Unimolecular decay paths of electronically excited species. VI. The Ã2E state of NH+3

Nonadiabatic nuclear dynamics of the ammonia cation studied by surface hopping classical trajectory calculations

The Landau-Zener (LZ) type classical-trajectory surface-hopping algorithm is applied to the nonadiabatic nuclear dynamics of the ammonia cation after photoionization of the ground-state neutral molecule to the excited states of the cation. The algorithm employs a recently proposed formula for nonadiabatic LZ transition probabilities derived from the adiabatic potential energy surfaces. The evolution of the populations of the ground state and the two lowest excited adiabatic states is calculated up to 200 fs. The results agree well with quantum simulations available for the first 100 fs based on the same potential energy surfaces. Three different time scales are detected for the nuclear dynamics: Ultrafast Jahn-Teller dynamics between the excited states on a 5 fs time scale; fast transitions between the excited state and the ground state within a time scale of 20 fs; and relatively slow partial conversion of a first-excited-state population to the ground state within a time scale of 100 fs. Beyond 100 fs, the adiabatic electronic populations are nearly constant due to a dynamic equilibrium between the three states. The ultrafast nonradiative decay of the excited-state populations provides a qualitative explanation of the experimental evidence that the ammonia cation is nonfluorescent.

The Jahn-Teller effect in the electron momentum spectroscopy of ammonia

The 1e and 3a(1) bands of the ammonia molecule have been studied using the high-resolution electron momentum spectroscopy at impact energies of 1200 and 600 eV. Several slices of 1e and 3a(1) bands in the different binding energy ranges were selected, and their electron-momentum distributions were carefully compared. The discernable difference among the distributions of the selected slices of the 1e band shows that the Jahn-Teller effect indeed influences the electron momentum distribution of the 1e orbital of ammonia.

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 nu2+ bending vibrational levels of the ground (x2A'') electronic state by two-photon resonant, three-photon ionization of NH3(ND3), via several different nu2' levels of the and ' Rydberg states. Subsequent excitation to the A2E 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 ground-state neutral) and, as previously, can be rationalized in terms of excitation to the Jahn-Teller distorted state, rapid radiationless transfer via one or more conical intersections linking the and 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.

Photoionization-induced dynamics of ammonia: ab initio potential energy surfaces and time-dependent wave packet calculations for the ammonia cation

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH3+ have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5 fs time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20 fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100 fs.