Vibrational Spectra and Structures of Neutral Si_mC_n Clusters (m + n = 6): Sequential Doping of Silicon Clusters with Carbon Atoms

Vibrational spectra of mixed silicon carbide clusters SimCn with m + n = 6 in the gas phase are obtained by resonant infrared–vacuum-ultraviolet two-color ionization (IR–UV2CI for n ≤ 2) and density functional theory (DFT) calculations. SimCn clusters are produced in a laser vaporization source, in which the silicon plasma reacts with methane. Subsequently, they are irradiated with tunable IR light from an IR free electron laser before they are ionized with UV photons from an F2 laser. Resonant absorption of one or more IR photons leads to an enhanced ionization efficiency for SimCn and provides the size-specific IR spectra. IR spectra measured for Si6, Si5C, and Si4C2 are assigned to their most stable isomers by comparison with calculated linear absorption spectra. The preferred SimCn structures with m + n = 6 illustrate the systematic transition from chain-like geometries for bare C6 to three-dimensional structures for bare Si6. In contrast to bulk SiC, carbon atom segregation is observed already for the smallest n (n = 2)