Populations of $B{}^2{\mathrm{\Sigma }}_u^{+}$ and $X{}^2{\mathrm{\Sigma }}_g^{+}$ electronic states of molecular nitrogen ions in air determined by fluorescence measurement

We carry out a combined experimental and numerical investigation on the population distribution between the ground state and the excited state of molecular nitrogen ions in an 800-nm laser field. Efficient population transfers between the ground state ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ and the excited state ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ occur with a 400-nm time-delayed pulse, resulting in an enhanced fluorescence at 391 and 428 nm. The population distribution between the ground state ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ and the excited state ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ is extracted from the fluorescence enhancement factor. The results show that the population of ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ decreases when the laser exceeds 2.1 mJ in our experiment. Our study provides a method to check the population information of both excited and ground states experimentally in a strong-field-induced plasma of ionized nitrogen molecules.

Figure 1

(a) The forward emission and (b) typical fluorescence spectra, measured under both 800- and 400-nm pulses (black solid line), only the 800-nm pulse (red dashed line), and the 400-nm pulse (blue dotted line). The energy of the 800-nm pump pulse is 2.7 mJ and the time delay is 1 ps.

Figure 2

$I_{800+400}$ (black square), $I_{800}$ (red circle), and $I_{800+400}-I_{800}$ (blue upper triangle) of (a) 391 nm and (b) 428 nm as a function of 800-nm pump energy.

Figure 3

Energy levels of ${\mathrm{N}}_2$ and ${\mathrm{N}}_2^{+}$, and the near-resonant interaction of a 400-nm pulse and $B{}^2{\mathrm{\Sigma }}_u^{+}\text{−}X{}^2{\mathrm{\Sigma }}_g^{+}$ two-level system.

Figure 4

(a) The temporal evolution of the relative population difference $w_2$ under the 400-nm laser field with an intensity of $4.7\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. (b) The final $w_2$ as a function of $w_1$.

Figure 5

(a) The enhancement factor $\eta$ (black square) and initial relative population difference $w_1$ (red circle). (b) The initial relative population distribution of ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ (black square) and ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ (red circle) extracted from the luminescence measurement as a function of 800-nm laser energy. The relative population of ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ (black box) and ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ (red circle) is calculated by the PPT model as a comparison.

Figure 6

The initial populations of ${\mathrm{N}}_2^{+}(B{}^2{\mathrm{\Sigma }}_u^{+},{\nu }^{\prime }=0)$ (black square), ${\mathrm{N}}_2^{+}(X{}^2{\mathrm{\Sigma }}_g^{+},\nu =0)$ (red circle), and the sum (blue triangle) at a fluorescence level as a function of 800-nm pump energy.