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 Si_mC_n 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. Si_mC_n 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 Si_mC_n 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 Si_mC_n 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).