The measurement of energy gain spectra of projectile is an effective method to study state-selective cross sections in ion-atom collisions in recent decades.For pure electron capture process, we can get the energy gain of the reaction:Q= -v0Pz-(nv02)/2 (a. u.) , by measuring the recoil-ion longitudinal momentum Pz, where -v0 is the velocity of the projectile, and n is the number of captured electrons. Theoretically, using the extended MCBM(Molecular Coulombic over Barrier Model) , i. e. reaction window theory, one can get a Gaussian distrubution of the differential cross section in a certain electron capture process about the Q value. That is The measurement of energy gain spectra of projectile is an effective method to study state-selective cross sections in ion-atom collisions in recent decades. For pure electron capture process, we can get the energy gain of the reaction: Q = -v0Pz- ( nv02) / 2 (au), by measuring the recoil-ion longitudinal momentum Pz, where -v0 is the velocity of the projectile, and n is the number of captured electrons. Theoretically, using the extended MCBM (Molecular Coulombic over Barrier Model) , ie reaction window theory, one can get a Gaussian distrubution of the differential cross section in a certain electron capture process about the Q value. That is