Characterization of a pulsed supersonic beam of ammonia monomer and clusters using the hexapole electric field

An intense pulsed supersonic beam of ammonia monomer is focused and state selected by a hexapole electric field. The intensity of the focused beam is estimated to be ∼10¹⁹ molecules sr^-1 s^-1. The state-selected monomer beam is characterized by comparison of its intensity with the numerical calculation of trajectories according to the Stark effect for two nearby levels, and the rotational state distribution is obtained. Ammonia clusters with n members (n = 2-10) are found to be unfocused by the hexapole field. Thus, neither a symmetric-top structure nor a very large electric dipole moment is expected for ammonia dimer and larger clusters. The analysis on the nozzle-stagnation pressure dependence of the focusing behavior shows a method to estimate the fraction of clusters in the beam by the hexapole technique.