Low-temperature rotational relaxation of N2 in collisions with Ne

The rotational relaxation of nitrogen molecules in collisions with neon has been studied in supersonic expansions. N-2 rotational temperatures have been determined from resonance-enhanced-multiphoton-ionization (REMPI) spectra performed in a series of supersonic molecular beams of N-2 diluted in Ne. Terminal flow velocities and translational temperatures for the two expansion partners have been obtained from mass-selected dine-of-flight measurements. From the measured data, the approximate isentropic behavior of the expansion could be verified and the thermal cross section for N-2/Ne rotational relaxation at very low temperatures could be derived. The cross section increases from a value of approximate to 35 Angstrom (2) at 5 K, goes through a maximum of about 55 Angstrom (2) between 10 and 20 K, and then decreases smoothly to approximate to 25 Angstrom (2) for T = 60 K. The relaxation cross section for N-2/Ne collisions is larger than that for N-2/N-2 collisions below roughly 20 K and smaller above this temperature.