Optical diagnostics for temperature measurement in a DC arcjet reactor used for diamond deposition

In the plume of a de arcjet used for the chemical vapor deposition (CVD) of diamond, we determine the gas temperature from laser induced fluorescence measurements (LIF) of the rotational distribution of the X-state of nitric oxide which is seeded into the plume. This temperature is compared with LIF measurements of the rotational distributions of the CH X-state and the C-2 a-state in this plume. LIF measurements of CH are complicated by the presence of C-3 and we discuss strategies to deal with this interference. The gas temperature describing the rotational distributions obtained for NO, C-2 and CH agree within experimental error. Optical emission measurements indicate that the rotational and vibrational distributions of the excited A-state of CH and a-state of C-2 are characterized by vibrational and rotational populations which are at least 1000 K above the ground state distributions. The excited states are collisionally quenched before their population distributions equilibrate with the gas temperature. We also determine relative populations of the ground and excited states along the axis of the plume between the arcjet nozzle and the substrate and relative populations for a cross section of the jet, midway between the nozzle orifice and the substrate. The measured relative ground and excited state populations for both CH and C-2 show different trends along the plume axis, indicating that the ground and excited states of these molecules are products of different chemical mechanisms; such mechanisms are discussed.