Control of $text{N}_2^+$ Air Lasing

A near-infrared laser generates gain on transitions between the $text{B}^{text{2}} Sigma_{text{u}}^{text{+}}$ and $text{X}^{text{2}} Sigma_{text{g}}^{text{+}}$ states of the nitrogen molecular cation in part by coupling the $text{X}^{text{2}} Sigma_{text{g}}^{text{+}}$ and $text{A}^{text{2}} Pi_{text{u}}$ states in the V-system. Traditional time resolved pump-probe measurements rely on post-ionization coupling by the pump pulse to initialize dynamics in the $text{A}^{text{2}} Pi_{text{u}}$ state. Here we show that a weak second excitation pulse reduces ambiguity because it acts only on the ion independent of ionization. The additional control pulse can increase gain by moving population to the $text{A}^{text{2}} Pi_{text{u}}$ state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on $text{X}^{text{2}} Sigma_{text{g}}^{text{+}}$ to $text{A}^{text{2}} Pi_{text{u}}$ transitions may prevent the formation of a coherent rotational wave packet in the ground state in our experiment, but the control pulse can reverse impulsive alignment by the pump pulse to remove rotational wave packets in the $text{B}^{text{2}} Sigma_{text{u}}^{text{+}}$ state.