Formation Dynamics of Excited Neutral Nitrogen Molecules inside Femtosecond Laser Filaments

Nitrogen molecules are promoted to excited neutral states during femtosecond laser pulse filamentary propagation in atmosphere, leading to a characteristic UV fluorescence. Using a laser-induced fluorescence depletion technique, we measure the formation dynamics of these excited neutral nitrogen molecules with femtosecond time resolution. We find that the excited neutral molecules are formed in an unexpected ultrafast timescale of $\sim 4\mathrm{ps}$ at 1 bar and $\sim 120\mathrm{ps}$ at 30 mbar pressure. From this observation we deduce that the excitation of neutral ${\mathrm{N}}_2$ occurs via multiple collisions with hot free electrons. Numerical simulations based on rate equations reproduce well this ultrafast formation time and its dependence on gas pressure, and thus support this interpretation.

Figure 1

(a) Energy levels of the nitrogen molecules and the principle of probe pulse (396 nm) induced fluorescence (337.1 nm) depletion. (b) The femtosecond pump and probe pulses are focused by a convex lens ($f=500\mathrm{mm}$) into a gas chamber filled with nitrogen gas or air at different pressures. The fluorescence from the air plasma filament is detected in the side with a spectrometer as a function of the pump-probe time delay $\tau$.

Figure 2

Side fluorescence of the nitrogen gas plasma without (black line) and in the presence of (red line) of the probe pulse. The nitrogen pressure was 1 bar.

Figure 3

Side fluorescence at 337.1 nm as a function of the pump-probe delay for different nitrogen gas pressures.

Figure 4

Side 337.1 nm fluorescence as a function of the pump-probe delay for different pump laser polarization (a), and comparison of air and pure nitrogen gas (b). In (a), the pressure of nitrogen was 1 bar.

Figure 5

(a) Experimentally measured (square dot) and calculated (solid circle) formation time of the $C^3{\mathrm{\Pi }}_u^{+}$ state molecules as a function of the nitrogen gas pressure. For 1 bar (b) and 30 mbar (c) nitrogen, calculated formation dynamic of the $C^3{\mathrm{\Pi }}_u^{+}$ state molecules based on the electron impact mechanism (blue line) and the dissociative recombination mechanism (red line).