  Delineating the dominant processes responsible for nanomaterial synthesis in
a plasma environment requires measurements of the precursor species
contributing to the growth of nanostructures. We performed comprehensive
measurements of spatial and temporal profiles of carbon dimers (C$_2$) in
atmospheric-pressure carbon arc by laser-induced fluorescence. Measured spatial
profiles of C$_2$ coincide with the growth region of carbon nanotubes and vary
depending on the arc operation mode, which is determined by the discharge
current and the ablation rate of the graphite anode. The C$_2$ density profile
exhibits large spatial and time variations due to motion of the arc core. A
comparison of the experimental data with the simulation results of
self-consistent arc modeling shows a good agreement. The model predicts well
the main processes determining spatial profiles of carbon dimers (C$_2$).
