Direct evidence of hydrogen-atom tunneling dynamics in the excited state hydrogen transfer (ESHT) reaction of phenol–ammonia clusters

Abstract

The photodynamics of phenol–ammonia clusters, PhOH(NH₃)_3–5, after one UV photon absorption, has been investigated using velocity map imaging of the NH₄(NH₃)_2–4 cluster products. The dependence of the NH₄(NH₃)₂ translational energy distributions on the available energy reveals three dynamical regions in close correspondence with the photodissociation of bare phenol. At low excitation energies (between 282 and 260 nm), the NH₄(NH₃)₂ distribution mirrors the hydrogen-atom passage through the 1¹ππ*–1¹πσ* barrier, constituting the first evidence of hydrogen-atom tunneling dynamics in an excited state hydrogen transfer (ESHT) reaction. At excitation wavelengths below 260 nm, the product distributions are consistent with two separate barrierless dissociation processes associated, respectively, with excitation to the 1¹ππ* and 2¹ππ* excited electronic states. Similar conclusions can be derived from the velocity map imaging results on the larger NH₄(NH₃)_3,4 cluster products.