Proton tunnelling assisted by the intermolecular vibration excitation in solid state

The two-dimensional potential energy surface (PES) of the quasi-symmetric OHO fragment suggested earlier is used for treatment of proton transfer dynamics. The PES describes semi-quantitatively the main experimental regularities for a strong hydrogen bond. Strong coupling of modes (proton movement and O⋯O vibration) and dynamic asymmetry of the PES are taken into account. The respective wave equation was solved numerically with adiabatic separation of the "fast" (proton) and "slow" (O⋯O vibration) subsystems. Quantum jumps between vibrational levels of both subsystems under the random force action of the environment are assumed to realize the proton transfer from one well into another. The tunnel transitions between the "slow" subsystem levels. Corresponding to the proton localization in different wells, are most important, their probabilities depending strongly on the O⋯O equilibrium separation. At large O⋯O distances (≈2.64 Å) the total tunnel transition probability from definite O⋯O vibrational levels (m) of one well into all possible levels of the other well is shown to increase with m. Such a promotion of the proton tunnelling was observed by several authors at the laser selective vibrational excitation of the "slow" subsystem. For smaller O⋯O separation (≈2.52 Å) no vibrational assistance of the proton tunnelling occurs. The microscopic mechanism of the process for these two cases is interpreted in terms of the respective matrix elements. © 1992.