A forgotten channel in the excited state dynamics of phenol–(ammonia)_n clusters: hydrogen transfer

Small phenol–(NH₃)_n clusters have been studied through two-color two-photon and one-photon VUV ionization, in order to disentangle the contributions of various dissociation or reaction paths in the excited and ionic states. The most striking result of the two-color experiment is that (NH₄)⁺(NH₃)_n=1,5 fragments are observed with large delays (up to a few hundred nanoseconds) between the excitation and ionization lasers, whereas these same fragments are not observed in the VUV one-photon ionization experiment. In order to account for these findings, a new deactivation channel in the excited state of phenol–(NH₃)_n clusters has to be introduced: the hydrogen atom transfer PhOH(S₁)–(NH₃)_n→PhO^•+(NH₄)(NH₃)_n−1. In this case, the delayed (NH₄)⁺(NH₃)_n−1 signals correspond to direct ionization of the (NH₄)(NH₃)_n−1 clusters produced in the excited state.