Abstract Measurements of work functions on the electrodes in plasma diodes of the thermionic energy converter (TEC) type are commonly made by studies of the voltage-current characteristics. The plasma in such converters is a low temperature cesium plasma, between two electrodes at different temperatures, around 1500 and 800 K respectively. We have recently reported on new phenomena in such plasmas, giving very strong electron emission from the cold to the hot electrode. This type of behaviour is related to the formation of large densities of excited states, and we explain the observations as due to a condensed phase of excited cesium atoms, which we call Rydberg matter. This type of matter was recently predicted theoretically by Manykin et al. An analysis of the diode measurements gives very low work functions for the excited matter, less than 0.7 eV and probably less than 0.5 eV. This low work function agrees with the jellium model, since the density of atoms in Rydberg matter is very low.