Excited state hydrogen transfer dynamics in phenol–(NH3)2 studied by picosecond UV-near IR-UV time-resolved spectroscopy

Abstract

Time-evolutions of excited state hydrogen transfer (ESHT) in phenol (PhOH)–(NH₃)₂ clusters have been measured by three-color picosecond (ps) ultraviolet (UV)-near infrared (NIR)-UV pump–probe ion dip spectroscopy. The formation of a reaction product, ˙NH₄NH₃, is detected by its NIR absorption due to a 3p–3s Rydberg transition. The ESHT reactions from all of the vibronic levels show biexponential time-evolutions, even from the S₁ origin. Based on the biexponential time-evolution, it is suggested that there is a second reaction path via the triplet πσ* state, which gives the slow component. The fast time-evolution of the ESHT reaction from the S₁ origin is measured to be 268 ps, which is 10-times slower than that in PhOH–(NH₃)₃, and a higher barrier between the ππ* and reactive πσ* states is suggested. The size dependence of the ESHT reaction rates is discussed based on a potential distortion due to the proton transferred state in the ππ* potential surface.