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Production of non-stoichiometric SF6 cluster anions by electron attachment to SF6 clusters
Volume 201, number 1,2,3,4
CHEMICALPHYSICSLETTERS
1 January 1993
Production of non-stoichiometric SF6 cluster anions by electron attachment to SF6 clusters T. Rauth, 0. Echt ’ and T.D. Mix-k Institurfiir lonenphysik, Leopold Franzens Universitiil, Technikerstrasse 25, A-6020 Innsbruck, Austria
Received 20 July 1992;in final form 2 I September 1992
We report here on the detection of (SFs),,.X- (X= SFs, SF+ SFs) ions which form by electron attachment to SF6clusters in a molecular beam. The formation of some species, e.g. SFs,SFF with resonances at 6 and 10 eV, is unexpected because attachment to the bare SF, molecule does not yield the analogous monomer fragment ion in this energyrange.
1. Introduction Electron attachment to sulfur hexafluoride molecules in the gas phase has been the subject of numerous investigations. Most reports have focused on the energy dependence of the ion yield at low energies, because the use of SF6 as an effective electron scavenger in high-voltage devices is related to its exceptionally large capture cross section for thermal electrons [ 1-3 1.Likewise, electron attachment to free SFs clusters revealed a strong 0 eV resonance for formation of ( SFs); [ 4-61. However, intramolecular fragmentation into (SFs):SFc was reported to be completely absent, although SF, is a ubiquitous companion of SF; in gas phase studies of SF6 at low energies. A trace amount of (SF,);SF,was reported at an electron energy of 2.5 eV, but its intensity was negligible in comparison with that of (SFr,); [ 71. No other fragment ions were observed #I, although electron attachment to SF6 generates a rich spectrum of fragment ions with resonances in the range 2-l 3 eV [ 2,3 1. Quenching of intramolecular fragmentation is a well-documented phenomenon in electron attach-
’ Permanent address: Department of Physics,University of New Hampshire, Durham, NH03824-3568,USA. *r Associativeion-molecule reactions may lead to the formation of cluster ions, see, e.g., ref. [ 8 1.
ment studies of molecular clusters, making possible the observation of stoichiometric cluster ions (0,); 191, (Cod; [10,111, (SOA [121, and ( Hz0 ); [ 131, to name a few. However, the total absence of intramolecular fragment anions in previous studies of SF6 clusters is unprecedented. In this report, we re-investigate the yield of ions obtained by electron attachment to SF6 clusters in a molecular beam, with emphasis on the energy range l-20 eV. We observe a variety of fragment ions ( SFs). *X-, with X=SF5, SF4, SF,. These findings complement a recent report on SF? (i.e. SF6eF-) from our laboratory [ 141. The electron energy resonances of some of these ions closely parallel the yield of fragment ions from SF6 molecules. However, there are some notable differences: (i) Some ions which are formed from bare SF6 (e.g. F,- at 4.5 and 11 eV) do not find analogs in this cluster study. (ii) Some cluster ions, e.g. SF,*SF, at 6 and 10 eV, and SF,.SFr at 1O-l 5 eV, do not have analogs in attachment studies of bare SFs. (iii) In contrast to the situation of dissociative attachment to SFs, where the various relative abundances are quite different for different fragment ions, the various non-stoichiometric SF6 cluster ions are produced with approximately similar probabilities. These differences between attachment to SF6 and ( SFs). are tentatively attributed to intra-cluster ion-molecule reactions or charge-transfer reactions, and to the possibility that intramolecular fragmentation or autodetachment
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
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may be quenched in the transient cluster anion.
2. Experimental The crossed molecular beam/electron beam-mass spectrometer system has been described in detail [ 91. Neutral SF6 clusters are formed by expanding a gas mixture of SF6 at a typical partial pressure of 100 Torr and Ar at a typical partial pressure of 2 bar at -70°C through a 20 pm nozzle into vacuum. The expanding supersonic gas beam passes a skimmer and is crossed in a Nier-type ion source at right angles by an electron beam of variable energy. If necessary, the cluster beam can be blocked with a beam flag between skimmer and ion source. In this case only monomers will be present in the ion source as a stagnant gas target. The electron beam is guided by a weak magnetic field (4x lo-* T) and has a measured energy spread (fwhm) of x 0.5 eV [ 15 1, After electron attachment to the neutral target systems the resulting anions are extracted at right angles to both the cluster and electron beam by a weak electric field. The possible influence of this field on the electron energy and the measured cross section has been discussed previously [ 151. The extracted and focused ions are accelerated to 2.5 keV, pass a Iieldfree region and are analyzed in a double focusing sector field mass spectrometer of reversed geometry. Moreover, additional ion beam deflection plates (ydirection, that is, perpendicular to the mass spectrometer entrance slit) between ion source and mass spectrometer entrance (see fig. 2 in ref. [ 151) may be used to record the ion beam profiles at the mass spectrometer entrance. Shape and position of these ion beam profiles are directly related to the different starting conditions (e.g. kinetic energy, starting angle) of the ions in the ion source, i.e. it is possible to distinguish between ions generated from neutral clusters of different sizes due to the fact that neutral clusters produced by supersonic expansion are generated all with the same rather narrow velocity distribution and thus with kinetic energies depending on the cluster size (for details see refs. [ 15,161). Moreover, it is possible by using different beam deflection voltages U,, to distinguish for instance between ions produced by a primary resonance attachment process to a neutral cluster and ions produced 346
1January 1993
by possible secondary ion-molecule reactions in the background gas (e.g. see ref. [ 161). The electron energy scale was calibrated using the known cross section curves for the production of SF; and F- by electron attachment to SF6 molecules reported by Fenzlaff et al,. [ 3 1.These resonances reported by ref. [ 31 are approximately 0.3 eV higher in energy than those given by Kline et al. [ 21.
3. Results and discussion In the course of this study, we have observed a variety of cluster ions consisting of (SFs);X(X= SFs, SF4, SF3, F). Previous studies of electron attachment [4-61 or electron transfer [7] to SF6 clusters reported on the observation of ions ( SFa); (n 3 1) and SF, at or near thermal electron energies (EE< 1 eV). No anions originating from clusters were observed at higher energies, with the exception of (SF,);, which was formed indirectly, via inelastic scattering and subsequent capture (autoscavenging) [ 171. By contrast, long-lived SF,-, SF,, SF,-, F, and F- ions are formed by resonant attachment to SF6 in the energy range l- 15 eV [ 2,3 1. Our present data reveal that some of these fragment ions X-, and also their cluster analogs, (SF6);X(n= 1, 2)? do form via electron attachment to SF6 clusters. Moreover, ions such as (SFs);SFs (n =0, 1, 2) are formed at energies above 1 eV even though attachment to the molecule does not yield SF, in this energy range. It is essential to determine if a given ion arises from a neutral cluster or from the free molecule. Obviously, ( SFs);X- originates from clusters if nb 1, because ion-molecule reactions, e.g. X-+(SF,),+(SF,);X-+(m-n).SF,, are excluded under our experimental conditions (i.e. by studying the dependence of the respective ion signals on electron current, background gas pressure, cluster production conditions, ion extraction conditions, and on the beam flag position). The origin of “monomer ions” X- is more difficult to assess; we shall discuss them case by case. Most of the (SF,);Xions observed exhibit two resonances, one peaking at approximately 6 eV, the other at lo-15 eV. The latter is possibly composed
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of two or more independent resonances. Relative attachment cross section curves are shown in fig. 1 ((SF,);SF;,n=l,2),fig.2 ((SF,);SF:,n=O, 1, 2), and fig. 3 ( (SFs);SF~, II= 1, 2), respectively. The attachment spectrum of SFe*F- (i.e. the superhalogen ion SF, ) has been reported and discussed recently; it exhibits resonances at 6 and at 13 eV
[141. The low-energy resonance in the ion yield of (X=SF5, SF4, F) shows a number of common features: It peaks at or slightly below 6.0 eV, it is relatively narrow (fwhm z 2 eV) as compared to the high-energy resonance, and its intensity is high only if the deflection voltage U, is high (cf. figs. 1alc and figs. 2b and 2~). As explained in section 2, this indicates that the precursors of these ions are relatively large. The 6 eV resonance appears also in the attachment spectrum of (SFs)2SF~ (fig. Id). We speculate that its intensity is low because the deflection voltage was not optimized. It is conceivable (by comparison of the behavior of the relative intensity of the two resonances on CJ,in figs. 1 and 2), although we cannot prove it, that a similar resonance would be observable in the spectra of ( SF6)2,SFr and (SF6);SF~ (n=l, 2) if U, were set to higher values. The resonance also shows up in spectra of SF; (fig. 2a) and F- (not shown here, see fig. la in ref. [ 141). Finally, an isolated measurement at 5 eV reveals the presence of SF;. How do these features compare with electron attachment to SF6? In the range 3-8 eV, only F-, SF,- and F, are observed, with a maximum yield at 5.5, x 6, and c 5 eV, respectively [ 2,3 1. Their relative peak intensities differ by factors of ~5, Fbeing the strongest and F? the weakest. Other ions are conspiciously absent in this energy range. Hence, the monomer ion SF? observed at 5 eV in our experiment is definitely produced by electron attachment to clusters. F- and SF; may partially arise from attachment to SFs, but the fact that these ion signals maximize only if the deflection voltage is set to high values (cf. figs. la-lc) clearly shows that they also arise from attachment to clusters. This is confirmed by the fact that their intensity rises dramatically if the cluster beam is switched on. It has been suggested that SF,-, Fi and F- produced by electron attachment to SF6 derive from the same ion state in this energy range, and that the difSF6.X-
0 4
W',W-
2
-
E ; 0
(SF,%-
(4 3-
2, 2-
s si
b f
O, 1
0 5 corrected
4
10 electron ‘,
15
20
energy
25
(eV)
Fig. 1. Ion yield of SF,.SF; and (SF&SF; ionsproduced via electron attachment to a SF6cluster beam. Ions extracted from the ion source are deflected in front of the mass spectrometer entrance slit with the followingdeflection voltages U,: (a) 1.3V, (b) 4.0 V, (c) 6.1 V, and (d) 3.1 V.
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(4
9 6
(SF,SF,)-
7
6 5 4 3 2 1 0
5
1(b)
(SF,SF,)-
-I
i (b)
0
(SF,SF,)-
@FdzW) -
s
5
corrected
IO
electron
15
20
energy
I
25
..
J
(eV)
Fig. 3. Ion yield of SF,.SF? and (SF,),*SF; ions produced via electron attachment to a SF6 cluster beam. Ions extracted from the ion source are deflected in front of the mass spectrometer entrance slit with the following deflection voltages U,: (a) 4. I V, and (b) 6.7 V.
5
!
(4
((W)zSF,)-
ferences in their resonance energies stem from the differences in their heat of formation (referenced to SF; ) [ 3 1. The nature of the ion state in SF; remains unknown, although a state at 2.5 eV (symmetry a,,), which is known to exist from electron scattering experiments in SF, [ 181, is a possiblecandidate. Note that the reaction SF6Se-+SFi
+F2
is endothermic by AHa2.9 eV, formation of FF + SF, is endothermic by AH= 1.3 eV [ 191#*. corrected
electron
-energy
(eV)
Fig. 2. Ion yield of SFi, SFG’SFi and (SF,)s’SFi ions produced via electron attachment to a SF, cluster beam. Ions extracted from the ion source are deflected in front of the mass spectrometer entrance slit with the following deflection voltages U;(a) 1.4V,(b)3.6V, (c)6.3V,and(d)7.1V.
348
#* Heat of reactions are computed based on published disaociation energies D (SF,-F) [ 191 and electron affinities of SF, [ 201. Some of these data may be in error by several tenths of an electron volt. Also note that the heat of reaction may be somewhat different if it occurs in the cluster.
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Summarizing this subsection, the observation of SF7 and F- and its cluster analogs may not be surprising, but the appearance of (SF,);SF, , ~0, 1, 2, is completely unexpected. In the following we propose several mechanisms which may be responsible for the formation of these latter ions: ( 1) Ion-molecule reactions. The reaction SF,- +SFg-SFs
+SF5
is exothermic by m= - 0.5 eV [ 191. Therefore, dissociative attachment to (SF,), (ma 2) may yield (SF,);SF, (n< m), followed by the above ionmolecule reaction. This reaction cannot occur in studies of isolated SF, molecules, thus explaining the absence of SF,- at higher electron energies. However, due to the fact that SF,SFr is observed with large abundance in the present experiment, this process is a rather unlikely mechanism. Another possible ion-molecule reaction would be SF;+Fr+SF,+F
(mx-2.9eV
[19]),
This reaction is qualitatively different from the preceeding one, because caging of the fragmentation counterpart is required; therefore the neutral precursor has to contain many molecules (see below). Due to the high exothermicity the reaction will be followed by strong intermolecular fragmentation/ decomposition and high U,,voltages are required for detection of ions from this production mechanism. (2) Charge exchange reactions. F-+SF5+SF~
+F
e.g., is exothermic by A?i= -0.3 eV [ 191. Again, it would hardly occur unless the escape of SF* is quenched. ( 3 ) Quenching offragmentation (caging). It is well known that bimolecular reactions may be greatly enhanced if they occur in the stabilizing environment of a cluster [ 2 11. V-V as well as V-T transfer may occur. The stabilization will be particularly efficient if potential reactants are “caged” in a sufficiently large cluster [ 221. Clusters may also efficiently quench the dissociation of a primary, highly excited ion such as (CO, )*, formed upon vertical electron attachment [ 111. Therefore, another possibility of explaining the occurrence of SF: would be that the cluster quenches the fragmentation of highly excited (SFc)* into F-+SF4 (A~Yo2.5 eV). In any case,
1January I993
the cluster may have to accommodate a large amount of excess energy, and it may rapidly eject several SF6 molecules before its size is actualIy determined in the mass spectrometer. (4) Quenching of autodetachment. It appears conceivable that a short-lived SF, is one of many products from vertical electron attachment to SFs at 56 eV, but that it autodetaches (M~3.7 eV) in addition to its possible fragmentation into F-SSF, (LvI= 2.5 eV) or other, more endothermic products. Neutral SF5 would, of course, remain undetected. Efficient V-V transfer between (SF, )* and intra- or inter-molecular vibrational modes would not only prevent fragmentation, but it also would quench autodetachment. While the relevance of this mechanism may be doubtful in case of SF,, because the detachment channel would be significantly more endothermic than the fragmentation channel, it may become important in other cases. At elevated electron energies, above 8 eV, the following ions are formed by electron attachment to SF6 clusters: ( SFs);SFs (n= 1, 2; see fig. I), (SF,);SF,(tr=O, 1, 2; see fig. 2), (SF6);SF~ (n= 1, 2; see fig. 3), and (SFs);F(not shown; cf. ref. [ 141). An isolated measurement at 10 eV establishes the generation of SF: from SF6 clusters, while our experiments do not conlirm, nor do they rule out, the possible formation of F- or SF, from SFs clusters (i.e. these ions are known to originate from attachment to isolated SF6). For all ions this high energy resonance peaks at energies ranging from 10 to 15 eV. The exact location and shape of the ion yield depends on the stoichiometry and size of the cluster anion. Furthermore the maximum of the ion yield is seen to depend on the y-deflection voltage, cf. figs. 2b and 2c. One possible explanation is that the value of U, indirectly selects the size of the neutral precursor of, say, SF,,SFT, because the dissociation dynamics will depend on precursor size. However, the energetic shift of the resonance in fig. 2 would be unusually large if it was due to the size dependence of the solvation energy. Another, more likely explanation is that more than one ion state contributes to the apparent resonance at 1O-l 5 eV. Again, the fragmentation dynamics would (probably) be different for different ion states, hence their relative contribution to the ion yield would depend on U,. The following observations support this latter 349
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interpretation: (i) The cross section curves of some of these cluster anions (e.g. SF,.SF, , see fig. 1a) exhibit shoulders. (ii) Electron attachment above 8 eV to bare SF6 generates mainly F-, F? , and SF, (with decreasing yield, in this order), with main resonances at about 11 V and minor resonances at about 9 eV. Another, very weak resonance is observed for SF?, at 12 eV [ 2,3 1. The origins of these resonances are unknown. A more detailed comparison of the resonances observed for bare SF6 and for SF6 clusters is not warranted at this point. However, the absence of ( SF6)n.F? in the cluster study is remarkable. On the other hand, the formation of (SFs),sSFy (n= 1,2), and (SF,);SF,(n= 0, 1, 2) above 8 eV contrasts with the complete absence of SF, and SF; in studies of the bare molecule. One possible mechanism for the formation of SF;, e.g., from electron attachment to SF6 clusters is: SF,te--t(SFr)*+2F -SF,
t F t 2F, in the isolated molecule ,
SF,+e-+(SF,-)*tZF fSF6
SF,- + 2F, in the cluster complex .
We have already listed a number of possible mechanisms for the formation of anions of unexpected stoichiometry at the low energy resonance like SF,*SF? (see above); we abstain from suggesting any other ion-molecule or change-transfer reactions which might occur within the cluster ion at these higher electron energies, Suffice it to repeat that the present investigation demonstrates that clusters appear to suppress intramolecular fragmentation and1 or autodetachment of single anionic cluster constituents with high efficiency, whereas ion-molecule reactions cannot account for the observed energy resonances of the non-stoichiometric SF6 cluster anions.
Acknowledgement Work partially supported by the ostereichischer Fonds zur Fiirderung der Wissenschaftlichen Forschung, Wien.
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References [ 1] C.L. Chen and P.J. Chantry, I. Chem. Phys. 7 1 ( 1979) 3897; D. Klar, M.-W. Ruf and H. Hotop, Chem. Phys. Letters 189 (1992) 448, and references therein. [2] L.E. Kline, D.K. Davies, CL. Chen and P.J. Chantry, J. Appl. Phys. 50 (1979) 6789. [ 31 M. Fenzlaff, R. Gerhard and E. Illenbcrger, J. Chem. Phys. 88 (1988) 149. [41 M. Knapp, 0. E&t, D. Kreisle, T.D. M8rk and E. Recknagel, in: Physics and chemistry of small clusters, eds. P. Jena, B.K. Rao and S.N. Khanna, (Plenum Press, New York, 1987) p. 693. [ 510. Echt, M. Knapp, C. S&wan and E. Recknagel, in: Large finite systems, eds. I. Jortner, A. Pullman and B. Pullman (Reidel, Dordrecht, 1987) p. 179. [61 A. Stamatovic, P. Scheier and T.D. MXrk, J. Chem. Phys. 88 (1988) 6884. [71 K. Mitsuke, T. Kondow and K. Kuchitsu, J. Phys. Chem. 90 (1986) 1552. [ 81 R.N. Compton, D.R. Nelson and P.W. Reinhardt, Intern. J. Mass Spectry. Ion Phys. 6 (197 I ) 117; R.N. Compton, private communication (1992). [9] T.D. M&rk, K. kiter, W. Rittcr and A. Stamatovie, Phys. Rev. Letters 55 (1985) 2559; Intern. J. Mass Spectry. Ion Processes 74 (1986) 265. [lo] A. Stamatovic, K. Leiter, W. Ritter, K. Stephan and T.D. M&k, J. Chem. Phys. 83 (1985) 2942. [ 111 M. Knapp, 0. Echt, D. I&isle, T.D. M8rk and E. Recknagel, Chem. Phys. Letters 126 (1986) 225. [ 121 T.D. M&k, P. Scheier and A. Stamatovic, Chem. Phys. Letters136(1987) 117. [ 131 M. Knapp, 0. Echt, D. Kreisle and E. Recknagel, J. Phys. Chem 91 (1987) 2601. [ 141 T. Rauth,V.Grill, M. Foltin, P. Scheier and T.D. M&k, J. Chem. Phys. 96 ( 1992) 9241. [IS] K. Stephan, H. Helm and T.D. M&k, J. Chem. Phys. 73 (1980) 3763. [ 161 A. Stamatovic, P. Scheier and T.D. MilrL, Z. Physik D 6 (1987) 351. [ 171 T. Rauth, M. Foltin and T.D. Miirk, J. Phys. Chem. 96 (1992) 1528. [ 181 J.L. Dehmer, J. Siegel and D. Dill, J. Chem. Phys. 69 ( 1978) 5205, and references therein. [ 191 J.T. Herron, J. Phys. Cbem. Ref. Data 16 (1987) 1. [20] D.R. Lide, ed., CRC handbook of physics and chemistry, 71stEd. (1990-1991). [ 2 1] M. Henchman, Proc. EPCLP’9 1, eds. T.D. MIrk and R.W. Schrittwieser (Kilhtai, Inssbruck, 199 1) p. 126. 1221 F.G. Amar and L. Perera, 2. Physik D 20 ( 1991) 6720; M.L. Alexander, N.E. Levinger, M.A. Johnson, D. Ray and W.C. Lineberger, I. Chem. Phys. 88 (1988) 6200.
CHEMICALPHYSICSLETTERS
1 January 1993
Production of non-stoichiometric SF6 cluster anions by electron attachment to SF6 clusters T. Rauth, 0. Echt ’ and T.D. Mix-k Institurfiir lonenphysik, Leopold Franzens Universitiil, Technikerstrasse 25, A-6020 Innsbruck, Austria
Received 20 July 1992;in final form 2 I September 1992
We report here on the detection of (SFs),,.X- (X= SFs, SF+ SFs) ions which form by electron attachment to SF6clusters in a molecular beam. The formation of some species, e.g. SFs,SFF with resonances at 6 and 10 eV, is unexpected because attachment to the bare SF, molecule does not yield the analogous monomer fragment ion in this energyrange.
1. Introduction Electron attachment to sulfur hexafluoride molecules in the gas phase has been the subject of numerous investigations. Most reports have focused on the energy dependence of the ion yield at low energies, because the use of SF6 as an effective electron scavenger in high-voltage devices is related to its exceptionally large capture cross section for thermal electrons [ 1-3 1.Likewise, electron attachment to free SFs clusters revealed a strong 0 eV resonance for formation of ( SFs); [ 4-61. However, intramolecular fragmentation into (SFs):SFc was reported to be completely absent, although SF, is a ubiquitous companion of SF; in gas phase studies of SF6 at low energies. A trace amount of (SF,);SF,was reported at an electron energy of 2.5 eV, but its intensity was negligible in comparison with that of (SFr,); [ 71. No other fragment ions were observed #I, although electron attachment to SF6 generates a rich spectrum of fragment ions with resonances in the range 2-l 3 eV [ 2,3 1. Quenching of intramolecular fragmentation is a well-documented phenomenon in electron attach-
’ Permanent address: Department of Physics,University of New Hampshire, Durham, NH03824-3568,USA. *r Associativeion-molecule reactions may lead to the formation of cluster ions, see, e.g., ref. [ 8 1.
ment studies of molecular clusters, making possible the observation of stoichiometric cluster ions (0,); 191, (Cod; [10,111, (SOA [121, and ( Hz0 ); [ 131, to name a few. However, the total absence of intramolecular fragment anions in previous studies of SF6 clusters is unprecedented. In this report, we re-investigate the yield of ions obtained by electron attachment to SF6 clusters in a molecular beam, with emphasis on the energy range l-20 eV. We observe a variety of fragment ions ( SFs). *X-, with X=SF5, SF4, SF,. These findings complement a recent report on SF? (i.e. SF6eF-) from our laboratory [ 141. The electron energy resonances of some of these ions closely parallel the yield of fragment ions from SF6 molecules. However, there are some notable differences: (i) Some ions which are formed from bare SF6 (e.g. F,- at 4.5 and 11 eV) do not find analogs in this cluster study. (ii) Some cluster ions, e.g. SF,*SF, at 6 and 10 eV, and SF,.SFr at 1O-l 5 eV, do not have analogs in attachment studies of bare SFs. (iii) In contrast to the situation of dissociative attachment to SFs, where the various relative abundances are quite different for different fragment ions, the various non-stoichiometric SF6 cluster ions are produced with approximately similar probabilities. These differences between attachment to SF6 and ( SFs). are tentatively attributed to intra-cluster ion-molecule reactions or charge-transfer reactions, and to the possibility that intramolecular fragmentation or autodetachment
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
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may be quenched in the transient cluster anion.
2. Experimental The crossed molecular beam/electron beam-mass spectrometer system has been described in detail [ 91. Neutral SF6 clusters are formed by expanding a gas mixture of SF6 at a typical partial pressure of 100 Torr and Ar at a typical partial pressure of 2 bar at -70°C through a 20 pm nozzle into vacuum. The expanding supersonic gas beam passes a skimmer and is crossed in a Nier-type ion source at right angles by an electron beam of variable energy. If necessary, the cluster beam can be blocked with a beam flag between skimmer and ion source. In this case only monomers will be present in the ion source as a stagnant gas target. The electron beam is guided by a weak magnetic field (4x lo-* T) and has a measured energy spread (fwhm) of x 0.5 eV [ 15 1, After electron attachment to the neutral target systems the resulting anions are extracted at right angles to both the cluster and electron beam by a weak electric field. The possible influence of this field on the electron energy and the measured cross section has been discussed previously [ 151. The extracted and focused ions are accelerated to 2.5 keV, pass a Iieldfree region and are analyzed in a double focusing sector field mass spectrometer of reversed geometry. Moreover, additional ion beam deflection plates (ydirection, that is, perpendicular to the mass spectrometer entrance slit) between ion source and mass spectrometer entrance (see fig. 2 in ref. [ 151) may be used to record the ion beam profiles at the mass spectrometer entrance. Shape and position of these ion beam profiles are directly related to the different starting conditions (e.g. kinetic energy, starting angle) of the ions in the ion source, i.e. it is possible to distinguish between ions generated from neutral clusters of different sizes due to the fact that neutral clusters produced by supersonic expansion are generated all with the same rather narrow velocity distribution and thus with kinetic energies depending on the cluster size (for details see refs. [ 15,161). Moreover, it is possible by using different beam deflection voltages U,, to distinguish for instance between ions produced by a primary resonance attachment process to a neutral cluster and ions produced 346
1January 1993
by possible secondary ion-molecule reactions in the background gas (e.g. see ref. [ 161). The electron energy scale was calibrated using the known cross section curves for the production of SF; and F- by electron attachment to SF6 molecules reported by Fenzlaff et al,. [ 3 1.These resonances reported by ref. [ 31 are approximately 0.3 eV higher in energy than those given by Kline et al. [ 21.
3. Results and discussion In the course of this study, we have observed a variety of cluster ions consisting of (SFs);X(X= SFs, SF4, SF3, F). Previous studies of electron attachment [4-61 or electron transfer [7] to SF6 clusters reported on the observation of ions ( SFa); (n 3 1) and SF, at or near thermal electron energies (EE< 1 eV). No anions originating from clusters were observed at higher energies, with the exception of (SF,);, which was formed indirectly, via inelastic scattering and subsequent capture (autoscavenging) [ 171. By contrast, long-lived SF,-, SF,, SF,-, F, and F- ions are formed by resonant attachment to SF6 in the energy range l- 15 eV [ 2,3 1. Our present data reveal that some of these fragment ions X-, and also their cluster analogs, (SF6);X(n= 1, 2)? do form via electron attachment to SF6 clusters. Moreover, ions such as (SFs);SFs (n =0, 1, 2) are formed at energies above 1 eV even though attachment to the molecule does not yield SF, in this energy range. It is essential to determine if a given ion arises from a neutral cluster or from the free molecule. Obviously, ( SFs);X- originates from clusters if nb 1, because ion-molecule reactions, e.g. X-+(SF,),+(SF,);X-+(m-n).SF,, are excluded under our experimental conditions (i.e. by studying the dependence of the respective ion signals on electron current, background gas pressure, cluster production conditions, ion extraction conditions, and on the beam flag position). The origin of “monomer ions” X- is more difficult to assess; we shall discuss them case by case. Most of the (SF,);Xions observed exhibit two resonances, one peaking at approximately 6 eV, the other at lo-15 eV. The latter is possibly composed
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of two or more independent resonances. Relative attachment cross section curves are shown in fig. 1 ((SF,);SF;,n=l,2),fig.2 ((SF,);SF:,n=O, 1, 2), and fig. 3 ( (SFs);SF~, II= 1, 2), respectively. The attachment spectrum of SFe*F- (i.e. the superhalogen ion SF, ) has been reported and discussed recently; it exhibits resonances at 6 and at 13 eV
[141. The low-energy resonance in the ion yield of (X=SF5, SF4, F) shows a number of common features: It peaks at or slightly below 6.0 eV, it is relatively narrow (fwhm z 2 eV) as compared to the high-energy resonance, and its intensity is high only if the deflection voltage U, is high (cf. figs. 1alc and figs. 2b and 2~). As explained in section 2, this indicates that the precursors of these ions are relatively large. The 6 eV resonance appears also in the attachment spectrum of (SFs)2SF~ (fig. Id). We speculate that its intensity is low because the deflection voltage was not optimized. It is conceivable (by comparison of the behavior of the relative intensity of the two resonances on CJ,in figs. 1 and 2), although we cannot prove it, that a similar resonance would be observable in the spectra of ( SF6)2,SFr and (SF6);SF~ (n=l, 2) if U, were set to higher values. The resonance also shows up in spectra of SF; (fig. 2a) and F- (not shown here, see fig. la in ref. [ 141). Finally, an isolated measurement at 5 eV reveals the presence of SF;. How do these features compare with electron attachment to SF6? In the range 3-8 eV, only F-, SF,- and F, are observed, with a maximum yield at 5.5, x 6, and c 5 eV, respectively [ 2,3 1. Their relative peak intensities differ by factors of ~5, Fbeing the strongest and F? the weakest. Other ions are conspiciously absent in this energy range. Hence, the monomer ion SF? observed at 5 eV in our experiment is definitely produced by electron attachment to clusters. F- and SF; may partially arise from attachment to SFs, but the fact that these ion signals maximize only if the deflection voltage is set to high values (cf. figs. la-lc) clearly shows that they also arise from attachment to clusters. This is confirmed by the fact that their intensity rises dramatically if the cluster beam is switched on. It has been suggested that SF,-, Fi and F- produced by electron attachment to SF6 derive from the same ion state in this energy range, and that the difSF6.X-
0 4
W',W-
2
-
E ; 0
(SF,%-
(4 3-
2, 2-
s si
b f
O, 1
0 5 corrected
4
10 electron ‘,
15
20
energy
25
(eV)
Fig. 1. Ion yield of SF,.SF; and (SF&SF; ionsproduced via electron attachment to a SF6cluster beam. Ions extracted from the ion source are deflected in front of the mass spectrometer entrance slit with the followingdeflection voltages U,: (a) 1.3V, (b) 4.0 V, (c) 6.1 V, and (d) 3.1 V.
341
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(4
9 6
(SF,SF,)-
7
6 5 4 3 2 1 0
5
1(b)
(SF,SF,)-
-I
i (b)
0
(SF,SF,)-
@FdzW) -
s
5
corrected
IO
electron
15
20
energy
I
25
..
J
(eV)
Fig. 3. Ion yield of SF,.SF? and (SF,),*SF; ions produced via electron attachment to a SF6 cluster beam. Ions extracted from the ion source are deflected in front of the mass spectrometer entrance slit with the following deflection voltages U,: (a) 4. I V, and (b) 6.7 V.
5
!
(4
((W)zSF,)-
ferences in their resonance energies stem from the differences in their heat of formation (referenced to SF; ) [ 3 1. The nature of the ion state in SF; remains unknown, although a state at 2.5 eV (symmetry a,,), which is known to exist from electron scattering experiments in SF, [ 181, is a possiblecandidate. Note that the reaction SF6Se-+SFi
+F2
is endothermic by AHa2.9 eV, formation of FF + SF, is endothermic by AH= 1.3 eV [ 191#*. corrected
electron
-energy
(eV)
Fig. 2. Ion yield of SFi, SFG’SFi and (SF,)s’SFi ions produced via electron attachment to a SF, cluster beam. Ions extracted from the ion source are deflected in front of the mass spectrometer entrance slit with the following deflection voltages U;(a) 1.4V,(b)3.6V, (c)6.3V,and(d)7.1V.
348
#* Heat of reactions are computed based on published disaociation energies D (SF,-F) [ 191 and electron affinities of SF, [ 201. Some of these data may be in error by several tenths of an electron volt. Also note that the heat of reaction may be somewhat different if it occurs in the cluster.
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CHEMICALPHYSICSLETTERS
Summarizing this subsection, the observation of SF7 and F- and its cluster analogs may not be surprising, but the appearance of (SF,);SF, , ~0, 1, 2, is completely unexpected. In the following we propose several mechanisms which may be responsible for the formation of these latter ions: ( 1) Ion-molecule reactions. The reaction SF,- +SFg-SFs
+SF5
is exothermic by m= - 0.5 eV [ 191. Therefore, dissociative attachment to (SF,), (ma 2) may yield (SF,);SF, (n< m), followed by the above ionmolecule reaction. This reaction cannot occur in studies of isolated SF, molecules, thus explaining the absence of SF,- at higher electron energies. However, due to the fact that SF,SFr is observed with large abundance in the present experiment, this process is a rather unlikely mechanism. Another possible ion-molecule reaction would be SF;+Fr+SF,+F
(mx-2.9eV
[19]),
This reaction is qualitatively different from the preceeding one, because caging of the fragmentation counterpart is required; therefore the neutral precursor has to contain many molecules (see below). Due to the high exothermicity the reaction will be followed by strong intermolecular fragmentation/ decomposition and high U,,voltages are required for detection of ions from this production mechanism. (2) Charge exchange reactions. F-+SF5+SF~
+F
e.g., is exothermic by A?i= -0.3 eV [ 191. Again, it would hardly occur unless the escape of SF* is quenched. ( 3 ) Quenching offragmentation (caging). It is well known that bimolecular reactions may be greatly enhanced if they occur in the stabilizing environment of a cluster [ 2 11. V-V as well as V-T transfer may occur. The stabilization will be particularly efficient if potential reactants are “caged” in a sufficiently large cluster [ 221. Clusters may also efficiently quench the dissociation of a primary, highly excited ion such as (CO, )*, formed upon vertical electron attachment [ 111. Therefore, another possibility of explaining the occurrence of SF: would be that the cluster quenches the fragmentation of highly excited (SFc)* into F-+SF4 (A~Yo2.5 eV). In any case,
1January I993
the cluster may have to accommodate a large amount of excess energy, and it may rapidly eject several SF6 molecules before its size is actualIy determined in the mass spectrometer. (4) Quenching of autodetachment. It appears conceivable that a short-lived SF, is one of many products from vertical electron attachment to SFs at 56 eV, but that it autodetaches (M~3.7 eV) in addition to its possible fragmentation into F-SSF, (LvI= 2.5 eV) or other, more endothermic products. Neutral SF5 would, of course, remain undetected. Efficient V-V transfer between (SF, )* and intra- or inter-molecular vibrational modes would not only prevent fragmentation, but it also would quench autodetachment. While the relevance of this mechanism may be doubtful in case of SF,, because the detachment channel would be significantly more endothermic than the fragmentation channel, it may become important in other cases. At elevated electron energies, above 8 eV, the following ions are formed by electron attachment to SF6 clusters: ( SFs);SFs (n= 1, 2; see fig. I), (SF,);SF,(tr=O, 1, 2; see fig. 2), (SF6);SF~ (n= 1, 2; see fig. 3), and (SFs);F(not shown; cf. ref. [ 141). An isolated measurement at 10 eV establishes the generation of SF: from SF6 clusters, while our experiments do not conlirm, nor do they rule out, the possible formation of F- or SF, from SFs clusters (i.e. these ions are known to originate from attachment to isolated SF6). For all ions this high energy resonance peaks at energies ranging from 10 to 15 eV. The exact location and shape of the ion yield depends on the stoichiometry and size of the cluster anion. Furthermore the maximum of the ion yield is seen to depend on the y-deflection voltage, cf. figs. 2b and 2c. One possible explanation is that the value of U, indirectly selects the size of the neutral precursor of, say, SF,,SFT, because the dissociation dynamics will depend on precursor size. However, the energetic shift of the resonance in fig. 2 would be unusually large if it was due to the size dependence of the solvation energy. Another, more likely explanation is that more than one ion state contributes to the apparent resonance at 1O-l 5 eV. Again, the fragmentation dynamics would (probably) be different for different ion states, hence their relative contribution to the ion yield would depend on U,. The following observations support this latter 349
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interpretation: (i) The cross section curves of some of these cluster anions (e.g. SF,.SF, , see fig. 1a) exhibit shoulders. (ii) Electron attachment above 8 eV to bare SF6 generates mainly F-, F? , and SF, (with decreasing yield, in this order), with main resonances at about 11 V and minor resonances at about 9 eV. Another, very weak resonance is observed for SF?, at 12 eV [ 2,3 1. The origins of these resonances are unknown. A more detailed comparison of the resonances observed for bare SF6 and for SF6 clusters is not warranted at this point. However, the absence of ( SF6)n.F? in the cluster study is remarkable. On the other hand, the formation of (SFs),sSFy (n= 1,2), and (SF,);SF,(n= 0, 1, 2) above 8 eV contrasts with the complete absence of SF, and SF; in studies of the bare molecule. One possible mechanism for the formation of SF;, e.g., from electron attachment to SF6 clusters is: SF,te--t(SFr)*+2F -SF,
t F t 2F, in the isolated molecule ,
SF,+e-+(SF,-)*tZF fSF6
SF,- + 2F, in the cluster complex .
We have already listed a number of possible mechanisms for the formation of anions of unexpected stoichiometry at the low energy resonance like SF,*SF? (see above); we abstain from suggesting any other ion-molecule or change-transfer reactions which might occur within the cluster ion at these higher electron energies, Suffice it to repeat that the present investigation demonstrates that clusters appear to suppress intramolecular fragmentation and1 or autodetachment of single anionic cluster constituents with high efficiency, whereas ion-molecule reactions cannot account for the observed energy resonances of the non-stoichiometric SF6 cluster anions.
Acknowledgement Work partially supported by the ostereichischer Fonds zur Fiirderung der Wissenschaftlichen Forschung, Wien.
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