Raman spectra and calculated vibrational frequencies of size-selected C₁₆, C₁₈, and C₂₀ clusters

The surface plasmon polariton-enhanced Raman spectra of size-selected C₁₆, C₁₈, and C₂₀ clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C₂₀ are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm^-1 region for C₁₆, C₁₈, and C₂₀ suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C₂₀. The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions.