Abstract
Coalescence of C20(H)n fullerenes with n=0–4 was studied by laser desorption and ionization in a time-of-flight mass spectrometer (337 nm N2 excimer laser). The fullerenes were produced by elimination of bromine from extensively brominated dodecahedranes (mixture, mean composition [C20H1Br13]≡Y, and mixtures with somewhat higher hydrogen content [C20H2Br12]≡X) accessible by photobromination of the C20H20 cage hydrocarbon. For Y probes at 80 mJ/cm2 laser fluence, coalescence of the generated C20 fullerenes was identified as the dominant process providing a series of (C20)k+ oligomers (k=1–13); there is evidence for the formation of C60+ from C20. At lower fluences (30 mJ/cm2) in the mass spectra measured near the desorption threshold or in the low ion velocity regime, the bromine substituents were not (totally) eliminated, the original degree of bromination has even been raised. High laser fluences (1500 mJ/cm2) primarily effect the bromine content, generating C20HnBrm ions with m ranging from 0 to 18, fullerene coalescence is suppressed. For X probes the only slightly higher hydrogen content inhibits C20 coalescence; a C36H4+ ion is interpreted as fullerene.
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