You may also wish to search for items by Palm.
Only one matching reference was found.
Signorell, R.; Palm, H.; Merkt, F., Structure of the ammonium radical from a rotationally resolved photoelectron spectrum, J. Chem. Phys., 1997, 106, 16, 6523, https://doi.org/10.1063/1.473653 . [all data]