Room-temperature ablation of a composite graphite-metal target, at 248 nm, in O2, produces large-channeled (50 - 150 nm) multiwalled carbon nanotubes. We find that the formation of these carbon nanotubes is dependent on the ambient gas employed during ablation. Such structures are not produced in inert atmospheres of Ar or in high vacuum. High-resolution, in-situ, time-resolved emission spectroscopy has been used to track the evolution of species (C2, C3, Ni/Co) in the ablation plume, in different ambient gas atmospheres. Spectral fits on low and high-resolution spectra reveal time-dependent vibrational-rotational temperatures for C2 that are different in O2 compared to Ar. Spectral modeling shows that the vibrational-rotational temperatures for C2 produced in O2 remain at ~ 5000 K for nearly 20 &mgr;sec, but drop rapidly in Ar. The key role of exothermic reactions occurring in the plume, and that of radiative cooling will be discussed.