A new hypermetallic molecule LaOMn generated by laser ablation

Article abstract of DOI:10.1016/S0009-2614(98)01383-9

A new hypermetallic oxide LaOMn and its positive ion involving two heterometal atoms were observed and identified in the 532 nm laser ablation of a La_0.67Ca_0.33MnO₃ target using both time-resolved quadrupole mass spectrometric and time-of-flight mass spectrometric techniques. The dependence of LaOMn⁺ yield on the laser fluence also confirmed the formation of the ionic hypermetallic species. Theoretical calculations were carried out to predict the stability and the geometric structures of these new molecules. The calculations suggest that the LaOMn and LaOMn⁺ molecules might be formed via secondary reactions of the neutral and ionic MnO with La or La⁺ in the laser ablated plasma.

Full text of DOI:10.1016/S0009-2614(98)01383-9

12 February 1999
Ž
.
Chemical Physics Letters 300 1999 739–744
A new hypermetallic molecule LaOMn generated by laser
ablation
)
Xue-feng Wang, Hai-jun Dang, Zhen-ning Gu, Qi-zong Qin
Laser Chemistry Institute, Fudan UniÕersity, Shanghai 200433, China
Received 7 May 1998; in final form 5 November 1998
Abstract
A new hypermetallic oxide LaOMn and its positive ion involving two heterometal atoms were observed and identified in
the 532 nm laser ablation of a La0.67Ca0.33MnO3 target using both time-resolved quadrupole mass spectrometric and
time-of-flight mass spectrometric techniques. The dependence of LaOMnq yield on the laser fluence also confirmed the
formation of the ionic hypermetallic species. Theoretical calculations were carried out to predict the stability and the
geometric structures of these new molecules. The calculations suggest that the LaOMn and LaOMnq molecules might be
formed via secondary reactions of the neutral and ionic MnO with La or Laq in the laser ablated plasma. q 1999 Elsevier
Science B.V. All rights reserved.
w
x Ž .
1. Introduction
6,8–12,16 , and the time-of-flight TOF mass spec-
trometry and Knudsen-effusion mass spectrometry
were employed to identify these molecules. In the
theoretical studies, ab initio calculations have proven
to be a powerful method to predict the structure,
stability and other physical properties of these new
The investigations of the hypermetallic molecules
such as AnO, AnS, AnC and AnCN, with metal
atom, AsLi, Na, K, Mg, or Al, have attracted
considerable attention since the early 1980s 1–8 .
Many hyperlithiated molecules, including closed-
shell Li6C molecule and open-shell radicals, such as
Li3O, Li3S and Li4 P, have been studied experimen-
tally by Kudo and his coworkers 8–12 . Previous
studies provided evidence that the main character-
istics of these molecules are their unusual stoichiom-
etries and unexpected hypervalent bonding with ther-
w
x
w
x
molecules 8,17 .
Recently, a Glasgow group reported the observa-
tion of a hypermetallic molecule K2CN generated by
w
x
Ž
.
266 nm pulsed laser ablation of solid K3Fe CN ,
6
both the K2CNq ions and the neutral K2CN
molecules have been detected by a reflection TOF
w
x
mass spectrometer 18 . They argued that these hy-
permetallic molecules have many features of struc-
tures similar to those of Li2CN investigated by Kudo
et al. 8 . In addition, they reported the readily
generation of a new molecule Si2 N by laser ablation
w
x
modynamic stability 13,14 . The hypermetallic
molecules have been generated by laser ablation 15
and high-temperature reactions in a Knudsen cell
w
x
w x
)
w
x
of Si3N4 target 15 . Although many hypermetallic
molecules have been investigated experimentally and
Corresponding author. Fax: q86 21 6510 2777; e-mail:
qzqin@fudan.ac.cn
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 98 01383-9

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740
X.-f. Wang et al.rChemical Physics Letters 300 1999 739–744
theoretically, there is no report so far on a novel
molecule which consists of two heterometal atoms
and one electronegative atom. Recently we have
studied the mass and velocity distributions of the
laser ablated species from a colossal magnetoresis-
with a 10 MHz transient recorder interfaced to a PC
computer. The signal intensities or relative yields of
the ablated species were obtained by integrating the
corresponding TOF spectra.
A laser ablationrionization linear TOF-MS, which
consists of an ablation chamber and a flight tube of
1.3 m length, was employed to detect the ablated
positive ions. The TOF-MS system was pumped by
two turbomolecular pumps to a base pressure of
10y5 Pa. The 532 nm laser beam was incident along
the surface normal and focused onto the sample
surface. The ablated ions passed through an orifice
leading to the extraction field, where they were
extracted perpendicularly by a pulsed high voltage
Ž
.
w x
tant La0.67Ca0.33MnO3 La–Ca–Mn–O target 19 .
Novel molecules LaOMn and Mn2O as well as their
positive ions were observed by time-resolved
quadrupole mass spectrometry.
In this Letter we report the experimental observa-
tion of the existence of the LaOMn and LaOMnq
molecules involving two hetero-metal atoms. Both
Ž
.
time-resolved quadrupole mass spectrometric QMS
Ž
.
and TOF mass spectrometric TOF-MS techniques
were employed to identify these new hypermetallic
molecules. The structure and stability of LaOMn
molecule and LaOMnq ion were predicted theoreti-
cally, and their possible formation pathways were
also discussed.
Ž
.
1800 V . After drifting over the flight tube, the ions
were detected by a dual microchannel plate detector
with a gain of 107, and the ion signals were trans-
ferred to a 100 MHz transient recorder for measuring
the TOF mass spectra. The time delay between the
ablation laser pulse and high-voltage extraction pulse
was controlled by a pulse delay generator. Data
acquisition and storage were performed with a PC
computer.
2. Experimental
The La–Ca–Mn–O target pellet was prepared by
uniaxially compressing a mixture of MnO2 , La2O3,
The apparatus used for laser ablation and QMS
w
x
Ž
.
detection has been described previously 20 . Briefly,
a stainless steel ablation chamber was evacuated
down to a pressure of 10y4 Pa, and a rotatable target
holder was situated at the center of the chamber. A
and CaO AR with a nominal composition of
La0.67Ca0.33MnO3 and calcinated at 8008C for 5 h.
Ž
quadrupole mass spectrometer QMS, ULVAC
3. Results and discussion
.
MSQ-400 was housed in a detection chamber which
was sitting in the ablation chamber and was evacu-
ated to 10y8 Pa by two ion pumps. The distance
between the ionizer of QMS and the target sample
surface is 18 cm. The laser beam at 532 nm was
provided by the second harmonic frequencies of a
The typical mass spectra of ionic and neutral
ablated species generated from 532 nm laser ablation
of a La–Ca–Mn–O target at a fluence of 1.5 Jrcm2
are shown in Fig. 1. It can be seen that both ionic
and neutral LaOMn at mres210 are observed. It is
interesting that another hypermetallic molecule
Mn2O at mres126 is also observed. Since the
natural isotopic abundance of 55 Mn, 139 La and 16O
for these elements are 100%, 99.91% and 99.76%,
respectively, it is reasonable to suggest that the mass
peaks at mres210 and mres126 should be as-
signed to the hypermetallic molecules LaOMn and
Mn2O, respectively. It should be noted that another
possible ablated species with mres210 is CaLaO2 ,
but it seems very difficult for a tetra-atomic molecule
to be formed under our experimental conditions. To
Ž
Q-switched Nd:YAG laser Spectra-Physics GCR-
.
190 and focused onto the sample surface with an
angle of 458 to the surface normal. The laser was
operated with a pulse width of 6 ns and a repetition
rate of 10 Hz. The laser intensity was measured by a
powermeter with a pyroelectric detector. The mass
distributions of the neutral and ionic ablated species
were determined with the QMS. For detecting the
genuine ionic species, the ionizer of QMS was
switched off, and both ablated ionic and neutral
species were detected with the ionizer switched on.
The TOF spectra of ablated species were recorded

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X.-f. Wang et al.rChemical Physics Letters 300 1999 739–744
741
Ž
.
Ž
.
Fig. 1. The mass distributions of ionic solid and neutral shadow laser ablated species from a La–Ca–Mn–O target at 532 nm. Insert:
Ž . Ž .
TOF spectra of mres210 species measured with ionizer off 1 and on 2 , respectively.
exclude the possibility of the CaLaO2 formation, a
similar ablation experiment was carried out using a
which are close to those for other ablated species
such as MnOq and MnO 19 .
w
x
Ž
.
CaOrLa2O3 1:1 mole ratio target and no signal
was detected at mres210. As shown in Fig. 1, the
relative signal intensities of positive ions LaOMnq
and Mn2Oq are higher than those of neutral
molecules LaOMn and Mn2O. But it is difficult to
expect that the neutral LaOMn and Mn2O molecules
are less stable than the ionic species because the
neutral ablated species are detected with the ionizer
of QMS switched on, and the LaOMn and Mn2O
molecules might be cracked by 70 eV electron im-
pact although there are no information on the crack-
ing cross section of these new molecules.
The laser ablationrionization TOF-MS technique
was also employed to identify the hypermetallic
oxide LaOMnq generated from the ablation of a
La–Ca–Mn–O target. Fig. 2 shows a TOF mass
spectrum of ablated ions at 532 nm with a fluence of
0.6 Jrcm2. It can be seen that besides the metallic
ions Caq, Mnq and Laq, ionic species LaOq and
The TOF spectra for both ionic and neutral
LaOMn molecules were measured by a time-re-
solved QMS with the ionizer switched off and on,
respectively. As shown in the insert of Fig. 1, the
TOF spectrum of ionic LaOMnq curve 1 shows
Ž
.
only one component peaked at 45 ms, while there
Ž
appear two components for neutral LaOMn curve
.
2 , corresponding to a fast one and a slow one. It can
be seen that the peak position of the fast component
is close to that of the genuine ions and due to the
contribution of LaOMnq ions, and the slow and
broad component peaked at ;300 ms might be
contributed by the neutral LaOMn species. From the
TOF spectra, the most probable velocities of the
ionic LaOMnq and neutral LaOMn are estimated to
be 4.0=103 mrs and 0.6=103 mrs, respectively,
Fig. 2. A TOF mass spectrum of ionic ablated species from laser
ablation of a La–Ca–Mn–O target at 532 nm.

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X.-f. Wang et al.rChemical Physics Letters 300 1999 739–744
LaOMnq are observed with comparable signal inten-
sities. The long tails of the peaks for heavy ablated
species such as LaOq and LaOMnq ions suggest
that these ionic species have a broader velocity
distribution and are probably formed via the sec-
ondary reactions between the ablated species in the
laser plasma plume.
function of the laser fluence. It is found that the
measured LaOMnq signal increases linearly with the
increase of the laser fluence up to 0.50 Jrcm2 and
seems to be saturated in higher fluence region. This
result is evident to clarify the existence of the ionic
LaOMnq, and implies that there exists a competing
reaction pathway against the formation of LaOMnq
at higher laser fluence.
In order to predict the stability and the structures
of the LaOMnq and LaOMn molecules, the theoreti-
cal calculations were performed to provide the geo-
metric parameters and structure features of the new
hypermetallic molecules. Geometry of both the ionic
LaOMnq and the neutral LaOMn molecules were
To further confirm the formation of LaOMnq, the
laser fluence effect on the signal intensity of
LaOMnq was examined. Fig. 3a gives mres210
signals measured at different laser fluence. It can be
seen that the signal intensity of LaOMnq is en-
hanced by increasing the laser fluence. The ablation
yield of LaOMnq is obtained by integrating the peak
of mres210 in the TOF mass spectrum, and Fig.
3b presents the ablation yield of LaOMnq as a
Ž
.
optimized at MP2 FU rLANL2DZ to find the global
minima using GAUSSIAN94 package of computer
w
x
codes 21,22 . Electron correlation was considered
via Møller–Plesset perturbation theory to the second
Ž
.
order MP2 . The D95 basis set built in the program
was employed for oxygen atoms and LANL2DZ
basis set with an Ar-like effective core potential
Ž
. w
x
ECP 21–24 was used for La and Mn atoms.
Generally, the Mn and La atom basis sets are de-
scribed with an explicit 7 and 3 electrons valence
shell, respectively, with the chemically less active
core electrons pictured with effective potentials. Due
to the degeneracy of the ground state and low-lying
excited states for the investigated systems, the virtual
orbitals were allowed to mix into the occupied space
Ž
.
in the self-consistent field SCF using fractional
occupations of orbitals close to the Fermi level.
Fundamental frequencies were obtained with the cal-
culated complete quadratic force constant matrices at
the optimized structures. The MP2 equilibrium ge-
ometries were used to evaluate electron correlation
by Møller–Plesset perturbation theory to full fourth
Ž
.
order MP4SDTQ with the same basis sets.
Geometric parameters and vibrational frequencies
of the LaOMnq and LaOMn molecules are listed in
Table 1. Since the quartet of OMnO and sextet of
MnO were found to be the most stable states in ESR
w
x
and IR measurements 25–27 , it is reasonable to
infer that the most stable states of LaOMn and
LaOMnq are higher multiplicity. Our calculation
result shows that the quintuplet state for LaOMn is
found while the sextet state is converged for
LaOMnq. As shown in Table 1, the bond length of
the Mn–O bond is little shorter than that of the
Fig. 3. The measured signal of ionic LaOMnq species mres
Ž
.
Ž .
210 at different laser fluence a , and the plot of the integrated
Ž .
intensity versus laser fluence b .

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X.-f. Wang et al.rChemical Physics Letters 300 1999 739–744
743
Table 1
Geometries, total energies
level
q
Ž
.
E
and unscaled vibrational frequencies for LaOMn, LaOMn and other correlated ablated species at the MP2
a
Ž
.
.
Species
Multiplicity
5
E MP4
Ž
Geometry
Frequency
y1
Ž
cm
.
hartree
b
Ž
.
.
.
.
LaOMn
y209.166652
y209.043777
RLaO s2.19 A
RMnO s1.84 A
As174.68
RLaO s2.12 A
RMnO s1.88 A
As180.08
Rs2.031 A
Rs2.018 A
Rs1.939 A
Rs1.844 A
875.6 489.0
Ž
.
.
271.9 8.5
Ž
184.9 4.2
LaOMnq
6
805.1 659.8
Ž
Ž
.
246.9 2.9
Ž
.
114.6 29.4
Ž
LaO
LaOq
MnO
MnOq
O
2
1
6
7
1
2
1
6
7
y105.930360
y105.796919
y178.131544
y177.942003
y74.754957
y30.8507510
y30.630842
y103.220977
y103.073228
822.4 339.9
Ž
.
.
728.0 12.1
Ž
627.2 53.6
Ž
708.0 103.8
La
Laq
Mn
Mnq
aCalculated using the MP2 optimized geometries and frequencies.
b
y1
Ž
.
.
IR intensity in parentheses km mol
w
x
La–O bond in these molecules. The bond angle of
the ionic triatomic molecule LaOMnq is 1808; how-
ever, the bond angle of the neutral LaOMn is 174.68,
which might be expected as a result of Jahn–Teller
distortion. In addition, the calculated frequency of
LaO was found to be 822.4 cmy1, in agreement to
experimental value of y8.3 eV 30 . Then we calcu-
lated the energy changes D E for different possible
pathways to form hypermetallic molecule LaOMn
and its positive ion based on the calculated total
energies of LaOMn and other species listed in Table
1. The calculated results are summarized in Table 2.
the available experimental value of 811.6 cmy1
.
Energetically, the highly exothermic reactions 1 ,
Ž .
Ž .
Ž .
Accordingly, our theoretical calculations give more
evidence for the existence of the ionic and neutral
hetero-hypermetallic oxides LaOMnq and LaOMn.
The formation of the LaOMn can be simply de-
scribed by the secondary reactions underlying a laser
ablation of a La–Ca–Mn–O target. It is well known
that the ablated metal atoms and ions can easily react
with the O atoms to form binary metal oxides in the
laser plasma plume near the target. In order to
evaluate the accuracy of the calculated energy change
D E values for the hypermetallic oxide formation,
the energy changes for the following processes:
2 and 3 are more favorable to form the LaOMn
and LaOMnq molecules. These secondary reactions
probably have low activation barriers, and the colli-
sional probabilities of the ablated La and MnO
species play an important role for the formation of
these new molecules. Because of the lack of exten-
sive information on the reaction products and inter-
mediates, a detailed examination of the potential
Table 2
Possible reaction pathways and reaction energy changes
Mn 6S qO 1D ™MnO 6 S ,
Reaction
D E
Ž
.
Ž
.
Ž
.
Ž
eV
.
Mnq 7S qO D ™MnOq 7 S ,
1
Ž
.
Ž
.
Ž
.
q Ž1
.
Ž6
S qMnO S ™LaOMn
.
q Ž6
.
Ž .
La
S
y7.658
y9.506
y5.017
y0.416
y0.703
y1.093
1
Ž2
.
q Ž7
.
q Ž6
.
Ž .
La D qMnO
S ™LaOMn
S
2
were calculated and found to be y3.78 and y3.1
eV, respectively, which are close to the experimental
Ž2
.
.
.
Ž6
.
Ž5
S
q Ž6
.
.
Ž .
La D qMnO S ™LaOMn
S
3
Ž6 Ž2 Ž5
.
Ž .
Mn S qLaO S ™LaOMn
4
w
x
values y3.7"0.17 eV 28 and y2.53"0.6 eV
Ž6
q Ž1
.
.
.
Ž .
Mn S qLaO
S ™LaOMn
S
S
5
q Ž7
Mn
.
Ž2
.
q Ž6
Ž .
6
w
x
29 . In addition, the calculated D E for the forma-
tion of LaO is y8.8 eV, also in agreement with the
S qLaO S ™LaOMn

(
)
744
X.-f. Wang et al.rChemical Physics Letters 300 1999 739–744
w x
Ž .
H. Kudo, Nature 355 1992 432.
10 H. Kudo, K. Yokoyama, C.H. Wu, J. Chem. Phys. 101
9
x
enrage surfaces is difficult. Apparently, the reaction
mechanisms for the formation of LaOMn and its ion
are complicated and the details of the reaction path-
ways should be further investigated.
w
Ž
.
1994 4190.
w
w
x
Ž
.
11 H. Kudo, K.F. Zmbov, Chem. Phys. Lett. 87 1991 77.
x
Ž
.
12 C.H. Wu, H. Kudo, H.R. Ihle, J. Chem. Phys. 70 1979
1815.
w
w
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x
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.
.
13 C.J. Marsden, J. Chem. Soc., Chem. Commun. 1989 1356.
x
4. Conclusions
Ž
14 J. Ivanic, C.J. Masden, J. Am. Chem. Soc. 115 1993 7503.
x
15 S.L. Wang, K.W.D. Ledingham, W.J. Jia, R.P. Singhal,
Both time-resolved QMS and TOF-MS techniques
are successfully used to identify the new hetero-hy-
permetallic molecule LaOMn and its positive ion
generated by pulsed laser ablation of a La–Ca–Mn–
Ž
.
Appl. Surf. Sci. 93 1990 205.
w
w
w
w
w
x
Ž
.
16 J.W. Chinn Jr., R.J. Lagow, J. Am. Chem. Soc. 106 1984
3694.
x
17 A.I. Boldyrev, I.L. Shamovsky, P.v.R. Schleyer, J. Am.
Ž
.
Chem. Soc. 114 1992 6469.
O target. The theoretical calculations suggest that the
x
18 S.L. Wang, K.W.D. Ledingham, R.P. Singhal, J. Phys. Chem.
Ž5
.
Ž6
.
q
LaOMn S and LaOMn
S are stable, and their
Ž
.
100 1996 11282.
geometric structures nearly belong to a linear
molecules. Based on the energetic consideration, both
the LaOMn and LaOMnq molecules might be pro-
duced mainly via the secondary reactions in the laser
x
19 H.J. Dang, Z.H. Han, Q.Z. Qin, Science in China A 41
Ž
.
1998 1309.
x
Ž
.
20 Q.Z. Qin, Z.H. Han, H.J. Dang, J. Appl. Phys. A 83 1998
6082.
w
w
x
x
Ž
.
21 P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 1985 270.
Ž2
.
Ž6
D qMnO S ™
.
ablated plasma such as La
22 M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G.
Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A.
Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-
Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J.
Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challa-
combe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L.
Andres, E.S. Replogle, R. Gomoerts, R.L. Martin, D.J. Fox,
J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-
Gordon, C. Gonzalez, J.A. Pople, GAUSSIAN94, Revision
B2, Gaussian, Pittsburgh, PA, 1995.
Ž5 Ž2
LaOMn S .
.
.
.
q Ž7
.
D q MnO
LaOMn
S
and La
S ™
qŽ6
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