Theoretical Study On The Low-lying Excited States And Charge-transfer Collisions In Several Molecular Systems

As the dissociation product by ultraviolet radiation, bromine-containing radicalshave large influence on the ozone sphere. The bromine-containing radicals haveattracted the extensive attention. Therefore, the electronic structures, spectrum andtransition properties of bromine-containing systems have attracted a lot of researchinterests. The importance of bromine-containing systems in application has beenperceived. As the product of photodissociation for iodine generation of methyl iodide,the excited I atom could be used as the operation material of new type high-energychemical laser. CH₃I is the simplest Iodine generation of methyl iodide, of which theionization and dissociation mechanism have significant utility practical value andguiding significance.The analysis of dissociation channel and energy provides the theoretical supportsfor improving the quantum yield of excited I atom and plays an important role infurther development of laser technology. Moreover, the construction of reasonablemodel that explains the dissociation of CH₃I has the important significance inexploring the related polyatomic molecules. Based on the adiabatic potential energyfunction and coupling between excited states, the collision charge dynamics is one ofthe important topics in atomic and molecular physics. Charge transfer is also theimportant dynamic processes in life sicence. The study charge transfer ofion-molecular collision has signicance in understanding the physical and chemical reaction in living bodies.In consideration of the important significance of property of electronic states anddynamic, we select the bromine-containing diatomic molecule CBrq+（q=0,1,2）,polyatomic molecule system CH₃Iq+（q=0,1,2） and collision systems [Be²⁺+H₂O] and[H++CH] as the theoretical research object. Our specific work is as follows:Part one, the multi-configuration interaction method including Davidson（+Q）was used to study the bromine-containing systems including CBr、CBr+and CBr²⁺.The spin-orbit coupling has been taken into account. The PECs of the Λ-S states andstates were present and spectrscopic constants corresponding to the bound stateswere obtained. We also discussed the curve crossings of Λ-S states, predissociationmechanism and spin-orbit coupling effect on the potential energy curves andspectrscopic constants. For CBr+, the transition properties including Franck-Condonfactors and radiative lifetimes have been investigated as well. The potential energycurves of12Λ-S states for CBr²⁺ion were calculated. The obtained results are ingood agreement with the experimental and theoretical results, which are conducive tounderstand the crossing of potential energy curves, avoided crossing, predissociationand provide the theoretical foundation for exploring chemical reaction in atmosphericozonosphere.In part two, the spin-orbit multi-reference configuration interaction method withall-electronic basis set was employed to calculate the potential energy curves in thedissociation of CH₃I and the transition moment from ground state to excited states,through which the relaxation effect of the alkyl radical was considered. Our calculatedresults were in good agreement with the previous theoretical and experimental dates.Then, the dissociation with the relaxation effect of the alkyl radical along the C-Ibond of the CH₃I+was performed. All the electronic states related to the first ninelowest dissociation limits have been investigated, and the dissociation limit CH₃+（3A）/I （2P） and nine undetected bound states were firstly reported. Position of thebound states, kinetic energy releases of the exclusive states, and the possibledissociation channels of CH₃I+were calculated and analyzed. Finally, the dissociationchannels of CH₃I²⁺were investigated. The structures and energies corresponding to the steady structures and the transition states of CH₃I, CH₃I+and CH₃I²⁺were presentin our work. The calculated first and second ionization energies were agreement withexperimental results, and the ground state of the CH₃I²⁺is a triplet state3A2. On basisof the calculated structures and the energies, the two-body and three-body dissociationprocesses of the CH₃I²⁺have been analyzed and discussed in detail. Our calculationsindicate that the two-body dissociations CH₃I²⁺（1A）�'CH₃++I+（1D）/HCI++H²⁺/CI+（1+）+H3+（1A1）/CH²⁺+HI+（2A1） and CH₃I²⁺（3A2）�'CH₃++I+（3P） and the three-bodydissociations CH₃I²⁺（1A）�'CH²⁺+H+I+（1D）/HCI++H+H+/CI+（3Σ+）+H²⁺+H, CH₃I²⁺（3A2）�'CH²⁺+H+I+（3P）/CI++H²⁺H+are exergonic, nevertheless, two processes CH₃I²⁺（3A2）�'CI+（3+）+H3+（1A1） and CH₃I²⁺（3A2）�'H²⁺+H+CI+are most difficult to achieve.In the last part, the charge transfer in collision systems Be²⁺-H₂O and H+-CHhave been undertaken employing the multi-reference configuration interaction （MRCI）method and quantum-mechanical molecular orbital close-coupling methods. Total andpartial charge-transfer cross sections for the collisions in the energy range of1eV/u-1000eV/u were present. The mechanism of the charge transfer process, inparticular its anisotropy, has been investigated in detail in connection withnon-adiabatic interactions around avoided crossings between states involved in thereaction. With comparison of the charge transfer process under different geometricalstructure, the charge transfer process in collisions of Be²⁺ions with the H₂O moleculewas more likely to happen under a non-coplanar geometry. The anisotropy incharge-transfer cross sections in the1eV/u-500eV/u was analyzed. Analogously, thecharge transfer dynamics in [H++CH] collision was investigated in this work. In orderto study the anisotropy of this process, a series of calculations have been performedwith different target orientations. The potential energy curves, associated radial androtational coupling matrix elements, total and partial charge-transfer cross sectionswere calculated to analyze the mechanism of the charge transfer process. It’s foundthat the rotational coupling gives significant contributions to the charge transfer crosssections in the high energy region. The calculations on the charge transfer dynamics inBe²⁺-H₂O and H+-CH collisions are helpful to understand the collision-induced chargetransfer reactions in different research fields and the anisotropy affect on the charge transfer process. Moreover, the atomic and molecular parameters of thecharge-transfer cross sections in [H++CH] collision could supply the physicaldistributions to thermonuclear fusion plasma research.