Population dynamics of molecular nitrogen initiated by intense femtosecond laser pulses

Tunneling ionization is one of the fundamental processes for molecules in intense laser fields, and the tunnel-ionized molecules are in various electronic states. Here, we report an experimental study of the population dynamics of ${{\mathrm{N}}_{2}}^{+}(B{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+})$ and ${{\mathrm{N}}_{2}}^{+}(X{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+})$ generated in intense femtosecond laser fields by simultaneously measuring the fluorescence emission spectra and the transmission spectra. The results reveal that the population inversion exists between ${{\mathrm{N}}_{2}}^{+}(B{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+})$ and ${{\mathrm{N}}_{2}}^{+}(X{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+})$. But the population difference is much smaller than the absolute population of ${{\mathrm{N}}_{2}}^{+}(B{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+})$. The present study provides insight into understanding the observation of the generation of an air laser from the tunnel-ionized nitrogen molecules, which has been attributed to the population inversion between ${{\mathrm{N}}_{2}}^{+}(B{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+})$ and ${{\mathrm{N}}_{2}}^{+}(X{\phantom{\rule{0.28em}{0ex}}}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+})$ in intense femtosecond laser fields at 800-nm wavelengths.