The Experimental Investigation Of Minimum Structure Of High Order Harmonic Spectra From Aligned O₂ And N₂ Molecules

During the process of interaction between atomic and molecular systems and femtosecond lasers,many non-linear phenomena including above-threshold ionization and high-order harmonics and so on are produced.High-order harmonics are the special method to obtain attosecond pulses and XUV coherent light sources,and are of great significance to the study of ultrafast imaging of molecular orbital structures.The information of the rotating wave packet,such as the structure and phase,and the molecular orbital structure characteristics are encoded in the harmonic spectrum.Therefore,we could extract the information to analyze ultrafast physics process of strong fields.It has been found that the minimum structure can reflect the molecular orbital structure and ultrafast electronic dynamics process,so the minimum structure of high-order harmonic spectra has attracted widespread attention.Firstly,the high-order harmonic spectra of O₂ and N₂ molecules with different time delays of the laser pulses has been studied.The results show that when the molecules are at the alignment point and the anti-alignment point,an obvious minimum structure could be observed in the harmonic spectrum,however for the randomly oriented molecules,the minimum structure in the harmonic spectrum is weaker.The fact illustrates that the harmonic spectrum of the aligned molecules encodes richer molecular structure information.Secondly,the minimum structure of the high-order harmonic spectra for O₂ and N₂ molecules has been studied.The results show that the multi-orbital interference minimum has been found in the harmonic spectrum for the aligned O₂ molecules.As the laser intensity changes,the position of the minimum changes.The Cooper-like minimum has been found in the harmonic spectrum for the aligned N₂ molecules.The position of the minimum does not move with the variation of laser intensity,and is only related to the electronic structure of the molecules.In addition,we also found that when the molecules at the alignment point and the anti-alignment point,due to the different contributions of the HOMO and HOMO-1 orbitals to the harmonic spectrum,the position of the high-order harmonic spectra minimum for the N₂ molecules moves slightly.The analysis of the generation mechanism of the minimum structure in the harmonic spectrum can help us intuitively reveal the structure characteristics of the molecules,and deeply understand the physical process of the interaction between the laser and the molecules.