| 研究実績の概要 |
In this academic year, we focus on clarifying the post-ionization excitation mechanism in nitrogen molecular ions on the rotational levels. We developed a theoretical model, which includes the electronic, vibrational, rotational degrees of freedom, so that the time-dependent dynamics of the complete molecular wave functions of nitrogen molecular ions can be calculated. There are two main achievements in this academic year:
1. We have demonstrated that the population inversion can be achieved in a specific range of rotationally excited levels without population inversion between the vibrational states. This result was published in Physical Review A 101, 053412 (2020.05) with a title, “Rotationally induced population inversion between the B2Σu+ and X2Σg+ states of N2+ exposed to an intense laser pulse.”
2. Through a pump-probe experiment, we have further understood that the electronic, vibrational and rotational dynamics in the emission at 391 nm and the rotational coherence in the B state. The result of this study is to be submitted to Physical Review A soon.
I attended several conferences to disseminate above achievements:【国内】分子科学討論会：Rotational coherence in R-branch transition of air lasing at 391 nm (Oral 2020.09.15) 日本化学会第101春季年会：Rotational coherence created in the B state of N2+ in air lasing at 391 nm (Oral 2021.03.20)【国外】The 22nd International conference on ultrafast phenomena: Population inversion the B2Σu+ and X2Σg+ states of N2+ assisted by rotational excitation (Poster 2020.11.16)
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
In our simulation, we have shown that a sufficiently high electric field can result in the population inversion between the B(v = 0) and X(v = 0) states, which is consistent with our previous results on the vibrational excitation as well as the population inversion between rotational levels achieved at specific electric field intensity without net vibrational population inversion. This indicates that rotational transitions will be critical in determining the threshold of the population inversion and designing optimal experimental conditions.
Further, we have performed a pump-probe measurement of the rotational structure of the emission spectrum of the B(v = 0)-X(v= 0) lasing and simulated the temporal variation of the emission of N2+ at around 391 nm as a function of the pump-probe delay time using the model in which the rotational degrees of freedom are explicitly included in addition to the vibrational and electronic degrees of freedom.
We have found in both the results of the experiment and the results of the theoretical calculation that a splitting structure appears in the temporal evolution of the R-branch emission spectrum and that the peak position of the R-branch transitions shifts towards the longer wavelength region. This is because the emission from the B(v = 0) state to the X(v =0) state exhibits different time dependences depending on the rotational quantum number K, originating from the rotational coherence created in the B(v=0) state by the pump and probe pulses.
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| 今後の研究の推進方策 |
Based on the results of the theoretical simulations described above, we plan to explore the mechanism of the significant enhancement of the lasing signal at 391 nm achieved when the polarization states of the pump and probe pulses are modified.
Besides the post-ionization excitation process, I will include the strong-field ionization process in our current theoretical model on the basis of the single active electron (SAE) approximation by developing a python program code. In addition to SAE, the Multi-Configurational Time-Dependent Hartree Fock (MCTDHF) method and the time-dependent two-electron reduced density matrix (TD-2RDM) method are also candidates by which I can simulate the ionization rate and time-dependent electronic, vibrational and rotational excitation during the ionization process.
In the coming academic year, I expect that we will be able to obtain a more complete simulation code for time-dependent dynamics of molecules or molecular ions in an intense laser field by including the ionization and the post-ionization processes.
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