Time-resolved visible spectroscopy of plasmas produced by laser breakdown of air using femtosecond Nd laser pulses (50-300 mJ, 500 fs) reveals features not observed with nanosecond laser pulses. Emission is initially dominated by molecular lines, specifically the second positive system of N<sub>2</sub> and the first negative system of N<sub>2</sub><sup>+</sup>. This is followed by continuum emission with a growth time of ~3 ns and a decay time of ~30 ns. Atomic lines of N and O emerge from the decay of the continuum and last up to 1 μs; only faint ionization lines are observed. Several of the atomic lines are initially strongly broadened, narrowing over a period of 100 ns.
F. Martin, R. Mawassi, F. Vidal, I. Gallimberti, D. Comtois, H. Pépin, J. C. Kieffer, and H. P. Mercure, "Spectroscopic Study of Ultrashort Pulse Laser-Breakdown Plasmas in Air," Appl. Spectrosc. 56, 1444-1452 (2002)