The Generation Of High-Order Harmonic And Attosecond Dynamics From Hydrogen Molecule In Intense Laser Fields

With the development of laser technology,scientists have made further research on atomic and molecular dynamics.In particular,the generation of the attosecond pulse plays a very important role in understanding the microcosmic world and detecting the electronic dynamics process.We know that the high harmonic is an important way to generate the attosecond pulse.The harmonic spectrum is characterized by a plateau following a fast decrease in the low-energy region and followed by a sharp cut-off in both experimental and theoretical studies.In 1993,the semi-classical three-step model was presented to further describe the HHG process.The bound electrons in atoms or molecules are excited into the continuum states,which are the ionization process;then the electrons are accelerated by the laser field;finally,the electrons recombine with parent nucleus,thus,emitting the high-energy photons.Nowdays,many efforts have been paid to broaden the bandwidth of HHG by using multi-color laser fields and inhomogeneous field.In this paper,the high harmonic emission of hydrogen molecule and the generation of isolated attosecond pulse under the interaction of multi-color and inhomogeneou laser field are presented.The main work of this paper includes the following aspects:Firstly,we theoretically research the HHG and attosecond pulses of 1D H₂molecule in one-color laser field,two-color laser fields and three-color laser fields.The nuclear and electronic probability density of the electron was further investigated to explain the underlying physical mechanism of HHG.We find that the harmonic spectra have more modulation and the platform do not change.When we use the two-color and the three-color laser fields,the harmonic spectra platform has a great extension and the harmonic spectra become more smoothly.The three-step model and time-frequency analysis show that only short trajectories contribute to harmonic spectra so that the single quantum path control is realized.Then,in order to further study this phenomenon,the wave packet distribution and the probability density distribution of the nucleus are given.In the duration of.125o.c.?t?.15o.c.and.175o.c.?t?.20o.c.,the electron ionizes respectively along the positive-z₁ and negative-z₁ and recombine with the nuclei?around t=1.75o.c.and t=2.5o.c.?.Therefore,these processes contribute significantly to harmonic emission.The probability density distribution of the coupled electron nuclear?R-Z₁Zspace?wave packet for the time evolution is applied to illustrate the physical mechanism of the electronic and nuclear dynamics in the Non-Born-Oppenheimer Approximation?NBOA?case.At the same time,we found at t=1.5 o.c.and t=1.75 o.c.,the electron is distributed along the positive-z₁ direction and distinct interference fringes also can be found.By superposing harmonics from 90th to 120th order,the isolated attosecond pulses with duration 75 as and 54 as are obtained in the Born-Oppenheimer Approximation?BOA?and NBOA cases respectively.Secondly,we study the high harmonic and isolated a-second pulse generation in inhomogeneous field under 1D fixed-nuclear model of H₂.The intensity of the harmonic spectra does not change with different laser envelope.We find that the harmonic platform is extended with the increasing inhomogeneous parameters,and the harmonic spectra become more smooth.The inhomogeneous parameters???is0.003,the harmonic spectra platform is extended with 50th.The inhomogeneous parameter???increase to 0.005,the harmonic spectra platform is extended with 100th and the cut off region become more smooth,which is benefit to generate isolated attosecond pulse.The time-frequency analysis of H₂ molecule withsin² envelope shows that only short trajectories contribute to harmonic spectra when the inhomogeneous parameters is 0.005.Finally,by superposing several orders of harmonics,the isolated attosecond pulses with duration of 120 as can be obtained.