Spectroscopic study of laser-induced tunneling ionization of nitrogen molecules

Tunneling ionization is one of the fundamental processes for molecules in intense laser fields. Depending on the ionizing molecular orbitals, molecular ions are in ground or excited electronic states. Here, we report an experimental study of tunneling ionization of nitrogen molecules using spectroscopic methods. The molecular ions in the excited electronic state were detected through the fluorescence spectra. The molecular ions in the ground electronic state were detected through the laser-induced-fluorescence (LIF) spectra. The gas-pressure dependences of fluorescence and LIF intensities demonstrated the collision-induced population redistribution in these electronic states.

Figure 1

Fluorescence spectra of pure nitrogen molecules irradiated by intense laser pulses at three different gas pressures (a) 1.4 × 10${}^{-5}$ mbar, (b) 2.5 × 10${}^{-1}$ mbar, and (c) 1.0 × 10${}^3$ mbar. The laser has a pulse duration of 110 fs, a central wavelength of 800 nm, and an intensity of a few 10${}^{15}$ W/cm${}^2$.

Figure 2

(a) Fluoresce spectra and (b) and (c) LIF spectra of pure nitrogen molecules irradiated by 110-fs 800-nm laser pulses at an intensity of a few $10{}^{15}$ $\mathrm{W}/\mathrm{cm}{}^2$. The wavelength of the nanosecond laser was set at (b) 391.5 nm and (c) 358.2 nm and was represented by the blue dashed-dotted lines (see text).

Figure 3

Fluorescence lifetimes of ${{\mathrm{N}}_2}^{+}(B{}^2{\Sigma }_u^{+},v^{\prime }=0\to X{}^2{\Sigma }_g^{+},v^{\prime \prime }=1)$ at different gas pressures. The fluorescence lifetimes were obtained through fitting the experimental data with an exponential decay function, which are represented by the black solid lines.

Figure 4

Fluorescence and LIF decay at different gas pressures. The intensities have been normalized with the fluorescence intensity. In the measurement, the nanosecond laser wavelength was set at 391.5 nm, and the collecting fluorescence wavelength was set at 427.8 nm. The time delay between the femtosecond laser and the nanosecond laser was 200 ns. For visual convenience, the decay curves have been shifted backward 30 ns relative to the moment of the femtosecond laser irradiation.