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Position dependent microwave transitions in Rydberg helium were driven to investigate anomalous systematic effects in the waveguide apparatus used to measure the n = 2 fine structure of positronium [1]. The Stark broadening of the resulting line shapes was used to map out stray electric fields [2] along the beam axis and characterize frequency shifts dependent on the microwave propagation direction. As an independent test of the line shape model used to analyse the data, free space microwave spectroscopy was performed in well-defined electric fields external to the waveguide and the results of the two methods have been compared.
[1] R. E. Sheldon, T. J. Babij, S. H. Reeder, S. D. Hogan, and D. B. Cassidy “Precision microwave spectroscopy of the positronium 2 3S1 → 2 3P2 interval”. Phys. Rev. Lett. 131, 043001 (2023)
[2] A. Osterwalder and F. Merkt, “Using high Rydberg states as electric field
sensors,” Physical Review Letters, 82, 1831, (1999).
