Mode selective control of unimolecular dissociations: Survey, and model simulations for HDO → H+DO, D+HO

Abstract

A survey of mode selective bond fissions, decay rates and vibrational structures in unimolecular dissociations is followed by model simulations of dissociative HDO resonance decays HDO∗( m a n b)→ H+DO, D+HO. Accordingly, the system yields extreme variations of dissociative lifetimes from ps to > ns andfissions of specific bonds, either (a) OH or (b) OD, depending on selective investments of similar energies for dominant excitations of m a plus n b quanta in the stretches of bonds a and b, in the electronic ground state. Both effects result from the decay mechanism of vibrational Feshbach resonances: diabatic transfers of preferably few vibrational quanta from the “spectator” bond that survives as product hydroxyl towards the “promoting” bond to be broken. For example, preferable transfers of single quanta from spectator bonds b and a of near-degenerate local modes resonancesHDO* (17 a1 b) and HDO* (1 a22 b) induce exclusive fissions of “promoting” bonds a (OH) and b (OD), respectively, on the picosecond-time scale. In contrast, dissociation of the near-degenerate hyperspherical-type resonance HDO*(9#a#7#b#) would require rather improbable transfers of several (>/7) vibrational quanta to be accumulated in a single bond, implying much longer (>ns) decay times. Mode selective dissociation dynamics correlate with vibrational structures: local and hypersphericalotype modes have wavefunctions which extend along the potential valleys of dissociative bonds and along ellipsoidal arcs, respectively. The nodal patterns correlate well with the quantum numbers m a n b of HDO* ( m a n b). These as well as complementary, e.g., toroidal structures are also observed for excited bound states HDO( m a n b). In general, mode selectivity in HDO is more extreme than in the symmetric reference H 2O, allowing even better control of reaction rates, and novel control of exclusive product branching ratios induced by selective bond fissions. The results are derived for the simple Thiele-Wilson model of HDO, neglecting effects of bending, rotations, and potential couplings. The simulations employ fast Fourier-transform propagations of wavepackets prepared bysuperpositions of products of Morse oscillator wavefunctions adapted to bonds of asymmetric dihydrides.