Multiphoton Ionization and Oxidation Processes of Mg(NH3)n

Metal ions play important roles in chemical and biological systems and its oxidation and reduction processes trigger many catalytic reactions in solution. In these systems, the solvation of metal ions is a key issue to understand the chemical behavior. In order to clarify the microscopic aspect of solvation process under a limited number of solvent molecules, solvated metal ion clusters are considered as the good target for modeling the solvation process. We have previously reported the nanosecond photodissociation experiments and ab initio calculations for Mg(NH3)n (n = 1 4); the formation of the ion-pair state, [Mg-(NH3)n], and the size-dependent reactivity have been found [1]. In the present work, the femtosecond photoionization and pump-probe experiments for Mg(NH3)n are conducted in order to gain further insight into the details of oxidation modes including multiphoton ionization process. In the photoionization of Mg(NH3)n, the evaporation is the dominant process at lower laser intensity, while the oxidation reaction to produce H-atom elimination product, MgNH2(NH3)m, becomes predominant at higher laser intensity. The latter products have been found to produce through the higher excited state populated by multiphoton excitation. In addition to these fragment ions, doubly-charged ions, Mg(NH3)m, are observed for n ≥ 2 at the laser intensity higher than 10 W/cm. To gain further insight into the multiphoton ionization process, we examine the relaxation process of Mg(NH3)4 by using the pump-probe technique with the femtosecond laser. The fundamental (800 nm) of a Ti-sapphire fs laser is used as the pump and probe pulses. The decay time profiles are obtained by plotting the fragment ion signals in the mass spectra as a function of the delay times between the pump and probe pulses. The pump-probe curves for MgNH2(NH3)m (m = 1, 2) exhibit the decay time of 0.8 ps, which give the lifetime of the first excited state of Mg(NH3)4. From the comparison with the results on the isoelectronic systems such as Na(NH3)n and NH4(NH3)n, the ultrashort lifetime of the first excited state of Mg(NH3)4 is ascribed to the enhancement of nonradiative transition, which is induced by the strong vibronic interaction associating with the formation of ion-pair state. On the other hand, the time profile for Mg(NH3)3 exhibits a bleaching of the absorption, which gives the recovery time of the initial state as 1.2 ps. From the previous ab initio calculations, Mg(NH3)4 have four isomers such as two (4 + 0) structures and two (3 + 1) structures with similar energy [1]: Only the (4 + 0) structure has the absorption band at 800 nm. Therefore, the observed time profile is ascribed to the recovery of the population for the (4 + 0) structure depleted by the pump pulse. The recovery time is considered to include the internal conversion and intracluster vibrational relaxation in the first excited state and as well as the isomerization between the closely lying (4 + 0) and (3 + 1) structures in the ground state.