Multiphoton ionization and oxidation processes of Mg–ammonia clusters

Abstract

Multiphoton ionization and oxidation processes of ammoniated magnesium clusters are investigated by the multiphoton ionization method with an intense femtosecond laser. In the photoionization of mass-selected Mg⁺(NH₃)_n, evaporation dominates at lower laser intensity, while the oxidation reaction to produce H-atom elimination products, MgNH₂⁺(NH₃)_m, becomes predominant at higher intensity. In addition to these fragment ions, doubly-charged ions are observed for n ≥ 2 at the laser intensity higher than 10¹² W cm⁻². We also examined the femtosecond pump–probe experiments for Mg⁺(NH₃)₄ by monitoring these reaction products. The lifetime of the first excited state is determined as 0.8 ps from the temporal profile of MgNH₂⁺(NH₃)_m. On the other hand, the time profile of the evaporation products exhibits a bleaching of the absorption, which gives the recovery time of the initial state as 1.2 ps. Multiphoton excitation of Mg(NH₃)_n with the femtosecond laser at 800 nm gives doubly-charged ions with n > 3 in addition to singly-charged cluster ions and H-atom elimination products such as MgNH₂⁺(NH₃)_m. The absence of small doubly-charged ions is ascribed to a charge reduction reaction followed by Coulomb explosion. On the basis of these results, the dynamics of the solvation and oxidation reaction processes of Mg(NH₃)_n is discussed.