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Theoretical aspects of cluster formation by keV bombardment of rare-gas solids
THEORETICAL
17 hlay 1983
CHEhIICAL PHYSICS LEl-l-ERS
Volume 97, number 4.5
ASPECTS OF CLUSTER FORhlATION
BY keV BOMBARDhlENT
OF RARE-GAS SOLIDS B.J. GARRISON and N. WINOGRAD Department of Chemistry. i%e Pennyhaniix State Universit_v, 152 Dare-y Loboratory. University Pnrk-.Pennsyh*ania 16802. USA Received 19 July 1982:in
final form 17 hfarcll
19S3
The mechanism of energy dissipation in arson crystals bombarded by 100 and 500 eV Ar+ has been e.\amincd using &ssic.d dynamics. The study indicates that very high yields may be expected and that large molecular Ar clus~srs. Ar,,. may form via recombination in the near surface region. The results are compared qualitatively to recent matrix-isolation SIMS ekpcrinlents on a vxiety of inertgas solids.
1_ Introduction
Rare-gas solids provide unique matrices for trapping a variety of reactive atoms and molecules. In recent studies it has also been found that the secondary ion mass spectra (SIMS) of compounds stabilized in solid Ar. Kr and Xe yield important structural information about the isolated material [ I-31 _In these experiments, the rare-gas solid is bombarded with a O-5-5 keV primary ion, often Ar+. The information about the matrix-isolated species is then obtained from the mass spectrum of the material ejected from the solid. The mechanism of energy dissipation of the primary ion and subsequent ejection of material has not been explored for rare-gas solids but has beeu investigated in detail for a variety of ion-bombarded metals such as Ni [4] and Cu [5] using well-established classical dynamics techniques. These techniques have also been employed to establish a generalized theory of the molecular
cluster
metal surfaces
ejection
process
on chemically
reacted
[6,7]. It is not clear, however, whether
these concepts are applicable to rare-gas solids where cohesive energies are more than an order of magnitude smaller. The SlhlS spectra of solid Ar is characterized by an intense Ar+ peak and a variety of Arz cluster ions [2] _The largest clusters are characterized by II < 25 as limited only by the mass range of tile detector.
Their relative intensities are found to decrease nexly esponentidily with increasing II. In addition. the Linetic energy distribution of ejected .4r+ ions is extremely narrow (O-l 2 eV) relative to those distrihutions found from metals [Sl. 11 is importam IL>understand these observations if matrix-isolation SlhlS is to become a viable technique. In this work. then. we examine the mechanism of energy dissipatkw in ion-bombarded Ar crystals and compare the results crf classical dynamics calculations to those obraiued on metals. The results show tltat very large yields may be r‘kpetted and that in contrast to metals, a sigm!k.mt fraction of tire atoms may originate from tbr scwiid or third layer. Luge clusters of Ar,, llsvtt heeu ob-
served in the calculations. Tile atom in thr‘ clustzt need not consist of atoms tbs: were originally contiguous in tlie lattice, altbougli many of tllem are ejected from highly localized regions of the solid. in general, we believe our model &culstious 1 icld 3 realistic picture of the rner=’ dissipation in crystalline Ar and should aid in the interpretation of matrixisolated SlhlS spectra.
2. Description
of the calculation
The basic scheme for computing
tlw dynamics
of 381
Solurw
97.
UlUUtwr
1.5
CHEhtlCt\L
p.wtcle and the Ar substrate h&sbeen &~~bed m .I lcccnt series of articies I&7]. Briefly. die pwtm~ md ffiwmit3 of all the partxles we drvri~~p~~iirt Iunc by mtcgr,~tin_stiamilton’s equations 513mktwt. The Iindl positions and momenta of the wcrt-d p&urrclcs CM rfxn he ttsed 10 determine yields, ~wrrg .md ,H@C di~tr~lrwnrrs. and poss~hle cluster the twt~k:~rti~~tg
!iilltlJtl~~ll
L’hct~~fii~t~r soikl IS tliotietcd hy 3 finite microcrysI.ithtr. fti this k-.tFeswsisrmg of ,i ~~ce~~litere~i-cubic .i11.1\r)l‘At :itrws. Tfic biLe of the cryst.tllirc is chosen ~1 1h.11 1111IIWI IIIC~LISL'S ill the nwnlwx of ;IKUIISdo w)t ~1llhldi1l14lly .Iltcr rtie pedicttxl 0bservablcs. Be, .tuw wtttl ..\t 1~ .ttt cwrwwly slnttll ht of subhmatofu (0 OS cV). .S~tystallrte ~$7 layers ~8th = 180 .1101i1> ~L*I1,1yr1(Ii&. 1~) ($1total u!‘ 1,765 9fw~s) is irrcdzd 1~1LWI~~.I~I~ 1hr procccws involved when 4 pri-
PHYSICS LE-ITERS
27 May I983
mary particle bomkwds the surface with only100eV. The cakulstion of one Ar impact on this crystal requires =5D times more computer time than a similar calcufation on a 230 atom Ni microcr~st~lit~. Due to this difficulty, only 21 Ar impacts have been determined for each set of c&xl&ions. This number is too mall IO produce statistically reliable quantitative resuits so our analysis is restricted to a mechanistic investigaGon of the ejection of Ar atoms and clusters. For al1cafculations. the Ar* ion isincident at an angle of 45O from the surface normal along a (100) azimuth of the (001) crystal face and is given a kinetic energy of 100 or 500 eV. For the latter case. the crystal is not large enough to contain all the action. although the catculations do SHOWfor a qualitaGve indication
of the effect of primdry ion energy on the process. Smcc Ar has a closed-shell electronic confipura-
Volume 97.
CHEhilCAL
number 4.5
PNYSICS
tion the interaction among the atoms in the solid can be reasonably approximated by a pahwise additive font IS]. For the short-range repulsive interaction a Born-Mayer or exponential function fit to values determined by ab initio electronic structure techniques [S.9] is used for the potential. This interaction has the form R < 2.20 A,
V(R) = 3400 esp(-327R),
(1)
where R is the separation distance between two Ar atoms in a and the potential V(R) is in eV. Considerable effort has been expended to develop the longrange attractive interaction between two Ar atoms. with many different function forms and sets of parameters being tested [Sl. Since the processes esamiued in this study involve collision energies from tenths to tens of eV. the differences among the various forms should not he as hnportant as for the esauk~tion of extremely low-energy processes. Thus the hlorse potential [IO] 113sbeen chosen as follows: V(R)=
X (exp [ - 1_425(R - 4.04)\
- 2) .
The yields of species calculated to eject from an Ar(001) crystal bombarded by 100 and 500 eV .Ar+ are summarized in table 1. The average uumhcr of species ejected per trajectory is not sho\w due to the relatively sn~~sll number oftrz$ectories that were deter-
Table 1
(2)
where the potentkd isgiven in eV. ForR between 3.20 and 2.92 ,& cubic spline is used to smoothly connect the two forms in eqs. (1) and (2). For R greater than 6.3s A the potential is set equal to zero. The lattice constant of fee Ar is 5.311 A. To check for the formation of clusters. the relative kinetic energy, Tr, plus potential energy. r’, from eq. (2) for all psirs of atoms is cticulated after the collision cascade has been ternlinated. If the total energy of the dinier b-dimer = Tdinier + v tot r
(3)
is negative, then the tested ditner is bound. For many of the impact points several bound ditners rare detected. From these the possibility of linked or overlapping dhners is checked. If this condition is found. then nI
3. Results and discussion
Yield of species ejected from an Ar bomhuded Ar( 100) cr.\s-
2.92
tot
uate the possibility of fomilng a cluster. As in the dimer analysis, if E:F for the atoms in the linkage is less than zero then this set of n atoms is considered to be a cluster. This definition of cluster formation using the restriction of linked dimers underestimates the number of a_ggregates of atoms whose E;c$ster is negative. This overly stringent detiuition has been chosen because the interaction potmthl for each cluster size is not known and because the possibility ofunimolecular dissociation of the cluster on the way to the detector has not been taken into account.
(R -3.0-t)]
0.011-I esp[-1.425
,@nstcr= Tchster
27 Nsy 19s.;
LETTERS
+
1
I1
c c
i=l
i>i
v(R__)
v
’
with II being the number of atoms in the cluster, is recalculated for all of the atoms in the linkage to eval-
la1 3)
_-_.-.
----------
energy (eV) of the primary paticle b, number of primary Xr impacts total number of Ar ejwted number of species ejectrd Aq Arz
-______-___ 100 11 1161
SO0 11 132-a
773 63
s53 s9 19 12 3 6
AT3
30
Ar6
4
i\ra Ar5
12 1
At7
1
1
4
1
3
Arg
1
2
-ho
1
-Art2
Aw
1 I!
-hS
I
1
Ar2s Ar-JAr1
1 -
O-OS --l__l_
0.10
3 The yields presented here are ftx neutral species. For diea cxmqsrison x\ith SlhlS experiments an ionic interaction
needs to be included. b)The primary particle is initially at 3 poLu angle of lSJ ~irh respect to the surface normal in the (100) azimuthal dirertion.
C1lEMCXL PHYSICS LETTERS
27 hiiiy 1983
l-able 2 Percentago contribution cluster yield La) cr nwnber
of each layer to the total yield and -
Energy of
the
primary
tooev
tot31 ciiistrr yield Yield _- _“_______-__
1 2 3 >a -----.-I_
-----_
61 23 9 7
45 s-3 11 6
_~--_-
particie
500 eV -total yield
cluster yield
57 25 12 6
46 $1 1s 6
le. 500 eV Ar+ ion bombardment results in the ejection of 250 atoms. Such high action is found to occur r.ither iill*reqiiciitly. but when it does happen. the ejecting atoms ate in a favorable position to form lsgc shifters. in this case. the atoms noted in fig. le have formed an Arz5 species. The mechanism of formation is very similar to that found for Cu iiiultimers in that the atoms thai form the cluster arise from a fziirly locxlizrd rc$& of the surf&e altlxxiglt they need nor be cottti~uous. The concept of recoiilbiiiatioti discussed previously [7] isckrtrly appkable to thissituatiofi.
In
this
niechanism.
even
thou@
the
atoms
in
tlte
from 3 fairly localized region of the crystal. the collisioi~ processes which give rise to the ejection of the various ci)tlst~tueiit atoms are indeycndent. This is in contrast to the ejection of a molecular species such as lwtzeile \\here generallv one collision cjccrs rhe efltire 13amm cluster f 1 11. For Ar crystals, it does ,ippex. that third- and fourth-layer atoms may be involved ill cluster formation. In fig. le. fur exdmpk. e&lit atoms in tfie cluster arise from the third Lipr and thretl (xl01 &0wn) arise from the fourth layer, Similar for~clusioris iire rfrawn by examining the trajectory shitw~~ ill fig. If where tfle Ar17 cluster consisis of two first-layer atoms. four second-ktyer Adonis, five third-layer atoms and one f’ourth-layer atom. Note in this situation that none of the firstlayer atoms above third-layer atoms that eject and foml part of the cluster are found to form part of the cluster. In three of the cases. the first-layer atom is not even found to eject. In both of the examples shown in figs. le and 1f. then, it is clear that these large species do not form by direct fragmentation of the Ar lattice. cluster
w3ginstc
CHEMICAL
Volume 97, number 4,s
As the energy of the primary ion increases there is a greater propensity for the ejected atoms to be aggregated in clusters. This is reflected in the Ar,/Ar ratio as shown in table 1. This observation is in qualitative agreement with recent experiments where solid argon was bombarded by He+ ions at JO0 and 2000 eV. In this case the Ar;/Af went from =O.Ol to aO.1 as the energy was increased [?I. The kinetic energy distribution of the ejected Ar atoms is of interest from a mechanistic standpoint. The peak in the energy distribution curve is characteristic of the surface binding energy of Ar in the crystal and the presence of a high-energy tail is indicative of a collision cascade, rather than of a thermal, process. The calculated energy distribution for Ar is given in fig. 2a. The peak in the curve occurs at only 0.06 CV and the width is extremely narrow - on the order of 0.1 eV. From fig. 2b. however. the presence of the high-energy tail is clearly observable. There are no experimental results which are directly comparable to our calculations, although the distributions of Ar+ have been measured using a quadrupole mass spectrometer equipped with a Bessel box energy
PHYSICS LETTERS
17 Nay 19s3
analyzer [2]. In these experiments, the energy distributions werk found to exhibit a full-width at halfmaximum of =6 eV as well as the espected hightnergy tail characteristic of a collision cascade process. Although this energy spectrum is quite narrow relative to those obtained from metals. it is still considerably broader than the calculated spectra. There are several possible explanations for this devistion including charging. bandwidth of the analyzer and differences between ion and neutral trajectories (121. Ide.4y. it would be of interest to directly measure the distributions of the nentrals, perhaps using Doppler-shifted laser fluorescence [ 131 or multiphoton ionization techniques [ 141, to obtain a direct comparison with our predictions.
4. Conclusions The results of this study have raised several important points. First. it has been demonstrated that extremely high yields may be obtained from certain systems using the classicr\l dynamicsapproxh. For the case of solid argon. the reason for these hi& yields is simply that the very small cohesive energy in the solid allows a large number of low-ener~y atoms IO eject. This approach should now be useful to examine anomxlously high sputter& rates in other qstecx such ;1s ice [ 1 S] _Secondly, we have demonsrnred rhar ir is possible to produce very large clusters via the recombination mechanism. This mechanism should be apphcable to other situations where very I.ugc clusters have been observed 116) without having to conclude that they form via direct lattice fragmenration. Finally, the results for crystalline Ar should provide a quslitative basis for understanding the mechanism of energ> dissipation in matrix-isolation SIMS experiments,
Acknowledgement
1
I -I
25 ENERGY(eV1 Fig. 2. Calculated argon energy distribution. rqimc.
513
(a) Low-energy
The resolution is 0.01 eV. (b) High-enegy
resolution is 5 eV.
tail. The
The financial support ofthe National Science Foundation, the Office of h’aval Research. the Air Force Office of Scientific Research and the Petroleum Research Foundation is greatly acknowledged. One of us (BJG) is grateful to the Alfred P. Sloan Foundation and the Camille and Henry Dreyfus Foundation for support. The authors also appreciate srimulsting discussions and encouragement from Josef hlichl.
Solun~e 97, number 4.5
CHEMICAL
References [ I] H.T. Jonkman and J.Michl. J. Am. Chem. Sot. 103 (1981) 733. [Zj R-G. Orth, H.T_ Jonkman, D.H. Por\eiI and J. hlichl, J- Am. Chem. Sot. 103 (1981) 6026. f 3 1 H.T_ Jo&man and J_ hiichJ. J_ Chem. Sot. Chem. Commun. (1978) ?51_ 141 N. K’iiograd. B.J. Garrison and D-E. Harrison Jr., J_ them. Phys. 73 (1980) 3473. 1st D.E. Harrison Jr., P-W. KeJJy, B.J. Garrison and N. Winopnd, Surface Sci. 76 (1978) 311. [6] B.J. Garrison. N_ Wino8rad and D.E. Harrison Jr., J. Chein. Phys. 69 (1978) 1440. [ 7 J N. Winograd and B.J. Garrison, Accounts Chem. Res. 13 (1980)406.
PHYSICS
LETTERS
27 May 1983
[8] &f-L. Klein and J-k Venabfes, e&, Rare gas solid &ademic Press, New York, 1976). [9] T-L. Gilbert and AC Wabl, 3. C&em. Phys. 47 (1967) 3425. [IO] P-D. Kono~alowandS.Carr&. Phys.Fluids 8 (1965) 1585 [11] B.J. Garrison, J. Am. Chem. Sot. 102 (1980) 6553. 1121 R.A. Gibbs, S-P. Holland, II-E. Foley, B-J. Garrison and PI. Wino8rad. Phys_ Rev. 324 (1981) 6178. [ 131 Xf.J.PeUin,R.B.WrightandD.hf. C&en. J.Cbem,Phys. 74 (1981) 6448. [ 141 N. Winograd,Y.P_ BaxterandF.M. Kiiock. Chem. Phys. Letters 88 (1982) 581. [IS] P.K. Haff, C.C. Watson and Y.L. Yung, J. Geophys. Res. 86 (1981) 6933. [ 161 J-E. Camparm, T.&i. BarJak. R-J. Cotton, J-J. DeCorpo, J.R. Wyatt and B-1. Dunlap, Phys. Rev. Letters 47 (1981) 1046.
17 hlay 1983
CHEhIICAL PHYSICS LEl-l-ERS
Volume 97, number 4.5
ASPECTS OF CLUSTER FORhlATION
BY keV BOMBARDhlENT
OF RARE-GAS SOLIDS B.J. GARRISON and N. WINOGRAD Department of Chemistry. i%e Pennyhaniix State Universit_v, 152 Dare-y Loboratory. University Pnrk-.Pennsyh*ania 16802. USA Received 19 July 1982:in
final form 17 hfarcll
19S3
The mechanism of energy dissipation in arson crystals bombarded by 100 and 500 eV Ar+ has been e.\amincd using &ssic.d dynamics. The study indicates that very high yields may be expected and that large molecular Ar clus~srs. Ar,,. may form via recombination in the near surface region. The results are compared qualitatively to recent matrix-isolation SIMS ekpcrinlents on a vxiety of inertgas solids.
1_ Introduction
Rare-gas solids provide unique matrices for trapping a variety of reactive atoms and molecules. In recent studies it has also been found that the secondary ion mass spectra (SIMS) of compounds stabilized in solid Ar. Kr and Xe yield important structural information about the isolated material [ I-31 _In these experiments, the rare-gas solid is bombarded with a O-5-5 keV primary ion, often Ar+. The information about the matrix-isolated species is then obtained from the mass spectrum of the material ejected from the solid. The mechanism of energy dissipation of the primary ion and subsequent ejection of material has not been explored for rare-gas solids but has beeu investigated in detail for a variety of ion-bombarded metals such as Ni [4] and Cu [5] using well-established classical dynamics techniques. These techniques have also been employed to establish a generalized theory of the molecular
cluster
metal surfaces
ejection
process
on chemically
reacted
[6,7]. It is not clear, however, whether
these concepts are applicable to rare-gas solids where cohesive energies are more than an order of magnitude smaller. The SlhlS spectra of solid Ar is characterized by an intense Ar+ peak and a variety of Arz cluster ions [2] _The largest clusters are characterized by II < 25 as limited only by the mass range of tile detector.
Their relative intensities are found to decrease nexly esponentidily with increasing II. In addition. the Linetic energy distribution of ejected .4r+ ions is extremely narrow (O-l 2 eV) relative to those distrihutions found from metals [Sl. 11 is importam IL>understand these observations if matrix-isolation SlhlS is to become a viable technique. In this work. then. we examine the mechanism of energy dissipatkw in ion-bombarded Ar crystals and compare the results crf classical dynamics calculations to those obraiued on metals. The results show tltat very large yields may be r‘kpetted and that in contrast to metals, a sigm!k.mt fraction of tire atoms may originate from tbr scwiid or third layer. Luge clusters of Ar,, llsvtt heeu ob-
served in the calculations. Tile atom in thr‘ clustzt need not consist of atoms tbs: were originally contiguous in tlie lattice, altbougli many of tllem are ejected from highly localized regions of the solid. in general, we believe our model &culstious 1 icld 3 realistic picture of the rner=’ dissipation in crystalline Ar and should aid in the interpretation of matrixisolated SlhlS spectra.
2. Description
of the calculation
The basic scheme for computing
tlw dynamics
of 381
Solurw
97.
UlUUtwr
1.5
CHEhtlCt\L
p.wtcle and the Ar substrate h&sbeen &~~bed m .I lcccnt series of articies I&7]. Briefly. die pwtm~ md ffiwmit3 of all the partxles we drvri~~p~~iirt Iunc by mtcgr,~tin_stiamilton’s equations 513mktwt. The Iindl positions and momenta of the wcrt-d p&urrclcs CM rfxn he ttsed 10 determine yields, ~wrrg .md ,H@C di~tr~lrwnrrs. and poss~hle cluster the twt~k:~rti~~tg
!iilltlJtl~~ll
L’hct~~fii~t~r soikl IS tliotietcd hy 3 finite microcrysI.ithtr. fti this k-.tFeswsisrmg of ,i ~~ce~~litere~i-cubic .i11.1\r)l‘At :itrws. Tfic biLe of the cryst.tllirc is chosen ~1 1h.11 1111IIWI IIIC~LISL'S ill the nwnlwx of ;IKUIISdo w)t ~1llhldi1l14lly .Iltcr rtie pedicttxl 0bservablcs. Be, .tuw wtttl ..\t 1~ .ttt cwrwwly slnttll ht of subhmatofu (0 OS cV). .S~tystallrte ~$7 layers ~8th = 180 .1101i1> ~L*I1,1yr1(Ii&. 1~) ($1total u!‘ 1,765 9fw~s) is irrcdzd 1~1LWI~~.I~I~ 1hr procccws involved when 4 pri-
PHYSICS LE-ITERS
27 May I983
mary particle bomkwds the surface with only100eV. The cakulstion of one Ar impact on this crystal requires =5D times more computer time than a similar calcufation on a 230 atom Ni microcr~st~lit~. Due to this difficulty, only 21 Ar impacts have been determined for each set of c&xl&ions. This number is too mall IO produce statistically reliable quantitative resuits so our analysis is restricted to a mechanistic investigaGon of the ejection of Ar atoms and clusters. For al1cafculations. the Ar* ion isincident at an angle of 45O from the surface normal along a (100) azimuth of the (001) crystal face and is given a kinetic energy of 100 or 500 eV. For the latter case. the crystal is not large enough to contain all the action. although the catculations do SHOWfor a qualitaGve indication
of the effect of primdry ion energy on the process. Smcc Ar has a closed-shell electronic confipura-
Volume 97.
CHEhilCAL
number 4.5
PNYSICS
tion the interaction among the atoms in the solid can be reasonably approximated by a pahwise additive font IS]. For the short-range repulsive interaction a Born-Mayer or exponential function fit to values determined by ab initio electronic structure techniques [S.9] is used for the potential. This interaction has the form R < 2.20 A,
V(R) = 3400 esp(-327R),
(1)
where R is the separation distance between two Ar atoms in a and the potential V(R) is in eV. Considerable effort has been expended to develop the longrange attractive interaction between two Ar atoms. with many different function forms and sets of parameters being tested [Sl. Since the processes esamiued in this study involve collision energies from tenths to tens of eV. the differences among the various forms should not he as hnportant as for the esauk~tion of extremely low-energy processes. Thus the hlorse potential [IO] 113sbeen chosen as follows: V(R)=
X (exp [ - 1_425(R - 4.04)\
- 2) .
The yields of species calculated to eject from an Ar(001) crystal bombarded by 100 and 500 eV .Ar+ are summarized in table 1. The average uumhcr of species ejected per trajectory is not sho\w due to the relatively sn~~sll number oftrz$ectories that were deter-
Table 1
(2)
where the potentkd isgiven in eV. ForR between 3.20 and 2.92 ,& cubic spline is used to smoothly connect the two forms in eqs. (1) and (2). For R greater than 6.3s A the potential is set equal to zero. The lattice constant of fee Ar is 5.311 A. To check for the formation of clusters. the relative kinetic energy, Tr, plus potential energy. r’, from eq. (2) for all psirs of atoms is cticulated after the collision cascade has been ternlinated. If the total energy of the dinier b-dimer = Tdinier + v tot r
(3)
is negative, then the tested ditner is bound. For many of the impact points several bound ditners rare detected. From these the possibility of linked or overlapping dhners is checked. If this condition is found. then nI
3. Results and discussion
Yield of species ejected from an Ar bomhuded Ar( 100) cr.\s-
2.92
tot
uate the possibility of fomilng a cluster. As in the dimer analysis, if E:F for the atoms in the linkage is less than zero then this set of n atoms is considered to be a cluster. This definition of cluster formation using the restriction of linked dimers underestimates the number of a_ggregates of atoms whose E;c$ster is negative. This overly stringent detiuition has been chosen because the interaction potmthl for each cluster size is not known and because the possibility ofunimolecular dissociation of the cluster on the way to the detector has not been taken into account.
(R -3.0-t)]
0.011-I esp[-1.425
,@nstcr= Tchster
27 Nsy 19s.;
LETTERS
+
1
I1
c c
i=l
i>i
v(R__)
v
’
with II being the number of atoms in the cluster, is recalculated for all of the atoms in the linkage to eval-
la1 3)
_-_.-.
----------
energy (eV) of the primary paticle b, number of primary Xr impacts total number of Ar ejwted number of species ejectrd Aq Arz
-______-___ 100 11 1161
SO0 11 132-a
773 63
s53 s9 19 12 3 6
AT3
30
Ar6
4
i\ra Ar5
12 1
At7
1
1
4
1
3
Arg
1
2
-ho
1
-Art2
Aw
1 I!
-hS
I
1
Ar2s Ar-JAr1
1 -
O-OS --l__l_
0.10
3 The yields presented here are ftx neutral species. For diea cxmqsrison x\ith SlhlS experiments an ionic interaction
needs to be included. b)The primary particle is initially at 3 poLu angle of lSJ ~irh respect to the surface normal in the (100) azimuthal dirertion.
C1lEMCXL PHYSICS LETTERS
27 hiiiy 1983
l-able 2 Percentago contribution cluster yield La) cr nwnber
of each layer to the total yield and -
Energy of
the
primary
tooev
tot31 ciiistrr yield Yield _- _“_______-__
1 2 3 >a -----.-I_
-----_
61 23 9 7
45 s-3 11 6
_~--_-
particie
500 eV -total yield
cluster yield
57 25 12 6
46 $1 1s 6
le. 500 eV Ar+ ion bombardment results in the ejection of 250 atoms. Such high action is found to occur r.ither iill*reqiiciitly. but when it does happen. the ejecting atoms ate in a favorable position to form lsgc shifters. in this case. the atoms noted in fig. le have formed an Arz5 species. The mechanism of formation is very similar to that found for Cu iiiultimers in that the atoms thai form the cluster arise from a fziirly locxlizrd rc$& of the surf&e altlxxiglt they need nor be cottti~uous. The concept of recoiilbiiiatioti discussed previously [7] isckrtrly appkable to thissituatiofi.
In
this
niechanism.
even
thou@
the
atoms
in
tlte
from 3 fairly localized region of the crystal. the collisioi~ processes which give rise to the ejection of the various ci)tlst~tueiit atoms are indeycndent. This is in contrast to the ejection of a molecular species such as lwtzeile \\here generallv one collision cjccrs rhe efltire 13amm cluster f 1 11. For Ar crystals, it does ,ippex. that third- and fourth-layer atoms may be involved ill cluster formation. In fig. le. fur exdmpk. e&lit atoms in tfie cluster arise from the third Lipr and thretl (xl01 &0wn) arise from the fourth layer, Similar for~clusioris iire rfrawn by examining the trajectory shitw~~ ill fig. If where tfle Ar17 cluster consisis of two first-layer atoms. four second-ktyer Adonis, five third-layer atoms and one f’ourth-layer atom. Note in this situation that none of the firstlayer atoms above third-layer atoms that eject and foml part of the cluster are found to form part of the cluster. In three of the cases. the first-layer atom is not even found to eject. In both of the examples shown in figs. le and 1f. then, it is clear that these large species do not form by direct fragmentation of the Ar lattice. cluster
w3ginstc
CHEMICAL
Volume 97, number 4,s
As the energy of the primary ion increases there is a greater propensity for the ejected atoms to be aggregated in clusters. This is reflected in the Ar,/Ar ratio as shown in table 1. This observation is in qualitative agreement with recent experiments where solid argon was bombarded by He+ ions at JO0 and 2000 eV. In this case the Ar;/Af went from =O.Ol to aO.1 as the energy was increased [?I. The kinetic energy distribution of the ejected Ar atoms is of interest from a mechanistic standpoint. The peak in the energy distribution curve is characteristic of the surface binding energy of Ar in the crystal and the presence of a high-energy tail is indicative of a collision cascade, rather than of a thermal, process. The calculated energy distribution for Ar is given in fig. 2a. The peak in the curve occurs at only 0.06 CV and the width is extremely narrow - on the order of 0.1 eV. From fig. 2b. however. the presence of the high-energy tail is clearly observable. There are no experimental results which are directly comparable to our calculations, although the distributions of Ar+ have been measured using a quadrupole mass spectrometer equipped with a Bessel box energy
PHYSICS LETTERS
17 Nay 19s3
analyzer [2]. In these experiments, the energy distributions werk found to exhibit a full-width at halfmaximum of =6 eV as well as the espected hightnergy tail characteristic of a collision cascade process. Although this energy spectrum is quite narrow relative to those obtained from metals. it is still considerably broader than the calculated spectra. There are several possible explanations for this devistion including charging. bandwidth of the analyzer and differences between ion and neutral trajectories (121. Ide.4y. it would be of interest to directly measure the distributions of the nentrals, perhaps using Doppler-shifted laser fluorescence [ 131 or multiphoton ionization techniques [ 141, to obtain a direct comparison with our predictions.
4. Conclusions The results of this study have raised several important points. First. it has been demonstrated that extremely high yields may be obtained from certain systems using the classicr\l dynamicsapproxh. For the case of solid argon. the reason for these hi& yields is simply that the very small cohesive energy in the solid allows a large number of low-ener~y atoms IO eject. This approach should now be useful to examine anomxlously high sputter& rates in other qstecx such ;1s ice [ 1 S] _Secondly, we have demonsrnred rhar ir is possible to produce very large clusters via the recombination mechanism. This mechanism should be apphcable to other situations where very I.ugc clusters have been observed 116) without having to conclude that they form via direct lattice fragmenration. Finally, the results for crystalline Ar should provide a quslitative basis for understanding the mechanism of energ> dissipation in matrix-isolation SIMS experiments,
Acknowledgement
1
I -I
25 ENERGY(eV1 Fig. 2. Calculated argon energy distribution. rqimc.
513
(a) Low-energy
The resolution is 0.01 eV. (b) High-enegy
resolution is 5 eV.
tail. The
The financial support ofthe National Science Foundation, the Office of h’aval Research. the Air Force Office of Scientific Research and the Petroleum Research Foundation is greatly acknowledged. One of us (BJG) is grateful to the Alfred P. Sloan Foundation and the Camille and Henry Dreyfus Foundation for support. The authors also appreciate srimulsting discussions and encouragement from Josef hlichl.
Solun~e 97, number 4.5
CHEMICAL
References [ I] H.T. Jonkman and J.Michl. J. Am. Chem. Sot. 103 (1981) 733. [Zj R-G. Orth, H.T_ Jonkman, D.H. Por\eiI and J. hlichl, J- Am. Chem. Sot. 103 (1981) 6026. f 3 1 H.T_ Jo&man and J_ hiichJ. J_ Chem. Sot. Chem. Commun. (1978) ?51_ 141 N. K’iiograd. B.J. Garrison and D-E. Harrison Jr., J_ them. Phys. 73 (1980) 3473. 1st D.E. Harrison Jr., P-W. KeJJy, B.J. Garrison and N. Winopnd, Surface Sci. 76 (1978) 311. [6] B.J. Garrison. N_ Wino8rad and D.E. Harrison Jr., J. Chein. Phys. 69 (1978) 1440. [ 7 J N. Winograd and B.J. Garrison, Accounts Chem. Res. 13 (1980)406.
PHYSICS
LETTERS
27 May 1983
[8] &f-L. Klein and J-k Venabfes, e&, Rare gas solid &ademic Press, New York, 1976). [9] T-L. Gilbert and AC Wabl, 3. C&em. Phys. 47 (1967) 3425. [IO] P-D. Kono~alowandS.Carr&. Phys.Fluids 8 (1965) 1585 [11] B.J. Garrison, J. Am. Chem. Sot. 102 (1980) 6553. 1121 R.A. Gibbs, S-P. Holland, II-E. Foley, B-J. Garrison and PI. Wino8rad. Phys_ Rev. 324 (1981) 6178. [ 131 Xf.J.PeUin,R.B.WrightandD.hf. C&en. J.Cbem,Phys. 74 (1981) 6448. [ 141 N. Winograd,Y.P_ BaxterandF.M. Kiiock. Chem. Phys. Letters 88 (1982) 581. [IS] P.K. Haff, C.C. Watson and Y.L. Yung, J. Geophys. Res. 86 (1981) 6933. [ 161 J-E. Camparm, T.&i. BarJak. R-J. Cotton, J-J. DeCorpo, J.R. Wyatt and B-1. Dunlap, Phys. Rev. Letters 47 (1981) 1046.