Tunneling ionization is one of 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.