Real-time observation of formation and relaxation dynamics of NH4 in (CH3OH)m(NH3)n clusters.

The formation and relaxation dynamics of NH 4 (CH 3 OH) m (NH 3 ) 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, NH 4 + (CH 3 OH) m (NH 3 ) n , the kinetic model has been constructed, which consists of sequential three-step reaction: ultrafast hydrogen-atom transfer producing the radical pair (NH 4 -NH 2 )*, the relaxation process of radical-pair clusters, and dissociation of the solvated NH 4 clusters. The initial hydrogen transfer hardly occurs between ammonia and methanol, implying the unfavorable formation of radical pair, (CH 3 OH 2 -NH 2 )*. 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 NH 4 by solvation. The dependence of the time profiles on the probe wavelength is attributed to the different ionization efficiency of the NH 4 (CH 3 OH) m (NH 3 ) n clusters.