Control of N2+ air lasing

A near-infrared laser generates gain on transitions between the B-2 Sigma(+)(u) and X-2 Sigma(+)(g) states of the nitrogen molecular cation in part by coupling the X-2 Sigma(+)(g) and A(2)Pi(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)Pi(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)Pi(u) state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on X-2 Sigma(+)(g) to A(2)Pi(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 Sigma(+)(u) state.