Molecular beam diagnostics with internal state selection: Velocity distribution and dimer formation in a supersonic Na/Na 2 beam

Abstract

The velocity distribution parallel and perpendicular to the molecular beam axis has been determined for molecules in well defined quantum states using TOF — via optical pumping and the Doppler-shift method. It has been found that the flow velocity as well as the speed ratio changes with the internal energy of the molecule. The flow velocity increases with increasing internal energy at low pd values ( p is the pressure in the oven, d is the nozzle diameter) while the opposite is true at high pd values. The parallel speed ratio is smaller for molecules in vibrationally excited states and the perpendicular velocity distribution shows excessive tails that are more pronounced for molecules in higher lying levels. The population of individual levels has been monitored via laser induced fluorescence. It does not change monotonically with pd. The population distribution is not in thermal equilibrium and can only be approximately described by a temperature of T υ, ≈ 150 K. On the basis of these results a simple model for the influence of the recombination of atoms on the expansion is derived: Molecules are initially neither formed in the υ = 0 vibrational level nor with high internal excitation but probably with ⩾ 1000 cm −1 of internal energy. The recombination leads to fast atoms and molecules. It is the incomplete deceleration of these fast particles together with an efficient quenching process for the internal energy that determines the flow velocity of molecules in individual quantum states at low pd values. At high pd values the acceleration of molecules with much internal energy is incomplete because those molecules have necessarily made only few collisions.