1552
J . Phys. Chem. 1986, 90, 1552-1556
Formation of Negative Cluster Ions in Collision of SF, Clusters with Krypton Rydberg Atoms Koichiro Mitsuke, Tamotsu Kondow,* and Kozo Kuchitsu Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo I 1 3, Japan (Receiued: September 23, 1985)
Neutral SF6clusters formed in a supersonic beam seeded in He were subjected to impact of either high-Rydberg atoms (RAI) or electrons with an average energy of =2.5 eV (EI). Negative ions, (SF,),- ( 1 In 5 I I ) , produced by RAI were observed by mass spectrometry,while SF5-and a trace of (SF6),,SF5- (2 In 5 4) were also observed by EI. The intensities of (SF,); ( n 1 3) produced by RAI decreased less rapidly with increasing n than those produced by EI. This difference in the size distributions is ascribed to that in the degree of evaporation of SF6 molecules from the cluster ions: In RAI the evaporation does not extensively influence the distribution, whereas in E1 several molecules are removed from the cluster ion in order to release the excess energy given by the electron trapped in the cluster. The absence of ( S F ~ ) ” - I S F(~n-1 2) in RAI is probably caused by the smallness of the excess vibrational energy localized in the SF6- component in (SF6)[. The small cross section for the corresponding dissociative electron attachment in E1 suggests that the energy of the attached electron is transmitted more efficiently to the intermolecular vibrations of the cluster ion than to the intramolecular vibrations of the SF6- component in (SF6)n-.
Introduction Much attention has recently been directed to properties of negatively charged clusters. In particular, the bond energies and other thermochemical values related to association reactions of neutral molecules to ions have been determined for many systems of chemical importance by high-pressure mass spectrometry.’,* On the contrary, very little has been studied on the processes and mechanisms of electron attachment to clusters. Existing experimental and theoretical results suggest that the adiabatic electron affinity of a neutral cluster in general increases with the cluster size because the attached electron is stabilized by a collective effect of intracluster interaction^.^-^ However, systematic studies have scarcely been made on fundamental problems such as the size dependence of the vertical electron affinities of clusters, the cross sections for electron attachment to clusters, and the lifetimes of temporary negative-ion states with respect to autodetachment. Quantitative measurements of these values should contribute significantly to understanding of various electronic properties, such as inelastic scattering of electrons by phonons and electron trapping by formation of small polarons in the condensed phase. The evaporation from (or fragmentation of) negative cluster ions presents another problem of particular interest in contrast with similar processes of positive cluster ions produced by electron or photon impact, for which substantial disruption of van der Waals bonds is known to Because of the great importance for determining the cluster size distribution by mass spectrometry, fragmentation probabilities of positive cluster ions have been studied by laser spectro~copy,’~ laser bolometry,I4 and beam For instance, evaporation from argon cluster ( I ) Mark, T. D.; Castleman, A. W.. Jr. Ado. Atom. Mol. Phys. 1985, 20, 65. (2) Kebarle, P. Annu. Reo. Phys. Chem. 1977, 28, 445. (3) Knapp, M.; Kreisle, D.; Echt. 0.;Sattler, K.; Recknagel, E. Surf Sci. 1985, 156, 313. (4) Klots, C. E.; Compton, R. N . J . Chem. Phys. 1977, 67, 1779. (5) Haberland, H.; Ludewight, C.: Schindler, H.: Worsnop, D. R. Z . Phys. A 1985, 320, 151. ( 6 ) Rossi, A. R.; Jordan, K. D. J . Chem. Phys. 1979, 70, 4422. (7) Bowen, K. H.; Liesegang, G. W.; Sanders, R. A,; Herschbach, D. R. J . Phys. Chem. 1983, 87, 557. (8) (a) Lee, N.; Fenn, J. B. Rev. Sci. Instrum. 1978,49, 1269. (b) Gentry, W. R. Ibid. 1982, 53, 1492. (c) Fenn, J. B.; Lee, N.Ibid. 1982, 53, 1494. (9) Haberland, H. Surf. Sci. 1985, 156, 305. ( I O ) Peterson, K. I.; Dao, P. D.; Farley, R. W.; Castleman, A . W., Jr. J . Chem. Phys. 1984, 80, 1780. (11) Grimley, R. T.; Forsman, J. A,; Grindstaff, Q. G. J . Phys. Chem. 1978, 82, 632. (12) Dehmer, P. M.; Pratt, S. T. J . Chem. Phys. 1982, 76, 843. (13) Geraedts, J.; Stolte, S.; Reuss, J . Z . Phys. A 1982, 304, 167. (14) Gough, T. E.; Miller, R. E. Chem. Phys. Lett. 1982, 87, 280. (15) Buck, U.; Meyer, H. Surf. Sci. 1985, 156. 275. (16) Buck, U.; Meyer, H. Ber. Bunsenges. Phys. Chem. 1984, 88, 254.
0022-3654/86/2090-1552$01.50/0
ions produced by electron impact has been explained by a temperature increase due to the excess energy released by the formation of a valence bond between Arf and Ar in the ~ l u s t e r , ~ , ~ ’ and this model has been supported by molecular-dynamics calculations. ] * , I 9 Much less information has been obtained on the corresponding processes with negative ions, probably because of the following c o m p l e ~ i t i e s :(i)~ The ~ ~ ~extent ~ ~ of evaporation depends greatly on the kinetic energy of the attached electron, because the electron kinetic energy has to be taken into the cluster and contributes to the excess energy. (ii) A negative cluster ion undergoes competing processes for self-stabilization, such as dissociation of a chemical bond (dissociative electron attachment) and autodetachment in addition to evaporation, because the negative-ion states of a single molecule in the cluster to which the impinging electron is first trapped are very often unstable. (iii) Though the electron kinetic energy is small in magnitude, ion-molecule half-reactions can frequently occur after formation of a negative ion, by which extra energy may be released in the cluster. The dynamical behavior of negative ions is by itself an important problem of chemical physics, but it may also provide a useful means for determination of the size distribution of neutral clusters, if studied complementarily with mass spectrometry of positive cluster ions. From this standpoint, we have made a study of the formation of (SF,); (1 In I11) by impact of high-Rydberg rare gas atoms and low-energy electrons on the SF6clusters. High-Rydberg atoms are used as carriers of thermal or subthermal electrons,23so that the excess electron energy transmitted to the target system can be minimized. Furthermore, high-Rydberg atoms are expected to have large cross sections for negative-ion formation. The electron attachment to SF6monomer has recently been investigated extensively, because it is the best-known scavenger of slow electrons with a large cross section for attachment and is also a gaseous d i e l e c t r i ~ . ~ It~ -has ~ ~been reported that, if SF,- is produced by (1 7 ) Birkhofer, H. P.; Haberland, H.; Winterer, M . ; Worsnop, D. R. Ber. Bunsenges. Phys. Chem. 1984, 88, 201. (18) Soler, J. M.; SBenz, J. J.; Garcia, N.; Echt, 0. Chem. Phys. Lett. 1984, 109, 71. (19) SBenz, J. J.; Soler, J. M.; Gar& N. Chem. Phys. Lett. 1985, 114, 15.
(20) Klots, C. E.; Compton, R. N . J . Chem. Phys. 1978, 69, 1636. (21) Stamatovic, A.; Stephan, K.; Mark, T. D. Int. J. Mass Spectrom. Ion Processes 1985, 63, 37. (22) Klots, C. E.; Compton, R. N. J . Chem. Phys. 1978, 69, 1644. (23) Matsuzawa, M. In ’Rydberg States of Atoms and LMolecules”; Stebbings, R. F., Dunning, F. B., Eds.; Cambridge University Press: Cambridge, U.K., 1983: p 267. (24) Christophorou, L. G. In “Electron and Ion Swarms”: Christophorou, L . G., Ed.; Pergamon Press: New York, 1981; p 261. (25) Klein, G. W.: Schuler, R. H . J . Phys. Chem. 1973, 77, 978.
0 1986 American Chemical Society
Collision of SF6 Clusters with Krypton Rydberg Atoms
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The Journal of Physical Chemistry, Vol. 90. No. 8. 1986
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IV
Gc
GB
GA Mass
Analyzer
F N 8 c I8 IL QP IC D Figure 1. Schematic diagram of the apparatus. Four vacuum chambers are differentially pumped. A sample gas was expanded from a nozzle (N). Supersonic cluster beam was extracted through a skimmer (S) and a collimator (C). The cluster ions produced in a concentric triple-grid ion source (IS)were focused by ion lenses (IL) and mass-analyzed by a quadrupole mass spectrometer (QP).Negative ions were converted by an ion conversion dynode (IC) and detected by a Ceratran electron multiplier (D).
attachment of an electron with energy lower than about 0.1 eV, its lifetime is as long as a typical ion flight time in a conventional mass ~pectrometer?~'~ and no indication of ion-molecule reaction between SF; and SF, has been presented. Therefore, SF, clusters are thought to be one of the most suitable systems for our present study of the energetics and the dynamics of negative-ion formation. Experimental Section A schematic diagram of the apparatus is shown in Figure I . The apparatus consists of (i) a supersonic beam source with four stages of differential pumping, (ii) a triple-grid ion source where neutral clusters are ionized, (iii) a quadrupole mass filter with a negative ion detector, and (iv) a CAMAC system based on an LSI-I 1/23 computer. The beam source consists of a sonic nozzle having a thin platinum aperture and a conical skimmer (Beam Dynamics) with an entrance hole of 0.31 mm diameter; the nozzle-skimmer distance is adjusted externally lo be typically 5 mm. The nozzle orifice has a diameter of 50 pm and a channel length of 0.2 mm. The sample gas, SF, (Takachiho, 99.99%pure), was seeded in helium gas with a stagnation pressure of 3.5 atm, and the nozzle temperature was maintained at 293 K. A gas inlet system made of stainless steel was baked under vacuum before use. A mixture gas of H e with SF, was expanded from the nozzle into a beam expansion chamber, and a cluster beam was formed. The chamber was evacuated by a 6-in. diffusion pump so that the chamber pressure did not exceed torr when the beam was introduced. The partial pressure of SF, was adjusted so a s to maximize the intensity of SF6SF,+produced by electron impact. The intensity of this ion depended strongly on the mole fraction of SF, in He. The optimum fraction of SF, was found to be 0.05-0.1. The clusters in the supersonic beam were introduced through the skimmer into a collimation chamber pumped by a 4-in. diffusion pump to IW5-l0-6 torr when the beam was on. The beam was further collimated into a reaction chamber, where a concentric triple-grid ion source was placed. The ion source had a housing of 20 mm length and 60 mm diameter. in which three concentric cylindrical grids and filaments were mounted as shown in Figure 2. The grids G,, G , and Gc were made of stainless-steel mesh with a transmittance of 80%. The central region surrounded by G , was 20 mm in length and I O mm in diameter. All grids were insulated by ruby balls of 3 mm diameter. The housing was isolated from the grids by spacers made of talc porcelain. Four pieces of helical filaments made of thoriated tungsten wire of 0.15 mm diameter formed a regular (26) f i a y . E.; Mogcnscn, 0. E. Chew. Phys. 1980, 53, 131. (27) Christophorou, L. G.: Mathis. R. A.: James, D. R.; MeCorkle, D.L. 3. Phys. D 1981. 14, 1889. (28) Collins, P. M.; Christophorou, L. G.; Chancy. E. L.; Carter, J. G. Chem. Phys. L d l .
1970,4,646.
(29) Christophorou. L. G.: Hadjiantaniou. A,; Carter, J. G. J. Chem. Soe. 1973, 69. I 7 13.
Faraday Trans. 2
.H. Kr Figure 2. A side view (right) and a front view (left) of the ion source. Krypton gas is excited by impact of electrons emitted from four filaments (F). Three concentric grids, G,. G 8 . and G,. are installed in a housing (H) of the ion source for preventing ionic species and electrons from entering the central region surrounded by G,. A cluster beam passes along the axis of the ion source and collides with high-Rydberg krypton atoms in the central region.
square. A typical filament current and voltage were 5 A and 7 V, respectively. The cluster beam passing through the central region was ionized either by impact of krypton atoms, Kr**, in high-Rydberg states (a) or by electron impact (b). ( a ) Rydberg Atom Impact ( R A I ) . Krypton gas with 99.95% purity was admitted to the ion source through a stainless-steel pipe of 4 mm diameter. The pushing pressure was typically 0.05-0.2 torr. The Kr gas was excited in the exterior of GBby electrons having an impact energy of less than 50 eV. Ionic species and electrons were deflected back by application of appropriate potentials to the three grids so that only neutral species including Kr** were allowed to enter the central region. The applied voltages were -20, -150. -50, and -100 V for GA.GB.Gc. and the filaments, respectively. The reaction chamber was evacuated by a 6-in. diffusion pump with a liquid nitrogen trap to I X torr. When the Kr gas was admitted to the ion source, the pressure torr. The principal quantum numbers, was increased to 3 X np. of Kr** were estimated to range from 25 to 35; Kr** with np 2 35 were ionized by the field o f 4 3 0 V/cm between G, and G,, and Kr** with np 5 25 could not reach the central region because of their short radiative lifetimes?0 The following observations confirmed that the observed negative ions originated from the electron transfer from Kr** to the clusters by collision: (1) the intensity of (SF,); depended linearly on the pressure of Kr; (2) the intensity showed a rapid rise with increasing electron energy in the vicinity of the ionization potential of Kr; and (3) the signals disappeared almost completely by application of -420 V to G,; this observation was interpreted as the field ionization of Kr" a t 1.3 keV/cm. ( b ) Eleclron Impact (€0. In this experiment the electrons emitted from the filaments were accelerated by the three grids into the central region, where the cluster beam was bombarded by electrons. The average electron energy, 1, was estimated from the difference between the potential of GA and that of the filaments. The energy spread (fwhm), mainly due to the potential difference across each filament, was estimated to be about 2 eV a t z = 4 eV by a comparison ofthe measured cross section curve for the 0-formation from CO, with that reported in the literature?' The nezative cluster ions thus vroduced were extracted through an entrance slit into a quadrupole mass spectrometer (Extranuclear. 162-b) installed in a detection chamber. This chamber uas ewcuatcd by a +in. diffusion pump u i t h a liquid nitrogen trap; the pressure w35 3pproxirnately 5 X IO-' torr when the ion bcam was sdmilted. The mass-selected ionc were detected by a Ccratron (Mumta. t.MS-IOb1R) equipped w i t h an ion convcrsinn dynode made of a stainless-steel disk of 22 mm diameter so placed ab to (30) Gallagher, T. F. I n 'Rydberg Stales of Atoms and Molecules": Stebbings. R. F.. Dunning. F. E.. Eds.;Cambridge University Press: Cambridge. U.K.. 1983; p 165. (31) Chantry. P. J. J . Chem. Phys. 1972, 57, 3180.
1554 The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
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Mitsuke et al.
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0
1
1
cluster s i z e n
Figure 4. The size distributions of (SF,); produced by R A I and EI, and (SF,),_,SF,+ produced by EI. The relative intensities of these ions are normalized at the dimer signals.
500
1000 MASS NUMBER
1500 (mlz)
Figure 3. Mass spectra of the negative ions produced by R A I (a) and E1 (b), and a spectrum of the positive ions produced by E1 (c). Numbers above the ion peaks indicate the sizes of cluster ions, n. Off-scaled ion counts are given in parentheses.
make its normal perpendicular to the beam axis. The time of flight between the ion source and the Ceratron was about 30 1.1s for SF;. The dynode, by which the negative ions were converted to positive ions before the detection, was found to improve the signal-to-noise ratio by 3 orders of magnitude. The maximum mass-to-charge ratio of the analyzed ions was m/z 1650, and the mass resolution, m/Am, was about lo2 at m / z 146 and about 300 at m / z 1460. The transmission and detection efficiencies of the mass spectrometer were calibrated by the known fragmentation patterns of the positive and negative ions produced from perfluorokerosene (PFK) by the E1 method.32 The mass-to-charge ratios were also calibrated by the fragment ions of PFK below m / z 800 and by the cluster ions of COz above m / z 800. The signals from the detector were preamplified (ORTEC, 9301) and registered in a multichannel analyzer (Canberra, 3 100). The mass spectrometer was operated by a mass programmer so as to select an ion having a desired mass-to-charge ratio. The measurement system was controlled by a CAMAC-crate-mounted LSI-11/23 microcomputer. The intensity of the mass-selected ion was determined by the area of the peak in the calibrated mass spectrum. The reproducibility of the intensity was better than f10% over a period of several hours. Background noises were negligible in the spectrum obtained by EI, while dark counts of 15-20 cpm were observed in that of RAI. The signal-to-noise ratio for (SF,)"- (n L 2) was 10-20.
Resu1ts Figure 3 shows a typical mass spectrum of the negative ions produced by E1 and RAI in comparison with that of the positive ions produced by E1 at F = 50 eV. The relative intensities of the negative and positive ions, normalized at their dimer signals, are plotted against the cluster size in Figure 4. The following features are observed in the mass spectra and their size distributions obtained under various experimental conditions: (1) Only the negative ions, (SF,); (1 I n I1 I), are observed by RAI, whereas SF5- and a trace amount of (SF,),-,SF,- (2 I n I 4) are observed in addition to (SF,),- (1 I n I11) by E1 at E = 2.5 eV. (32) Gohlke, R. S.; Thompson, L. H . Anal. Chem. 1968, 40, 1004
(2) The relative intensity of SF,- produced by RAI is about 5 times as high as that produced by EL (3) The relative intensities of (SF,); (n L 3) produced by RAI are higher than those of the corresponding ions produced by E1 by a factor of 2-3. (4) A pronounced intensity drop from n = 6 to 7 is observed for both RAI and EI; in the latter case the distribution at n = 6 makes a peak. These anomalies in the size distributions are independent of the stagnation pressure (2.5-3.5 atm) and the average electron energy for E1 (2.5-10 eV). ( 5 ) Positive ions, (SF6)blSF5+(1 I n I 11) and a small amount of (SF,),SF,+ (0 I p I3 and 1 Iq I 4), are detected by E1 at E = 50 eV. (6) The relative intensities of the positive ions, (SF6)n-,SFS+ with n 2 3, are much lower than those of the negative ions, (SF,);, produced under the same experimental conditions. ( 7 ) The size distribution of (SF6)b1SF5+observed in the present study from SF, seeded in He is similar to that obtained by using the SF, clusters formed in a supersonic expansion of unseeded SF,. On the other hand, the relative intensities of (SF6)b1SF5+ (n I 3) are much lower than those measured by using the SF, clusters formed in an expansion of SF, seeded in Xe or Kr.33-35 (33) Echt, 0.;Reyes-Flotte, A,; Knapp, M.; Sattler, K.; Recknagel, E. Ber. Bunsenges. Phys. Chem. 1982, 86, 860. (34) Ding, A.; Cassidy R. A,; Hesslich, J. In "Contributions of the I l l Symposium on Atomic and Surface Physics, Maria Alm/Salzburg, Austria, 1984"; Howorka, F., Lindinger, W., Mark, T. D., Eds.; Institut fur Atomphysik der Universitat Innsbruck: Innsbruck, Austria, 1984; p 228. (35) The observations described in feature 7 imply that the mechanism of formation of neutral clusters, (SF,), ( m 2 3), depends on the carrier gas used. When a sample, M, is seeded dilutely in Ar, Kr, or Xe, a dominant process for the cluster formation is expected to be two-body displacement reactions:
(Rg = Ar, Kr, or Xe) [(a) Casassa, M. P.; Bomse, D. S.; Janda, K. C. J . Chem. Phys. 1981, 74, 5044. (b) Vasile, M. J.; Stevie, F. A. J . Chem. Phys. 1981, 75, 2399.1 However, this mechanism cannot be applied to the formation of SF, clusters in He, because the concentration of H e clusters is insufficient to become substantial precursors and because the van der Waals bonding between He and SF6 is very weak. Instead, a mechanism involving collisional association of monomers to SF6 clusters is considered to be more plausible for the production O f (SF6), as follows:
Here, the He atom only acts as a third body and stabilizes the metastable SF, clusters. In the high-pressure region downstream the nozzle, collisions of preexisting dimers in the source with monomers and other dimers may also be an effective process. [(c) Thompson, D. L. J . Chem. Phys. 1982, 77, 1269. (d) Breen, J. J.; Kilgore, K.; Stephan, K.; Hofmann-Sievert, R.; Kay, B. D.; Keesee, R. G.; Mark, T. D.; Castleman, A. W., Jr. Chem. Phys. 1984, 91, 305.1
The Journal of Physical Che,wistry, Vol. 90, No. 8 , 1986
Collision of SF6 Clusters with Krypton Rydberg Atoms
Discussion A. Formation of (SF,),- in Collision with Kr**. When the electron occupying the Rydberg orbital of Kr** atom (Rydberg electron) is attached to a neutral cluster, (SF,),, the electron is expected to be trapped in a single SF6 molecule and forms a vibrationally excited SF; component in the cluster ion, (SF6)m-.36 Accordingly, the excess energy generated by the electron attachment is nearly equal to the sum of the electron affinity of SF6 and the polarization energy due to attractive charge-induced dipole interactions. This excess energy, E,,(m), is estimated to be 4-15 times as large as the sublimation energy of the (SF,),-, i.e., the energy which is necessary to remove one SF6molecule from the ion3' (see Appendix). Nevertheless, evaporation of SF, molecule(s) from the cluster surface does not necessarily occur, because this excess energy is partitioned to the intramolecular vibrations of the SF,- component and the intermolecular motions of the constituent SF6molecules (vibrations involving van der Waals bonds). The extent of the evaporation can be estimated from the effective vibrational temperature of the (SF,),- ion, which is essentially equal to the increase in the effective temperature, AT(m),resulting from Eex(m),because the vibrational temperature of the neutral (SF,), cluster in the supersonic beam is negligible. If Ee,(m) is assumed to be partitioned statistically to the vibrational modes of the cluster ion, A T ( m ) can be evaluated by
A T ( m ) = Eex(m)/N(m)kB
(1)
where N ( m ) is the number of the vibrational modes taking part in the energy partitioning and kBis the Boltzmann constant. As described in the Appendix, A T ( m ) decreases monotonically with the cluster size, m, and becomes lower than 200 K for m k 5. Then the effective vibrational temperature of the cluster ion is lower than the effective sublimation temperature of (SF,),-, at which the evaporation is appreciable, because the latter is estimated to be higher than the sublimation temperature of solid SF,, 210 K.38939 Therefore, it is unlikely that substantial evaporation occurs after the negative-ion formation by collision of Kr** with (SF,), ( m k 5) produced in a supersonic beam. The ionization process, generally written as M,
+ A**
-
Mi
+ A' + ( m - n)M
(2)
can be simplified in this case as
(SF,),
+ Kr**
(SF,),-
+ Kr'
( m k 5)
(3)
A more quantitative treatment requires the rate constant of the reaction
by which the extent of the evaporation for each m during the flight time of the cluster ions to the detector can be estimated. For example, an RRKM calculation may be applied to this estimation if the intermolecular vibrational frequencies of (SF,),- can be obtained from a multidimensional intermolecular potential surface of the cluster ion;@unfortunately, however, no reliable information is available at present. The general features listed in the results in regard to the mass spectrum and its size distribution can be explained as follows. Feature 1 . No negative ions, SFS- and (SF,),SF,- ( n 2 l ) , formed by disruption of chemical bonds are observed in the spectrum of RAI. This suggests that the activation energy for dissociation into (SF,),,SF< and F exceeds the sum of the energy of the captured electron and the vibrational energy of the neutral (SFdm.41 ~
~
~~
(36) Jortner, J. Ber. Bunsenges. Phys. Chem. 1984, 88, 188. (37) Brand, K. P.; Jungblut, H. J . A m . Chem. Phys. 1983, 78, 1999. (38) Cady, G. H. Adu. Inorg. Chem. Radiochem. 1960, 2, 105. (39) In general, the sublimation energy of a van der Waals cluster ion is expected to be higher than that of the neutral condensed phase of the same material, because ion-neutral interactions are stronger than neutral-neutral interactions. [Lee, N.; Keesee, R. G.; Castleman, A. W., Jr. J . Colloid Interface Sci. 1980, 75, 555.1 (40) Sunner, J.; Kebarle, P. J . Phys. Chem. 1981, 85, 327.
1555
Feature 2. The relative intensity of the SF6-ion produced by RAI is higher than that produced by E1 for the following reasons: (i) The lifetime of SF,- produced by E1 with respect to autodetachment is reported to be about 30 1s.28,29Since this lifetime is comparable with the ion-flight time for our mass spectrometer, a large fraction of the SF6- ions produced are expected to be autoionized before detection. On the other hand, most of the SF,ions formed by RAI are likely to survive because they have a longer lifetime than that for the SF6-produced by EI. Similar trends have been reported by several according to Klots, SF; may be stabilized by an interaction with the ion core of the colliding Rydberg atom.43 (ii) For both methods of ionization the cluster ions, (SF,); ( n 1 2), have much longer lifetimes than that for SF,-, because the excess energy of the sF6- component can be transmitted to the intermolecular vibrations. B. Evaporation from Cluster Ions Produced by EI. Feature 3. The relative intensities of the (SF,),- ( n L 3) produced by E1 are noticeably lower than those produced by RAI. The most probable interpretation is that the evaporation from the (SF,),ion produced by E1 is more extensive than that produced by RAI (see section A). A portion of the (SF,),- ( m 1 3) produced by E1 is likely to release a few SF, molecules and to be detected as SF6-or (SF&. This process is plausible because the electrons have kinetic energies of a few electronvolts in contrast with those of the Rydberg electrons (10-20 meV).23 The (SF,),- ion is heated above its effective boiling temperature by transmission of this energy, and SF6 molecules are evaporated. Similar observations have been reported for (CO,); and (CS2);.20,21,46 In the former case the stagnation-pressure dependence for ( C 0 2 ) , formation by E1 indicates that a neutral cluster larger than the trimer is required to yield (C02)2-.20-2' An alternative interpretation is that the size dependence of the cross sections for electron attachment by E1 is different from that by RAI, but this interpretation seems unlikely because, if it were the case, a reverse trend would be expected for the following reason: The cross section for the attachment of the Rydberg electron to (SF,), is expected to depend only weakly on the cluste size, because the cross section, u(SF6), for the electron attachment to the SF6 monomer is estimated to be close to the maximum cross section, urnax, for s-wave capture below 100 meV. This has been confirmed e x p e r i m e r ~ t a l l y ~and ~ , ~explained ~ - ~ ~ by a model based on the assumptions that at low collision energy the s-wave dominates the interaction of the incident electron with SF,, and that the electron capture is caused by a nonadiabatic coupling with nuclear motion^.^' On the other hand, the cross section for electron attachment to (SF,), by E1 is expected to increase with the cluster size, because an increase in the internal degrees of freedom of the cluster should contribute effectively to decelerating the incoming electron before the trapping to one of the SF, components (see section c),and because u(SF,) increases rapidly with decreasing electron en erg^.^^,^^ ~
~~~
~
(41) The SF, molecule excited to the u 2 2 vibrational states of the q-mode is known to be capable of producing SF5- from SF, by attachment of a zero-energy electron.56 However, most of the SF6 molecules in (SF,), ( m 2 1) are considered to be in the ground state of the u,-mode (948 cm-'), because the vibrational temperature of SF6 in a supersonic beam seeded in He is estimated to be lower than 200 K. [Luijks, G.; Stolte, S . ; Reuss, J. Chem. Phys. 1981, 62, 217.1 (42) Foltz, G. W.; Latimer, C. J.; Hildebrandt, G. F.; Kellert, F. G.; Smith, K. A,; West, W. P.; Dunning, F. B.; Stebbings, R. F. J . Chem. Phys. 1977, 67, 1352. (43) Klots, C. E. J . Chem. Phys. 1977, 66, 5240. (44) Astruc, J. P.; Barb&,R.; Schermann, J. P. J . Phys. B 1979, 12, L377. (45) Dimicoli, I.; Botter, R. J . Chem. Phys. 1981, 74, 2355. (46) Kondow, T.; Mitsuke, K.; Kuchitsu, K. In "Rarefied Gas Dynamics"; Oguchi, H., Ed.; University of Tokyo Press: Tokyo, 1984; Vol. 2, pp 833. (47) McCorkle, D. L.; Christodoulides, A. A,; Christophorou, L. G.; Szamrej, I. J . Chem. Phys. 1980, 72, 4049. (48) Kline, L. E.; Davies, D. K.; Chen, C. L.; Chantry, P. J. J . Appl. Phys. 1979, 50, 6789. (49) Zollars, B. G.; Smith, K. A.; Dunning, F. B. J . Chem. Phys. 1984, 81, 3158. (50) Chutjian, A.; Alajajian, S . H . Phys. Reo. A 1985, 31, 2885 (51) Gauyacq, J. P.; Herzenberg, A. J . Phys. B 1984, 17, 1155.
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The Journal of Physical Chemistry, Vol. 90, ,Vo. 8, 1986
Mitsuke et al.
Feature 4. A prominent peak at n = 6 in the size distribution for E1 indicates that (SF& is anomalously populated in the evaporation process. Such “magic numbers” encountered in the formation of the positive cluster ions33.53.54 produced by E1 have been discussed extensively in relation to the stable structures of cluster i o n ~ . ~ ~ l By * ~analogy, ~ ~ - ~ *the peak at n = 6 seems to be related to the thermochemical stability of (SF,),. Features 5 and 6. Defluorinated positive ions, (SF6)n_lSFc+, are dominantly produced by E1 at c = 50 eV, but the relative intensities of (SF6)n-lSF5+( n I3) produced by E1 are an order of magnitude lower than those of (SF,),- ( n L 3) produced by RAI. These trends indicate that (SF,),’ formed initially by E1 is likely to lose one F atom and several SF, molecules before being stabilized and detected as (SF,),-,SF5+. Indeed, more than 90% of (SF,)? and (SF,), are reported to be detected as SF5+ after E1 at ‘t = 100 eV.13 C. Dissociative Electron Attachment to SF, and ( S F 6 ) n zA. strong peak of SF,- is observed in the mass spectrum by El, but the intensities of the corresponding cluster ions, (SF,),_,SF,- ( n I2), are negligible in comparison with those of (SF,),- ( n L 2) (feature 1). Therefore, dissociative electron attachment to the SF, clusters
This mechanism explains the observed feature 1, because the electron energy is primarily transmitted to the intermolecular vibrations of (SF,),- in step a instead of the intramolecular vibrations of the SF; component formed in step b. A similar observation has been reported by Klots et al. for the negative-ion formation of CCI4 clusters by EI; they found the CCI4- ion produced from CCI,-CO, but never from CC1, itself.“,?
(SF,),
+e
-+
(SF,),-,SF,-
+F
(W
L 2)
+e
-
SF,-
Appendix Estimation of the A T ( m ) of (sF6)ti. The excess energy of the (SF,),- produced by the collisional ionization of (SF,), with Kr**, eq 3, is given by Eex(m)= EA + E d m ) - Ebo(m) (AI) Here, the electron affinity of SF,, EA, is reported to be about 0.5 ev;63.64 &(m) and Ew(rn)are the binding energies of (SF,),- and (SF,),,,, respectively, written as
(5)
appears to have very small cross sections, and the monomer fragment ion is formed by59
SF,
Acknowledgment. We gratefully thank Professor K. Shobatake for the use of his quadrupole power supply and Professor M. Matsuzawa and Dr. Y. Ozaki for fruitful discussions. This work was supported by a Grant-in Aid from the Japanese Ministry of Education, Science and Culture.
and 1?7
+F
(6)
A simple-minded consideration on the energetics would have predicted a different trend. The activation energy for process 5 is expected to be smaller than that for process 6, 0.2-0.6 eV,60,61 because the SFj- component in (SF,),_,SF,- gains more stabilization energy from surrounding SF, molecules than does an SF, component in (SF,),. Thus the SF; component in (SF,),- would dissociate rapidly into SFS-and F to generate (SF6),,-,sF5-, unless (i) the electron is delocalized among the SF, components of the (SF,), cluster immediately after the attachment and (ii) the kinetic energy of the incoming electron is transmitted efficiently to the vibrational modes of the van der Waals bonds in (SF6)nl. Therefore, the following mechanism may be proposed for the electron attachment to the SF6 clusters by EI. (a) The incoming electron approaches (SF,),, which retains its original geometry, and transmits the electron energy to the internal modes of the cluster. (b) The electron is trapped to one of the molecules to form a vibrationally excited SF6-component, which then transmits its energy to the intermolecular vibrational modes. (c) The excess energy is released by evaporation of SF, molecules (see sections A and B). ~
~
~~~~~~~
~~~
(52) Christophorou, L. G.; McCorkle. D. L.; Carter. J . G . J . Chem. Phys. 1971, 54, 253. (53) Echt, 0.;Sattler, K.; Rechnagel, E. Phys. Reo. Letr. 1981. 47, 1121. (54) Ding, A.; Hesslich, J. Chem. Phys. Lett. 1983, 94, 54. (55) Echt, 0.;Dao, P. D. Morgan. S.; Castleman. A . W., Jr. J . Chern. Phys. 1985, 82, 4076. (56) Harris, I. A.; Kidwell, R. S.; Northby, J. A. Phys. Rev. Leri. 1984, 53. 2390. ( 5 7 ) Worsnop, D. R.; Buelow, S. J.; Herschbach, D. R. J . Phys. Chem. 1984. 88. 4506. (58) Hermann, V . ; Kay, 9. D.; Castleman, A. W., Jr. Chenr. Ph,ks. 1982, 72, 185.
(59) Even when the stagnation pressure, Po, was decreased to 0.3 atm, where no clusters were produced, the intensities of SF; and SF; observed by E1 were as high as about 10% of those measured at Po = 3.5 atm. This observation indicates that at Po = 3.5 atm these ions are produced mainly from the SF, monomer. Here, the ratio, N I s / N s , of the number density of the monomer in the ion source to that at the skimmer can be assumed to be independent of Po, because the speed ratio, defined by the hydrodynamic speed divided by the most probable random speed perpendicular to the hydrodynamic velocity. is so large at the skimmer entrance (340-560 at Po = 0.3-3.5 atm) that the effect of Mach-number focusing on N l s / N s can be disregarded. [Sharma, P. K.; Knuth, E. L.; Young, W. S. J. Chem. Phys. 1976, 64, 4345.1 (60) Fehsenfeld, F. C. J. Chem. Phys. 1970, 53, 2000. (61) Chen, C. L.; Chantrj, P. J. J . Chem. Phys. 1979, 7 / , 3897.
E,o(m) = C E d 3 J=2
(‘43)
where E,(j) and Es0G)are the sublimation energies of (SF,); and (SF,),, respectively. The number of the vibrational modes, N ( m ) ,which participate in the dissipation of the excess energy, E,,(m), can be calculated as a sum of the (6m - 6) intermolecular modes of (SF,),- and 15 intramolecular modes of the SF,- component to be N ( m ) = 6m 9 (A4)
+
Therefore, the increase in the effective temperature
l T ( m )= Eex(m)/N(m)k, (1) is expected to decrease monotonically with the increasing cluster size, m, because E,(j) decreases with increasing J and becomes nearly equal to Eso(j)and, consequently, Ee,(m) approaches a constant value. In the case of m = 5, for example, Eb(5)can be calculated by using the Es(j) (2 Ij I 5) estimated from the ion-molecule interaction potentials proposed by Brand et aL3’ On the other hand, E,(m) can be estimated as proportional to the number of near-neighbor van der Waals bonds, as shown in the case of rare gas since the anisotropy of the SF,-SF6 interaction is negligible because of the spherical structure of SF,. Under the assumption that (SF,), has an equilibrium geometry of a trigonal bipyramid, as in the case of Ar,,,’ one obtains an approximate relation, Eb0(5)E 9Eb0(2), where EbO(2) is derived from the potential minimum of the Lennard-Jones 6-12 potential for SF,.,, As a result, E,,(5) is calculated by use of eq A1 to be about 0.7 eV, which leads to A T ( 5 ) of about 200 K according to eq 1 and A4. (62) In order to get further information, we studied the negative ions produced by E1 on (CC14)elclusters. Though negative ions, (CC14)[ ( 1 5 n 5 3), were clearly observed, being consistent with the reports by Klots et the intensities of (CC14),- were 2 orders of magnitude lower than those of (CC14),,.,Cl- formed by rupture of one of the C-CI bonds. This observation implies that the potential energy surface of the monomer negative ion placed in the cluster controls the branching into two alternative processes: rapid transmission of the electron energy to the intermolecular vibration [as in (SF,),] or to the intramolecular vibration of the monomer component [as in (CCI4),]. The details will be reported in a separate publication. (63) Compton, R. N.; Reinhardt, P. W.; Copper, C. D. J . Chem. Phys. 1978, 68, 2023. (64) Streit, G. E. J . Chem. Phys. 1982, 77, 826. (65) Hoare, M. R.; Pal, P. AdG. Phys. 1975, 24, 645. (66) Hirschfelder, J. 0.;Curtiss, C. F.; Bird, R. 9. “Molecular Theory of Gases and Liquids”; Wiley: New York, 1954; p 1 I 1 I .
