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Formation, Fragmentation and Structures of YO (x=1,2, y=1-13) clusters: Collision Induced Dissociation Experiments and Density Functional Theory Calculations Pavle Glodi#, Claudia Mihesan, Emmanouel Klontzas, and Michalis Velegrakis J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11560 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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The Journal of Physical Chemistry

Formation, Fragmentation and Structures of YxOy+ (x=1,2, y=1-13)

clusters:

Collision Induced Dissociation Experiments and Density Functional Theory Calculations

Pavle Glodić1,2 , Claudia Mihesan1 , Emmanouel Klontzas2, and Michalis Velegrakis1,a) 1

Institute of Electronic Structure and Laser, Foundation for Research and

Technology−Hellas, Heraklion 700 13, Crete, Greece 2

Department of Chemistry, University of Crete, Heraklion 71003 Greece

ABSTRACT: Yttrium oxide cluster cations have been experimentally and theoretically studied. We produced small, oxygen-rich yttrium oxide clusters, Yx O +y (x=1,2 , y=1-13), by mixing the laser produced yttrium plasma with a molecular oxygen jet. Mass spectrometry measurements showed that the most stable clusters are those consisting of one yttrium and an odd number of oxygen atoms of the form YO +2 k +1 (k=0-6). Additionally, we performed collision induced dissociation experiments, which indicated that the loss of pairs of oxygen atoms down to a YO+ core is the preferred fragmentation channel for all clusters investigated. Furthermore, we conduct DFT calculations and we obtained two types of low energy structures: one containing an yttrium cation core and the other composed of YO+ core and O2 ligands, being in agreement with the observed fragmentation pattern. Finally, from the fragmentation studies, total collision cross a) Author to whom correspondence should be addressed. Electronic mail: [email protected], Telephone: (+30) 2810 391122.

1

ACS Paragon Plus Environment


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sections are obtained and these are compared with geometrical cross sections of the calculated structures.

1. INTRODUCTION The research field of transition metal clusters, especially transition metal oxide clusters has exhibited a rapid development in both experiment and theory in the past decades. The clusters have attracted much attention because of wide applications in many areas, such as catalysis, material science, nanotechnology and microelectronics.1-3 Among them, yttrium oxide clusters have been a subject of many theoretical and experimental studies. For example, yttrium doped vanadium oxide cluster cations are investigated in order to demonstrate the local charge effects on methane activation for catalytic purposes.4 Another study has shown that nanoparticles composed of cerium oxide or yttrium oxide could have possible applications in medicine.5 For studies of free clusters (in gas phase), well established experimental procedures - by coupling of ablation laser with a mass spectrometer - have been used,6 providing information on molecular, fragment and cluster ion formation in the ejected plume. Laser ablation of simple oxides ( Y2O3 ) or Y-M-Cu-O metal-composite oxides (M = Ba, Sr, Ca, Mg)6-9 (for example YBa 2 Cu 3O7− x ) in high vacuum led to formation of pure yttrium oxide cluster ions YO(Y2 O 3 ) n + or mixed cluster ions of various sizes and compositions. The laser vaporization experiments of Becker and al.6 have shown Y2O3 and YO to be the building units of generated cluster ions and the authors suggest that

2


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these clusters are products of condensation in gas phase rather than direct emission from the solid. Recently, there have been several studies on electronic properties of small yttrium oxide clusters. Knickelbein probed the photoionization spectra and measured electron vertical ionization potentials (IP) of Yx O clusters (x=2-31).10 Wu and Wang11 performed the photoelectron spectroscopy studies (PES) on YO y (y=1−5) clusters. Pramann et al. − produced YxO y (x=2-10, y=1-3) clusters, presented their PES spectra and measured the

electron affinities.12 + Yttrium oxide clusters of the form YxO y , produced by laser vaporization, have

also been investigated by Reed and Duncan, with time-of-flight (TOF) mass spectrometry and mass-selected photodissociation, providing insight in their fragmentation mechanism (fragmentation channels) and identifying the most stable stoichiometries.13 Moreover, formation mechanisms, structures and relative stability of transition metal oxide clusters, have been studied in our laboratory employing the collision-induced dissociation (CID) method, for mass selected titanium,14 iron15-16 and niobium-oxide clusters cations.17 With regards to the yttrium oxide clusters, the CID method was employed earlier to investigate some smaller species at collision energies 30-110eV and 170 eV by Kahwa et al.18 Their CID spectra have shown higher thermodynamic stability for the clusters with general formula Ya O+(3a −1)/2 , where a is an odd number (e.g. YO + ,

Y3O4+ , Y5O7+ , Y7O10+ ). The progress in the experimental studies has motivated some theoretical calculations of the yttrium-oxide clusters, most of which were focused on monoxides. 3



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Hybrid density functional theory (DFT) calculations were performed to study the structural (geometry) and electronic properties of neutral, anionic and cationic Y3O clusters19 and Y4O clusters.20 Yang and Xiong extended theoretical work on monoxides with their study on structural and electronic properties of Yx O (x=2-14) clusters.21 Theoretical reports on other types of yttrium-oxide clusters, containing more oxygen + atoms, started with the investigation of the relative stabilities and structures of YxO y

cations and their neutrals.13 In another study, DFT calculations were performed to investigate electronic and magnetic properties of Yx O 2 and Yx O−2 (x=1-8) clusters.22 In a recent study, electronic and geometrical structures of neutral and charged YO y (y=2-12) clusters were investigated using various functionals.23 However, up to now, the structure and stability of yttrium-oxide clusters, especially oxygen-rich clusters, have not been investigated sufficiently. In this + contribution we study mass spectra, structure and stability of Yx O y (x=1,2, y=1-13). In

our experimental studies we used an alternative CID method based on crossed molecular beams,15-16 which enables fast measurement of fragmentation processes without the need of mass selection of individual clusters. We also systematically investigated the structures of the prominent series of + clusters with an odd number of oxygen atoms, YO y (y=2k+1, k=0-6), and their

electronic properties and energetics, using density functional theory (DFT) calculations, as previously done for iron oxide clusters.15 Finally, comparison between theoretical and experimental results are performed and discussed. 4


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2. EXPERIMENTAL APPARATUS AND METHODS 2.1 Setup The cluster source and the basic experimental setup have been described in detail elsewhere24-26 and only the general features will be presented here. Yttrium oxide clusters are produced in a first vacuum chamber (source chamber), where a rotating pure yttrium target is placed just in front of an oxygen ( O2 ) pulsed nozzle (diameter 0.5 mm, backing pressure 4 bar). The fundamental (1064 nm) of a Nd:YAG (neodymium-doped yttrium aluminum garnet) laser (Spectra Physics DCR-11) is employed to vaporize the yttrium target. The plasma plume ejected from the target surface during the ablation process expands in vacuum and is crossed perpendicularly, a few millimeters above the target surface, with the pulsed O2 supersonic jet. Yttrium oxide clusters are formed due to reactions of yttrium species present in the plasma plume with O2 molecules. The generated clusters pass through a 4 mm diameter skimmer and enter the two-field acceleration region of a linear time-of-flight (TOF) mass spectrometer. The cluster ions beam is accelerated to the laboratory-frame kinetic energy ELab=z·e·Vacc eV, where z is the charge number of the ion, e is the elementary charge and Vacc is the acceleration potential. In our case, for singly ionized species, ELab=1500 eV. The cluster ions traverse a field-free region of the TOF mass spectrometer, where they are separated in arrival time according to their mass over charge ratio and detected directly in a linear arrangement using a multichannel plate (MCP) detector placed at the end of the flight tube. The TOF spectra produced by every laser shot are acquired and averaged with a 150 MHz

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computer-controlled digital oscilloscope (LeCroy 9410). The recorded signal is obtained by averaging several hundred laser shots and reflects the initial distribution of cluster ions taken directly from the source chamber. The time of flight spectra are transferred through a GPIB interface to a PC. Software developed in our laboratory is used for data acquisition and processing. 2.2 Fragmentation cross section Measurements of the fragmentation cross sections of yttrium oxide clusters were done using the crossed molecular beams method, used in some of our previous studies.1516

Briefly, in this method, we have introduced in the field-free zone of the TOF mass

spectrometer a secondary, neon (Ne) beam expanding from a nozzle (0.2 mm diameter and 3.5 bar backing pressure) with the beam pulse duration of ~600 µs that crosses the primary cluster beam. For CID experiments the mass spectra are recorded with the overlap between the primary cluster beam pulse and the secondary Ne beam pulse. This situation is denoted as “beam on”. In order to calculate the fragmentation cross section it is also necessary to record mass spectra without the overlap between the two beams, denoted as “beam off”, by adding a time delay of 1 ms to the secondary beam nozzle. A retarding field analyzer (RFEA), consisting of two grids, is inserted in front of the MCP detector and is used to selectively filter ions. The first grid is grounded, while the second one (closer to the MCP) is at a variable retarding potential (Vgrid) and repels all ions possessing kinetic energy lower than z·e·Vgrid. The two series of mass spectra, “beam on” and “beam off”, are recorded at various Vgrid in the range from 0 V to a voltage significantly higher than the accelerating voltage (Vgrid ≫ Vacc) and are used to measure total intensities of each particular cluster and its fragments. Assuming single collision 6


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conditions for our experiments, the fragmentation cross section Q can be determined from the equation:

Q=−

1 I ln( ) , NL I 0

(1),

where I and I0 are the transmitted intensities of the ion beam with and without the presence of the secondary beam, respectively, N is the number density of the target gas, and L is the length of the beams crossing area (N and L are not known values, but are the same for all clusters). In order to determine I and I0, all fragments are rejected (by setting Vgrid close but slightly lower than the acceleration energy of the parent ion), and the measured intensity is corrected for the neutrals contribution (measured with Vgrid much higher than the acceleration potential, so that none of the charged species – parent or fragment – reaches the detector).17 Using this experimental configuration, no mass selection of a particular cluster from the cluster beam is necessary and all clusters’ cross sections can be measured in one experiment run. 2.3 DFT calculations In order to further investigate the structure and stability of the YO+2 k +1 (k=0-6) clusters identified in the mass spectra, we have performed theoretical density functional (DFT) calculations with the Gaussian 03W program package,27 to obtain the lowest energy structures, Mulliken population analysis and energetics of these clusters. As previously seen, the B3LYP hybrid exchange-correlation functional28-30 in combination with LanL2DZ basis sets has been proven to be efficient for the prediction of structure and different electronic properties of various yttrium-oxide clusters Y3O ,19

Y4O ,20 Yx O 2 and Yx O−2 .22 In addition, the use of a larger polarized Quadruple-ζ basis set 7



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def2-QZVP31 instead of effective core potential LanL2DZ basis set provides more accurate DFT calculations of bond parameters and frequencies.23 The geometry optimization calculations on YO+2 k +1 (k=0-6) clusters have been performed using B3LYP hybrid functional and general gradient approximation functional BPW91.28,

32

Yttrium atom was treated with def2-QZVP basis set,31 while the full

electron basis set 6-311G(d,p) was used for oxygen. Two growth patterns were considered for determining the ground-state structures and low lying isomers of the clusters. (1) We first optimized YO + and then we continued with the sequential addition of O2 . In order to locate the most stable geometrical configurations during calculations, several initial geometries were attempted for each

YO+2 k +1 cluster by adding one O2 molecule in various positions, in the vicinity of the previously optimized structure of the YO+2 k −1 (k=1-6) cluster. Some of these initial structures were built according to a certain symmetry group, however no symmetry restrictions were considered in the calculations in order to ensure a more thorough exploration of the potential-energy surfaces of the clusters. (2) We also considered the possible structures reported for the same yttrium oxide cluster ions by Venkataramanan.23 For each of the initial structures, we tested different spin multiplicities (S) to find the most stable electronic state. Finally, harmonic vibrational frequencies by numerical differentiation of analytical gradients were computed on the optimized geometries (structures) at the same level of theory in order to characterize the stationary points and obtain zero point energy (ZPE) corrections. No correction factors were applied for the calculation of the frequencies or the ZPE correction. The stability of the obtained 8


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wavefunction for the ground state structures was checked performing single point energy calculations with the same method and criteria, including the keyword Stable in the input file. The optimized geometric structures from the DFT calculations were used to calculate geometrical cross sections of the clusters, which were then compared with the experimentally determined collision cross sections to validate the obtained ground state structures of the YO+2 k +1 (k=1-6) clusters. The geometrical cross sections of the theoretical structures were calculated as the average of structures projections under different orientations using the projection model.16,33 The radii of the atoms were evaluated using the universal potential model at the center of mass collision energy (radius Y+ = 0.85 Å and radius O = 0.60 Å).16, 34-35

3. RESULTS AND DISCUSSION 3.1 Mass spectra + A typical TOF mass spectrum of the Yx O y clusters obtained in our apparatus is

+ shown in Fig. 1. The intensity peaks of Yx O y clusters are labeled as x,y ranging from 1,1

to 3,4. The spectrum reveals the formation of two main cluster series containing one

(YO +y ) and two yttrium atoms (Y2O+y ) . The YO +y series consists of members with an odd number of oxygen atoms while those with an even number of oxygen are absent. The maximum number of oxygen atoms observed for these two series was 13 for YO +y and 6

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for Y2O+y under our experimental conditions. In addition, presence of water impurities in the gas inlet system led to the formation of hydrated clusters (marked by triangles in Fig. 1), with the general formula YO+y (H2O) (y=1, 3, 5, 7). Accordingly to the spectrum in Fig.1, for the series of cluster ions containing an odd number of O atoms, YO+2 k +1 (k=1-6), we assume a formation mechanism by subsequent physisorptions of O2 molecules to the initially formed YO + ion. Clusters containing more than one Y atoms are also formed, but in small amounts, due to the particularity of our open cluster source,16 and the most abundant cluster containing two yttrium atoms is Y2O3+ , where the formal oxidation states of Y in the cluster ion can be represented in combination of +3 and +4. Although in relatively small amounts, a peak at mass 331 amu in our mass spectrum is attributed to the Y3O+4 cluster, as this cluster appears with high stability in the experiments of Reed and Duncan,13 and it is also the most abundant species produced by Kang and Bernstein.36 Also, YO+ and

Y3O4+ are identified as stable species and as preferred dissociation products of larger clusters by other groups.18

10


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1,1 1,9 1,7 Intensity (a.u.)

1,11

1,5

1,3

2,3 2,4

YO(H2O)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2,2

2,5

2,6

1,13

3,4

∆ ∆

100

∆

∆

150

200

250

300

m/z (amu)

+ FIG. 1. Mass spectra of YxO y clusters (labeled as x,y) obtained at Vgrid=0, with beam

crossing (dashed line, red) and without beam crossing (solid line, black). The ∆ indicates the hydrated clusters (see text for details).

3.2 Fragmentation channels We have measured ion intensities versus retarding potential for several YO+n and

Y2O+y clusters using the procedure described in Sec. 2.2. The potential on the retarding electrode Vgrid was increased from 0 to a value significantly higher than the accelerating + voltage, i.e. ~2100 V, and the results are shown in Fig. 2(a)-(d) for YO y (y = 5, 7, 9 and

11



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11) clusters respectively. In the case of collisions (“beam on”), as the Vgrid increases, attenuation of the cluster’s intensity is observed, due to gradual rejection of fragment ions. This happens because fragmentation of a parent ion mP accelerated to laboratory kinetic energy ELab will lead to a fragment mF possessing kinetic energy EF, which is a fraction of the parent’s kinetic energy: EF = (ELab) (mF /mP)

(2)

As Vgrid increases, fragments with EF≤qVgrid are rejected and this appears as a sudden signal intensity decrease. This value of Vgrid is to derive the mass mF from Eq. (2). For + each of the YO y cluster investigated, the only fragmentation channels we could identify

were those corresponding to loss of one or more pairs of O atoms (possibly molecules) and are seen as well-pronounced steps in Fig. 2(a)-(d). As a typical example, there are four possible fragmentation channels in the case of YO9+ (Fig. 2(c)) which lead to the

YO7+ , YO5+ , YO3+ and eventually YO + products.

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YO

+

YO +

YO3

50

relative intensity

80

+

60

Vacc

(a)

+

YO

3

60

(b)

+

YO

5

V

40

acc

40

30 20

20 10 0

0 80

0

500

+

1000

YO + YO 3

relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

1500

2000

0

YO

+

1000

2000

YO

(d)

+ 5

+

YO

7

7

+

YO

V

9

acc

40

1500

+

YO 60

YO

+

3

(c)

+ 5

500

YO

80

V

40

20

acc

20

0

0 0

500

1000

1500

2000

0

Vgrid(volts)

500

1000

1500

2000

Vgrid(volts)

+ FIG. 2. Relative ion intensity of (a) YO5+ , (b) YO7+ , (c) YO9+ and (d) YO11 versus retarding

potential, with beam crossing (circles) and without beam crossing (squares). The arrows indicate the laboratory kinetic energies ELab of the corresponding fragment ions. The kinetic energy distribution of the ions is obtained by taking the negative differential. Similar measurements were performed for clusters containing 2 yttrium atoms. The Y2O2+ fragmentation (results not shown) produces only one fragment ion, YO+ , in agreement 13



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with Ref.

18

. The graphs in Fig. 3(a) and (b) show plots of ion intensities vs. retarding

potential for two representative clusters containing two yttrium atoms: Y2O3+ and Y2O4+ , respectively. From Fig. 3(a), we observe that Y2O3+ releases O to produce Y2O2+ , or undergoes fragmentation by loss of YO2 to produce YO+ . The center of mass collision energy of Y2O3+ is 124 eV, and the main CID product is YO+ , also in agreement with Ref.

18

, where formation of YO+ was identified as the dominant product at collision

energies higher than 50 eV.

FIG. 3.Fragmentation of (a) Y2O3+ and (b) Y2O4+ , with beam crossing (circles) and without beam crossing (squares). The fragmentation channels of the ion Y2O4+ , depicted in Fig. 3(b), are most likely the following two: a) Y2O4+ → Y2O2+ + O2 14


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b) Y2O4+ → YO+ + YO + O2 Although we cannot directly identify the neutral fragments, the products accompanying the formation of YO+ in reaction b) are most likely YO and O2 , as YO3 is not expected + to have a stable structure.13 All fragmentation channels of the Y2Oy clusters and their

fragmentation products are summarized in the Table 1.

+ Table 1. Fragmentation channels of Y2Oy clusters and their fragmentation products

Cluster ion

Fragment ion

Neutral(s)

Y2 O+2

YO + Y2 O+2 YO + Y2 O+2 YO +

YO

Y2 O3+ Y2 O

+ 4

O

YO 2 O2 YO+O

3.3 DFT calculations To gain further insight in the formation and stability of the clusters, we performed DFT calculations on the prominent species in our mass spectra, YO+2 k +1 (k=0-6). For each cluster investigated, we optimized numerous initial geometries and multiplicities in the search for the ground state structure (and multiplicity). The calculation of the total binding Eb energy, which corresponds to the attaching of all O2 units to the YO + ion, YO+ + kO2 = YO2+k +1 , was done according to the following formula:

Eb (YO+2 k +1 ) = Eel (YO+ ) + k ⋅ Eel (O2 ) – Eel (YO2+k +1 ) , 15



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where all the electronic Eel energies were corrected by adding the zero point energy (ZPE) to the calculated values: Eel = Ecalc + E ( ZPE ) . The optimized geometries which correspond to the ground states and low-lying isomers of YO+2 k +1 (k=1-6) clusters, along with their multiplicities, are presented in Fig. 4. The stable structures are designated by ya and yb, where y is the number of oxygen atoms, while labels a and b correspond to the ground and first low-lying states, respectively (according to the total binding energy Eb from high to low). Relative energies (RE), which represent the difference in total binding energy Eb between the energetically lowest structures, are also given in the Fig. 4. The relative stability of the clusters usually correlates with differences in the binding energies of the successive clusters, calculated with the relation:

∆Eb = Eb (YO+2 k +1 ) – Eb (YO2+k –1 ) . In Table 2, we summarize the multiplicities of the most stable and some low-lying cluster isomers, the calculated electronic Eel energies for these configurations, the total Eb binding energies for all O2 ligands, and the binding energy differences ∆Eb of the

YO+2 k +1 clusters.

16


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FIG. 4. The optimized cation ground-state and low-energy structures for the YO2+k +1 (k=1-6) clusters along with bond lengths, multiplicities and relative energies RE. 17



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Table 2. Calculated structures (symmetries), multiplicities, total binding energies Eb, relative energies RE, and binding energy differences ∆Eb of YO+2 k +1 clusters obtained at the DFT/B3LYP level of theory.

Symmetry Eb cluster

group

RE ∆Eb (eV)

multiplicity (eV)

(meV)

(isomer)

YO3+

YO

+ 5

YO7+

YO

+ 9

+ YO11

Cs (3a)

triplet

2.217

0

2.217

Cs

singlet

0.624

1593

-

Cs (5a)

quintet

4.408

0

2.191

Cs (5b)

triplet

4.253

155

-

C3v (7a)

septet

6.577

0

2.169

C1 (7b)

quintet

6.548

29

-

C1

triplet

6.546

31

-

C1 (9a)

triplet

8.699

0

2.122

C1

septet

8.693

6

-

C1

quintet

8.693

6

-

C4v (9b)

nonet

8.676

23

-

Cs (11a)

septet

10.819

0

2.120

C1

triplet

10.807

12

-

C5 (11b)

11-et

10.614

205

-

18


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+ 13

C1

septet

10.590

229

-

C2v (13a)

nonet

12.795

0

1.976

C2v

11-et

12.787

8

-

C1(13b)

13-et

12.457

338

-

C1

11-et

12.030

765

-

YO

The global minimum of YO + is a singlet state, in agreement with Ref.

23

. The

calculated length of the yttrium-oxygen bond is 1.747 Å, which also agrees with the value of 1.745 Å computed at the same B3LYP level in the same study. For YO3+ , a planar geometry with Cs symmetry is predicted, where the oxygen atoms are bound to the yttrium to form one oxo ligand (terminal oxygen atom) and an end-on bonded η1- O2 ligand (superoxo ligand), presented as the structure 3a in Fig. 4. In this study, terminal oxygen atom and terminal Y–O bond will be denoted as Ot and Y–Ot, respectively. The distance between Y and the superoxo ligand is quite long (2.513 Å) compared with the length of the Y–Ot bond (1.756 Å). For this cluster we predicted a triplet ground state. Another stable structure obtained for YO3+ is a singlet (not shown in the Fig. 4), also with Cs symmetry. In this structure, the distance between Y and superoxo group is significantly longer (2.603 Å). However, this isomer structure is higher in energy then the triplet with 1593 meV and based on the large energy difference we rule out this structure. The predicted minimum energy structure 3a is comparable with the observation of YO+ (CO) instead of Y+ (CO2 ) .37

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In the case of YO5+ cluster we obtained two stable structures, 5a and 5b, corresponding to a quintet and a triplet state, respectively, with quite different structure. The quintet state has a nonplanar structure corresponding to Cs symmetry, with one Ot atom and two end-on bonded superoxo ligands coordinated around central yttrium, where the bond lengths are slightly longer than in the case of YO3+ cluster. In the triplet state the oxygen atoms form side-on bonded peroxo (η2-O2) and ozonide (η2-O3) ligands similarly to the structure presented in Ref. 23. The quintet state is energetically lower than the triplet for 155 kcal/mol, and represents the ground state of YO5+ . The ground electronic state of YO7+ is a septet with C3v symmetry (7a), with an Ot atom and three superoxo ligands coordinated around yttrium. All bond lengths are around 0.01 Å longer than in the case of the ground state YO5+ structure (5a), except

O − O bond of the superoxo ligands, whose length remains unchanged (around 1.203 Å), and is very close to the calculated value of 1.207 Å for O2 . The calculated bond length is in a very good agreement with the precise experimental value of 1.2074 Å.38 Additionally, two stable structures were found, 29 and 31 meV higher in total binding energy than the ground state, corresponding to a quintet and a triplet state, respectively, thus representing two degenerate states. These two states are very similar superoxoperoxo ozonide structures, with close geometrical parameters, thus only the quintet state is presented (7b). In the case of the YO9+ we obtained three degenerate states: a triplet, a quintet and a septet, with the triplet state being the most stable, while the later two isomers are ~6

20


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meV above the triplet. All three structures have C1 geometry and very similar values of structural parameters, therefore only ground triplet state is presented (9a). These cluster structures have two superoxo, one peroxo and one ozonide groups connected to the central yttrium. Another stable structure (9b) was obtained, with one oxo and four superoxo ligands, placed approximately in the same equatorial plane normal to the axis of the YO + unit (the axis of the cluster) - the angles between the YO + axis and the coordinated superoxo ligands are around 60°. Although the optimized structure has C1 symmetry, by putting a higher cutoff level below which structural parameters are considered to be equal, the YO9+ cluster can be approximated with a higher C4v symmetry group. + Of several different stable isomeric structures obtained in the case of YO11

cluster, the ground state and the first low-lying isomer have similar geometrical structure and bond parameters, and are found to have septet (11a) and triplet spin multiplicity, respectively. They are predicted to be (nearly) degenerate, with the ground septet state being only 12 meV more stable than the triplet (not presented in the Fig. 4). These two isomers have a geometrical structure similar to the one of the most stable YO9+ isomer (9a), however with one additional superoxo ligand. In addition, there also exists a lowlying isomer structure 11b (205 meV higher than the ground state) with the multiplicity of 11, possessing C5 symmetry with five oxo ligands distributed around the YO + unit. + For the largest cluster theoretically studied, YO13 , we found two degenerate

structures corresponding to the ground state (13a) and the first low-lying state with a multiplicity of 9 and 11, respectively, where the later one is 8 meV higher. The two 21



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structures have four superoxo, one peroxo and one ozonide groups, and posses C2v symmetry. Similarly to the previous clusters, for this cluster size we have also found lowlying structures with a YO + unit – with a multiplicity of 13 (338 meV above the ground state) and 11 (765 meV above the ground state), respectively. These two isomers were found to have C1 symmetry with one oxo, five superoxo ligands in pentagonal arrangement and one additional superoxo ligand placed approximately on the same axis with the YO + unit. These two isomers have very similar structures (13b), with the bond lengths differing by less than 0.01 Å. Of the six bonds between yttrium and superoxo ligands, the five in the “equatorial” plane are around 2.630 Å, whereas the on-axis bond is quite elongated (2.925 Å). From the Fig. 4. we observe a series of cluster isomers with n≥3, possessing a shorter Y–Ot bond, and significantly longer bonds between central yttrium and the weakly coordinated superoxo ligands. Here the optimized Y–Ot bond distance increases with the cluster size, between 1.747 Å in the case of YO + and 1.854 Å for the largest + YO13 . For the same structures, the bonds between yttrium and the superoxo ligands range

+ from 2.513 Å in the case of YO3+ to 2.731 for YO13 . These structures strongly imply the

existence of a chemically bound YO+ cluster core whose formation can be explained by the fact that the initial phase in the growth of these clusters, the reaction Y + + O 2 → YO + + O , is exothermic.39 The preference for formation of clusters with an

odd number of O atoms can also be explained by the stability of YO + core, where yttrium takes its stable +3 oxidation state13 corresponding to the bulk yttrium oxide, Y2O3 .40-41 22


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We observe another series of cluster isomers without the YO+ core, starting from the size y≥5, containing superoxo, peroxo and ozonide oxygen. All the bonds connecting yttrium and these ligands show slight increase in the bond lengths with the cluster size. The bond lengths between yttrium and superoxo ligands range between 2.487 and 2.591 Å, the bond lengths between yttrium and peroxo ligands are between 2.165 and 2.213 Å, while the bonds between yttrium and ozonide ligands vary from 2.167 and 2.363 Å. In Fig. 5. we plot the binding energy difference ∆Eb of the ground energy state (ground state structures of) YO+2 k +1 (k=1-6) cluster ions, corresponding to the reactions + YO+2k –1 + O2 → YO2+k +1 , leading to the formation of YO13 .

2.25

YO + + O 2 → YO3+ YO5+

2.20

YO7+ 2.15 ∆ Eb (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


YO9+

+ YO11

2.10

2.05

2.00

+ YO13

1.95 0

1

2 3 4 number of O2 lignads

23


5

6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FIG. 5.Variation of the binding energy difference ∆Eb of the YO+2 k +1 , (k=1-6) cluster ions with the number of O2 ligands.

Fig. 5. shows that as the number of O2 attached to the YO+ core increases, the sequential reactions gradually become less energetic favorable. The sequential bonding energy + + , and then it shows a sudden decrease for YO13 . shows only a slight decrease until YO11

As a consequence, the formation of clusters with more than five O2 ligands becomes thermodynamically less favored. These results are in agreement with our experimental results for the cluster abundances shown in the mass spectrum (Fig. 1), where the a series of relatively intense clusters containing up to five O2 ligands is followed by a much + lower YO13 peak. This observation is similar to the case of iron oxide clusters, where

attaching up to five O2 molecules to a Fe cation is a thermodynamically favored process, while adding one more O2 is endothermic, which resulted in dominant production of

Fe(O2 )+n (n=1-5) and much weaker peaks for clusters with more than ten O atoms.15 In contrast to the theoretical results of Venkataramanan,23 which indicate that there are no thermodynamically stable structures for clusters with n>8, our experimental + + results have shown formation of significant amounts of YO9+ , YO11 and YO13 clusters.

Our calculations showed that for each YO2k+1 (k=0-6) cluster there are two types of structures, close in energy. Based on the current state of the calculations, we are not able to rule out any of them; probably a mixture exists in the cluster beam since the cluster

24


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formation starts at relatively high temperature (≥ 300 K) in the gas-plasma mixing region. Therefore more calculations are needed in order to gain information on these aspects.

3.4 Collision cross section measurements For the experimental determination of the fragmentation cross section we employ the procedure described in Sec. 2.2 as it is outlined in detail in Ref. 17. Eq. (1) can be rewritten in the following way:

(3), where ION and IOFF are signal intensities at appropriate voltages with (ON) and without (OFF) the secondary beam. Using Eq. (3), the quantity QNL, which we call relative fragmentation cross section is determined from the graphs in Fig. 2(a)-(d) for the individual YO +y clusters. The results corresponding to experiments performed at the acceleration voltage 1500 V (laboratory kinetic energy =1500 eV) as a function of the cluster size, i.e. number of oxygen atoms y are displayed in Fig 6. Measurements were also performed at acceleration voltage of 1000 V, and these results are also plotted in Fig. 6. The cross sections show similar trends, within the experimental errors (estimated at + around 20 %). The cross-sections for YO y +Ne collisions increase with y, as seen in Fig.

6. The relative minimum observed for y=9 can be explained by the special stability of this cluster, as noticed also in the mass spectrum in Fig. 1, where it is seen as the most prominent cluster. A similar observation was made for the iron oxide cluster containing 10 Oxygen atoms.15

25



+ The center of mass frame collision energy for the smallest YO y cluster

investigated, YO3+ is 193 eV and decreases with the number of O down to 106.1 eV in + the case of YO11 , while for comparison, dissociation energy of YO is ~7.4 eV.23

Therefore, in collisions with the target gas, for every cluster of the series the energy available for conversion to internal energy is much higher than the dissociation threshold and therefore it is expected to cause efficient fragmentation. In the situation of high energy collisions, where all collisions lead to dissociation, the measured fragmentation cross sections can be considered as the lower limit of the total collision cross section. Since, the present experiments are performed under relative large center of mass collision energy; the collision cross section can be approximated with the orientationaly averaged geometrical cross section, which reflects directly the geometrical structure of the involved species.

1.8 1.6 1.4 1.2

QNL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0 0.8 0.6

1500V 1000V

0.4 0.2 3

5

7

9

11

13

number of oxygen atoms, n

26


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+ FIG. 6. Comparison of relative fragmentation cross sections of YO y clusters, obtained

using different acceleration voltages Vacc, as a function of number of oxygen atoms, y. Based on these considerations a validation for the theoretical geometrical structures from Section 2.3, is the comparison of their geometrical cross sections with the experimentally determined collision cross sections. In Fig. 7 they are shown as a function of the number of oxygen atoms, y. The experimental values are normalized and displayed on the same figure for comparison.

16

Collision Cross Sections Geometrical Cross Sections

14

12 2

2 QQ[Å [A ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

8

6

4 3

5

7

9

11

13

number of oxygen atoms, y

FIG. 7. Comparison between the experimental (squares) and theoretical (circles) cross sections at 1500 eV laboratory energy. The experimental values have been normalized to the theoretical ones. The general evolution trend is similar for the two sets of data and a good correspondence exists between the experimental and theoretical values. While the theoretical values show an almost linear trend, a change of slope is observed at YO9 for the experimentally 27



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Page 28 of 35

determined cross sections. This discrepancy can be due to the fact that we compare the measured fragmentation cross section with the theoretically calculated geometrical one. While generally the two values are very similar, in the case of clusters with special stability, the fragmentation cross section can be smaller. This was also observed in the cases of FeO10 and NbO10).16, 17

4. CONCLUSIONS We produced oxygen rich yttrium oxide clusters by mixing the laser-ablated yttrium plume with a pure oxygen molecular jet. Mostly clusters containing one metal atom are formed, with reduced quantities of clusters containing more Y atoms. Contrary +

+

to other transition metal oxide clusters produced in our laboratory ( FeO y , NbO y ), the

YO+y clusters have odd values of y, due to the negative standard enthalpy of formation of YO, while for FeO and NbO (TiO, TaO, VO) is positive.42 Additionally to the TOF mass + spectrometry, the YO y clusters have been studied by CID and their relative collision

cross sections were measured. We used an experimental configuration developed in our laboratory, based on the interaction of the ionic clusters beam with an atomic beam of noble gas and the subsequent separation of the reaction products (fragments) using an energy filter. In this way, we identified the fragmentation channels of the clusters after collision with noble gas atoms as the successively loss of oxygen molecules (or pairs of oxygen atoms) down to a YO+ core. Therefore, based on the fragmentation patterns of

28


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the clusters, we propose that the clusters are formed via sequential attaching of O2 molecules to the initially formed YO + cluster core. Furthermore, DFT calculations were performed in order to identify the clusters’ structures. Starting from YO5+ , two different series of structures are observed. In the case of the first series of cluster isomers, O2 molecules bind to the YO + core, to form end-on bonded superoxo ligands. In the second series of cluster isomers, the oxygen is in superoxo, peroxo and ozonide states. Although the energy difference between the isomers of the same size is relatively small, there might exist a high energy barrier for the isomerisation reaction, which leaves most of the cluster in one of the isomer forms. Further theoretical work is needed in order to calculate different configurations and to explore the configuration space of these systems.

ACKNOWLEDGEMENTS PG acknowledges support from ITSSUED/ERCO3 grant funded from Greece and EU under NSRF2007-2013. CM acknowledges support from the BIOSYS research project, Action KRIPIS, project no. MIS-448301, funded by the GSRT, Ministry of Education, Greece and the European Regional Development Fund (Sectoral Operational Programme:

Competitiveness

and

Entrepreneurship,

NSRF 2007-

2013)/European Commission. PG also acknowledges Dimitris Zaouris for very fruitful discussions on theoretical calculations.

29



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For Table of Contents Only

35


Formation, Fragmentation, and Structures of Y - American

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