Waveform-dependent air fluorescence from neutral and ionic nitrogen molecules.

Laser-induced air fluorescence in the ultraviolet regime is primarily attributed to transitions between the C and B states in excited neutral [Formula: see text] molecules and between the B and X states in [Formula: see text] ions. However, the mechanism underlying the former remains contentious, as direct population to the C state by light fields is forbidden by electron spin constraints. In this work, we investigate the mechanism of air fluorescence from excited neutral [Formula: see text] molecules by carrier-envelope phase-stabilized sub-4 femtosecond pulses. Our results show that fluorescence from [Formula: see text] ions reaches a maximum with cosine-like pulses, while fluorescence from excited neutral [Formula: see text] molecules peaks with sine-like pulses. In addition, by scanning the chirp of the driving pulse, we find that ionic fluorescence is maximized with chirp-free pulses, whereas neutral fluorescence favors negatively chirped pulses. These observations, supported by classical trajectory Monte Carlo simulations, support the mechanism of intersystem crossing from excited spin-singlet states, which are populated via recollision-induced strong-field excitation.