Theoretical Study On The Mechanism Of The Mode Specificity And Electronic Spectra In The Photoexcitation （?）¹A₁←（?）¹A₁ Of H₂C₃H⁺ And D₂C₃D⁺

Molecular spectroscopy is an important approach to study atoms and molecules in the microscopic world,by means of which the structure and internal motion information of atoms and molecules can be obtained.A potential energy surface（PES）is the cornerstone for the quantum mechanical simulation of molecular spectrum.However,the construction of the accurate full dimensional（12D）PESs of both the ground and excited states of propargyl cation（l-H₂C₃H⁺）from Ab initio calculations is a demanding and challenging task.In this thesis,we constructed an efficient and accurate reduced-dimensional quantum model,which is based on the normal coordinate displacement calculated by means of the equilibrium structures and normal modes of the ground and excited states of the molecule.Based on the constructed quantum model,we calculated the electronic spectrum of the propargyl cation,and further reinterpreted the vibrational structure in the spectrum.The main contents of this thesis appear as follows:1.In this work,the ground and excited states of H₂C₃H⁺were first optimized at the level of coupled-cluster singles,doubles and triples（CCSDT-3）and equation-of-motion coupled-cluster singles,doubles and triples（EOM-CCSDT-3）,respectively.Their harmonic frequencies were further calculated at the same computational level.The calculated equilibrium structures of the ground and excited state belong to the point group C_2v symmetry,of which the geometric parameters are consistent with those reported in the literature,and the frequencies are in good agreement with experimental results.Furthermore,theΔQ of the twelve modes of H₂C₃H⁺notably is calculated based on the calculated differences of the geometric coordinates from the ground state to the excited state and the vibrational normal modes of the excited state.The results showed that:only the v₅ and v₃ modes have relatively large displacement in the calculatedΔQ of all the twelve modes of H₂C₃H⁺,which suggests that v₅ and v₃ modes are active modes.Therefore,they need to be included in the construction of an accurate and effective quantum model to simulate the spectra.2.A reduced-dimensional quantum model is constructed based on the normal coordinate displacement（ΔQ）,and the B¹ A₁ absorption spectra of the H₂C₃H⁺and D₂C₃D⁺were further calculated.The results showed that:（1）The assignment of the strong progression is v₅ mode,and the weaker progression is 3¹5ⁿ.The calculated B¹ A₁ spectrum of l-H₂C₃H⁺is in good agreement with experiment,which validates the accuracy of the scheme of identifying the active modes in the photoexcitation processes based onΔQ.（2）For the absorption spectrum of l-D₂C₃D⁺,the calculated one is in good agreement with experiment as well.Consistent with experiment,the overall shapes of the spectra of H₂C₃H⁺and D₂C₃D⁺are quite similar,which suggests that the isotopic effect is very small and the active modes are not related to the motions of the H/D atoms.