Coalescence of C-20 fullerenes

Coalescence of C-20(H)(n) fullerenes with n=0-4 was studied by laser desorp tion and ionization in a time-of-flight mass spectrometer (337 nm N-2 excim er laser). The fullerenes were produced by elimination of bromine from exte nsively brominated dodecahedranes (mixture, mean composition [C20H1Br13]equ ivalent toY, and mixtures with somewhat higher hydrogen content [C20H2Br12] equivalent toX) accessible by photobromination of the C20H20 cage hydrocarb on. For Y probes at 80 mJ/cm(2) laser fluence, coalescence of the generated C-20 fullerenes was identified as the dominant process providing a series of (C-20)(k)(+) oligomers (k=1-13); there is evidence for the formation of C-60(+) from C-20. At lower fluences (30 mJ/cm(2)) in the mass spectra meas ured near the desorption threshold or in the low ion velocity regime, the b romine substituents were not (totally) eliminated, the original degree of b romination has even been raised. High laser fluences (1500 mJ/cm(2)) primar ily effect the bromine content, generating C20HnBrm ions with m ranging fro m 0 to 18, fullerene coalescence is suppressed. For X probes the only sligh tly higher hydrogen content inhibits C-20 coalescence; a C36H4+ ion is inte rpreted as fullerene. (C) 2001 American Institute of Physics.