Molecular beam diagnostics with internal state selection. II. Intensity distribution of a Na/Na2 supersonic beam

The profile of an Na₂ molecular beam is measured with internal state selection via laser induced fluorescence behind narrow collimating slits (slit width 0.2 to 0.5 mm). A flexible connection between the rotatable detector and the laser as well as the photomultiplex is provided by optical fibres. It is found that for pd < 0.5 Torr cm (p is the pressure in the oven, d is the nozzle diameter) the profile is approximately of trapezoidal form independent of the internal energy of the molecules. For pd > 0.5 Torr cm excessive wings appear. The population of the wings increases with pd and with the internal energy of the molecules. It is shown that scattering processes of molecules at the centerline do not cause the wings. The appearances of the wings is correlated with the onset of dimer formation in the beam outside the nozzle. Therefore, it is assumed that a large percentage of those molecules, that are formed in the free jet in intermediate energy levels with excess kinetic energy, diffuse rapidly off the centerline. Some of them are eventually deflected towards the collimating slit. An estimate of the total number of collisions shows that the relaxation of the internal energy along an off-axis path will be less complete than along the centerline. Thus, molecules that are already present in the oven or are formed in the nozzle channel are predominantly found in the narrow central part of the beam profile. A relatively large percentage of those molecules formed in the free jet are found in the wings. The cooling of the internal degrees of freedom is less efficient for the latter.