A soft-landing experiment on organometallic cluster ions: Infrared spectroscopy of V(benzene)2 in Ar matrix

Article abstract of DOI:10.1016/S0009-2614(00)01450-0

Vanadium (V)-benzene cluster ions, produced by laser ablation with reaction toward benzene vapor, were size-selected and deposited into a low-temperature Ar matrix. Infrared spectrum of V₁(benzene)₂ in the Ar matrix was measured after one-hour deposition with the deposition energy of 20 eV. The spectrum was in agreement with both the reported spectrum and our theoretical calculations, showing that (1) V₁(benzene)₂, prepared in the gas-phase reaction, takes a sandwich structure and that (2) the ions were soft-landed onto the Ar matrix and were neutralized by charge transfer from a metal substrate without fragmentation.

Full text of DOI:10.1016/S0009-2614(00)01450-0

9 February 2001
Chemical Physics Letters 334 (2001) 277±284
www.elsevier.nl/locate/cplett
A soft-landing experiment on organometallic cluster ions:
infrared spectroscopy of V(benzene)2 in Ar matrix
Ken Judai a, Kentaro Sera a, Shin-ichi Amatsutsumi a, Keiichi Yagi a,
Tomokazu Yasuike a, Satoshi Yabushita a, Atsushi Nakajima a,*, Koji Kaya b
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
a
b
Institute for Molecular Science, Myodaiji, Okazaki, 444-8585, Japan
Received 25 September 2000; in ®nal form 20 November 2000
Abstract
Vanadium (V)±benzene cluster ions, produced by laser ablation with reaction toward benzene vapor, were size-se-
lected and deposited into a low-temperature Ar matrix. Infrared spectrum of V1(benzene)2 in the Ar matrix was
measured after one-hour deposition with the deposition energy of 20 eV. The spectrum was in agreement with both the
reported spectrum and our theoretical calculations, showing that (1) V1(benzene)2, prepared in the gas-phase reaction,
takes a sandwich structure and that (2) the ions were soft-landed onto the Ar matrix and were neutralized by charge
transfer from a metal substrate without fragmentation. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
controlled deposition of the size-selective clusters
in the gas phase, at surfaces would provide a
promising route. When cluster ions are size-selec-
tively prepared, both the kinetic energy and the
neutralisation energy of the ion, i.e., the ionisation
energy of the neutral cluster, should be dissipated
to ensure soft-landing of the clusters. Then, the
synthesis of the nanostructure surfaces requires
low kinetic energies to be released during the de-
position to ensure a nondestructive deposition in
which the nanoclusters maintain their individual
characteristics. Very recently, Heiz et al., have
succeeded in the controlled deposition of mon-
odispersed metal clusters directly onto a substrate,
and have investigated catalytic oxidation of car-
bon monoxide on monodispersed platinum clus-
ters on the substrate [2]. Harbich, Buttet and
co-workers [3,4], on the other hand, have suc-
ceeded in soft-landing of size-selected silver clus-
ters on a substrate with preadsorption of noble-gas
The study of small elemental clusters is cur-
rently a very active area, and there have been a
number of reports on their formation and stability,
physical and chemical properties, and structure [1].
In particular, it is important to provide informa-
tion about the relationship between isolated at-
oms/molecules and bulk solids/liquids in order to
understand the change in physical and chemical
properties as the size increases from atoms to the
bulk. Another motivation for these studies is to
®nd out clusters with useful properties. From the
viewpoint of their application to fabricating new
materials, nanostructure formation, using the
* Corresponding author. Fax: +81-45-566-1697.
E-mail address: nakajima@sepia.chem.keio.ac.jp (A. Nak-
ajima).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 4 5 0 - 0

278
K. Judai et al. / Chemical Physics Letters 334 (2001) 277±284
®lms. In fact, it has been shown by molecular±
dynamics (MD) simulations that a thin rare-gas
®lm on top of a surface helps a cluster ion to be
deposited nondestructively [5,6].
combined with the formation of the organome-
tallic cluster consisting of transition-metal and
benzene (M-Bz) for vanadium (V)-Bz. High cur-
rents of the organometallic cluster cations were
prepared with size-selection, and the infrared (IR)
spectroscopy was applied for identi®cation of the
species and also for clarifying the conditions for
the nondestructive deposition. Together with the-
oretical calculations of density functional theory
(DFT), the vibrational features in the V-Bz com-
plex are analysed, and ®nally the perspective of the
electron spin material based on the organometallic
clusters will be discussed.
When nanostructured material is synthesized
through the assembly of clusters deposited from
the gas phase, it is crucial to develop a new
methodology for depositing them onto the sub-
strate. Some of the clusters can be expected to
work as a functional unit such as a catalyst.
Among the functional molecular units, intrachain
or multi-layered organometallic clusters would be
one of the promising candidates, because metal
atoms chained by organic ligands have been in-
spiring chemistry and physics for a long time [7±9].
Furthermore, synthesis of organometallic com-
pounds in the gas phase presents a novel guideline
for the production of new organometallic com-
pounds because they can be prepared without
solvent and oxidation in air. Especially, the ap-
plication of the laser vaporisation technique to the
gas-phase synthesis of organometallic compounds
enables us to prepare the constituents in consid-
erable density in a short time because there are no
interfering e€ects of solvents, aggregation phe-
nomena, and counter ions. This new approach
should open up an entirely di€erent aspect of or-
ganometallic chemistry and physics, which can be
studied quite nicely in the gas phase [10±15].
In this Letter, we will report on the develop-
ment that the soft-landing technique has been
2. Experimental
Fig. 1 shows a schematic diagram of the ex-
perimental set-up. The apparatus used in this
Letter consists of a cluster ion source, ion optics
with mass selection, and a deposition room for IR
spectroscopy. To generate organometallic clusters,
the cluster ion source, introduced by Hiez et al.
[16], was slightly modi®ed. A plasma was pro-
duced by focusing the output of a Nd3‡; YAG
laser (532 nm, 100 Hz; In®nity 40±100 Coherent)
on the surface of a rotating disk of vanadium (V)
and the plasma is entrained in a He carrier gas
(stagnation pressure ˆ 3 atm) from a piezo-driven
pulsed valve [17]. The vapor of the V atoms was
reacted with benzene molecules that were injected
Fig. 1. Schematic diagram of the experimental apparatus in this Letter.

K. Judai et al. / Chemical Physics Letters 334 (2001) 277±284
279
in synchronisation with the laser vaporisation. The
organometallic clusters were co-axially expanded
towards the incoming vaporisation laser: The laser
was introduced through the hole of the cluster-
beam exit, which reduces the leakage of the He
carrier gas from the source assembly.
to observe the vibrations of the clusters with a
commercial FTIR spectrometer (Magna 760,
Nicolet). The chamber windows for the IR radia-
tion were KBr and the re¯ected IR radiation at a
polished Au substrate was observed by a mercury-
cadmium-tellurium (MCT) detector, with which
The produced cluster ions were guided by a
series of ion optics; electrostatic ion lens, a quad-
rupole de¯ector, a quadrupole mass ®lter, and an
octapole ion guide. The cluster ions were acceler-
ated and focused to the Q-pole de¯ector through a
couple of ion lenses, and were bent perpendicularly
at the Q-pole de¯ector. This de¯ection can sepa-
rate the cations from neutrals, anions, and elec-
trons. Going through another set of ion lens, the
cluster cations were mass-separated with a com-
mercial Q-pole mass spectrometer (4±4000 amu;
9:5 Â 200 mm rod, Extrel). The size-selected clus-
ter cations were further guided onto the deposition
substrate by two sets of the octapole ion guides,
which have larger transmission rate, compared to
the Q-pole. The apparatus was partitioned into
the IR absorption spectrum can be measured in
1
the range of 700±6000 cm
.
3. Calculations
To assign the peaks in the IR spectra, DFT
calculations for M(benzene)2 [M ˆ Ti, V, and Cr]
have been performed using the GAUSSIAN 98
program [19]. The optimized structures with the
B3LYP functional [20] and the MIDI basis set [21]
had the D6h symmetry for these three kinds of
clusters, and all the obtained harmonic vibrational
frequencies were real.
For discussion of the nature of the chemical
bond of the clusters, we use Mayer's bond order
index [22] obtained with the GAMESS program
[23]. As shown by the previous work [24], this in-
dex describes properly the nature of the chemical
bonds in M-benzene systems. For calculating this
index, we use the RHF/ROHF wave functions
with the same basis set.
four di€erentially pumped units. The deposition
10
chamber with a base pressure of 3 Â 10
Torr
was evacuated by a turbo molecular pump (1500 l
s
1) and a titanium getter pump (400 l s 1).
The deposition substrate was a polycrystalline
gold (Au), which was cooled down to 18 K by a
liquid He cryostat (GFS750, Oxford Instrum.).
Before the deposition, the substrate was exposed
to Ar gas to make an Ar layer of ꢀ30 MLs on the
Au substrate. The Ar layer works as a cushion for
the soft-landing of the cluster ions, and also as a
spectroscopically inert media for IR spectroscopy.
The deposited cluster cations are successively
neutralized by charge tunneling through the thin
Ar layer [3,18]. The ion current can be monitored
by the neutralizing charge with an electrometer
(6517A, Keithley) and the number of deposited
clusters was obtained by integrating the measured
current on the substrate over the deposition time.
The deposition energy was controlled by a di€er-
ence of the bias voltages between the cluster source
and the substrate, which was typically 20 eV. For
the deposition energy below 50 eV, the fragmen-
tation of the clusters can be excluded as mentioned
in Section 4. For selected clusters, Fourier trans-
form infrared (FTIR) studies have been performed
4. Results and discussion
‡
4.1. Production of Vnꢁbenzene† with size selection
m
A
mass spectrum was measured for
‡
Vnꢁbenzene† produced in the reaction between
m
laser-ablated V atoms and benzene vapor, and the
organometallic ions were produced predominantly
with the composition of ꢁn; m† ˆ ꢁn; n ‡ 1; n ˆ
1
3†. There were some minor peaks that can be
assigned as Vnꢁbenzene† ions. This result is in
n
agreement with the time-of-¯ight (TOF) mass
spectrum reported previously [25,26]. As reported
in the previous work, the pattern in the mass
spectrum of V-benzene can be explained by the
sandwich structure along with the results on the
chemical reactivity toward CO molecules. For
the ꢁn; n ‡ 1† cluster, V atoms are sandwiched

280
K. Judai et al. / Chemical Physics Letters 334 (2001) 277±284
by benzene molecules, whereas for the ꢁn; n†, one
of the terminal benzene molecules is missing.
When CO molecules were additionally added, the
ꢁn; n ‡ 1† cluster exhibited no adsorption reaction,
while three CO molecules coordinated the ꢁn; n†
cluster. This is because the three CO molecules
correspond to one benzene molecules in the coor-
dination chemistry. However, this qualitative ex-
planation needs some spectroscopic information to
con®rm the geometric structures, and for this
purpose IR spectroscopy was performed on the
size-selected organometallic clusters deposited on
the Ar matrix.
Fig. 2. Infrared spectrum of V(benzene)2 in the range of 700±
‡
1200 cm 1. The size-selected Vꢁbenzene† ion (8 nA) was de-
The transmission eciency of ions in the
Q-mass spectrometer strongly depends on its res-
olution. In the deposition experiments, the mass
resolution was set to be as low as possible to in-
crease the ion current under higher transmission
eciency; the high mass resolution of 1±2 amu was
only adopted for mass identi®cation. For
2
posited onto a gold substrate (18 K) covered with a thin ®lm of
Ar for 1 h. Distinct absorptions were observed at 748, 958, and
1
990 cm
.
distinct peaks were observed at 748, 958, and 990
1
cm as shown in the Fig. 2. A useful reference
‡
experiment for this cluster deposition is the ther-
mal vapor condensation of chemically synthesized
V(benzene)2 into an Ar matrix [27,28]. The series
of absorption peaks in our work correspond
closely with those of the chemically synthesized
V(benzene)2 having a sandwich structure that has
been revealed by X-ray crystalline analysis [29].
Thus, the IR spectrum shows that (1) the cluster
ions were nondissociatively deposited into the Ar
matrix and that (2) the neutralized cluster has a
sandwich structure.
When the Ar layer are thin below 50 MLs, the
deposited cluster ions can be neutralized by the
charge tunneling from the Au substrate [18]. For
more than 100 MLs of Ar, however, the monitored
current gradually decreased with the deposition
time due to pile-up of the charge on the substrate.
The current was immediately recovered when the
Ar matrix is removed by heating the substrate.
This technique for neutralisation gives an ex-
perimentally important advantage; the ion current
can be monitored directly by the tunneling current
from the metal substrate beneath the matrix. This
real-time monitoring of the ion current is an im-
portant technique to keep the source conditions
constant during the deposition, because the ion
current is often ¯uctuated during the deposition,
primarily depending on a delicate charge balance
Vnꢁbenzene†m, the cluster ions were produced at
ꢁn; m† ˆ ꢁn; n ‡ 1† and ꢁn; n†. Since the produced
ions appear every several tens amu in the mass
spectrum (the masses of V and benzene are 51 and
78 amu, respectively), the mass resolution of 30±50
amu was enough to choose the target ions from the
other ions. This low mass resolution made the ion
current several ten times higher compared to that
with mass resolution of 1±2 amu: the ion current
‡
for Vnꢁbenzene† were typically 8 nA for (1,2) and
m
1 nA for (2,3) at the deposition energy of 20 eV.
The energy distribution (full-width at half maxi-
mum) was around 5 eV. It should be noted that the
current of 8 nA equals to 5 Â 1010 ions per second
(or 1:8 Â 1014 ions per hour), which yields 0.06 lg
of the V1(benzene)2 product per hour. Similarly,
the laser ablation of a titanium (Ti) and a chro-
mium (Cr) target provides the size-selected ions of
‡
‡
2
Tiꢁbenzene† (7 nA) and Crꢁbenzene† (5 nA),
2
respectively.
4.2. IR spectrum of V(benzene)2 in Ar matrix
Fig. 2 shows the IR absorption spectrum of
V(benzene)2 deposited onto the Au substrate with
the thin Ar layer for 1 h. The IR spectrum was
1
taken at 1 cm resolution with 1024 scans aver-
aged over the region from 700±6000 cm 1. Three

K. Judai et al. / Chemical Physics Letters 334 (2001) 277±284
281
vibrations are IR active, consisting of seven ligand
modes and three metal-ring modes.
at the cluster source, for which space charge e€ect
is mainly responsible. To neutralize the deposited
ion in the Ar matrix, supply of thermal electrons
from a hot ®lament, an electron shower, could also
be used. However, it is extremely hard to estimate
the ion current during the deposition with the hot
®lament because the thermal electrons disturb the
exact detection of only the deposited ions. In ad-
dition, the hot ®lament located near the substrate
might make the Ar matrix warm. Therefore, the
tunneling electron from the substrate preferably
makes it easy to work with the low-temperature Ar
matrix.
The metal substrate in this Letter plays an im-
portant role in IR spectroscopy with the re¯ection
mode. As reported in a lot of studies [30], the re-
¯ection mode provides higher sensitivity than the
transmission mode, because the total path length
for the IR light passing through the sample is
longer in the former. For comparison, the IR ab-
sorption was measured in the transmission mode
for V(benzene)2 on a Ar-preadsorbed CsI which
was prepared with the deposition of the corre-
sponding ions under the electron shower. It turned
out that the sensitivity of the re¯ection mode was
roughly 3±5 times higher than that of the trans-
mission mode.
According to the study by Andrew et al., the
peaks observed at 742, 958, 988 cm 1 were assigned
as CH out-of-plane bending, breathing of ring, and
CH in-plane rocking modes, respectively [28]. Our
DFT results are consistent with the experimental
assignment and the three vibrational modes are il-
lustrated in Fig. 3. Only the antisymmetric
breathing vibration between two rings can be ob-
served in the IR spectrum. The breathing mode of
1
958 cm originates from m1 of free benzene (991.6
cm 1) which is a Raman active fundamental mode,
1
and the frequency shift of about 30 cm occurs.
Two explanations are possible for the shift: (1) the
splitting of the breathing vibrations into a sym-
metric and an antisymmetric mode and (2) the
change in the C±C bond strength due to an elec-
tronic cause. According to our DFT calculations,
1
however, only a small splitting of a few cm is
predicted; the symmetric and antisymmetric
breathing vibrations of the two rings are calculated
to be 974 and 971 cm 1, respectively.
To reveal the electronic cause, IR spectra for
the other bis(benzene)-metal complexes were
‡
measured. With the deposition of Crꢁbenzene†
2
‡
and Tiꢁbenzene† for 1±2 h, absorptions corre-
2
sponding to the breathing mode were observed at
1
1
949 cm for Ti(benzene)2 and at 970 cm for
Cr(benzene)2. Namely the vibrational frequency
increases in the order of Ti, V, and Cr, indicating
that the hardening of the breathing mode occurs
from Ti to Cr. This change can be rationalized by
the di€erence in the attractive interaction between
metal 3d orbitals ꢁddg† and unoccupied orbitals of
benzene ꢁpÃ; Ldg† as explained below.
4.3. Bonding in metal-benzene complexes
The assignments of IR absorption peaks of
bis(benzene) metal complexes, M(benzene)2 (M ˆ
Ti, V, and Cr) have been made based on a series of
total normal coordinate analyses [31]. The analyses
were done in terms of the symmetry coordinates of a
free benzene molecule under an eclipsed structure of
symmetry D6h with planar C6H6 ligands. Assuming
The metal±benzene complexes are formed
through two electronic interactions: the p electrons
of benzene are donated to the vacant metal d
orbitals (donation: Lpg ! dpg), and the electrons
of the metal d orbitals are donated to the unoc-
cupied pà orbitals of benzene (back-donation:
ddg ! Ldg). Both the donation and the back-do-
nation lead to a strong bond between the metal
and benzene and also to weakening of the C±C
bond of the benzene, making the breathing vi-
bration soft. For Ti-, Vi-, and Cr-benzene com-
plexes, the interaction between 3d-metal atom and
D
6h symmetry for bis(benzene)-metal complexes, 69
normal vibrational modes are expected to have
symmetry species,
Rvib ˆ 4A1g ‡ A2g ‡ 2B1g ‡ 4B2g ‡ 5E1g
‡ 6E2g ‡ 2A1u ‡ 4A2u ‡ 4A2u ‡ 4B1u
‡ 2B2u ‡ 6E1u ‡ 6E2u:
ꢁ1†
Among these modes only the A2u and E1u sym-
metry species are IR active due to the selection
rules within the D6h point group: 10 fundamental

282
K. Judai et al. / Chemical Physics Letters 334 (2001) 277±284
order indices for the C±C bond are 1.272, 1.293,
and 1.314 for M ˆ T, V, and Cr, respectively. Thus,
the change in the vibrational frequency of the
breathing mode can be rationalized by the di€er-
ence in the strength of the ddg ! Ldg interaction. If
the dominant interaction is Lpg ! dpg, the above
frequency change should show an inverse behavior.
Therefore, the observed tendency in the frequency
change gives an experimental evidence for the im-
portance of the ddg ! Ldg interaction in the
chemical bond of the M(benzene)2 complexes.
4.4. Soft-landing conditions for organometallic
cluster ions
As mentioned above, the soft-landing of the
size-selected ions is accompanied by energy release
in the two processes: (1) deposition, (2) neutrali-
sation of the cations. In this Letter, the IR spec-
trum of V(benzene)2 clearly shows the cluster ions
can be soft-landed in the processes of the deposi-
tion and the neutralisation. Since both the impact
energy for the deposition (20±50 eV) and the
ionisation energy (5.75 eV) are higher than the
binding energy of the sandwich cluster (2.42±2.55
eV for metal-benzene ring) [32], it is considered
that the excess energies are dissipated by vapori-
sation of the Ar matrix, resulting in the nonde-
structive deposition. Furthermore, although the
charge combination generates the excess energy
corresponding to the ionisation energy, the cold
Ar media seemingly prevents the hot cluster from
dissociating itself.
Fig. 4 shows three IR spectra of V(benzene)2
deposited with di€erent deposition energies of 20,
50, and 100 eV. Below the deposition energy of 50
eV, the IR spectrum is in agreement with that of
V(benzene)2 of the previous work [27], indicating
that the deposited cluster maintain its original
structure. At the impact energy of 100 eV, how-
ever, the IR absorption attributed to V(benzene)2
disappears completely, but instead, the peak as-
signable to m18 mode of a free benzene was ob-
served at 1038 cm 1. The IR absorption of the free
benzene shows that a major fragmentation process
in V(benzene)2 is the bond dissociation between
the metal atom and benzene. Although it is not
clear how the free benzene molecule is trapped into
Fig. 3. Optimized structures and normal modes obtained by
DFT calculations: (a) CH in-plane rocking; (b) breathing of
ring; and (c) CH out-of-plane bending. The calculated vibra-
tional frequencies are systematically shifted to higher values
compared to those measured in the experiment. This ®gure was
drawn with the MacMolPlt program by Bode and Gordon [33].
benzene can be described mainly by the back-do-
nation of ddg ! Ldg through e2 symmetry [24], and
hence the energy change in the atomic 3d level
should a€ect the ddg ! Ldg interaction. The 3d
level becomes deeper in energy in the order of Ti, V,
and Cr, and then it is expected that the ddg ! Ldg
interaction becomes smaller from Ti to Cr. In fact,
the bond order, estimated by Mayer's bond order
index, is consistent with this expectation: the bond

K. Judai et al. / Chemical Physics Letters 334 (2001) 277±284
283
optics, to Prof. T. Kondow, Dr. M. Ichihashi
(Toyota Technological Institute, Japan), and Prof.
T. Tahara (Institute for Molecular Science) for
information on an octapole ion guide, to Dr. M.
Kawai, Prof. J. Yoshinobu, (The Institute of
Physical and Chemical Research) and to Prof. M.
Nakata (Tokyo University of Agriculture and
Technology) for suggestion on IR spectroscopy
and a cold cryostat. This work is supported by a
program entitled ``Research for the Future
(RFTF)'' of the Japan Society for the Promotion
of Science (98P01203) and by a grant-in-aid for
scienti®c research on priority areas from the
Ministry of Education, Science, Sports, and Cul-
ture. K. Judai expresses his gratitude to research
fellowship from the Japan Society for the promo-
tion of Science for Young Scientists.
Fig. 4. Infrared spectra obtained by the size selective deposition
‡
of the Vꢁbenzene† ions with various deposition energies: (a)
2
20 eV; (b) 50 eV; and (c) 100 eV. At 100 eV, the both peaks
assigned as V(benzene)2 disappear, and instead the peak for
1
benzene appears at 1038 cm ꢁm18†.
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