An Improved Coupled-state Approach And Its Applications In Quantum Dynamics For Energy Transfer

Collision-induced energy transfer processes between molecules have drawn considerable attentions in the field of atmospheric chemistry,interstellar chemistry,ultra-cold chemistry and combustion chemistry.In the hydrogen fluoride chemical lasers,HF molecule plays an important role as working medium,so that the key of development of chemical lasers is acquirement of HF-relevant energy transfer dynamical data.It generally relies on theoretical calculations because of the difficulty in measuring state-to-state energy transfer rate coefficients.Since rate coefficients of many initial states are needed,the time-dependent wave packet approach is less preferred than the time-independent close-coupling?TICC?approach.However,rigorous TICC calculations are not computationally feasible,while the traditional approximation approach,coupled-state approximation?CSA?,is resulted with many errors,due to neglect all Coriolis couplings.In this work,an improved CSA approach which includes the nearest neighbor Coriolis couplings?CSA-NNCC?is proposed.Then,it is applied to study the dynamics of Ar–HF,H₂–HF,and HF–HF energy transfer systems and physical insights are discussed.It is believed that the newly developed approach can be further used to investigate molecular dynamics and the calculated data are helpful to the chemical lasers.The achievements in this work are summarized as follows.It is not feasible now to conduct dynamics calculations of tetra-atomic systems by using TICC approach,due to the nature of matrix operation.On the other hand,by neglecting all Coriolis couplings and each submatrix dealt with separately,the CSA approach introduces immeasurable errors.The CSA-NNCC approach is thus proposed,by which the non-zero and nearest couplings are included in each submatrix and further zero-valued one is neglected.The computational cost of CSA-NNCC is on the same scale as that of CSA,but more accurate because the consideration of non-zero couplings.Test calculations are carried out on H₂–H₂ scattering system.Taken the state-to-state probabilities by TICC as reference,the root mean square error?RMSE?of CSA and CSA-NNCC results are calculated,respectively.It shows that the CSA-NNCC RMSEs are systematically less than CSA one,especially in case of high internal angular momentum,and it thus demonstrates that CSA-NNCC has advantages in both small computational cost and high accuracy.Argon is the buffer gas in chemical lasers and the vibrational relaxation of Ar–HF is extensively studied.In this work,the cross sections and rate coefficients of pure rotational energy transfer and vibrational relaxation are calculated.Cross sections of?0,0???0,j?and?1,2???1,j?are both in good agreement with experiments.The calculated?v=1???v=0?relaxation rate coefficients are also in better consistent with experimental data than the previous theoretical study.Vibrational relaxation of H₂–HF also plays and important role in chemical lasers.The cross sections and rate coefficients of pure rotational and vibrational energy transfer processes are calculated.Our results of rotational relaxation rate is slightly different from previous work by rigid rotor approximation.Rate coefficient of?1;0???0;1?decreases initially and then increases from 100 to 1500 K,and agrees with most experiments.For vibrationally excited HF molecules,the basis set in the calculations is truncated appropriately.Calculated rate coefficients of?0;3???1;2?,?1;3???0;4?,and?1;3???2;2?are all reasonably consistent with experiments.The self-vibrational relaxation rate coefficient of HF is the key parameter of developing lasers.It is a great challenge of calculation dynamics of two non-hydrogen molecules in full dimensionality.In this work,the vibrational relaxation dynamics of the HF–HF system are studied for the first time.It is pointed by a well-established energy gap law that molecular collisions can be described by a simple hard-ball model and molecules tend to transit to the final states whose internal energy is near-conserved to the initial state.However,as the results of HF–HF state-to-state quantum dynamics shows,the leading final states are not several certain one,but hundreds which distribute broadly in an energy range.Further calculations indicate that the nature of breaking down energy gap laws originates from the deep potential well and the long-lived?HF?₂ complex.This makes the hard-ball model,on which the gap law is based,cannot well described the dynamics of the HF–HF system.