State-to-State Quantum And Quasiclassical Dynamics Of Water Molecule Scattering From Metal Surfaces

Chemical reactions at gas-surface interfaces are governed by the energy transfer and conversion taking place at the surface.Finding a way of channeling energy into the proper degrees of freedom will help to understand and guide new developments of heterogeneous catalysis.It is however remains less clear that how intramolecular vibrational modes couple with each other and energy flows among them during the collision process with metal surfaces.Fortunately,state-to-state molecular scattering can serve as a sensitive probe of the molecule-surface interaction,providing very valuable information on the fate of initial energy deposited into a specific mode.However,due to the complexity of state-to-state quantum dynamics(QD)calculations,existing theoretical studies have primarily focused on diatomic molecules scattering from metal surfaces.In this thesis,the state-to-state molecular scattering dynamics on metal surfaces is extended from diatomic to polyatomic by developing the fully-coupled six-dimensional(6D)Chebyshev wave packet dynamics method.And,the accuracy of the quasiclassical trajectory(QCT)method and site-averaging model in moleculesurface state-to-state scattering dynamics is verified by comparing with the exact quantum results.The specific research contents are as follows:The 6D state-to-state QD of polyatomic molecule H2O/HOD scattering from Cu(111)surface is studied,and the intramolecular mode-and bond-selective vibrational energy transfer mechanism in these two molecules is revealed.When H2O in its antisymmetric stretching vibration(1v3)collides the surface,we find that the intramolecular vibrational energy redistribution is predominant from 1v3 to the symmetric stretching state(Iv1),while rather inefficient to the bending state(1v2).Similarly,the initial energy in 1v1 state of H2O can also transfer to the 1v3 state,with almost the same efficiency as v3-to-v1.In comparison,the energy transfer from the more localized lvOH or lvOD state to the other bond is much more difficult.This mode-and bond-selective vibrational energy transfer can be well understood in a local mode picture within the sudden limit.The mechanism proposed by us qualitatively explains that why the product v1/v3 ratio calculated here is～3 times over that recently measured for CH4 scattering on Ni(111).And we predict the possible vibrational energy transfer phenomena that may occur during the scattering of metal surfaces by other polyatomic molecules such as deuterated methane and ammonia.However,extending this 6D QD model to higher dimensions is extremely difficult,because of the exponential increase of the number of basis functions in the wavefunction-based quantum method.Therefore,we are also thinking about some alternative approximation methods.An effective approximation method is the QCT method,in which the initial state sampling and final state analysis of molecules are important.The traditional sampling method is to treat the polyatomic molecules as harmonic oscillators,that is,normal mode sampling.In this thesis,we adopt a more rigorous semi-classical adiabatic switching(AS)sampling method and the final state distributions of scattered molecules are obtained with either the standard histogram binning or the energy-based Gaussian binning(1GB)schemes.Through comparison of state-to-state scattering probabilities with the benchmark QD results,we find that the QCT method with AS sampling and 1GB can largely capture the mode-specific vibrational energy transfer of H2O/HOD scattering from Cu(111)surface.Therefore,our results provide more confidence for the future use of QCT method to study the state-to-state scattering dynamics in gas phase and gas-surface systems involving polyatomic molecules.Another method is to calculate the reduced dimensional QD on a series of fixed sites,and then weighted average the results of each site to approximate the fulldimensional(FD)results,which is the so-called site-averaging model.Since there are no FD state-to-state QD results of poly atomic molecules scattering from metal surface,choosing a simple system,H2/D2+Cu(100),we have tested the validity of the siteaveraging model in this system.By comparing the 6D rovibrational(in)elastic scattering probabilities with the weighted average of 4D results over fifteen fixed sites,we find that the site-averaging model reproduces vibrationally elastic scattering probabilities quite well,though less well for vibrationally inelastic results at high energies.Our results show that the site-averaging model can be applied to the state-tostate QD of heavy diatomic or polyatomic molecules scattering from metal surfaces in the future to reduce the high cost of FD calculations.