Raman spectroscopy of size-selected linear polyyne molecules C2nH2 (n=4-6) encapsulated in single-wall carbon nanotubes

Polyyne molecules (C2nH2, n = 4-6) encaged inside single-wall carbon nanotubes (SWNTs) are investigated by Raman spectroscopy. The Raman bands observed in the region of 2000-2200 cm(-1) are identified as due to axial-stretching fundamentals, nu(2)-nu(4) (sigma(g) in D-infinity h), of the linear carbon molecules. For C10H2 and C12H2, the overtone bands at similar to 4100 cm(-1) and combination bands of the polyyne and graphitic modes at similar to 3600 cm(-1) are also observed. The two-phonon excitations may be responsible for the observed resonance Raman peaks via new electronic states generated by the dispersion interaction exerted between encapsulated polyynes and nanotubes. The vibrational frequencies of the polyyne molecules in SWNTs increase as the diameter distributions of SWNTs decrease. The interaction is also responsible for the observed frequency shifts and broadening of the Raman bands.