Tunneling dynamics of the NH3 (Ã) state observed by time-resolved photoelectron and H atom kinetic energy spectroscopies.

Abstract

We have investigated the effects of quantum tunneling on the photodissociation dynamics of ammonia, following below and above barrier photoexcitation of low-lying levels of the ν'(2) umbrella mode of the NH(3) à state (NH(3) (Ã)). This barrier separates the local minimum of the vertical Franck–Condon region from the NH(3) (Ã)/NH(3) (X̃) conical intersection (CI) which can be accessed along the N–H stretch coordinate. Two complementary techniques, time-resolved photoelectron spectroscopy (TR-PES) and time-resolved total kinetic energy release spectroscopy (TR-TKER), have been utilized to directly measure, for the first time, vibrational level dependent excited state lifetimes and N–H dissociation time scales as well as photoproduct final energy distributions. Interestingly, ν'(2) even/odd dependencies are observed in the measured time constants and NH(2) internal energy spectra, which are attributed to tunneling through a barrier, whose magnitude is dependent on the planarity of NH(3) in the à state and direct versus indirect dissociation at the NH(3) (Ã)/NH(3) (X̃) conical intersection.