Optical emission studies were performed to investigate thermal and dynamical properties of a plume produced by laser ablation of a graphite target in a nitrogen atmosphere. Experimental spectra of $C_2(d^3{Pi}_g{ o}a^3{Pi}_u$, ${Delta}_V$=1) and CN ($B^2{Sigma}^+{ o}X^2{Sigma}^+,{Delta}_V=0)$ were simulated to obtain the vibrational and rotational temperatures of the electronically excited species at various laser fluences and distances from the target. The spectroscopic temperatures of both molecules were found to be nearly independent of the laser fluence. The temperature of CN molecules was peaked in the middle of the plume while that of $C_2$decreased with increase in the distance. At a given distance, the temperature of CN molecules was clearly higher than that of $C_2$.