Detection And Manipulation Of Dissociation Pathways Of Diatomic Molecules Driven By Circularly Polarized Laser Field

The interaction of ultrashort laser pulses with molecules leads to many novel physical phenomena,such as strong-field ionization and strong-field dissociation.A deeper understanding of the strong-field dissociation process will provide an important basis for manipulating chemical reaction channels at the molecular scale.Compared with linearly polarized laser,circularly polarized laser provides more degrees of freedom for the manipulation of molecular dissociation reaction channels because the magnetic quantum number of photons is not zero.Therefore,this thesis investigates the detection and manipulation of dissociation channels of diatomic molecules in circularly polarized laser fields by computational and experimental methods.The main work of this thesis is as follows:A scheme based on wave packet interference to extract phase information to detect dissociation dynamics is proposed.The ion wave packet interference method employs a delayed laser pulse to generate a probe wave packet interfering with the target wave packet.The phase information can be extracted from the momentum-space interference fringe to reveal the dissociation channel information.The reference ion wave packet used for detection is compeletly separated from the target wave packet in the time domain.Therefore,the method can be used to study the nuclear wave packet motion before the end of the laser pulse,which is difficult to be detected by traditional pump-probe scheme.The calculations show that the ion wave packet interference method can well detect the conversion of the three-photon pathway into a net-two-photon pathway,and a delay of a few femtosecond is found at the peak of the dissociated wave packet versus the peak of the laser pulse.Further study shows that the probability of three-photon pathway converting to net-two-photon pathway increases with increasing laser intensity and pulse width.The mixed-photon dissociation pathway has been studied theoretically.The mixed-photon dissociation pathways in the bicircularly polarized laser pulses with frequency ratios of 1:2 and 1:3 are simulated by solving the two-dimensional Schr?dinger equation.The dissociative fragments show a characteristic angular distribution on the momentum spectrum due to the mixed-photon dissociation pathway.To solve the problem that the dissociation fragments from different pathways carry the same kinetic energy and thus cannot be distinguished in the kinetic energy release spectrum,this thesis further proposes a method to accurately weigh the pathways from the contrast of the interference fringes.The double ionization dissociation process of nitrogen molecules molecules was investigated by experimentally measuring the ratio of the yield of N⁺and N₂²⁺.tt is confirmed that the sequential ionization dissociation of nitrogen molecules is mainly through electronic excited state dissociation.The effect of the molecular orbital configuration of nitrogen on the double ionization dissociation process was calculated and analyzed.Based on the analysis results,the manipulation of the double ionization dissociation pathway was experimentally achieved by aligning the nitrogen molecules,thereby controlling the ratio of the N⁺and N₂²⁺.