Control of N2+ air lasing

A near-infrared laser generates gain on transitions between the Bςu+2 and Xςg+2 states of the nitrogen molecular cation in part by coupling the Xςg+2 and AΠu2 states in the V system. Traditional time resolved pump-probe measurements rely on postionization coupling by the pump pulse to initialize dynamics in the AΠu2 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 AΠu2 state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on Xςg+2 to AΠu2 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 Bςu+2 state.