Contribution of nitrogen atoms and ions to the luminescence emission during femotosecond filamentation in air

During femtosecond filamentation in air, nitrogen molecules and corresponding molecular ions undergo dissociation due to the high intensity of laser pulses, generating nitrogen atoms and atomic ions. The generated atoms and atomic ions emit luminescence in the UV range, which superposes on those emissions for the neutral and ionic nitrogen molecules. Here we report on a significant difference between the emission behavior of the 391-nm line and the other spectral lines under different pump laser polarizations. We attribute this difference to the contribution of the atomic ions to the luminescence emission around 391 nm. The difference becomes more evident in tightly focusing cases, providing an indirect but effective evidence for the dissociation of nitrogen molecular ions.

Figure 1

Schematic of experimental setup for the spectroscopy measurement of filament generated by the femtosecond pulses. F, optical fiber; G, Glan laser polarizer; H, half-wave plate; L, lens; Q, quarter-wave plate; R, rectangular diaphragm.

Figure 2

Spectra generated by the circularly (dashed red curves) and linearly polarized (solid black curves) pulses as the laser energy is (a) 2.0 mJ and (b) 3.1 mJ. The focal length is 100 cm.

Figure 3

Variation of the intensity of (a,a′) 337-, (b,b′) 380-, (c,c′) 391-, and (d,d′) 428-nm spectral lines along the propagation axis. The focal length is 100 cm, and the pulse energy is (a)–(d) 2.0 mJ and (a′)–(d′) 3.1 mJ.

Figure 4

(a) and (b) Intensity ratio $R$ of different spectral lines as the incident energy is 3.1 mJ and the focal length is 100 cm. (b) Interaction of nitrogen molecules with laser field, resulting in the formation of molecular nitrogen ions, and dissociation of molecular nitrogen ions into nitrogen ions and atoms.

Figure 5

Variation of the intensity of (a) 337-, (b) 380-, (c) 391-, and (d) 428-nm spectral lines along the propagation axis. (e) and (f) Intensity ratio $R$ of different spectral lines. The focal length is 40 cm, and the pulse energy is 3.1 mJ.