Ground state rotational and kinetic temperatures of C2 molecules in a laser-produced plasma

This study presents an experimental comparison between rotational and kinetic temperatures of molecular species in a laser-produced plasma, utilizing tunable laser absorption spectroscopy. We analyzed high-resolution and time-resolved absorption spectra of 31 rotational transitions within the Phillips A1Πu–X1Σg 3-0 band of the C2 molecule, measured from a laser-produced graphite plasma in ∼2 Torr argon, within the 771–781 nm spectral range. Spectral fitting provided quantitative state column densities of C2 rotational energy levels in the ground electronic and vibrational states. Boltzmann analysis revealed thermalized rotational distributions showing rotational temperatures decreasing from 3000 to 600 K within 5–500 μs following the onset of the laser plasma. The measured kinetic and rotational temperatures of C2 molecules are comparable, suggesting that collisional processes in the laser-produced plasma are sufficient to maintain thermal equilibrium between the C2 molecules and the Ar background gas.