The Structure of Gas Phase Tin Oxide Ions Generated by Laser Ablation: A Combined Fourier Transform Mass Spectrometry and Density Functional Theory Study

165

1040-7278/02/0600-0165/0 © 2002 Plenum Publishing Corporation

Journal of Cluster Science, Vol. 13, No. 2, June 2002 (© 2002)

The Structure of Gas Phase Tin Oxide Ions Generatedby Laser Ablation: A Combined Fourier TransformMass Spectrometry and Density FunctionalTheory Study

Phillip Jackson,1 , 2 Keith J. Fisher,3 Ian G. Dance,3 Gerard E. Gadd,4

1 Mass Spectrometry, Research School of Chemistry, Australian National University, Canberra,ACT 0200, Australia.2 Author to whom correspondence should be addressed; e-mail: [email protected] School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia.4 Physics, Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, NSW

2234, Australia.

and Gary D. Willett3

Received August 21, 2001; accepted January 15, 2002

The radical ion series (SnO)+2-6, (SnO)−2-6, (SnO)0-5Sn+ and (SnO)1-6O− have beengenerated by the high power laser ablation of SnO and SnO2 targets positionedinside an ICR cell. In all ablation spectra obtained, and for any particular size Snxcore, the tin-rich clusters (SnO)xSn+were more abundant than the correspondingoxygen-equivalent clusters (SnO)+x , while the oxygen-rich clusters (SnO)xO−werealways more abundant than the oxygen-equivalent clusters (SnO)−x . High yieldsof the ions (SnO)1,3Sn+, (SnO)3,6O− and (SnO)−6 suggest high stabilities for thesespecies. Low energy CID studies revealed that loss of neutral (SnO)x units is thepreferred, and for most ions investigated the exclusive, dissociation pathway.Global minima for the smaller cations and anions are proposed on the basis oflocal density functional theory (DFT) calculations. Calculated dissociationenergies for the neutral and charged clusters were found to compare well witheffusion cell and FTICR results. DFT also predicts that, for any cluster with thesame size Snx core, IE(SnO)x < IE(SnO)xSn and EA(SnO)xO > EA(SnO)x. A cor-relation between ion abundances and DFT heats of formation is evident, and theground state geometries provide insight into the evolution of structural versus sizetrends. Without assistance from the calculations, erroneous conclusions regardingthe structures of the experimentally-sampled clusters might have been drawn fromthe low energy CID results.

KEY WORDS: Tin oxide; mass spectrometry; clusters; structure; FTMS.

INTRODUCTION

Tin (IV) oxide is a well characterised n-type semiconductor. Doped thinfilms are utilised for their gas-sensing characteristics and opto-electronicproperties [1] and have been the subject of numerous experimental inves-tigations [2–5]. It has been proposed that defect sites and the formation ofnon-stoichiometric phases in thin films gives rise to the low resistivity thatis desirable for such applications [1].

By directing high-intensity 1064 nm laser radiation onto tin oxidetargets positioned within the FTICR-cell, it was possible to generate mono-charged cationic and anionic molecular clusters (SnO)0-5Sn+, (SnO)1-6O−

and (SnO)±2-6 which can be considered model microsurfaces due to theirhigh surface-to-bulk atom ratio. This method has been used in the past togenerate a number of interesting clusters from different solid precursors[6–11], and overcomes the requirement of an external high-pressure clustersource. In many cases, cationic and anionic cluster series can be generatedfrom the same precursor; reactivity studies with a carefully selected reagentcan then be used to assess charge, ligand or structural effects, with theultimate goal of predicting neutral cluster reactivity. An inspection of thepublished literature for anionic clusters suggests that they are either diffi-cult to generate experimentally, or their reactivity is implicitly predictable[12, 13].

Some of the ions reported in this article have been detected previouslyusing MS. For instance, the tin rich cation series (SnO)0-3Sn+ have beengenerated by the laser ablation of SnO2 powder using the second harmonicmode of a Nd:YAG laser [14]. The smaller oxygen rich anions (SnO)1,2O−

have also been detected by the same group [15]. Bombarding SnO andSnO2 powders with 8 keV Kr atoms yielded the ions (SnO)0-2Sn+, SnO+ andSnOH+ [2]. Relative intensities inferred decreasing molecular ion stabilityin the order (SnO)Sn+> (SnO)+2 > SnOH+> SnO+> (SnO)2Sn+. The clus-ters (SnO)+1-4 were first detected in the vapour above an Sn–SnO2 melt usinga Knudsen effusion cluster source and electron ionisation-MS [16].Using the third law method, thermochemical results for the oligerimisationreactions:

(SnO)x−1+SnO Q (SnO)x (1)

were derived.In related studies published by this group, laser ablation (LA) and

FTICR-MS has been used to generate and study the naked metal clustersGe−1-7 and Sn−1-6 from the respective solid metal precursors [17, 18]. The

166 Jackson et al.

cationic analogues Ge+1-7 were also detected, however, only Sn+ was gener-ated under the same conditions [18]. This could be due to a number offactors, including low cation cluster binding energies for Sn+2-7, a concen-tration effect ([Sn+] ± [Sn+2-7]) or a plasma temperature effect, which isinherently more difficult to ascertain. Density functional theory (DFT)ionisation energies for M+

2-5, M=Ge–Sn, together with experimental neutralcluster binding energies derived from effusion-MS results [19, 20], were laterused to show that the binding energies of Sn+3-5 are significantly lowerthan the values for the corresponding Ge-analogues, for example BE(Ge+3 )=BE(Sn+3 )+2.1 eV, and BE(Ge+5 )=BE(Sn+5 )+4.0 eV [21]. Overall, theDFT cluster geometries and binding energies (magnitude, size variation)were in good agreement with values obtained from experimental and moresophisticated ab initio studies [19, 20]. It is reasonable to suggest then, thatrelativistic effects (spin-orbit, inert pair) are not critical for an understand-ing of size-related trends for fourth row main group clusters, and eitherlocal or nonlocal DFT, with a double zeta basis set, will be sufficient for astudy of laser-generated tin oxide clusters.

EXPERIMENTAL METHOD

The mass spectra were obtained using a Spectrospin CMS-47 FTICR-mass spectrometer equipped with a 4.7 T superconducting magnet and anAspect 3000 computer with 24-bit memory. The instrument has beendiscussed in detail previously [7]. The powdered samples of tin(II)- andtin(IV)-oxides were purchased from The British Drug Houses (Broom Rd.,Poole, BH12 4NN, UK) and used without further purification. Thepowders were pressed into small disks inside the cavity of a stainless steelprobe tip. The stainless steel satellite, upon which the sample disk wasmounted, was positioned flush in the centre of the trapping plate furthestfrom the laser using a magnetic-propelled insertion arm (cylindrical ICRcell, h=dia.=60 mm, geometry factor sg=0.814).

The ablation was performed using a Spectra Physics Nd:YAG laseroperating at 1064 nm and focused to a spot size of 0.3 mm diameter. Thepower density was manipulated using neutral density filters and/or bychanging the laser pulse width from Q-switch (8 ns) to long pulse (230 ms).Typical ICR background pressures of 10−9–10−10 mbar were maintainedusing a 330 L/s turbo molecular pump backed by a 3-phase rotary pump.The trapping plate potential was set to ± 4 V.

The cloud of sputtered material formed post-ablation contains pre-dominantly neutral species (which are subsequently pumped away) plussome ions and electrons. The background pressure in the FTICR cell was

Structure of Gas Phase Tin Oxide Ions Generated by Laser Ablation 167

observed to increase up to 3 orders of magnitude (to 10−6 mbar) immedi-ately after a laser pulse. It is assumed that any ions formed in this plumemust undergo numerous collisions with neutrals prior to axialisation. If theFTICR-MS is operated in the negative ion mode, the electrons which resultfrom multi-photon ionisation of the sample are also trapped by the ICRfields, and become available for resonant capture by neutrals with positiveelectron affinities. Both broadband and narrowband spectra were recorded;the former were acquired for ion-molecule studies and abundance deter-minations, whereas the latter were acquired for ion identification from theisotopomer distribution.

Collision induced dissociation (CID) gases were leaked in through aheated inlet system to pressures of approximately 1.0 × 10−7 mbar. Ne, N2and Ar gases were used in these studies. Lower centre-of-mass collisionenergies were achieved with the former gases. To calculate the collisionenergy in the centre-of-mass reference frame, the quantity of energyimparted to the ion by resonant rf-excitation must first be calculated. Therelevant equations are discussed in the articles by Wilkins and Freiser[22, 23] and the recent review by Marshall [24].

COMPUTATIONAL METHOD

The highly numerical approach to DFT, as implemented in theprograms ‘‘Dmol versions 2.3.2 for Unix workstations and 096 for super-computers,’’ is discussed in the articles by Delley and Ellis and the booksection by Delley [25, 26].

Geometry optimisations were performed using the local spin densityexchange-correlation approximation [27, 28] until the rms gradient wasless than 10−3. All minima were located using a quasi Newton–Raphsonenergy searching procedure. Low energy structures were investigated forimaginary vibrational frequencies using central (4-point) differencing. Thenumerical basis sets that were used are equivalent in quality to analyticalsplit-valence 6-31G* basis sets [29–33], and include 5d and 3d polarisationfunctions for Sn and O respectively. The extra-fine grid density option wasused at all times for the exchange-correlation energy quadrature.

For smaller neutrals and ions, spin states of several candidate geome-tries were investigated in order to map the potential energy hypersurface.The study of larger clusters such as (SnO)5O were then restricted to themost probable geometries on the basis of these results. It is acknowledgedthat the accuracy of the predicted ionisation energies (IE) and electronaffinities (EA) at this level of theory is not clear; however previous studies[17, 18] generally indicate that the local-DFT values improve as the clus-ters become larger and the electron density more homogeneous, at which

168 Jackson et al.

point implementation of gradient corrections to the exchange and correla-tion energy is less critical. The cluster IE values are not expected to be assensitive to the use of a small basis sets as the EA values. Such problemsshould be most conspicuous for the smaller clusters, and can be evaluatedthrough a comparison of DFT and experimental and theoretical results[16, 34, 35]. All absolute reaction energies were derived from zero-pointcorrected molecular binding energies, unless stated otherwise.

The calculations were performed on a Silicon Graphics Indigo work-station and a Fujitsu VP-2200 system housed at the Lucas Heights ResearchFacility (ANSTO), Sydney.

RESULTS AND DISCUSSION

Laser Ablation of SnO and SnO2 Precursors

High power density ( > 500 MW · cm−2) laser ablation FTICR-massspectra obtained from SnO and SnO2 samples are presented in Fig. 1. Thedominant ion series, that is, (SnO)0-5Sn+ and (SnO)1-6O−, together with theoxygen-equivalent series (SnO)±2-6, can be produced by ablating either solidSnO or SnO2 at power densities in excess of 1 GW · cm−2; however, greatercluster yields for both cation series ((SnO)xSn+ and (SnO)+x ) were obtainedfrom solid SnO, and greater cluster yields for both anion series ((SnO)xO−

and (SnO)−x ) were obtained from solid SnO2. Reducing the incident laserpower to 1–100 MW · cm−2 produces only the oxygen/tin equivalent series(SnO)±x from either precursor, together with Sn+ (solid SnO, strong; solidSnO2, moderate), Sn− (solid SnO, moderate), and (SnO)O− (solid SnO2,moderate). Further reduction (100–500 kW · cm−2) enhances the yields of(SnO)±2,3 from both solids, while increasing the power density to greaterthan 500 MW · cm−2 favours the formation of larger species. The observa-tion of weak signals due to the SnO2 ablation products (SnO)3-5O

−2 at high

power densities suggests dioxygen is a component of SnO2 plumes. Clearly,the precursor oxygen content and the incident laser power (and possiblywavelength) influence the relative cluster abundances. Moreover, evidencesuggests that trapped atomic ions, such as Sn+ and O−, may play some rolein cluster formation, either in association reactions with neutral (SnO)xclusters or as nucleation centres. We refrain from a further discussion ofcluster formation mechanisms, which is beyond the scope of this article.

Representative cluster yields (signal averaged, 4 laser pulses) from high-powered laser ablation have been calculated using the formula:

[ion] 5 Iion= Cisotopomers

I (2)


Fig. 1. (a) Positive-ion FTICR-mass spectrum obtained from the laser ablation of solid SnOand (b) negative-ion FTICR-mass spectrum obtained from the laser ablation of solid SnO2.The oxygen equivalent series (SnO)±x can be seen on the low mass side of the (SnO)xO− series,and the high mass side of the (SnO)xSn+ series. The numbers on the diagram refer to ‘‘x’’ inthe tin- and oxygen-rich series (SnO)xSn+ and (SnO)xO−. Asterisks denote instrumental arte-facts.

170 Jackson et al.

Table I. Relative Cation Concentrations from Laser Ablation Experiments, Measured withan FTICR-Mass Spectrometer. Spectra were Acquired from SnO and SnO2 Precursors, and

Averaged over 4 Laser Pulses.

SnO SnO2

Precursor Normalised Normalisedion

Cisotopomers abundance


Sn+ 287 38 263 100SnOH+ 4 0.5 9 4(SnO)Sn+ 129 17 89 34(SnO)+2 46 6 39 15(SnO)2Sn+ 250 33 52 20(SnO)+3 58 8 20 8(SnO)3Sn+ 752 100 127 48(SnO)+4 57 8 24 9(SnO)4Sn+ 96 13 4 2(SnO)+5 4 0.5 < 0.5 0.2(SnO)5Sn+ 29 4 1 0.5(SnO)+6 22 3 0.6 0.2

from which relative ion stabilities can be inferred (Tables I and II). Apartfrom Sn+ and SnO−2 , the cluster stoichiometries (SnO)1,3Sn+, (SnO)3,6O−

and (SnO)−6 are most abundant, suggesting structures containing Sn3subunits possess enhanced stability [20]. This point is explored further inthe computational section. Not surprisingly, the (SnO)xO− anions are moreabundant than their (SnO)−x counterparts, as the additional oxygen atomshould increase the cluster electrophilicity. Similarly, addition of an Snatom to (SnO)x will make the cluster more metallic and reduce the IE,leading to more abundant (SnO)xSn+ in the positive-ion spectra.

COLLISION INDUCED DISSOCIATION

Due to low ion concentrations, CID studies of (SnO)±x clusters, withthe exception of (SnO)+2 , could not be performed. Although reasonableyields of (SnO)−6 were obtained, it could not be isolated cleanly from(SnO)6O−.

Ar was found to be an effective CID gas for the cation series, and wasused for all cation dissociation experiments. Electron detachment wasfound to dominate for anions using Ar, so Ne and N2 were also employed.Relative fragment products for specific cluster ions are presented inTable III.


Table II. Relative Anion Concentrations from Laser Ablation Experiments, Measured withAn FTICR-Mass Spectrometer. Spectra were Acquired from SnO and SnO2 Precursors, and

Averaged over 4 Laser Pulses

SnO SnO2

Precursor Normalised Normalisedion



Sn− 19 3 < 5 1SnO2H

−0,1 28 5 87 15

Sn−2 12 2 24 4(SnO)−2 27 5 32 6(SnO)2O− 53 9 241 42Sn−3 0 0 98 17(SnO)−3 51 9 21 4(SnO)3O− 234 40 580 100(SnO)3O

−2 0 0 52 9

(SnO)−4 23 4 < 5 < 1(SnO)4O− 54 9 130 22(SnO)4O

−2 0 0 55 10

(SnO)−5 12 2 < 5 < 1(SnO)5O− 76 13 266 46(SnO)5O

−2 0 0 20 3

(SnO)−6 199 34 47 8(SnO)6O− 581 100 285 49

Table III. Summary of the FTICR-CID Results for Charged Sn-O Clusters Generatedby Laser Ablation. The Abundance Ratios Follow the Order in which the Products Are

Presented in Adjacent Columns

Ion productsPrecursor ECM Abundanceion (eV) Major Minor ratio

(SnO)Sn+ 250 Sn+ SnO+ 6:1(SnO)+2 238 SnO+ –(SnO)2Sn+ 230 (SnO)Sn+, Sn+ (SnO)+2 2:3:1(SnO)3Sn+ 50 (SnO)Sn+ (SnO)2Sn+, Sn+ 3:1:1(SnO)4Sn+ 7 (SnO)3Sn+, (SnO)Sn+ (SnO)2Sn+, Sn+ 2:2:1:1(SnO)5Sn+ 12 (SnO)Sn+, (SnO)3Sn+ (SnO)2Sn+ 8:3:1(SnO)2O− 18–20 (SnO)O− –(SnO)3O− 15 (SnO)2O− (SnO)O− 2:1.0–1.5(SnO)4O− 9 (SnO)3O− (SnO)2O− 5:1(SnO)5O− 11 (SnO)3O− –(SnO)6O− 17 (SnO)5O−, (SnO)3O− (SnO)4O−, (SnO)2O−, (SnO)O− 10:6:1:2:1

172 Jackson et al.

Cations

CID of (SnO)Sn+ yielded the fragment ions Sn+ and SnO+, [Sn+] ±

[SnO+], consistent with Stevenson’s rule, that is IE(Sn)=7.34 eV < IE(SnO)=9.60 eV [34, 36].

For (SnO)+2 , loss of SnO was the exclusive fragmentation process. This

suggests either an extended or cyclic structure with OSnOSn+|||||||||

connectivity;an extended structure appears less likely as Sn+was not detected.

For (SnO)2Sn+, fragmentation yielding Sn+, (SnO)Sn+ and (SnO)+2 wasobserved. Such a product ion distribution is in accordance with either alinear or cyclic structure possessing an (SnO)2 core with Sn+ bonded to anoxygen atom. From the ratio of the dissociation products Sn+ and (SnO)+2 ,IE(SnO)2 > IE(Sn).

The fragmentation products of (SnO)3Sn+ included Sn+, (SnO)Sn+ and(SnO)2Sn+, with high yields of (SnO)Sn+. A candidate structure is a rhombic(SnO)2core with an Sn-O-Sn unit bridging them-O atoms through Sn-O bonds.

(SnO)4Sn+ fragmentation products included (SnO)3Sn+, (SnO)2Sn+,(SnO)Sn+ and Sn+. A very weak signal due to the fragment ion (SnO)+2 wasalso present in CID spectra, hence IE(SnO)2Sn < IE (SnO)2. The prolifera-tion of CID products renders interpretation difficult, however the closerelative abundances suggest a high-symmetry structure, e.g., a simple cubiccell (SnO)4 with Sn+ bridging O atoms of one face, or alternately a trigonalbipyramidal Sn5 core with four m3-O ligands.

Finally, fragmentation products of (SnO)5Sn+ included (SnO)3Sn+,(SnO)2Sn+ and (SnO)Sn+. The high abundance of the product (SnO)Sn+

suggests this unit might be associated with an (SnO)4 core.

Oxygen-Rich Anions

With the exception of (SnO)O−, which was not investigated, expulsionof SnO was observed for all of the anions. For the ion (SnO)2O−, this wasthe exclusive fragmentation channel.

For (SnO)3O−, small signals due to (SnO)O− and (SnO)2O− wereobserved in approximately equal amounts, suggesting high symmetry andnear-equivalent EAs. On the basis of this pattern, a likely structure is thesimple (SnO)4 cubic cell with a vacant metal site.

Loss of SnO was the dominant fragmentation channel for (SnO)4O−,however a minor signal due to (SnO)2O− was also detected, inferring asimple cubic cell structure with an extruding O adatom.

(SnO)5O− expels neutral (SnO)2 as the exclusive fragmentation process,implying a structure similar to that proposed for (SnO)5Sn+, but with[OSnO]− bridging (SnO)4.


Fig. 2. Dinitrogen-induced dissociation spectrum of (SnO)6O− after approximately onecollision. ECM increases from 6 eV in the top frame to 17 eV in the bottom frame. Oxygenequivalent ions result from dissociations of (SnO)−6 , which could not be completely ejectedfrom the ICR cell prior to the CID acceleration pulse.

174 Jackson et al.

The CID behaviour of (SnO)6O− is of interest as ablation spectrasuggest it is a cluster of high stability. N2 was the most effective target gasfor this ion, and the CID spectra obtained at approximate ECM’s of 6, 11and 17 eV are presented in Fig. 2. Expulsion of SnO is dominant, howeverthe ratio I(SnO)3O−:I(SnO)5O− increases with ECM. The dissociationpattern suggests an extruding SnO unit bonded to an Sn5O6 core, possibly(m3-O)6Sn5, where the Sn5 core is trigonal bipyramidal.

A general trend for both the dominant cluster series is the expulsion ofSnO, or multiples thereof, upon collisional excitation. For the dominantcations, the tin-rich fragment usually retains the charge, while for theanions the oxygen-rich fragment always retained the charge. The abun-dances of (SnO)Sn+ and (SnO)3O− as fragmentation (and ablation) pro-ducts is consistent with low heats of formation for particular structureswith these compositions.

COMPUTATIONAL RESULTS

SnO+/0/−

The DFT bond length, harmonic vibrational frequency and dissocia-tion energy for ground state SnO compare well with experimental photo-electron [34] and high-level ab initio [35] results (Table IV). Characteristicof local DFT, the dissociation energy is overestimated by 1.5 eV.

DFT appears to perform poorly for the states of SnO+, with the pre-dicted order seemingly incorrect [34] and the overbinding more pro-nounced. It is worth noting that the overlapping photoelectron bands [34]make an unambiguous ground state assignment difficult; also, DFT and abinitio calculations suggest a small geometry change accompanies IE 2S+P1S+, which should result in a short vibrational progression. DFT calcula-tions suggest a more dramatic energy change should occur for ionisationleading to 2P SnO+.

The adiabatic EA of SnO is small (0.6 eV), consistent with a dipole-bound electron. This value is not expected to be accurate as the basis setsused are not extensive, however the low value certainly explains the absenceof SnO− in negative-ion ablation spectra.

(SnO)O0/−

No published data exists for (SnO)O, its associated anion or any largercluster observed in these experiments. Both C2v and linear geometries wereinvestigated.


Tab

leIV

.C

lust

erP

rope

rtie

sfr

omL

ocal

DF

TC

alcu

lati

ons.

Exp

erim

enta

lV

alue

s(i

nIt

alic

s)fo

rSn

O,

SnO+

Are

from

[34,

35],

and

IEs

for

(SnO

) 2Sn

Are

Ver

tica

lVal

ues.

All

Oth

erE

As,

IEs

Are

Adi

abat

ic.B

ondl

engt

hsA

rein

Ang

stro

m,a

ndE

nerg

yU

nits

Are

ineV

Spec

ies

r SnO

r m-O-Sn

r m3-O-Sn

r SnSn

Ang

leDHat,0

IEE

AR×n

DH0

1 S+

SnO

1.85

6.9

10.3

0.6

1.94

3.7

8.8

1.83

5.510.1

2 S+

SnO+

1.85

4.25

1.96

2.5

2 PSn

O+

2.00

4.21

2.05

3.0

2 PSn

O−

1.91

6.1

1 S+ g

OSn

O1.

8318

0.0

10.8

2.4

1 OSn

OQ1 S

nO+1 O

4.9(

3)X2 A1

OSn

O−

1.91

130.

02 O

SnO−Q2 O−+1 S

nO4.

7a2

A1

OSn

O−

1.95

(4)

84.0

1 A1

SnO

Sn2.

032.

8790

.09.

86.

8(5)

1 SnO

SnQ3 S

nO+3 S

n5.

81 S

nOSn

Q3 S

n 2+3 O

6.7

3 S−

Sn2O

1.85

2.75

180.

08.

03 S

n 2O

Q3 S

n+1 S

nO1.

32 Pg

SnO

Sn+

1.96

180.

013

.52 S

nOSn+Q1 S

nO+2 S

n+6.

62 P

Sn2O+

1.83

2.96

180.

01 Ag

(m-O

) 2Sn2

2.02

3.00

17.3

8.6

1.2

1 (m

-O) 2

Sn2Q

21 S

nO3.

42 B2u

(m-O

) 2Sn+ 2

2.01

2.98

2 (m

-O) 2

Sn+ 2Q2 S

nO++1 S

nO5.

22 (m

-O) 2

Sn+ 2Q2 S

n++1 S

nO2

5.6

2 B3g

(m-O

) 2Sn− 2

2.04

3.07

2 (m

-O) 2

Sn− 2Q2 S

nO−+1 S

nO4.

02 (m

-O) 2

Sn− 2Q3 S

n+2 S

nO− 2

5.3

1 A1

(m-O

) 2Sn2O

1.84

2.03

,2.0

22.

9821

.52.

51 (m

-O) 2

Sn2O

Q1 S

nO+1 S

nO2

3.8

2 A1

(m-O

) 3Sn− 2

2.07

2.67

2 (m

-O) 3

Sn− 2Q1 S

nO+2 S

nO− 2

3.9

2 (m

-O) 3

Sn− 2Q1 (m

-O) 2

Sn2+2 O−

5.2

1 (m3-O

)(m

-O)S

n 32.

11(×

4)2.

332.

83(×

2)20

.17.

61 (m3-O

)(m

-O)S

n 3Q1 S

nO+1 S

nOSn

3.4

3 (m3-O

)(m

-O)S

n 32.

012.

16,2

.13

3.07

,3.1

319

.57.

13 (m3-O

)(m

-O)S

n 3Q1 (m

-O) 2

Sn2+3 S

n2.

23 (m

-O) 2

Sn3

2.00

3.11

19.5

6.7

2 (m

-O) 2

Sn+ 3Q1 (m

-O) 2

Sn2+2 S

n+2.

63 (m3-O

) 2Sn3

2.16

3.02

,3.0

419

.47.

02 (m

-O) 2

Sn+ 3Q2 S

nOSn++1 S

nO3.

0

176 Jackson et al.

1 (m

-O) 3

Sn3

2.00

26.9

(8)

7.8

1.9

1 (m

-O) 3

Sn3Q1 (m

-O) 2

Sn2+1 S

nO2.

81 (m3-O

)(m

-O) 2

Sn3

1.96

2.36

3.20

26.8

(8)

1 (m3-O

) 2(m

-O)S

n 32.

002.

283.

15,2

.95

26.7

2 (m3-O

)(m

-O) 2

Sn− 3Q2 (m

-O) 2

Sn− 2

+1 S

nO4.

02 (m3-O

)(m

-O) 2

Sn− 3

2.00

–2.0

62.

12,2

.22

3.15

,3.1

82 (m3-O

)(m

-O) 2

Sn− 3Q2 (m

-O) 3

Sn− 2

+3 S

n4.

82 (m3-O

) 2(m

-O)S

n−3

2.04

2.15

–2.1

93.

13,2

.90

2 (m3-O

)(m

-O) 2

Sn+ 3Q2 (m

-O) 2

Sn+ 2

+1 S

nO3.

62 (m3-O

)(m

-O) 2

Sn+ 3

2.00

2.12

–2.1

63.

162 (m3-O

)(m

-O) 2

Sn+ 3Q2 S

nOSn++1 O

SnO

5.5

2 (m3-O

) 2(m

-O)S

n+3

1.99

2.21

,2.1

03.

17,2

.86

2 (m3-O

)(m

-O) 2

Sn+ 3Q1 (m

-O) 2

Sn2O

+2 S

n+5.

4

1 (m3-O

) 2(m

-O) 2

Sn3

1.96

–2.1

22.

12–2

.17

2.81

31.7

(5)

3.5

1 (m3-O

) 2(m

-O) 2

Sn3Q1 (m

-O) 2

Sn2O

+1 S

nO3.

33 (m3-O

)(m

-O) 3

Sn3

2.01

2.14

3.10

31.6

(5)

1 (m3-O

) 2(m

-O) 2

Sn3Q1 (m

-O) 2

Sn2+1 O

SnO

3.7

2 (m3-O

) 2(m

-O) 2

Sn− 3

2.03

–2.0

62.

14–2

.21

2.90

–3.2

62 (m3-O

)(m

-O) 3

Sn− 3Q2 (m

-O) 3

Sn− 2

+1 S

nO4.

12 (m3-O

)(m

-O) 3

Sn− 3

2.02

–2.0

42.

15–2

.17

3.14

–3.1

52 (m3-O

)(m

-O) 3

Sn− 3Q2 O

SnO−+1 (m

-O) 2

Sn2

4.6

1 (m3-O

) 3Sn4

2.13

–2.1

43.

11–3

.25

30.9

(5)

7.0

1 (m3-O

) 3Sn4Q1 (m3-O

)(m

-O)S

n 3+1 S

nO3.

92 (m3-O

) 3Sn+ 4

2.12

–2.1

73.

27–3

.36

1 (m3-O

) 3Sn4Q1 (m

-O) 2

Sn2+1 S

nOSn

3.9

2 (m3-O

) 3Sn+ 4Q2 S

nOSn++1 (m

-O) 2

Sn2

3.8

2 (m3-O

) 3Sn+ 4Q2 (m

-O) 2

Sn+ 3

+1 S

nO4.

22 (m

-O) 2

Sn+ 3Q2 S

n++1 (m

-O) 3

Sn3

4.7

1 (m3-O

) 4Sn4

2.13

–2.1

53.

24–3

.26

38.3

8.1

1.1

1 (m3-O

) 4Sn4Q3 (m3-O

)(m

-O) 3

Sn3+3 S

n3.

12 (m3-O

) 4Sn+ 4

2.12

–2.1

53.

23–3

.26

1 (m3-O

) 4Sn4Q

21 (m

-O) 2

Sn2

3.7

2 (m3-O

) 4Sn− 4

2.14

–2.1

53.

22–3

.26

2 (m3-O

) 4Sn+ 4Q2 S

n++1 (m3-O

) 2(m

-O) 2

Sn3

2.7

2 (m3-O

) 4Sn+ 4Q2 (m

-O) 2

Sn+ 2

+1 (m

-O) 2

Sn2

4.2

2 (m3-O

) 4Sn+ 4Q2 (m3-O

) 3Sn+ 4

+3 O

4.2

2 (m3-O

) 4Sn− 4Q2 (m3-O

)(m

-O) 3

Sn− 3

+3 S

n0.

82 (m3-O

) 4Sn− 4Q2 (m3-O

)(m

-O) 2

Sn− 3

+1 S

nO3.

62 (m3-O

) 4Sn− 4Q2 (m

-O) 2

Sn− 2

+1 (m

-O) 2

Sn2

3.6

1 (m

-O) 5

Sn4

1.99

–2.0

13.

27–3

.46

40.8a

3.4a

1 (m

-O) 5

Sn4Q1 S

nO+1 (m3-O

) 2(m

-O) 2

Sn3

2.7

1 (m3-O

) 6Sn5

2.08

–2.2

63.

26–3

.46

52.6

1 (m3-O

) 6Sn5Q1 (m3-O

) 4Sn4+1 O

SnO

3.5

1 (m3-O

) 6Sn5Q1 (m3-O

) 2(m

-O) 2

Sn3+1 (m

-O) 2

Sn2

3.6

aV

alue

sba

sed

onpr

opos

edne

utra

land

anio

ngr

ound

stat

es,n

ohe

ssia

nan

alys

is.


The neutral ground state was found to be linear OSnO (1S+g ). C2vstarting geometries relaxed to the linear ground state without activation.Using the calculated atomisation energy of 1S+ SnO, D(OSn–O) is 3.9 eV,which is significantly less than D(SnO). In the absence of spin-orbit coupl-ing, the singlet ground state (1S+g ) must dissociate to either 1D O+1S+

SnO, or 3P O+3P SnO. Assuming the RECP-MRCI excitation energy,3.0 eV, for (3PP

1S+)SnO is accurate [35], the former asymptote appearsto be lower in energy, so D(OSn-O) is likely to be closer to 4.9(3) eV.

The hypersurface of (SnO)O− is more interesting with two inserted C2vminima located. The ground state, X2A1 OSnO− lies 0.55 eV lower inenergy than a second bent state, a2A1, with a more acute =OSnO angle(see Table IV). A D.h transition structure (rSnO=1.91 Å) was also found,lying 0.57 eV above the ground state. Super-oxide structures were notinvestigated, as loss of O2 was not observed in the CID experiments.

(SnO)Sn0/+

Three principal structures were investigated for both the neutral andcation: a C2v structure with m-O atom, and two linear structures, one with acentral O atom (D.h) and one with a terminal O atom (C.v).

Two minima were found for (SnO)Sn. The ground state is bent,1A1(m-O)Sn2, while the other isomer possesses C.v symmetry (3S−) and lies1.5 eV above the bent minimum. A singlet transition structure with D.hsymmetry was also found (0.56 eV above the ground state).

Two isomers were also found for (SnO)Sn+. The ground state is theD.h 2Pg structure [Sn-O-Sn]+. The other isomer has C.v symmetry, 2P[Sn–Sn-O]+, and is 2.5 eV less stable than the D.h isomer. The optimisedC2v structure was found to be a transition state lying only 0.04 eV abovethe ground state ( =SnOSn=104°, rSnO=2.00 Å, rSnSn=3.15 Å).

(SnO)−/0/+2

From the results for (SnO)Sn0/+, it appears that terminal oxygenatoms are unfavourable if there is more than one Sn atom. This is

confirmed for the D.h structures OSnOSn−/0/+|||||||||

which are 3.4, 4.0 and2.7 eV less stable than the respective anionic, neutral and cationic rhombicOSnOSn structures. At higher levels of theory, the D.h minima may notexist (anion, rSnSn=3.64 Å; neutral: rSnSn=4.41 Å; cation, rSnSn=3.60 Å). As the primary motivation of this study was to locate the groundstate, detailed structural investigations of (SnO)2 and its associated ionswere thus limited to the D2h structures (m-O)2Sn2. It is worth noting that

178 Jackson et al.

laser photoelectron results for silicon oxides [37] suggest extended struc-tures comprising (SiO)2 units are favoured for polyoxide group IV molecules,rather than closed or compact geometries.

The results are presented in Table IV. The bond lengths of the threestructures are similar, with electron attachment resulting in a greater per-turbation of the neutral structure than ionisation. The lowest dissociationreaction energies of (SnO)2 and other clusters are also presented in Table IV.The dissociation energies of (SnO)2 suggest loss of SnO should be theexclusive dissociation channel of the cation (confirmed), while only electrondetachment should be observed for the anion.

(SnO)2O−/0

The ground state structure of (SnO)2O, 1A1 (m-O)2Sn2O, possesses asingle, terminal O atom, which yields a mixed valence cluster; the two Snatoms should, as a consequence, exhibit contrasting reactivities. As men-tioned previously, the reactions of neutral species cannot be monitored inthese experiments.

Electron addition to 1A1 (m-O)2Sn2O results in barrierless rearrange-ment to the ground state of the anion, 2E (m-O)3Sn−2 , as 2(m-O)2Sn2O− is aconfirmed transition structure on the doublet anion surface.

(SnO)2Sn0/+

The potential hypersurface of the neutral molecule is characterised byat least 4 minima within 1 eV of the proposed ground state. All theseminima possess cyclo-Sn3 arrangements and have no terminal O atoms.

The ground state features O atoms in triple- and double bridgingpositions, 1(m-O)(m3-O)Sn3, and a triplet excited state of this structure liesapproximately 0.6 eV higher in energy. The isomer 3(m-O)2Sn3, lies approxi-mately 0.7 eV above the ground state. The structure is best described as aslightly puckered pentagon with the O atoms lying slightly above the Sn3plane. One other isomer was located, 3(m3-O)2m3, and is essentially iso-energetic with 3(m-O)2Sn3. The lowest energy linear structure, with D.hsymmetry and no terminal O atoms, was found to lie at least 3.4 eV abovethe ground state, so no frequency analysis was performed.

It is evident from the DFT results that the neutral triplet surface israther flat, and the oxygen conformations will be fluxional on this surface.Investigations of the structures (m3-O)2Sn3 and (m-O)2Sn3 on the singletsurface demonstrated that these conformations were unstable to rearran-gement at the local-DFT level of theory (to 1(m-O)(m3-O)Sn3).


Due to the complex nature of the (SnO)2Sn hypersurface and the rela-tively low level of theory used, exhaustive computational investigations of(SnO)2Sn+ were not undertaken; instead, we have used the vertical ionisa-tion energies of the neutral isomers as a crude indicator of relative energe-tics for the cation (see Table IV). These results suggest that 2A1 (m-O)2Sn+3is probably the ground state, and that Sn+ should be an abundant CIDproduct (observed).

(SnO)−/0/+3

Multiple low-lying minima are again characteristic of the potentialhypersurface of (SnO)3, with 3 confirmed minima within a 0.3 eV energy range(see Fig. 3). The ground state is a cyclic hexagonal structure 1(m-O)3Sn3with alternating Sn-O-Sn · · · bonding. Another isomer, 1(m3-O)(m-O)2Sn3,was found to lie less than 0.1 eV above the ground state. This structurepossesses mirror plane symmetry through the m3-O and apical Sn atoms,and resembles two (SnO)2 rhombii sharing a common edge. The m3-Oligand is displaced only slightly above the plane of the Sn3 core. The isomer1(m3-O)2(m-O)Sn3, lies approximately 0.3 eV above the ground state andpossesses C2v symmetry. The Sn–Sn bonds in this structure are shorter,with two bonds of 3.15 Å and one bond of 2.95 Å. A cyclic optimisedstructure with three terminal O atoms, Sn −, Sn',Sn'−-trioxocyclo-Sn3 (singlet,D3h symmetry), was found to lie 5.9 eV above the ground state. No linearstructures were investigated.

The doublet hypersurface of the anion is characterised by at least two(probably three) minima separated by 0.7 eV. The proposed ground statecorresponds to (m3-O)(m-O)2Sn−3 ( 2A − in Cs). The next lowest isomer is2(m3-O)2(m-O)Sn−3 (C2v); this structure closely resembles the ground statestructure of (SnO)2O−, that is (m3-O)3Sn−2 , with a m-Sn adatom bridgingtwo adjacent O sites. The 2(m-O)3Sn−3 structure lies 0.7 eV above theground state, but no frequency analysis was performed. 2Sn −,Sn',Sn'−-trioxocyclo-Sn−3 (D3h) was found to be 4.3 eV less stable than the groundstate.

The ground state on the doublet cation surface, 2(m3-O)(m-O)2Sn+3 , isseparated from the next lowest isomer, 2(m3-O)2(m-O)Sn+3 by almost 0.6 eV.The cyclic structure 2(m-O)3Sn+3 is a transition structure lying 0.4 eV abovethe ground state. 2(m3-O)(m-O)2Sn+3 possesses only two Sn–Sn bonds(3.16 Å), and structurally resembles its neutral analogue (common mirrorplane symmetry). 2(m3-O)2(m-O)Sn+3 (C2v) has comparable Sn–Sn bondlengths, but shorter m-O-Sn bond lengths and longer m3-O-Sn bonds.2Sn −,Sn',Sn'−-trioxocyclo-Sn+3 (D3h) is at least 7.5 eV less stable than theground state.

180 Jackson et al.

Fig. 3. Low energy ion and neutral cluster structures predicted by local DFT. See textand accompanying tables for more specific details. Energy differences between isomers aregiven in eV.


(SnO)3O0/−

Here, both cluster charge states are of interest because of the highabundance of the anion in the LA-mass spectra of both SnO and SnO2. Inaddition, (SiO)3O cluster has recently been studied both experimentally andtheoretically and proposed as a suitable model for defect sites on siliconoxide surfaces [37, 38].

Due to the increasing complexity of the potential hypersurfaces ofthese larger species (and ballooning computational effort), only a few can-didate geometries have been investigated. Structures with terminal O atomswere excluded, and from earlier results, the arrangement of four O atomsaround a cyclo-Sn3 core will probably adopt one of the following confor-mations: (m3-O)2(m-O)2Sn3 (A) or (m3-O)(m-O)3Sn3 (B), see Fig. 3.

For the neutral, it was discovered that isomer A was only 0.09 eVlower in energy than isomer B. Frequency analyses have confirmed bothstructures are minima. Whereas the ground state isomer A is closed shell(1A1 in C2v), B was found to be a triplet (3E in C3v). Attempts to locate alower energy singlet isomer of B were unsuccessful. Isomer A forms onlytwo Sn–Sn bonds, which are short and close to the neutral dimer bondlength of 3S−g Sn2 (LSD rSnSn=2.81 Å [18]). One extended structure wasinvestigated, OSn(m-O)Sn(m-O)SnO (Ci symmetry), however this optimisedstructure was 5.3 eV less stable than A. This structure did not undergorearrangement to Sn(m-O)2Sn(m-O)2Sn in unconstrained optimisations; theanalogous structure is the ground state on the (SiO)3O potential energysurface according to TOF-ZEKE and computational results [37, 38].

A− and B− are separated by only 0.21 eV on the doublet anionsurface, however B− is the more stable isomer in this case. Both structuresare confirmed minima, although the wavefunction of B− was significantlycontaminated from another state. Symmetry reduction was also noted forB−with respect to neutral B, while electron addition to A leads to a decreasein the weak/non-bonding Sn–Sn separation, from 3.37 Å to 3.26 Å, andyields a flatter structure (m3-O atoms shifted away from the m-O atoms andthe apical Sn, ie. r(m3-O-Snapical)=2.12 Å in A, r(m3-O-Snapical)=2.21 Åin A−).

(SnO)3Sn0/+

The high abundance of (SnO)3Sn+ in positive-ion LA-mass spectrasuggests a large negative heat of formation. Six neutral structures wereinvestigated in order to understand the energetics of various core andligand arrangements. Of these, vibrational analysis was only performed forthe lowest energy structure, as the nearest geometric structure energetically,1(m4-O)(m-O)2cyclo-Sn4, is at least 2.3 eV less stable.

182 Jackson et al.

The proposed neutral ground state possesses a Td Sn4 core, 1A1 (m3-O)3Sn4 in C3v, and resembles a simple cubic cell with an oxygen atomdefect.

In total, seven cation structures were investigated, with the 2(m3-O)3Sn+4isomer found to be the most favourable energetically. The minimised cationis significantly distorted with respect to the neutral (see Table IV). Thenearest structure energetically (1.9 eV less stable) to 2(m3-O)3Sn+4 was foundto be a near-planar, curved Sn-O-Sn-O · · · chain, 2(m-O)3cyclo-Sn+4 , whereasthe isomer 2(m-O)3Sn+4 with a tetrahedral Sn4 core and all three oxygenatoms aligned along the edges of a single Sn3 face, was 2.8 eV less stable.Unconstrained optimisations from starting geometries with a Td Sn4 coreand m-O atoms positioned along the edges of distinct Sn3 faces collapsed tothe ground state structure without activation.

(SnO)−/0/+4

Fragmentation from activated (SnO)4 or larger clusters cannot beexcluded as a major route to the formation of (SnO)3Sn+ and (SnO)3O−.Comparison of the heats of reaction for Sn, O loss from (SnO)4, and theground state cluster binding energy, could clarify which reaction channelsare important to the formation of these abundant ions.

Investigations of the singlet surface have revealed the simple rhombicunit cell structure 1(m3-O)4Sn4, is markedly more stable than any of theother structures investigated. Notably, planar ‘‘square’’ Sn4 arrangementspossessing m-O ligands were significantly less stable; square Sn4 arrange-ments with m4-O were, predictably, very unstable. Vibrational analysis wasonly performed for (m3-O)4Sn4, 1A1 in Td, which was confirmed to be aminimum on the singlet surface. Preliminary investigations revealedsimilarities in the anion and cation surfaces, so it was assumed that theanalogous optimised doublet structures were also the ground states of therespective ions.

(SnO)4O

On the basis of the relative abundance of this ion in LA-mass spectra,it probably has a more positive heat of formation than any of the otheroxygen-rich anions investigated. Three representative structures were ana-lysed for the neutral; two structures possessing planar cyclo-Sn4 coreswith three or four m-O, and either one or two m4-O, atoms respectively,1(m4-O)(m-O)4cyclo-Sn4 (A) and 1(m4-O)2(m-O)3cyclo-Sn4 (B), and a thirdstructure with a Td Sn4 core and five m-O atoms positioned along the Sn–Snbonds (tetrahedron edges), 1(m-O)5Sn4, (C).


The DFT study revealed that structure C was at was at least 0.7 eVlower in energy than the next most stable conformation, A. As it is notpossible to accommodate all five O atoms in m3-O positions on any fouratom core, and terminal O are unfavourable when the ratio Sn/O is closeto 1, we propose that C is the most stable conformation achievable.

If O− addition to ground state (SnO)4 is one of the major routes toformation of C−, the ion-molecule complex would require sufficientinternal energy to overcome the barriers associated with the m3-O Q m-Orearrangements, which could represent a significant bottleneck to ion for-mation and lead to poor yields (observed). According to results for theclusters with Sn3 cores, a single conversion requires ca. 0.1–0.5 eV, so thatthe ion-dipole potential alone could probably account for 1–2 of thesepseudorotations.

The detection of (SnO)4O−2 is readily rationalised on the basis of the

proposed ground state C, as the additional O atom can occupy the lastremaining m-O position on the cluster. Similar arguments can be used toexplain the detection of (SnO)3O

−2 , as the most stable structures of

(SnO)3O−/0 both possess a single, vacant binding site (Fig. 3).

(SnO)4Sn, (SnO)5O

Both trigonal bipyramidal and square pyramidal cores are capable ofaccommodating the four O atoms of (SnO)4Sn in m3-positions. Three dis-tinct structures are possible for four oxygen ligands constrained to m3-Opositions around a trigonal bipyramidal tin core; the first reduces possibleligand:ligand repulsions and distributes two O atoms above, and twobelow, the equatorial plane, giving rise to either aC2v (A) or aC1 structure (B).The other possibility distributes three O atoms above the equatorial plane,and one below, giving rise to a structure with Cs symmetry (C). For asquare pyramidal Sn5 core, only one isomer was investigated, with all Oatoms occupying m3-O positions centred over the Sn3 faces. This structure(D) has high symmetry, C4v.

Of the trigonal bipyramidal structures, B was found to be the mostenergetically favourable on the singlet surface, and was 0.4 eV more stablethan C. Structure A was found to collapse to structure D without activa-tion. Overall, D was the structure with the highest binding energy, and wasfound to be 0.4 eV more stable than B. The higher stability of D isattributed to the minimisation of O–O repulsions. On the basis of the smallenergy separation between B and D, it is likely that a mixture of cationisomers was sampled experimentally using FTICR-MS.

The only structure investigated for (SnO)5O was a trigonal bipyrami-dal Sn5 core with all O atoms occupying m3-O positions, 1(m3-O)6Sn5. This

184 Jackson et al.

structure was confirmed to be a minimum by vibrational analysis. Theoverall symmetry of this isomer is C2v.

CONCLUSIONS

The limitations of local DFT are well documented, and some havebeen mentioned in preceding sections. First, local DFT overbinds, andsecond, singlet-triplet energy differences are underestimated. As singletswere found to be the lowest states of all neutrals, with the exception of(SnO)3O, we can afford some optimism regarding the computational pre-dictions. Unfortunately, a comparison of experimental and calculatedreaction energies (Table V) suggests there is no simple relationship thatdescribes the scaling of the DFT overbinding. Nonetheless, structural, IEor EA trends should not be adversely affected by systematic errors.

The energies presented in Table IV indicate that loss of (SnO)x groupsis favoured for charged clusters, and expulsion of tin-rich neutrals fromthe hypermetallic cations and oxygen-rich neutrals from the hypometallicanions are the most energy-demanding processes [39]. This concurs withthe low-energy FTICR CID results. For (SnO)±4 , it is pertinent that thelowest energy dissociation channels are less energy demanding than for allother clusters, and that these channels lead to one of the most abundantions observed in the LA-spectra, notably, (SnO)3O−.

An interesting feature of the clusters with Snx > 3 is the preference forthe m3-O oxygen binding mode, which enables a Lewis acid/base interac-tion between the Sn and O atoms, and contributes to the stabilisation ofcertain metal cores, e.g., Td. This binding mode is also evident in the struc-ture of solid SnO2 [40]. Not surprisingly, constraining the oxygens tom4-positions is disfavoured energetically.

Table V. Comparison of Third-Law Oligomerisation Energies Derived from Knudsen Cell-MS Measurements from Reference [16] and DFT Values (This Work)

Energy (eV)

Reaction Reference [16] DFT

(SnO)2 Q 2 (SnO) 2.9 3.4(SnO)2 Q Sn2+O2 6.9 9.3(SnO)3 Q (SnO)2+SnO 3.0 2.8(SnO)3 Q 3 (SnO) 6.0 6.2(SnO)4 Q (SnO)3+SnO 3.1 4.4(SnO)4 Q 4 (SnO) 9.0 10.5(SnO)4 Q (SnO)2+(SnO)2 3.1 3.7


Without additional structural evidence, it cannot be stated that thelaser ablation of both SnO and SnO2 solids produces the same distributionof cluster states/geometries for a given composition, even though the CIDof isolated ion packets, generated from either precursor, did yield identicalresults (within the error margins of the experiments). This simply meansthat low energy CID, which tends to be ergodic in nature because of thelonger lifetime of the collision complex, is not sufficiently sensitive in thisinstance. As several low-lying isomers were located for some of the clustercompositions, with interconversion barriers far less energy demanding thandissociation, isomer discrimination is almost impossible.

The CID products detected suggest that (SnO)Sn, (SnO)3O and(SnO)3Sn might have high values for the binding energy per atom. Whilethis is true for (SnO)3Sn, the binding energy values for larger oxygen-richclusters fall off with increasing size, whereas the opposite is true of theoxygen equivalent clusters. This supports the proposal that dissociationsof (SnO)4 clusters contribute to the high abundance of (SnO)3O− in LA-spectra. Moreover, recent post-ablation neutral photoionisation studies[41, 42] have confirmed that the oxygen-equivalent neutrals are abundantin the laser plasma of SnO2.

Finally, comments concerning the high abundances of (SnO)−6 ,(SnO)6O− in the LA negative-ion spectra are warranted. If (SnO)6 is anabundant neutral, it follows that O− complexation would generate signifi-cant yields of (SnO)6O− (observed). Arguments against a high abundanceof neutral (SnO)6 are the poor yields of both (SnO)+6 and (SnO)5Sn+, andthe complete absence of (SnO)6Sn+ and (SnO)6O

−2 .

ACKNOWLEDGMENTS

P.J. wishes to thank AINSE Inc. for financial and computing support.G.D.W., I.G.D. and K.J.F. acknowledge financial assistance from theAustralian Research Council.

REFERENCES

1. K. L. Chopra, S. Major, and D. K. Pandya (1983). Thin Solid Films 102, 1.2. W. Unger and W. Pritzkow (1987). Int. J. Mass Spectrom. Ion Proc. 75, 15.3. M. J. Tarlov, J. F. Evans, and J. G. Newman (1993). App. Surf. Sci. 64, 115.4. T. Suzuki et al. (1989). J. Mat. Sci. Lett. 24, 2127.5. J. F. Geiger et al. (1991). Fresenius J. Anal. Chem. 341, 25.6. D. A. McCrery, E. B. Ledford, and M. L. Gross (1982). Anal. Chem. 54, 1435.7. P. F. Greenwood, G. D. Willett, and M. A. Wilson (1996). Org. Mass Spectrom. 28, 831.8. J. H. El-Nakat et al. (1991). J. Am. Chem. Soc. 113, 5141.9. J. H. El-Nakat et al. (1991). Inorg. Chem. 30, 2957.

186 Jackson et al.

10. J. H. El-Nakat et al. (1991). J. Chem. Soc. Chem. Commun. 746.11. J. H. El-Nakat et al. (1993). Inorg. Chem. 32, 1931.12. R. R. Squires (1987). Chem. Rev. 87, 623.13. For instance, weak ion currents for Nb−n in: (1997). Int. J. Mass Spectrom. Ion Proc. 161,

161.14. A. Mele et al., in P. E. Russell (ed.), Microbeam Analysis (Plenum Press, New York,

1989), p. 299.15. A. Mele et al. (1990). Int. J. Mass Spectrom. Ion Proc. 95, 359.16. R. Colin, J. Drowart, and G. Verhaegen (1965). Trans. Faraday Soc. 61, 1364.17. P. Jackson et al. (1997). Int. J. Mass Spectrom. Ion Proc. 164, 45.18. P. Jackson et al. (1996). Int. J. Mass Spectrom. Ion Proc. 157/158, 329.19. A. Kant and B. H. Strauss (1966). J. Chem. Phys. 45, 822.20. K. Gingerich, A. Desideri and D. L. Cocke (1975). J. Chem. Phys. 62, 731.21. P. Jackson (1998). Ph.D. Dissertation (University of New South Wales, Australia).22. R. L. White and C. L. Wilkins (1982). Anal. Chem. 54, 2211.23. R. B. Cody, R. C. Burnier, and B. S. Freiser (1982). Anal. Chem. 54, 96.24. A. G. Marshall, C. L. Hendrickson, and G. S. Jackson (1998).Mass Spectrom. Rev. 17, 1.25. B. Delley and D. E. Ellis (1982). J. Chem. Phys. 76, 1949.26. B. Delley and D. E. Ellis (ed.), Density Functional Theory of Molecules, Clusters and Solids

(Kluwer Academic, Amsterdam, 1995), pp. 221–254.27. U. von Barth and L. Hedin (1972). J. Phys. C 5, 1629.28. L. Hedin and B. I. Lundqvist (1971). J. Phys. C 4, 2064.29. R. Ditchfield, W. J. Hehre, and J. A. Pople (1971). J. Chem. Phys. 54, 724.30. W. J. Hehre, R. Ditchfield, and J. A. Pople (1972). J. Chem. Phys. 56, 2257.31. P. C. Hariharan and J. A. Pople (1974).Mol. Phys. 27, 209.32. M. S. Gordon (1980). Chem. Phys. Lett. 76, 163.33. P. C. Hariharan and J. A. Pople (1973). Theor. Chim. Acta 28, 213.34. J. M. Dyke et al. (1982). Chem. Phys. 67, 245.35. K. Balasubramanian and K. S. Pitzer (1983). Chem. Phys. Lett. 100, 273.36. J. B. Pedley and E. M. Marshall (1983). J. Phys. Chem. Ref. Data 12, 967.37. L. S. Wang et al. (1997). Z. Phys. D 40, 36.38. S. K. Nayak et al. (1998). J. Chem. Phys. 109, 1245.39. Not all calculated dissociation pathways are shown. A more complete table can be

provided upon request.40. W. H. Bauer (1956). Acta Cryst. 9, 515.41. S. A. Reid (1999). Chem. Phys. Lett. 301, 517.42. S. A. Reid, W. Ho and F. J. Lamelas (2000). J. Phys. Chem. B 104, 5324.


Documents

The Structure of Gas Phase Tin Oxide Ions Generated by Laser Ablation: A Combined Fourier Transform Mass Spectrometry and Density Functional Theory Study