Dynamics of hyperspherical and local mode resonance decay studied by time dependent wave packet propagation

Time dependent wave packet propagation of resonance states of ABA molecules is used to demonstrate the correlation between the directionality of the lobes of the wave functions and mode selectivity of the unimolecular decay. This correlation was inferred by Hose and Taylor. The molecule is modeled by the Thiele-Wilson coupled Morse oscillators. A near-degenerate pair of resonances with extreme motions is studied in detail: The local "bond" mode with lobes pointing towards the exit valleys of the potential decays about 30 times faster than the hyperspherical "restricted precession" mode with dominant lobe on the potential ridge. This is in close analogy to mode selectivity in the Hénon-Heiles system. The wave function propagation technique also yields detailed insight into the dissociation mechanism. Out of several choices, only a single lobe penetrates into the exit valley. For the local mode resonance vibrational predissociation starts out primarily from extended vibrationally excited diatomic configurations, A↔B(v* = 1)⋯A→AB(v′ = 0) + A. However, the hyperspherical mode resonance prefers compressed diatomic geometry just before dissociation, AB(v* = 3)⋯A→AB(v′ = 0) + A. The results imply some general criteria for mode selective unimolecular chemical reactions, as well as a successful numerical test of the preparation of resonance wave functions and their propagation by the Fourier method.