Observation of halos in rotationally inelastic scattering of Na2 from Ar

We have measured level to level differential cross sections for rotational level changing collisions in ground state Na2 with Ar: Na2 (v\=0, ji=7)+Ar→Na2 (v\=0, jf)+Ar. Measurements range in Δj(jf−ji) from −4 to 80 and in Θ to π. A seeded supersonic beam source produces rotationally cold Na2 with a narrow velocity distribution; the levels are isolated and the differential cross sections are measured with cw dye lasers, using a new technique, angular distributions using the Doppler shift. The differential cross sections display a sharp rise and slow decline, reaching a maximum at an angle which is roughly proportional to Δj. We term these features ’’halos’’. (AIP)