Structural evolution in (CO2)n clusters (n<103) as studied by mass spectrometry

The mass distributions of (CO2)n− produced by electron-impact ionization of a free jet have been measured up to n∼103. Several intensity anomalies observed in a small size range are ascribed to the stabilities of the negative ions. In contrast, a series of humps are clearly discernible in the range of n⩾100, which reflects the abundance of neutral (CO2)n. The intensity oscillations are analyzed in the context of ‘geometrical shell closings’. The analysis reveals that the (CO2)n clusters with n≳80 have truncated forms of an fcc crystal, most likely a cuboctahedral motif.