of t1 = 1.5 �s to the fluorescence
bands d/c appeared in small xenon clusters, and the
shorter lifetime of t2 = 150 ns to the fluorescence
bands b/a appeared in the larger clusters. This assign-
ment is in general agreement with dimer ion state
structure discussed in theoretical work by Wadt [21].
Similar repartitioning of the excited electronic states
is expected in xenon ion trimers and tetramers that ex-
plains our finding.
In contrast to the ionic chromophore subbands, rela-
tive fluorescence intensity originated from different
chromophores Xe 4
� and Xe3
� does almost not change in
the cluster size range m � 102. This result is directly
followed from Fig. 3 if we sum respectively the band
intensities d+b and c+a: each chromophore accounts
for about 50% of the IR intensity. Our measurements
confirm the charge localization on ionic trimer and
tetramer cores in small xenon clusters. Recently, Laar-
mann et al. [3] have shown that in free XeN clusters
the charge is preferentially repartitioned on Xe 4
� and
Xe3
� cores when N � 200, and localized on Xe2
� only
when N � 400. This explains in particularly why only
dimmer ionic core is observed in the bulk solids in
both absorption [21] and fluorescence [22]. Theoreti-
cal and experimental studies by Gasc�n et al. [23]
have shown that tetramer and trimer cores coexist in
small xenon clusters N � 30, which relative popula-
tion undergoes oscillatory variation with size. The
interconversion of these two isomers was also sug-
gested at temperatures of � 60 K. The temperature of
our embedded Xem clusters is considerably lower and
the interconversion may be prohibited. This may in
particularly explain the fact that the subband inten-
sity variation within each isomer with size is not ac-
companied by the intensity variation between two iso-
mers. The last is determined by the energy-relaxation
dynamics and not by the thermodynamic equilibrium
Ionic chromophores Xe p
� (p � 4) in multishell rare-gas clusters
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1429
0 50 100 150 200
m =160
m = 60
m = 40
m = 20
a
b
m
time, ns
t = 1.5 s1 �
t = 150 ns2
0.8
0.6
0.4
0.2
0
R
e
la
tiv
e
in
te
n
si
ty
o
f t
2
20 40 60 80 100 120 140 160
In
te
n
si
ty
,a
rb
.u
n
its
Fig. 4. IR fluorescence decay (a) and the fast component
relative intensity in XemNe7500 clusters as a function of
the size m (b) (Eexc = 17.7 eV).
d
c
b
a
R
e
la
tiv
e
in
te
n
si
ty
0.5
0.4
0.3
0.2
0.1
0
20 30 40 50 60 70
m, Xe atom/cluster
Fig. 3. Relative intensities of the IR emission bands due
to ionic chromophores (p = 2, 3, 4).
between the relaxed ionic cores. The isolation of Xe 4
�
and Xe3
� ionic isomers at low temperature of 10 K
makes their population independent on cluster size, in
contrast to what is observed in hotter free xenon clus-
ters (T = 79 K) [3]. We also remark that clusters with
preferential isomer population cannot be resolved in
our experiments because of a broad cluster size distri-
bution: Poisson distribution of the embedded xenon
clusters is convoluted with that lognormal of the host
neon clusters, issued of the nozzle expansion, resulting
in a somewhat larger width than m.
Our results show an importance of the small cluster
environment. Indeed, comparing with free xenon clus-
ters Xe 4
� and Xe3
� isomers contribute in fluorescence
spectra with almost equal weightings. Moreover, the
upper state population within each ionic core sensi-
tively depends on the cluster size m shown in Fig. 3.
This cannot be an effect of the cluster temperature
that is fixed at 10 K. Moreover, the temperature
would be expected to decrease with size m leading in
population of a deeper 2(1/2)u excited state. As a
consequence, the intensity of the 2(1/2)u � 1(1/2)g
transition would increase that is not the case. A pref-
erential population of the lower excited state related
to 2(1/2)g of Xe2
� in small clusters may be explained
by an effect of the interface formation similar to that
discussed in [13,14] or by the embedded atoms com-
pression [14]. According to Refs. 13, 14, the exciton
formation in embedded rare-gas clusters depends on
the electron affinity. In particular case of XemNeN
clusters a perturbation of the interface Xe–Ne layer is
strong and the bulk exciton of Xem only appears at
sufficiently large m � 50. The second case seems less
probable since the effect of compression was only ob-
served in small neon clusters about 40 < N < 90 [14].
However, the case of embedded ionized cluster atoms
is different from that of neutral isolated xenon atom
studied in Ref. 14 and requires further studies.
Xem(NF3)kNe7500 clusters
A competition between the ionization and
charge-transfer reaction can be studied in multishell
Xem(NF3)kNe7500 clusters. In fact excitation of the neon
cluster activates two reaction mechanisms. (1) Ioni-
zation of the internal Xem cluster takes place fol-
lowing the energy transfer process. The electron leaves
the cluster, whereas the hole is localized on excited
chromophores Xe p
� * (p = 2, 3, 4) that give rise to the
characteristic IR fluorescence bands shown in Fig. 2.
(2) The energy transferred from neon cluster triggers
charge-transfer reactions resulting in the excimer cre-
ation: XeF or Xe2F. In bulk xenon XeF relaxes into
Xe2F, therefore its observation would be an indication
of the excited-state desorption process [24,25].
The fluorescence spectra recorded after excitation
of multishell Xe50(NF3)kNe7500 clusters at 17.7 eV are
shown in Fig. 5 for different sizes k. At k = 0 only IR
fluorescence of Xe p
� * has been observed. However,
with an increase of NF3 doping the IR band shape
changes. It seems to follow the opposite tendency to
that earlier observed in the two-component XemNe7500
clusters: intensity of the subbands a and b decreases
relatively to that of the subbands c and d when k in-
creases. Moreover, at k � 30 new XeF excimer emis-
sion bands B–X in the UV at 3.5 eV and C–A in the
visible at 2.5 eV appear.
The observed excimer bands apparently belong to
the desorbed excited fragments illustrating the impor-
tance of the excited-state stimulated desorption pro-
cesses in cryogenic solids. Relatively weak spectral
features at 3.65 eV and 3.15 eV do not correspond to
the known NF3 fluorescence products [26]. They indi-
cate that a part of the expulsed excited products is hot
and possess significant vibrational excitation:
corespondingly v’ = 2–3 (B-state) and v’ = 6 ± 1
(C-state). The XeF excimers escape through the outer
shell of Ne7500 cluster. However, according to [12] vi-
brational fragments cooling can be particularly ineffi-
1430 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
A. Kanaev, L. Museur, T. Laarmann, and T. M�ller
k = 80
k = 53
x 5
k = 28
k = 0
In
te
n
si
ty
,a
rb
.u
n
its
hv, eV
XeF
C–A B–X
1.5 2.0 2.5 3.0 3.5 4.0
Xe m=3,4
+
Fig. 5. Fluorescence spectra after excitation of multishell
Xe50(NF3)kNe7500 clusters at 17.7 eV.
cient in neon clusters in view of a low Debye energy
(6.4 meV) compared to the vibration quantum (e.g.,
in the C-state
e = 346 cm–1). The main fluorescent
bands however are narrower than those earlier ob-
served in pure xenon clusters [24,25]. This may be an
effect of rotational cooling by the neon matrix.
The mechanism of the excimer state creation (chan-
nel (2)) may proceeds as following:
Ne Xe Xe
Xe NF
freeN m p
p
e e* **( ) *
* (
( )
( )
� � � ��� �
� ��� �
� � �
�
1
2
3) ( , )k B C�
� �
�
XeF
products
free
Firstly, the ionization of the internal Xem cluster
takes place. In XemNeN clusters this results to the
Xe p
� * center creation (channel (1)). However in
Xem(NF3)kNeN the electron may not escape and can
be trapped by the first-shell cluster (NF3)k. This hap-
pens if the size k is above some critical value of
k* � 30. The NF3 molecule accommodating excess
electron subjects to the dissociative attachment.
However it can be stabilized in clusters as NF3
� anion
and larger cluster ion units ( )NF3 1l�
� [24]. In such
case the Coulomb attraction between the localized
hole Xe p
� * and ( )NF3 k
� or lighter anionic fragment
can result in the XeF excimer formation. The excess
energy of the electronically excited center is appa-
rently so strong that the expulsion proceeds before
the relaxation into Xe2F takes place. Additionally,
the binding energy of the Xe p
� * core is much smaller
than that of the ground-state ion Xe p
� . This may pro-
hibit the Xe2F formation from xenon core states re-
lated to the 5p5 2P1/2 atomic limit. In fact, no signa-
ture of the of this trimer rare-gas halide emission has
been observed with 17.7 eV excitation.
On the other hand if the excitation energy is low-
ered below the ionization onset, the broad-band
intracluster emission of Xe2F appears, as shown in
Fig. 6. It is centered at 1.48 eV (838 nm) that is lower
compared to that reported in bulk solid Xe matrixes
— 1.60 eV (775 nm) [25] and in small free xenon clus-
ters — 1.54 eV (805 nm) [22]. The spectral width of
this emission EFWHM = 0.27 eV corresponds to that
observed in solids and somewhat smaller that in free
clusters (0.35 eV). A small red shift in the embedded
xenon clusters and a narrower bandwidth may be due
to the neon cluster compression. This issue requires
verification.
The dependence of the Xe p
� * fluorescence intensity
(IIR) on the size k of the first-shell cluster (NF3)k is
shown in Fig. 7. In this series the integral IR fluores-
cence of the excited chromophores (p = 2, 3, 4) was
recorded through the RG610 filter (red cut-off at
> 600 nm), which allowed higher sensitivity mea-
surements in a large size domain. The observed
semilogarithmic plot is characterized by two kinetics
domains separated by the cross-over region at
30 � k < 60. This transient region apparently relates to
an increase of the electron trapping rate by the NF3
cluster after the critical size of k � k* = 30. The fact
that the critical size k* does not appreciably depend
on the size of the embedded xenon cluster m (see
Fig. 7) indicate that the electron exit from the neon
cluster may not be direct but rather delayed process:
in case of the direct exit the electron trapping proba-
bility would be a function of the NF3 shell width
(k2/3) and not of the size k. An increase of the neutral
reaction channel relatively to molecular ionisation has
Ionic chromophores Xe p
� (p � 4) in multishell rare-gas clusters
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1431
c
b
a
In
te
n
si
ty
, a
rb
. u
n
its
Xe F2
Free Xe clustersN Solid Xe
hv, eV
1.2 1.4 1.6 1.8
Fig. 6. IR fluorescence spectra after excitation of multi-
shell Xe50(NF3)kNe7500 clusters at 8.551 eV for k = 50
(a), 25 (b), 14 (c).
103
104
105
106
m =25
m =40
m =60
k
Beginning of the XeF emission
Xe -clusterm
I IR
0 20 40 60 80 100 120 140
Fig. 7. Relative intensity of the IR fluorescence of
Xem(NF3)kNe7500 clusters as a function of the size k,
Eexc = 17.7 eV.
been recently observed in large neon clusters doped
with water molecules [12].
Additional remark concerns the excited-state reac-
tivity of Xe p
� *. The plot of ln (IIR) vs cluster size k
suggests the excited-states quenching process. More-
over as we can see in Fig. 5, the spectral lineshape of
the IR band changes with growing k: subbands b and a
weaken faster and the subbands d and c dominate flu-
orescence spectra at large k. As our results show, the
short-lived excited state 2(1/2)g is at the origin of
the subbands b and a, while the long-lived excited
state 2(1/2)u is at the origin of the subbands d and c.
This indicates a much higher reactivity of the 2(1/2)g
state in respect to the 2(1/2)u one. This may be re-
lated to its larger equilibrium distance that enables
more efficient potential curve-crossing between the
chromophore and the anion. Indeed, more localized
state 2(1/2)u may be centered inside the embedded
clusters while more delocalized one 2(1/2)g [2] is
readily set in contact with the negatively charged
halogen donor.
Conclusion
The present spectroscopy study of multishell Xem
and Xem(NF3)k clusters (m, k � 102) embedded in
large host Ne7500 clusters clear up the hole localization
process in small xenon clusters and evidences a compe-
tition between the ionization and charge-transfer reac-
tion. The IR emission bands due to ionic chromo-
phores (p = 2, 3, 4) as well as UV (B–X) and visible
(C–A) bands due to desorbed Xe+F– excimer mole-
cules have been observed. The cluster size effect of m
and k on the reaction dynamics was observed. The
delocalization of hole on trimer and tetramer ionic
cores is confirmed in xenon clusters of size m � 102.
Because of a low cluster temperature of 10 K the con-
figuration mixing is prohibited resulting in almost
equal relative ion isomer populations. However, ex-
cited state distribution within each isomer changes
with the size m, which may reflect the atoms compres-
sion either Xe–Ne interface effect. The free XeF
excimer emission shows an importance of the ex-
cited-state desorption processes in rare-gas clusters.
The charge-transfer reaction takes place when the
first-shell cluster size k is higher than the critical size
k* � 30. The triatomic Xe2F excimer is not observed at
17.7 eV excitation; this emission has only been ob-
served under low-energy excitation close to the lowest
atomic 3P1,2 states. This result shows that a large neon
cluster is not a good cage for rapid reaction fragments
that readily escape it.
This work was supported by the IHP-Contract
HPRI-CT-1999-00040 of the European Commission.
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