Assuming thermal equilibrium is fully achieved, at 1,000 K, the ion yields estimated according to equations ( ) and ( ) are 10 -10 and 10 -14 , respectively. These numbers are significantly lower than experimental values (~10 -4 ). At 2,200 K, the estimated ion yields are 10 -5 and 10 -6 according to equations ( ) and ( ), respectively. These numbers are close to experimental values. As discussed earlier, the plume temperature may be lower than 2,200 K, and thus, the estimated ion yields may be lower than the observed numbers. Therefore, thermal equilibrium may not be fully achieved in the plume, and a portion of the ions detected may be from the sample. In addition, equation ( ), where neutralization via proton transfer is impossible, may be responsible for the lower than observed ion yield.