Optical emission studies of atomic, molecular, and particulate carbon produced from a laser vaporization cluster source

Abstract

We perform optical emission studies on the species generated by the laser vaporization of graphite into a pulsed helium flow. With short flow channels prior to free-jet expansion the emission at short downstream distances is dominated atomic carbon lines and three band systems of C2: d 3∏g–a 3∏u (Swan), C 1 ∏g–A 1∏u (Deslandres–d’Azambuja), and D 1∑+u –X 1∑+g (Mulliken). Partially rotationally resolved spectra of the Δv=−1 Swan bands are analyzed using a band contour approach to obtain rotational temperatures and rough vibrational distributions for C2 d 3∏g. The direct emission spectrum from the short-channel source becomes a weaker, structureless continuum when viewed at long downstream distances. Similar long-lived continua are induced by a low-fluence laser at 266, 355, or 532 nm when it crosses the expansion emanating from a source with a long flow channel. We attribute these continua to the incandescence of hot carbon particles, T=2500–4000 K. The continua are fit to a model in which the Planck blackbody function is modified by the particle emissivity to determine particle temperatures and obtain crude estimates of particle sizes. These data provide the first direct evidence that carbon particles are produced in a laser-vaporization cluster source.