Coupling of N2+ rotational states in an air laser from tunnel-ionized nitrogen molecules

We report on experimental and theoretical investigations of dynamic evolution of individual R-branch laser lines from tunnel-ionized nitrogen molecules based on a pump-probe scheme. Our observation shows that the intensities of the R-branch laser lines exhibit rapid oscillations which can be decomposed into periodic oscillations at different frequencies. The oscillations can be attributed to the coupling between the rotational quantum states mediated either by the probe pulses or by the P-branch laser emission from the molecular ions.