For the case of the permanent salts such as BTPP-Cl, the experimentally measured ion yields, which were defined as the quotient of total number of ions detected by MS over the total number of salt molecules in the sample, are in the order of 10 -4 . The calculated value of 4.6 × 10 -5 at 1,000 K can be considered close to but slightly lower than the experimental values. One hypothesis for the discrepancy is related to the lucky survivor model, under which thermal equilibrium of plume is not fully achieved, and the distance between the ions in some of the ion pairs is longer than 1 nm due to shielding by other molecules, and remains so after the shielding molecules fall off. Another possibility is that the plume temperature is higher than 1,000 K. For example, at 1,200 K, the calculated ion yield is 2.4 × 10 -4 , which matches the experimental numbers well. In addition, for more accurate analysis, the plume pressure and frequency of particle collision that is required to transform translational kinetic energy to ion separation should also be considered.