Using a two sector field mass spectrometer in combination with a crossed beams ion source we have obtained direct experimental evidence that ${\mathrm{C}}_{60}$ fragment ions such as ${\mathrm{C}}_{58},{\mathrm{C}}_{56},{\mathrm{C}}_{54},\dots$ produced by electron impact ionization of ${\mathrm{C}}_{60}$ may be formed by unimolecular decay of the ${\mathrm{C}}_{60}$ parent ion involving sequential loss of ${\mathrm{C}}_2$. Moreover, by comparing experimental and theoretical breakdown graphs the overriding conclusion is that in the case of ${\mathrm{C}}_{56}^{z+}$ production (with $z=1,2,3\mathrm{}$) sequential loss of two ${\mathrm{C}}_2$ units dominates over the loss of a single ${\mathrm{C}}_4$ unit.

• Received 7 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.2654