Control of N 2 + air lasing

A near-infrared laser generates gain on transitions between the $B^{2}\mathrm{\ensuremath{\Sigma}}_{\text{u}}^{+}$ and $X^{2}\mathrm{\ensuremath{\Sigma}}_{\text{g}}^{+}$ states of the nitrogen molecular cation in part by coupling the $X^{2}\mathrm{\ensuremath{\Sigma}}_{\text{g}}^{+}$ and $A^{2}\mathrm{\ensuremath{\Pi}}_{\text{u}}$ states in the $V$ system. Traditional time resolved pump-probe measurements rely on postionization coupling by the pump pulse to initialize dynamics in the $A^{2}\mathrm{\ensuremath{\Pi}}_{\text{u}}$ 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 $A^{2}\mathrm{\ensuremath{\Pi}}_{\text{u}}$ state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on $X^{2}\mathrm{\ensuremath{\Sigma}}_{\text{g}}^{+}$ to $A^{2}\mathrm{\ensuremath{\Pi}}_{\text{u}}$ 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 $B^{2}\mathrm{\ensuremath{\Sigma}}_{\text{u}}^{+}$ state.