Reactive etching of positive and negative silicon cluster ions by nitrogen dioxide

Positively and negatively charged silicon cluster ions, Si _1-8 ⁺ and Si_1-6 ^-, react exothermically with NO₂. The predominant reaction is loss of a single silicon atom in the form of SiO. This reaction repeats sequentially such that the clusters are "etched" down to monatomic silicon ions. Charge transfer to form NO₂ ^- also occurs for Si _1-4 ^-. Previous results have shown that all Si _1-6 ^- exhibit electron transfer to WF₆. Together, these observations bracket the electron affinities of the neutral silicon clusters: EA (Si_2-4)≤2.6 eV and 2.6 eV≤EA (Si _5-6)≤3.7 eV. The reaction rates for the silicon cluster cations decrease slowly from 7.3±0.8x10^-10 to 0.4±0.2x10 ^-10 cm³ molecule^-1 s^-1 with increasing cluster size for Si_1-8 ⁺. Silicon cluster anion reaction rates show a slight decrease from 15±1x10^-10 to 5.0±0.1x10^-10 cm³ molecule^-1 s ^-1 with increasing cluster size for Si_1-6 ^-. The ionic silicon cluster reactivity is explained by a radical-radical coupling mechanism involving the unpaired electron on NO₂ and an unpaired electron on a silicon atom at a charged or trivalent center in the clusters. Involvement of a radical electron on the silicon cluster contrasts with ionic silicon cluster reactivity with CH₃SiH₃ which requires divalent, silylene reactive centers on the clusters. Previously identified "magic number" clusters, Si₄ ⁺ and Si ₆ ⁺, do not exhibit atypical reaction rates or products in spite of a theoretically derived 1-2 eV lower reaction exothermicity relative to the other cluster sizes.