Fragmentation process of size-selected aluminum cluster anions in collision with a silicon surface

Abstract

Dynamical processes involved in the collision of aluminum cluster anions, Al−N (4≤N≤25), with a silicon surface were investigated. Intact and fragment cluster anions, Al−n (n≤N), were produced upon the collision. The surf02ace-tangent and surface-normal recoil velocity components of these product a0n0ions were determined. The tangential recoil velocities of the fragment cluster anions were considerably slow, ranging from 5% to 30% of the velocity of the incident parent cluster anion, while the normal velocities were conserved relatively well. These results are explained in such a manner that the fragment cluster anion is evaporated from the parent cluster anion while it interacts with the surface and loses its tangential momentum. The dynamics and the energetics derived from these results show further that the fragmentation process involves not only sequential evaporation of aluminum atoms but also simultaneous production of several small fragment clusters. Comparison of the present result with that of the collision-induced dissociation by a rare-gas atom 88lends a further support on this nonsequential fragmentation 1/1mechanism.

Energy redistribution in cluster–surface collision: I2− (CO2)n onto silicon surface

“…2 The experimental study on the collision of (Ar) n against a graphite surface [13][14][15] has elucidated the dynamics of the energy and particle flows in the cluster-surface collision through the translational energy measurements of the fragments scattered from the surface. Similar information has been obtained in the collisional dissociation of (Al) 13 Ϫ against a silicon surface; the clusterimpact heating causes the cluster temperature to rise as high as 10 000 K. 16 We investigated energy redistribution in the impact of I 2 Ϫ ͑CO 2 ) n against a silicon surface through the measurements of the translational energy of its fragment anions. It was revealed that the surface-impact dissociation of I 2 Ϫ (CO 2 ) n proceeds via ͑1͒ preferential transmission of the collision energy to a particular vibrational mode such as the vibrational mode of I 2 Ϫ , and ͑2͒ rapid energy redistribution in the internal modes of the cluster anion and its neighboring surface atoms.…”

“…11,16,21 A supersonic expansion of CO 2 (1.5ϫ10 5 Pa) containing a͒ Author to whom correspondence should be addressed. I 2 ͑0.1-1 mol %͒ was bombarded by 350 eV electrons with an electron current of ϳ200 A.…”

“…In the present study, E col and ⌰ were maintained at 50 eV and 26°, respectively, for all the parent cluster sizes studied by adjusting V s and . 23 For 16 collision, for example, the uncertainties ͑onestandard deviation͒ in the E col and ⌰ values were estimated to be 8 eV and 4°, respectively, by use of Eqs. ͑1͒ and ͑2͒, since a full width at half maximum ͑FWHM͒ of E in was measured to be ϳ30 eV by using a retarding potential method.…”

“…Concurrently, many experimental studies have been performed to explore novel processes initiated by clustersurface impact. [11][12][13][14][15][16][17][18][19][20][21] In the collision of I 2 Ϫ (CO 2 ) n against a silicon surface, 11 the branching fraction of the I 2 Ϫ dissociation is found to change remarkably with the number of the solvent CO 2 molecules. This finding is interpreted as that the core ion, I 2 Ϫ , is mechanically split by a thrust of the CO 2 molecules at waist positions of the I 2 Ϫ core ion as if a piece of wood is split by a hammer thrust against a wedge ͑wedge effect͒, whereas the dissociation is retarded when the I 2 Ϫ core ion is embedded in a complete CO 2 solvent cage because the separating I Ϫ and I pair is efficiently recombined in the cage ͑cage effect͒.…”

Surface collisions of small cluster ions at incident energies 10–102eV

“…Fission of cluster ions into two fragments of about equal size was observed, together with evaporation, too. It was described for collisions of silicon anion clusters [22] and of certain metal anions and cations [23,24] at incident energies of tens of eV. Some alkali halide cluster ions were found to cleave to a series of two fragments of unequal size [25].…”