Mechanism of Population Inversion in N\(_2{^{+}}\)

  • Youyuan Zhang
  • Erik Lötstedt
  • Kaoru YamanouchiEmail author
Part of the Topics in Applied Physics book series (TAP, volume 136)


When intense femtosecond laser pulses are focused in air, unidirectional and coherent radiation called air-lasing is generated, originating from the population inversion processes between an electronically excited state and the electronic ground state of N\(_2\) and N\(_2{^{+}}\). In 2015, it was shown experimentally that the air-lasing at 391 nm, corresponding to the \(\mathrm{B}{}^2\Sigma _\mathrm{u}{^+}(v = 0)-\mathrm{X}{}^2\Sigma _\mathrm{g}{^+}(v = 0)\) emission of N\(_2{^{+}}\), is generated in an sub-10 fs near-IR laser field (Xu et al, Nat Commun 6:8347, 2015 [6]). The behavior that N\(_2{^{+}}\) generated in the pulse is exposed to the intense laser field immediately after its birth, is considered critical in leading to the population inversion between the B and X states. This chapter includes the demonstrations that the sudden exposure to an intense laser field could result in efficient population transition in a two-level system at off-resonance (Zhang et al, J Phys B: At Mol Opt Phys 50:185603, 2017 [32]), and that population inversion can be achieved in N\(_2{^{+}}\) by using sudden turn-on pulse (Zhang et al, J Phys B: At Mol Opt Phys 52:055401, 2019 [33]). This scenario of the population transfer to the excited state is expected to be universal and can be applied to an interpretation of population inversion of any kind of atomic and molecular ions created in a pulsed intense laser field.



This research was supported by JSPS KAKENHI grants no. JP15K17805, no. JP24245003, and no. JP15H05696.


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Copyright information

© Springer Nature Switzerland AG 2020

Authors and Affiliations

  • Youyuan Zhang
    • 1
  • Erik Lötstedt
    • 1
  • Kaoru Yamanouchi
    • 1
    Email author
  1. 1.Department of ChemistrySchool of Science, The University of TokyoTokyoJapan

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