Imaging studies of the photodissociation of NH3+ and ND3+ cations

Velocity map ion imaging methods have been used to study the photofragmentation dynamics of state-selected NH3+ and ND3+ cations. The cations were prepared in selected v(2)(+) bending vibrational levels of the ground ((X) over tilde (2)A '') electronic state by two-photon resonant, three-photon ionization of NH2(ND3) via several different v(2)' levels of the (B) over tilde and (C) over tilde Rydberg states. Subsequent excitation to the (A) over tilde E-2 state by absorption of a 207.6 nm photon resulted in N-H(D) bond fission and NH2+(ND2+) fragment ion formation. These fragments exhibit isotropic recoil velocity distributions, which peak at low kinetic energy but extend to the maximum allowed by energy conservation. Such findings accord with conclusions from earlier electron induced photoionization and photoelectron-photoion coincidence studies of NH3 at similar total energies (defined relative to the groundstate neutral) and, as previously, can be rationalized in terms of excitation to the Jahn-Teller distorted A state, rapid radiationless transfer via one or more conical intersections linking the (A) over tilde and (X) over tilde state potential energy surfaces (PESs) and subsequent unimolecular decay on the latter PES. Weak NH2+ and NH+ fragment ion signals are also observed when exciting with the ionization laser only; imaging these fragment ions provides some insights into their likely formation mechanisms.