Chapter 2
Mechanism of Population
Inversion in N 2
+
Youyuan Zhang, Erik Lötstedt, and Kaoru Yamanouchi
Abstract 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 B
2
u
+
(v = 0) − X
2
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.
2.1 Introduction
The concept of the standoff spectroscopy was introduced first in [1]. In 2011, a simple
approach of standoff spectroscopy, where the backward-directed lasing from air,
mostly from ionized oxygen molecules and ionized nitrogen molecules, was proposed
[2]. Since then, the air-lasing has been regarded as a promising phenomenon, with
Y. Zhang · E. Lötstedt · K. Yamanouchi (B)
Department of Chemistry, School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
e-mail: kaoru@chem.s.u-tokyo.ac.jp
Y. Zhang
e-mail: youyuan.zhang@chem.s.u-tokyo.ac.jp
E. Lötstedt
e-mail: lotstedt@chem.s.u-tokyo.ac.jp
© Springer Nature Switzerland AG 2020
K. Yamanouchi and D. Charalambidis (eds.), Progress in Ultrafast
Intense Laser Science XV, Topics in Applied Physics 136,
https://doi.org/10.1007/978-3-030-47098-2_2
21
Mechanism of Population
Inversion in N 2
+
Youyuan Zhang, Erik Lötstedt, and Kaoru Yamanouchi
Abstract 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 B
2
u
+
(v = 0) − X
2
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.
2.1 Introduction
The concept of the standoff spectroscopy was introduced first in [1]. In 2011, a simple
approach of standoff spectroscopy, where the backward-directed lasing from air,
mostly from ionized oxygen molecules and ionized nitrogen molecules, was proposed
[2]. Since then, the air-lasing has been regarded as a promising phenomenon, with
Y. Zhang · E. Lötstedt · K. Yamanouchi (B)
Department of Chemistry, School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
e-mail: kaoru@chem.s.u-tokyo.ac.jp
Y. Zhang
e-mail: youyuan.zhang@chem.s.u-tokyo.ac.jp
E. Lötstedt
e-mail: lotstedt@chem.s.u-tokyo.ac.jp
© Springer Nature Switzerland AG 2020
K. Yamanouchi and D. Charalambidis (eds.), Progress in Ultrafast
Intense Laser Science XV, Topics in Applied Physics 136,
https://doi.org/10.1007/978-3-030-47098-2_2
21