Tunneling Dynamics of the NH3 ((A)over-tilde) State Observed by Time-Resolved Photoelectron and H Atom Kinetic Energy Spectroscopies

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 nu(2)' umbrella mode of the NH3 (A) over tilde state (NH3 ((A) over tilde). This barrier separates the local minimum of the vertical Franck-Condon region from the NH3 ((A) over tilde)/NH3 ((X) over tilde) 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, 1.4 even/odd dependencies are observed in the measured time constants and NH3 internal energy spectra, which are attributed to tunneling through a barrier, whose magnitude is dependent on the planarity of NH3 in the (A) over tilde state and direct versus indirect dissociation at the NH3 ((A) over tilde)/NH3 (R) conical intersection.