Rotational relaxation of CO in He, Ar, N2 and CO free jets

We report terminal rotational state distributions for CO in free jets employing buffer gases of helium, argon, nitrogen, and carbon monoxide. These distributions are obtained from 2+2 resonance enhanced multiphoton ionization (REMPI) spectra of the CO A 1Π state. All of the distributions show significant non-equilibrium character having excess population which increases as J increases. In He and CO buffers, for which CO translational temperature information is available, the lowest J CO rotational states appear to be in equilibrium with the CO parallel translation. It is suggested that rotational state populations in high moment of inertia molecules provide a convenient means to probe translational temperatures in free jet expansions.