High Order Harmonic Generation In CO Molecules

The interaction of intense laser field with atoms,molecules and even solid medium will produce coherent radiation whose frequency is integer multiple of incident laser frequency,that is,high order harmonics.High order harmonics are known to be an effective means of generating attosecond pulses(1 attosecond=1×10^-18seconds),and provide an effective tool for detecting ultrafast electron dynamics in atoms and molecules.In this dissertation,we developed a theoretical method of strong field approximation based on quantum chemistry to calculate molecular wave function,which can be used to calculate the generation of high order harmonics of molecules.Further,we decomposed the phase of high order harmonics generation of asymmetric molecules.Finally,the dynamic mechanism of high harmonics generation of asymmetric carbon monoxide molecules is explored by using the improved quantized molecular strong field approximation model.The research work of this dissertation mainly includes:1.We use the strong field approximation model based on quantum chemistry to calculate the dipole moment,basis set and other related information of atomic and molecular systems from the output of quantum chemistry simulations,so as to easily extract the atomic or molecular orbital wave function.In addition,we extracted the molecular structure information of the dipole matrix element in Lewenstein model,so as to easily derive molecular wave functions from quantum chemistry software without expanding the dipole matrix element.This model requires less computation and is suitable for more complex molecular systems,which are often impossible to achieve by ab initio methods.Accurate analysis of the initial state wave function and high harmonic spectrum enables us to find the relationship between them,which is helpful to understand the underlying physical processes in the interaction between molecules and laser fields.2.Leweinsteins model is used for the calculation of high order harmonics,which is usually applied to the calculation of homonuclear diatomic molecules.Leweinsteins method based on quantum chemistry simulations is adopted,and the asymmetric molecular phase is modified and decomposed to make it suitable for accessing different channels of high order harmonics decomposition of CO molecules.The phenomenon of pure odd and even harmonics of CO molecule is also obtained in the strong field approximation method.By observing the effect of different nuclear distances on the even-order harmonics,it is found that the intensity of even-order harmonics decreases and then increases with the increased internuclear distance.It is pointed out that the phenomenon of even harmonic is caused by the interference between different channels.This phenomenon is of great significance for understanding the generation of higher harmonics in molecules.