Real-Time Observation of Formation and Relaxation Dynamics of NH4 in (CH3OH) m (NH3) n Clusters

The formation and relaxation dynamics of NH4(CH3OH) m (NH3) n clusters produced by photolysis of ammonia−methanol mixed clusters has been observed by a time-resolved pump−probe method with femtosecond pulse lasers. From the detailed analysis of the time evolutions of the protonated cluster ions, NH4 +(CH3OH) m (NH3) n , the kinetic model has been constructed, which consists of sequential three-step reaction: ultrafast hydrogen-atom transfer producing the radical pair (NH4−NH2)*, the relaxation process of radical-pair clusters, and dissociation of the solvated NH4 clusters. The initial hydrogen transfer hardly occurs between ammonia and methanol, implying the unfavorable formation of radical pair, (CH3OH2−NH2)*. The remarkable dependence of the time constants in each step on the number and composition of solvents has been explained by the following factors: hydrogen delocalization within the clusters, the internal conversion of the excited-state radical pair, and the stabilization of NH4 by solvation. The dependence of the time profiles on the probe wavelength is attributed to the different ionization efficiency of the NH4(CH3OH) m (NH3) n clusters.