Carbon Chain Molecules in Cryogenic Matrices - Polyynes: Synthesis, Properties, and Applications - page 11

FIGURE 1.7 Raman spectra of matrix samples bearing carbon molecules (532 nm
CW laser excitation). Most of the matrix material (neon) had been evaporated by
warming up, upon which the sample was cooled down again to 4K to carry out the
measurement (upper spectrum). Then the sample was warmed up to 200K and the
lower spectrum was taken. Notice the decrease in the relative height between
the broad peak at 2100 cm 1 relative to that at 1500 cm 1 . The former displays
the abundance of vibrating carbons with sp bonds while the latter displays that of
carbons with sp 2 bonds. At some point in the warming process, a virulent but,
judging from the Raman data, not entirely complete sp-C to sp 2 -C transition takes
place. This change is accompanied by flashes of light emission.
into disordered graphite-like networks. It may be mentioned that Casari
and co-workers obtained similar Raman spectra in their carbon cluster
deposition experiments [33]. The sp to sp 2 conversion thus seems to be a
very general phenomenon in the growing process from carbon clusters to
bulk carbon.
1.4 CONCLUSIONS
The uniqueness of carbon rests on the diversity of the atomic bond types
it can assume and the ease by which carbon can polymerize. Upon con-
densation from the vapor phase into matrices, carbon molecules initially
form chains and possibly rings. When these initial species become spatially
concentrated, an exothermic conversion into a graphite-like network takes
place. Under the cryogenic conditions prevailing during matrix sublimation,
the resulting network is highly irregular and distorted. The network seems
to still contain sp bonds which gradually disappear upon thermal annealing