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 N2 and the first negative system of N2+. 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)