The External Electric Field Effects On Ethylene And Dimethyl-siloxane And Excited States

The fundamental theory and model for treating matter-field interactions are explored, and various computation methods of excited states are compared. The external electric field effects on ethylene and dimethyl-siloxane molecules have been studied by using Gaussian program and Schrodinger program. Deformations and vibrations of diatomic molecules (BH, CH, LiH, H2 and Na2) with an applied static electric field have been studied by density functional theory using analytical potential energy function based on Morse model. Non-hermitian Hamiltonian is introduced to describe the ionization decay of atom coupled with laser. Several conclusions are obtained as follows:The effects of external electric field on the molecular structure of ethylene molecule using a hybrid method which combines the single-excitation configuration interactions (CIS) with density functional theory(DFT), i.e. CIS-DFT. It is found that the effects of electric dipole field on the molecular geometry (D2h, D2d and C2v), dipole moment, transition dipole moment, polarizability and, particularly, excitation energy of ethylene are rather remarkable. The advantages of hybrid CIS-DFT method are that it can determine the symmetry of molecule and the correct order of excitation as well as the Molecular Orbital(MO) electron configuration, thereby the electronic excitation states of ethylene are easily derived out, and most of them are in agreement with those obtained in experiments or references. It may be the firsttime the effects of external electric field on ethylene molecule have been considered. Compared with other ab initio methods, the CIS-DFT method is relatively accurate and low in computation cost. We expect that it can be used to study other closed-shell molecules.The ground states of dimethyl siloxane under different intense electric fields ranging from -0.04 to 0.04 a.u. are optimized using density functional theory DFT/B3P86 at 6-311+G(d,p) level. The excitation energies and oscillator strengths under the same intense applied electric fields are calculated employing the revised hybrid CIS-DFT method proposed by Grimme. The result shows that the electronic state, molecular geometry, total energy, dipole moment and excitation energy are strongly dependent on the field strength and behave asymmetry to the direction of the applied electric field. As the electric field changes from -0.04 to 0.04a.u., the bond length of Si-0 increases whereas the bond length of Si-C decreases because of the charge transfer induced by the applied electric field. The dipole moment of the ground state decreases linearly with the applied field strength. However, the dipole moment of molecule changes from positive to negative as the inverse electric field increase to -0.03a.u. Further increase of the inverse electric field results in an increase of the total energy of the molecule. The dependence of the calculated excitation energies on the applied electric field strength is fitting well to the relationship proposed by Grozema. The excitation energies of the first five excited states of dimethyl siloxane decrease as the applied electric filed increases because the energy gap between the HOMO and LUMO become close with the field, which shows that the molecule is easy to be excited under electric field and hence can be easily dissociated.The symmetry-adapted cluster-configuration interaction (SAC-CI) method proposed by Nakatsuji is introduced. As testing examples we employ this method for studying the ground state, singlet and triplet excited states, ionized doublet states, and anionic doublet states of Si(CH3)2O diradical molecule. Meanwhile, the vertical excitation energies and ionization spectra of C2H4 have been obtained.Comparison with experiment shows that the SAC-CI method has many advantages over the CIS or other ab initio methods in the studying of excited states.The ground state 1A1 and excited state 3A2 and 3B2 of dimethyl siloxane under intense electric field ranging from -0.06 to 0.06 a.u. are optimized using density functional theory DFT at 6-311++g(d,p) level. The results show that the electronic states, molecular geometry, dipole moments and energies are strongly dependent on the field strength. As the field increases the stability of 1A1, 3A2 and 3B2 states are enhanced with the former more stable than the later. However, when the field is larger than 0.0571a.u. the bond length of Si-C and molecular energy dipole moment increase abruptly, insulting the bond breakage or scission. When the reverse field is larger than 0.04 a.u., the 3B2 becomes more stable than lA\ state.The level structure of a 2-level atomic system coupled with a laser field is examined analytically and the results are compared with numerical calculations of time-dependent Schrodinger equation, showing that the conventional quantum mechanics can be extended to complex domain, i.e., a non-hermitian Hamiltoian with complex eigenvalues is reasonable. The real and imaginary parts of the complex eigenvalues represent the energy and the decay rate of the corresponding eigenvalues respectively.Deformations and vibrations of diatomic molecules with an applied static electric field have been studied by density functional theory using Morse model .It is found that for diatomic molecules, bond length and vibrational frequency as a function of the field canbe fit very well all the way up to the dissociation limit by an analytical formula derived from a Morse potential model including an additional external field term. Polyatomic molecules show more complex behavior with a single model becoming soft at the dissociation limit. The frequency of the soft mode near the critical breakup field is again described well by the analytical model. The vibrational analysis shows that in polyatomic molecules dissociation proceeds as a heterolytic fragmentation process, which can break the symmetry of the molecule in the applied field.