We present levelhyphen;tohyphen;level differential cross sections for the rotationally inelastic process Na2(thinsp;ji=7)+Arrarr;Na2(thinsp;jf)+Ar at center of mass energies 0.49 and 1.1 eV, with Na2in its lowest vibronic state. The energy was varied by seeding Na2in noble gas mixtures of varying atomic mass. The range of the final rotational quantum number thinsp;jfis 9le;thinsp;jfthinsp;le;47, corresponding to 2le;Dgr;jle;40. The measurements were made in crossed molecular beams. The initial rotational level was selected using optical pumping, and the final rotational level was detected by laser induced fluorescence. The angular distribution was determined by a new Doppler technique that gives good small angle resolution, and whose indifference to the direction of the initial relative velocity of the collision simplifies measurements at different energies.
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