are still questions relating to the performance of an iodine propellant important specifications of a thruster are efficiency input power thrust and specific impulse xenon based hall engines have exhibited efficiencies exceeding 50 and specific impulses higher than 1500 s the ionization potential and mass of the propellant have a strong effect on the thrust and required input power the lower the ionization potential the less energy is required to produce an ion electron pair the ionization potentials of both atomic 10 45 ev and molecular 9 4 ev iodine compare favorably to xenon 12 13 ev the atomic mass of iodine is only slightly lower than that of xe 127 versus 131 amu efficiency is largely determined by the discharge properties of the propellant in hall effect and ion thrusters high ionization efficiencies near 80 are sought at a minimal power input in this aspect there are some important differences between the properties of iodine and xenon whereas xenon is an atom iodine is a molecule that in addition to electronic internal energy states also has rotational and vibrational degrees of freedom since internal excitation of exhaust molecules signifies loss of translational thrust energy it is possible that previous searches for alternative propellants for hall effect and ion thrusters only.