Theoretical Studies On Quantum Dynamics Of Photodissociation From Triatomic Molecules Towards Polyatomic Molecules

Molecular reaction dynamics is at the heart of Chemistry,from which we gain the fundamental understandings of Chemical reactivity.Photodissociation,a bond breaking process induced by photons,is an important type of elementary reaction due to its strong connections to many other areas of chemistry,such as atmospheric chemistry,astrochemistry,environmental chemistry and so on.Starting from two triatomic molecules D2O and HCO,this thesis shows how first principle calculations provide insights into the dynamics based on good agreements with available experiments,using quantum wave packet method.Besides,a modified Gaussian wave-packet relaxation method is proposed as an improvement of G-MCTDH/vMCG.This modified mothed can calculate ground state wave function,providing a better preparation of initial state for photodissociation within the G-MCTDH/vMCG framework.This thesis is organized as follows:(1)The vibrational mediated photodissociation of D2O in the B band is studied,in which ground state OD(X)product distribution is found to be insensitive to initial vibrational state,while excited state OD(A)is sensitive.By vibrational pumping the molecule before photoexcitation,different portions of the excited-state potential energy surface(PES)can be explored,which may be an effective way to control the fragmentation dynamics toward a specific target.So the vibrational mediated photodissocation(VMP)dynamics of D2O has been studied in this prototype reaction.Being only quantitative different in the calculated product distribution and branching ratio between four different vibrational initial states,the main characteristics of dynamics are similar.However,differing from other three initial vibrational states,(0,1,0)state produces vibrational product of OD(X,v=1)which almost have the comparable population of OD(X,v=0)at a total energy of 9.59 eV.Previous works have revealed the relation between the vibrational distribution and which CI point leading to dissociation:ground product coming from DOD CI point and excited products coming from the other.This first one is dominant thus products are mainly vibrational ground state for most situations.So this work implies that more product coming out from the conical intersection at DDO configuration from(0,1,0)initial state.(2)An interference phenomenon is revealed and interpreted in the photodissocation of HCO in its first excited state.HCO is an important intermediate in combustion chemistry.The photodissociation in its first excited state provides an ideal prototype for studying Renner-Teller effect.Previous theoretical work found oscillations with odd-even alternations in rotational product distribution,indicating the possible existence of interference.Interference is a fundamental and cutting-edge subject in chemical reactions.So this very quantum phenomenon in the photodissociation of HCO awaits whole new investigations to answer whether the oscillations are caused by interference or not,more importantly,why it happens if so.We calculate absorption spectrum and rotational product distribution,which show good agreement with a new experiment performed by our collaborators.The new experiment also confirms oscillations in the rotational product distribution.Through a QCT study,we have found it is the special topology of ground state potential energy surface(PES)that leads to this very interference.The isomerization barrier on the ground PES deflects the trajectories with different angles,which has a turning point known as rainbow value.Different trajectories leading to the same destination can cause interference due to different varying associated phase.Thus the oscillations in the rotational product distribution is the reflection of interference.(3)A G-MCTDH/vMCG relaxation method is proposed for calculating the initial state wave function in photodissociation,which improves the accuracy and applicability for large systems.Despite the tremendous success of exact grid-based wave packet methods in characterizing quantum dynamics of triatomic molecular systems,it is difficult to extend them to larger systems.The amount of grids increases exponentially vs degree of freedoms(DOFs),which is known as exponential wall,thus making it almost impossible to apply to systems larger than five atoms.Gaussian wave-packet(GWP)based methods are potential to break the wall.It provides a simple idea of expanding wave function in terms of time dependent Gaussian functions rather than grids.Both the expanding coefficients and Gaussian parameters,Gaussian center and momentum namely,are propagated every time step.Among all kinds of GWP methods,G-MCTDH/vMCG is thought to be more accurate than others,as the motion of equation for Gaussian parameters is derived from Dirac-Freckel variational principle,bringing in quantum correlations.However,in the G-MCTDH/vMCG framework,the initial state for photodissociation is poorly described,using only one single Gaussian as an approximate ground state wave function.This shortcoming prevents its implements to quantitatively simulating real systems.Mainly model Hamiltonians were used.So in this work,we propose a modified Gaussian wave packet relaxation method to calculate the ground state wavefunction in a two-step procedure.In the first step,a multi-dimensional Gaussian product placed at the ground state equilibrium geometry is propagated in imaginary time.The relaxation optimizes the widths of the one-dimensional Gaussians.In the second step,additional Gaussian wave packets with the same widths are placed near the equilibrium geometry,and the corresponding expansion coefficients are optimized using the same relaxation method.This new algorithm is tested in photodissociation of NOC1 and NH3,and the results show satisfactory agreements with the exact results in the energy,wavefunction,and absorption spectrum.