J. Phys. Chem. 1995, 99, 6367-6373
6367
Collision Processes of Size-Selected Cluster Anions, (CsF&- (n = 1-5), with a Silicon Surface Tatsuya Tsukuda,t Hisato Yasumatsu, Toshiki Sugai? Akira Terasaki, Takashi Nagata,? and Tamotsu Kondow* Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan, and RIKEN, The Institute of Physical and Chemical Research, Wako, Saitama 351 -01, Japan Received: November 28, 1994; In Final Form: February 7, 1995@
Collision processes of a size-selected cluster anion, ( c 8 6 ) n - (n = 1 - 9 , with a silicon surface were investigated with use of a tandem time-of-flight (TOF) mass spectrometer. The impact of c86- on the Si surface at the collision energies of 0- 150 eV results in preferential formation of F-, giving a maximum yield at a collision energy of -0 eV. This observation can be explained in such a manner that F- is directly produced via the dissociation of c&,-in an electronically excited state. It is proposed that the electronic excitation of c86upon surface collision proceeds via electron-donation and back-donation processes between c86- and the surface. In the low-energy collision of (C86)n- (n = 1-5) with the Si surface, the relative yield of C6F5with respect to F- increases with the size of the parent cluster anion. The formation of F- becomes more favorable in a higher-energy collision of (C8.5)"- (n = 1-5) with the surface. These results lead us to conclude that the solvent molecules are intimately involved in the dissociation of the anionic core in the low-energy collision regime.
I. Introduction Recently, interaction of van der Waals (vdW) clusters with well-characterized solid surfaces has gained much interest in the research fields of cluster and surface It is well recognized that collision of a cluster with a surface results in such outcomes as scattering of ionic and neutral fragments, ejection of secondary particles from the surface (sputtering), chemical reactions within the cluster, and sticking reactions onto the surface. However, a full understanding of the clustersurface interaction has not been attained so far. The phenomenological outcomes of the cluster-surface collision may depend strongly upon the collision energy and the size of the cluster. In an energetic collision of the cluster with the surface (I 10 eV per particle), the repulsive nature of the interaction between the constituents of the cluster and the surface plays an important r ~ l e . ~The - ~ constituents in the bottom layer of the cluster which are already scattered off the surface subsequently collide with the incoming particles in the top layer. As a result, a large amount of mass and energy are piled up transiently at the boundary area. Because of the extreme condition, specific chemical processes, such as sputtering, may well proceed in a different manner from that governed by equilibrium kinetic^.^-^ In contrast, in a hyperthermal collision (20.1 eV per particle) where the collision energy is comparable to the interaction energy between the constituents in the cluster, it is expected that the collision processes are greatly influenced by the attractive nature of the interaction between the constituents of the cluster and the surface. The constituents of the cluster lose their kinetic energies during the multiple collision process and are finally trapped on the surface. Such an efficient sticking reaction of
* Correspondence to Tamotsu Kondow at The University of Tokyo. Present address: Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153, Japan. 4 Present address: Department of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan. @Abstractpublished in Advunce ACS Abstracts, April 1, 1995.
0022-365419512099-6367$09.00/0
an Ar cluster onto Pt(ll1) has been predicted by stochastic trajectory simulation^.^,^ Because of a short interaction time of the cluster with the surface, the cluster is excited as a simple particle in the surface collision. The internally excited cluster subsequently dissociates into characteristic fragments depending on the activation mechanism of the cluster, i.e., electronic a n d or rovibrational excitation. The size of the cluster also influences these dynamics of the surface collision because the extent of the multiple collision between the constituents is dependent upon the cluster size. In this regard, it is important to investigate the collision-induced processes while controlling these two parameters of the cluster-surface collision. This type of investigation gives us a clue to understand the complex interaction as well as to prepare a recipe to treat the surface in a controlled manner. To this end, a beam of a size-selected cluster ion is suitable because the two important parameters of the cluster-surface collision, the size of the impinging cluster and the collision energy, can be specified?JO In this paper, the collision-induced processes of cluster anions of hexafluorobenzene, (C6F6)n- ( n = 1-5), with a silicon surface were investigated by mass spectrometry. The size-selected cluster anion, (C6F&-, was allowed to collide with a Si surface at collision energies of 0- 150 eV, and secondary anions scattered from the surface were detected by mass spectrometry. The cluster anions of c& were used as a typical example for investigation of the collisioninduced processes because detailed information on the potential energy surfaces of c&- relevant to the dissociation is available in the literature.11+12In addition, it has been demonstrated that (C&)n- ( n = 1-8) is comprised of a monomeric anion core, c&5-, and solvent c& molecules by a photoelectron spectroscopic study.13 An activation mechanism of (C6F&- in the surface collision was examined with the aid of potential energy surfaces of c86- and the energetics relevant to the dissociation. It is demonstrated that weak interaction between the constituent molecules greatly influences the scattering processes especially 0 1995 American Chemical Society
6368 J. Phys. Chem., Vol. 99, No. 17, 1995
,
U -0.3 kV
*
Primary TOF
Tsukuda et al.
g““
Size-Sclccted
Anions
Si
7””
,
* Masr Spectra
Figure 1. Schematic diagram of the apparatus comprised of a cluster anion source, a primary TOF mass spectrometer, a cluster-surface collision assembly, and a secondary TOF mass spectrometer. In the cluster anion source, the cluster anions of are formed by intersecting a continuous electron beam with a pulsed free jet expansion. After traveling downstream, the cluster anions are extracted into the primary TOF mass spectrometer, where they are separated by their different arrival times. Single mass selection is achieved by a pulsed beam deflector (mass gate). A mass-selected cluster anion is allowed to collide with a solid surface in a decelerating electric field at a desired collision energy. Secondary anions from the surface are detected by the secondary TOF mass spectrometer.
at collisions with low energies, which are comparable to the binding energy of the constituents.
11. Experimental Section
A. General Description of the Apparatus. Figure 1 shows a schematic diagram of the experimental apparatus, which consists of six stages of differentially pumped vacuum chambers. These chambers contain a source of cluster anions, a time-offlight (TOF) mass spectrometer for selecting a primary cluster anion of a given size, a Si surface mounted in a homogeneous electric field, a secondary TOF mass spectrometer for product anions scattered from the surface, and a detector. The experimental apparatus used in the present study is similar in principal to those of Whetten et al.9 and Brenner et al.,I4 as described in detail below. B. Cluster Anion Source. The cluster anion source is composed of a pulsed valve with an orifice of 800-pm diameter, an electron gun assembly, and a set of samarium-cobalt magnets. A mixed gas made of 60 Torr of hexafluorobenzene and 400 Torr of N2 was expanded through the orifice at a repetition rate of 10 Hz. Cluster anions of hexafluorobenzene, (C6F&-, were formed by injecting 350-eV electrons into a supersonic expansion region at 1-4 mm downstream from the aperture.15 The electron beam was deflected by a pair of electrodes and confined spatially by a magnetic field of 0.05 T supplied by the permanent magnets.16 Typical filament and emission currents were 2.8 A and -100 pA, respectively. A source chamber was evacuated by a 14-in. oil diffusion pump backed by a mechanical booster pump. The base pressure of the source chamber was less than -1 x Torr. The chamber pressure increased up to -5 x Torr when the cluster beam was introduced. The cluster anion source mounted on an xyz-translation stage was adjusted externally so that a high-density region of the supersonic expansion was extracted by a skimmer (@ = 1 mm) to form an intense cluster anion beam. The cluster anion beam was introduced into the second chamber, which was evacuated by an 8-in. oil diffusion pump, and was further collimated by a hole of 10-mm diameter.
C. Primary TOF Mass Selector. The cluster anions, after traveling about 30 cm from the source, were extracted normal to the cluster anion beam by applying a pulsed voltage of -300 V with a rise time of -60 ns between a pair of electrodes with a gap of 30 mm. This electrode pair acts as the first acceleration region of the TOF mass spectrometer of a Wiley-McLaren type. l7 After passing through the 10-mm-diameter entrance aperture, the cluster anions acquired an additional kinetic energy of -4.0 keV over a distance of 37 mm in the second acceleration region of the primary TOF mass spectrometer. A series of guard rings was mounted to maintain a homogeneous field gradient in this region. Under the present experimental condition, the cluster anions were space-focused -4.6 m from the entrance aperture. A spatial spread of the cluster anion beam in the first acceleration region gives rise to a kinetic energy spread of the cluster anion beam; the energy spread was estimated to be 15 eV (fwhm) for the 1-mm-diameter skimmer (See section 1II.A). In the present experimental setup, the cluster anion source and the detection chamber are grounded. The ion beam with enough velocity to maintain good focusing characteristics was produced by incorporating “potential switch” into the apparatus.’* The voltage applied to a metal tube was maintained at a final acceleration voltage required for the Wiley-McLaren acceleration (+4 keV) until bunches of the cluster anion of a given mlz value entered it completely. By switching the tube bias voltage off quickly, the cluster anions inside the tube were allowed to maintain their velocities in traveling through the remainder of the apparatus referenced to the ground. Simultaneously, the potential switch served as a window for admitting ions in a certain mass range. By adjusting the time at which this voltage is switched off relative to the initial extraction pulse, this mass window can be swept in the entire mass range of the anionic species produced in the ion source. After traveling through the potential switch, the cluster anions were admitted into an einzel lens and an octapole beam deflector.19 The cluster anions were further separated into one particular ion packet having a given mlz value by a “mass gate”, a set of pulsed deflector plates: As an ion packet with a particular mlz value passes between the plates, the deflecting voltage on the plates is switched off to the ground during typically -2 ps so as to allow the cluster anions to travel toward the surface. After this ion packet has completely left the mass gate, the voltage is switched on so that the cluster anions with different mlz values are deflected off. The vacuum chamber for the pulsed extraction was evacuated by an 8-in. oil diffusion pump, while the vacuum chamber for the mass selection was evacuated by two turbo molecular pumps. The pressure of the extraction chamber was (2-4) x Torr in a typical operating condition. D. Assembly for the Cluster-Surface Collision and Secondary TOF Mass Spectrometer. The silicon surface was mounted in a “reflectron” which is applied with a retarding electric field tilted by a small angle, 8, off the beam axis (see Figure 1). The collision energy of a mass-selected cluster anion, Ej, is given by the difference between the kinetic energy, EO, and the surface potential energy, qU. It is more convenient to express the collision energy in terms of the normal and parallel components of the collision energy, E l and Ell, in order to understand the behavior of Ei as a function of U:
Ei = Eo - qU = ( E cos2 ~ = El
+ El,
o - qm + (E, sin2 0)
where E l and fil are defined as E l = EOcos2 8 - qU and Ell =
Collision Processes of (C&)n-
J. Phys. Chem., Vol. 99,No. 17, 1995 6369
EO sin2 6, respectively. For qU > EO cos2 6, i.e., EL < 0 eV, the cluster anion is reflected back to the detector without colliding with the surface. In a typical operating condition, the cluster anion with a kinetic energy of EO 4 keV was reflected by an electric field of -400 V/mm, where the reflectron provides a steep potential wall, which simply reverses the direction of the primary cluster anion beam. Therefore, the primary TOF mass spectrometer makes the cluster anions spatially compressed at the detector placed at a distance of -4.6 m from the first acceleration region. The resolution of the primary mass spectrometer, MIAM, was typically -600. In contrast, for qU < EOcos2 6 , Le., E l > 0 eV, the cluster anion collides with the surface. At a given angle of the reflectron, only the EL term can be varied by changing the surface bias voltage, U, whereas the Ell term remains unchanged. The cluster anion was decelerated to a desired collision energy, El, at a 12-mm distance between the grounded mesh and the solid surface, Le., in a field gradient of -370 Vlmm. Such a strong field gradient shortens the time required for secondary anions to be accelerated in the reflectron so that a collective efficiency of the secondary anions increases. Nevertheless, the secondary anions were scattered spatially and could not be fully collected because the parallel component of the secondary anion velocity is widely distributed. The secondary anions scattered off the surface acquire an additional kinetic energy of qU and are detected by the secondary TOF mass spectrometer. The m/z values of the secondary anions were determined from their flight times measured at a collision energy that was as low as possible. At a low-energy collision, the mlz values of the secondary anions can be determined unambiguously because their recoil velocities are negligible. In order to obtain information on "elasticity" for the scattering process, the flight time of a secondary anion with a given mlz value was measured as a function of U.9 The resolution of the secondary mass spectrometer was typically -60. The diameter of the primary cluster anion beam on the surface was measured to be -15 mm with a phosphorescent screen (Hamamatsu F2223-21P). The collision chamber was evacuated by a tandem turbo molecular pump. The base pressure was -7 x Torr without any baking of the chamber. The pressure of the collision chamber increased up to -1 x IOp9 Torr when the cluster beam was admitted into the chamber. The Si(100) surface (50 x 100 mm2) supplied from Toshiba Electric Co. was heated up to -200 "C by electron bombardment. With this treatment, thin oxide layers are left on the silicon surface so that charge neutralization of the incoming cluster anion is suppressed to some extent, but the surface is not significantly charged up. E. Data Acquisition. A tandem microchannel plate with a diameter of 4 = 14.5 mm (Hamamatsu F4655) was used for the particle detection; the gain of the channel plate is proportional to n112for (C&)n- at a given collision energy.20 Pulsed signals from the detector were amplified and processed by a 500-MHz digital storage oscilloscope (Tektronix TDS520) based on a personal computer (NEC PC9801FA) for storage and subsequent data analysis.
-
111. Results A. Kinetic Energy Analysis of a Primary Cluster Anion. In order to determine the initial kinetic energy of a primary cluster anion, EO, its intensity was measured as a function of the surface bias voltage, U. Figure 2 shows the plot of the integrated intensity of c& against U recorded at the reflectron angle of 6 = 4.2" with respect to the surface normal. As U is lowered, the intensity does not change appreciably until it
3800
4ooo
4200
4400
Surface Voltage, U (V)
Figure 2. Plots of the c&- intensity against the surface voltage, CJ, at the specular angle of the reflectron (0 = 4.2'). The signal falls off quickly at a surface voltage of 3980 V.
I -30eV
98 eV 150 eV
60
70 80 Time of Flight (ps)
90
Figure 3. TOF mass spectra for the product anions from the Si surface at different normal collision energies, E l . The top trace represents a mass spectrum of the mass-selected primary anion, c&-. The Fintensity reaches a maximum at E l 0 eV and decreases monotonously with an increase in EL. Detection of F- at E l = - 17 eV is due to the collision of C,&- with a larger kinetic energy than the averaged value.
-
reaches a critical voltage, UO, at which the intensity drops abruptly to almost zero as a result of change of the ion trajectory due to the collision of C6F6- onto the Si surface. This intensity drop is not attributable to deviation of the flight path in the reflectron simply due to the change of U, because the ion trajectory does not change significantly with U in this voltage range. This intensity-vs.-U curve gives UO of 3960 V. The initial kinetic energy of C6F6-, EO, was evaluated to be 3980 eV, from the relation, quo = EO cos2 8. The width of the derivatives of this curve provides an energy spread, AEo, of 15 eV (fwhm) in the initial kinetic energy of c&-. The initial kinetic energies and energy spreads of (C6F6)n- with n = 2-5 were also evaluated by the same procedure as stated above. B. Collision of Cas- with the Si Surface. Figure 3 represents a series of TOF mass spectra of secondary anions scattered off the s i surface in the C6F.5- collision at different
Tsukuda et al.
6370 J. Phys. Chem., Vol. 99, No. 17, 1995
71.4
3 71.3 %
v
3 M
2
71.2
c
8
G
71.1
71.0
0
Normal Collision Energy, E l (eV)
Figure 4. Ion yield of F- plotted as a function of the normal collision
1w
200
300
400
Normal Collision Energy, E l (eV)
energy, EL,of (28.5 against - the si surface. The yield exhibits a maximum peak at E l 0 eV and then decreases monotonously with
Figure 5. Comparison between the simulated and measured TOF for
El.
experimental data points agree well with the simulation curve on the assumption of totally inelastic scattering ( y = 0.0).
-
normal collision energies, E l . The mass spectra in Figure 3 were recorded at 8 = 7.3' where the total intensity of the secondary anions exhibits a maximum value at 0 eV < E l < 150 eV. At 8 = 7.3", the parallel collision energy remains as 41 = 64 eV regardless of the surface bias voltage, U. The following features are discernible in the mass spectra: (i) The scattering process involves dissociation of the C-F bond of C6F6- (dissociative scattering); even at E l 0 eV, any TOF peak associated with cf&-was not detected. The TOF peak for F- predominates in the mass spectra in the entire collision energy range studied.21 At this reflectron angle ( 6 = 7.3"), we detected F- scattered from the surface having a parallel velocity corresponding to -10% of that of the primary anion. The dissociative scattering process of C.&- is expressed as
-
C6F,-
F- scattered in the collision of c8.5- with the Si surface. The
C6FS'
Si(100)
F- i- (neutrals)
When E l exceeds -100 eV, the TOF peak assignable to Hemerges which may originate from hydrocarbon contaminants sticking to the surface. (ii) The peak width of F- indicates that F- is produced within a time scale of lo-' s, which is required for F- to be accelerated in the reflectron. A yield of F- at a given EL was obtained by dividing the area of its mass peak by that of the parent ion peak recorded at 8 = 4.2" and at a condition without any collision. The ion yield thus obtained is considered to be a lower limit because the secondary anions may not be fully collected by the detector at a given reflectron angle. The yield, however, can be used as a measure of the secondary anion yield because the ion intensity always exhibits the maximum at 8 = 7.3" at all the collision energies studied. Therefore, Figure 4 represents the behavior of the secondary anion yield as a function of the collision energy. The F- yield exhibits a maximum value of -10% at EL 0 eV and decreases monotonically with E l . This result indicates that F- is formed preferentially in a grazing collision of c&with the Si surface ( E l 0 eV and 41 = 64 eV). The nonzero yield of F- observed at E l < 0 eV originates from the collision of with kinetic energies higher than the average value. The elastic coefficient, y. which is defined as the ratio of the recoil velocity to the incident velocity, was obtained by measuring the flight time of F- as a function of El. In Figure 5 , the flight time of F- is plotted against E l , along with simulated curves which are obtained by assuming y = 1 (totally elastic), y = 0.5, and y = 0 (totally in el as ti^).^ In the
-
-
Time of Flight (ps/div)
Figure 6. TOF mass spectra for the product anions obtained in the impact of (C86)n- (n = 1-5) against the Si surface. The normal component of the collision energy per constituent molecule is Elo 8 eV for all the mass spectra. A branching fraction of C&- formation increases with the size of the primary cluster anion.
-
simulation, the elastic coefficient is assumed to be independent of E l . Evidently, the flight time-vs.-EL plots are well reproduced by the simulation with y = 0. C. Collision of (C6F& (n = 2-5) with the Si Surface. Figure 6 represents a series of TOF mass spectra of secondary anions scattered in the collision of (C86)n- (n = 1-5) with the Si surface (8 = 7.3') at a normal collision energy per constituent molecule, Elo E Elln = 8 eV, and Ell = 64 eV. The dissociative scattering process predominates in the surface collision of o d y the anionic fragments, F- and c&-, are scattered off the surface, but no parent cluster anion, (C6F&-. is. The dissociative scattering process of (C6F6)n- (n = 1-5) is expressed as follows:
+ (neutrals)
(3'4)
J. Phys. Chem., Vol. 99, No. 17, 1995 6371
Collision Processes of (C86)n-
............
Evidently, C85- is produced more preferably from a primary cluster anion with a larger size, n. In Figure 7, the yields of the total anions, F- and C&-, from (C6F&- (n = 1-5) are plotted as a function of Elo, the normal collision energy per constituent molecule. In the calculation of the ion yields, the intensities of the parent anions, (C&)n-, are scaled as n1I2 to compensate for the massdependent sensitivity of the detector.20 Figure 8 shows the branching fraction of the F- formation plotted as a function of Elo. The features derived from Figures 7 and 8 are enumerated as follows: (i) The yield of C@5- is larger in the collision of (C86)n- with a larger n in the entire collision energy range studied. (ii) The collision energy giving the maximum yields of F- and C85- increases with the cluster size. (iii) The branching fraction of the F- formation from (C86)n- increases with increase in Elo (see Figure 8). Figure 9 shows the flight times of F- and C6F5- scattered in the collision of (c6F6)4- as a function of Elo, along with simulated curves assuming elastic coefficients of 1 (totally elastic), 0.5, and 0 (totally inelastic). Here, the elastic coefficient is again assumed to be independent of Elo. The collision-energy dependence of the flight times of both F- and c6F5- is reproduced by assuming that they are scattered off the surface, losing almost all the initial kinetic energy. The flight times of F- and C&- produced in the collision of (C36)n- (n = 2, 3, and 5 ) also exhibit a similar behavior.
v
Y
+
are given as follows:
where the bond dissociation energy of D(C&-F) = 4.94 eV is available in the 1iteratu1-e.~~ The electron affinities of F and
12)
...., . . . . . . . .
121
.... .... ....
12 8 4 n "
50 100 150 0 Normal Collision Energy, Elo (eV) Figure 7. Plots of the yields of total anions (shaded circles), F- (closed circle), and C&- (open circle) for the collision of (C&)n- as a function of Elo.
IV. Discussion A. Mechanism of the Dissociative Scattering of C6F6-e It is often argued that the dissociative scattering of molecules and molecular ions from the surface proceeds via excitation of the projectile into (a) an electronically excited state22-28or (b) a vibrationally excited The internally. excited species temporarily formed in the surface collision will dissociate into characteristic fragments depending on the excitation process involved. In the collisional excitation of c@6-, the fragmentation reaction in Scheme l should occur. (a) Electronic Excitation, Fenzlaff and Illenberger have studied a dissociative electron attachment process of a free c86 molecule; the electronically excited anion, C6F6-*, which is temporarily formed by the attachment of a 4.5-eV electron, dissociates predominantly into F- together with a small amount of C6F5-.l2 No intact c86- anion has been produced by this resonant electron attachment channel. Hence, it is conceivable that c6;6-* formed in the surface collision also undergoes preferential dissociation into F-. (b) Vibrational Excitation. The following estimation shows that the electronic ground state of c&- correlates to the dissociation limit of F C&-. The heats of reactions for the dissociation of c&- yielding F- and C6F5-,
...
12)
M
.-
5 e m
;e
..............
..... ;,..
...* ....i o
i
0.6
0.4 0
--.-m=
- 50n=2100
0
50
100 0
50
100
Normal Collision Energy, E,o (eV)
Figure 8. Plots of branching fractions of F- formation in the collision of (C86)n- as a function of Elo.
c6F6 are reported to be 3.399 eV@ and 0.52 f 0.01 ev,41742 respectively. A lower limit of the electron affinity of C6F5 is reported to be EA(C&) 2 3.4 f 0.3 eV.43 Then, the heats of reaction for processes 4 and 5 are calculated to be A H 4 = 2.06 eV and A H 5 5 2.06 eV, respectively. Thus, the C@5- fragment and its precursor, c#6-, should be detected as main products if the vibrational excitation of c6F6- is operative in the surface collision. It is found from the present experiment that C85- is produced as a minor product21 and that the intact c&- does not survive in the surface collision. The F- anion, which is a characteristic fragment anion of c6F6-*, gives the maximum yield of 10% at a grazing collision condition ( E l 0 eV and Ell = 64 eV). These findings indicate that the unimolecular dissociation of C6F6-+ does not play a significant role in the dissociative scattering of c&-. It is more likely that c86- is electronically excited by the surface collision and directly dissociates into F-. Scattering of fragment ions characteristic of the electronic excitation has also been reported in the low-energy collision (
R2
Figure 10. Schematic potential energy surfaces with the asymptotic
+
+
+
limits of (C6F6 Si-), (c&Si), and (c&j-* Si) as a function of the distance between the molecule and the surface.
SCHEME 1 (a) electronic
F' C6F6-
c6F5-
excitation
R, of C6F6 from the s i surface. The difference in energy between the asymptotic limits of C6F6Si and c6F6 Siis given by the difference between the electron affinity of the s i surface, EA(Si), and that of c 8 6 , EA(C6F6) = 0.52 f 0.1 eV.41,42 The EA(Si) value is approximated by the energy difference between the work function, 4, and the band gap, .Eg, of the Si surface; namely, EA(Si) = q5 - .Eg = 3.7 eV. Thus, the asymptotic limit of the C6F6s i curve is located at an energy of 3.2 eV higher than that of the c6;6 -4- Si- curve. The stabilization energy for the C6F6- -k s i system is larger than that for the C6F6 4- Si- system, because the image force exerted in the C6F6 4- Si- system is reduced significantly by charge delocalization over the Si surface. Suppose that the C6F6- -k s i curve crosses with the c6F6 si- curve at R2 and the C6F6
+
+
+
+
+
+
J. Phys. Chem., Vol. 99, No. 17, 1995 6373
Collision Processes of (C86)n-
As a result, a sizable amount of energy is expected to be stored in the intemal degrees of freedom of the neutralized cluster. The time required for the neutralized cluster to dissociate into monomers is presumably in the range of 10-13-10-12 s, which is a typical time for Kr512 dissociation into Kr atoms at collision energies of 0.5-50 eV/~article.~ On the way leaving from the surface, one of the constituent molecules in the “dissociating” cluster captures one electron to form c&-*. The subsequent dissociation of c&-* is perturbed by the surrounding c& solvents, resulting in the formation of C&-. As the collision energy Elo increases, the dissociation of the neutralized cluster proceeds faster and the “isolated’ Cf& molecules capture the electron from the surface. This inference explains the experimental finding that the branching fraction of the F- formation increases and tends to level off toward the value observed in the dissociation of C6F6- (Figure 8). Because the recoil velocities of F- and C&- are negligible (see Figure 9), their precursor, c&-*, is scattered from the surface with a small kinetic energy and dissociates into the fragments with a small kinetic energy release. The small kinetic energy release in the C&-* decomposition is supported by the electron attachment study by Fenzlaff and Illenberger.’* The collision energy of (C&)n- is transferred to surface vibrational motion at the collision energy range studied. The decreasing trend in the total ion yield with the cluster size at a low collision energy (see Figure 7) is attributable to more efficient trapping of (CsF&- onto the surface in the lowenergy collision regime. The normal components of the recoiling velocities of the constituent molecules are lowered because of multiple collisions between the constituent molecules (multiple collision model4). As a result, the constituent molecules tend to be scattered in a tangential direction of the surface and the molecules are easily trapped by the attractive force between the molecules and the surface. The trapping probabilities of the cluster decrease in a high-energy collision as the constituent molecules are likely to gain recoil velocities sufficient to escape from the attractive force of the surface (see Figure 7).“ V. Summary Collision of a size-selected cluster anion, (C&)n- (n = I+), onto a Si surface was investigated in a high-vacuum collision chamber equipped with a tandem TOF mass spectrometer. The ( 2 8 6 - collision at a collision energy of -0 eV results in preferential formation of F-. This result can be explained in such a manner that F- is directly produced via the dissociation of C6F6- in an electronically excited state. The electronic excitation of c86- upon the surface collision proceeds via electron-donation and back-donation processes between c86and the Si surface. The effects of the solvent molecules on the dissociative scattering of C8.f- were examined, and the following phenomena were observed in the low-energy collision of (C&)n- (n = 1-5) with the s i surface: (i) the relative yield of C a 5 - with respect to F- increases with n and (ii) the total yield of the anionic fragments decreases with n. The change in the branching fraction with n arises presumably from quenching of the electronically excited state to the electronic ground state of the c&- surface in the presence of the solvent molecules. The small yield of the anionic products in the cluster-surface collision is explained by the surface trapping of the anionic products enhanced by multiple collision among the products. Acknowledgment. We are indebted to the ULSI laboratory of Toshiba Electric Co. for providing us with the pure Si specimens. Professor M. A. Johnson (Yale University) is greatly
acknowledged for his technical advice on the design of the potential switch used. Tatsuya Tsukuda was supported by the Fellowship of Japan Society for the Promotion of Science for Japanese Junior Scientists.
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