Abstract: 研究成果概述 (Brief summary of research outcome)
We have studied how to use the density matrix method to treat laser control of electron dynamics in chiral molecule. The molecular vibrations of the electronic ground and excited states are taken into account in this formulation using the density matrix method. We observe that the angular momentum and vibrational frequency spectra are strongly dependent on laser polarizations, which control the direction of the electron ring current.
We have also investigated how to induce the ultrafast electronic motion in a molecular ion by high power intense laser based on two different excitation mechanisms, i.e., one takes place during ionization, and the other one sequentially takes place after the ionization. The electronic excited states of hydrogen molecular ion are created during the ionization by taking the configuration interaction mixing of neutral molecule into account. Photoionization process is calculated by the generalized (Keldysh-Faisal-Reiss) KFR theory. We observe ultrafast oscillatory electronic motion between two atoms in a hydrogen molecular ion due to the creation of excited states during the ionization.
The mechanisms of ionization and dissociation of cyclohexanone (C6H10O) in a 90 fs, 788 nm
linearly polarized (LP) laser field ranging from 1013 to 1014 W/cm2 by a time of flight mass spectrometer (TOF_MS) have been investigated. The ion yields as a function of laser intensity have been measured experimentally. By comparison with the Ammosov−Delone−Krainov (ADK) theory based on a hydrogen like model, the ionization mechanism of cyclohexanone in this intense femtosecond laser field has been understood. Considering the importance of molecular nuclear motions, we propose that the Franck−Condon (FC) factor can provide the excess vibrational energy in the molecular ion. This energy is required for the decomposition of the molecular ion which finally results in the observed mass spectrum.
Ionization and dissociation mechanisms of pyrrolidine in intense 800 nm laser field (1013 to 1014 W/cm2) have been experimentally investigated by using a method of molecular beam and time-of-flight mass spectrometer. Singly charged parent ion and numerous fragment ions are observed in the mass spectra, which are investigated as a function of laser intensity and polarization. In order to understand the details of the ionization processes of pyrrolidine in intense femtosecond laser field, we quantitatively calculate the rate constants and ion yields by means of generalized KFR theory, and the excitation probabilities of the excited states are also calculated by using Floquet theory. The results suggest that the ionization might occur partially through the excited states of neutral pyrrolidine. Comparing with linearly polarized (LP) laser field, we observe some enhancement of fragmentation with a circularly polarized (CP) laser field above the saturation threshold intensity which might be explained by the active energies of the pyrrolidine molecular ions are different under CP and LP laser irradiated. To interpret the dissociation patterns of the pyrrolidine ions, we have used the Rice-Ramsperger-Kassel-Marcus (RRKM) theory with the potential surfaces obtained from the ab initio quantum chemical calculations.