The Theoretical Study On The Interference Effect And The Spectral Redshift Of High-order Harmonic Of H₂⁺ Molecule In Intense Laser Field

In the past few decades,due to the rapid development of the laser technology,the interaction between intense laser pulse and atom or molecule have attracted comprehensive attention.Many interesting phenomena can be observed,such as high-order harmonic generation,tunneling ionization,multiphoton ionization,above threshold ionization,coulomb explosion and so on.High-order harmonic generation is one of the effective means for generating the isolated attosecond pulses.The attosecond pulses have a great potential to probe the ultrafast electronic dynamics deep inside the atoms and molecules,which provide the possibility of the ultrafast dynamics.The high-order harmonic spectrum has an obvious feature,which has been widely investigated both theoretically and experimentally.Firstly,the intensity of harmonic start decline rapidly in the low order.Secondly,the harmonic spectrum has a broad plateau.Finally,the harmonic spectrum has a sharp cutoff in a certain order.Through continuous research,the high-order harmonic generation can be well understood by means of semi-classical three-step model.The electron tunnels through the potential barrier by the intense laser pulse;then the ionized electron accelerates and obtains energy in the laser field;in the end,the electron recombines with parent ion and generates high-order harmonic when the laser field changes direction.To investigate the feature of high-order harmonic generation,people have done a lot of work and provided many schemes.In my paper,the initial wave function is obtained by discrete variable representation?DVR?.The time-dependent wave function is obtained by the second-order splitting-operator fast-Fourier transform algorithm.Content of our researches is as following:First,the high-order harmonic emission of H₂⁺molecule in a 2200nm linear laser pulse is investigated.The result show that the interference minimum can be observed by changing the full width at half maximum?FWHM?of the laser field.Fort?28?7.34 fs,the interference minimum at around 60th order can be observed in high-order harmonic generation spectrum and time-frequency distribution of the high-order harmonic generation.When the full width at half maximum of the laser field is gradually increased,the interference minimum is gradually disappeared.After separating the contribution to HHG from two nuclei,it can be found that the harmonic spectrum overlap at 60th order when the HHG intensity from two nuclei is identical,which results in the interference minimum.Second,we present a combined laser scheme in theory to investigate the spectral redshift of high-order harmonic of the H₂⁺?D₂⁺?molecule.The result shows that the spectral redshift of high-order harmonic can be observed by adding a weak pulse in the falling part of the trapezoidal laser pulses.The main emission time is delayed and the peak of harmonic signals starts to shift.Meanwhile,we have calculated the high-order harmonic generation of the D₂⁺molecule,the phenomenon of the spectral redshift of the D₂⁺molecule is more obvious,comparing with the peak of harmonic signals of the H₂⁺molecule.Third,we investigate the high-order harmonic emission of H₂⁺molecule in an inhomogeneous laser field.The result shows that only odd harmonic with small redshift can be observed in the symmetric nonhomogeneous laser pulse.Therefore,we found that the odd and even harmonic with small redshift can be observed in the asymmetric nonhomogeneous laser pulse.The physical mechanism of the generation of even harmonic can be well understood by interference term of the high-order harmonic generation spectra.