Temporally resolved properties of femtosecond laser-induced plasmas in air

We present the temporally resolved measurements of electron number density and temperature over timescales on the order of 1 ns to 10 ns in a femtosecond (fs) laser-induced plasma (LIP). Additionally, we also report on the neutral gas number density and rotational temperature over timescales on the order of 1 ns to 1000 ns. The fs LIP was generated in ambient air and investigated using laser Thomson and rotational Raman scattering. Electron number densities were observed to decrease from $$\sim 4\times 10^{21}$$ $$\hbox {m}^{-3}$$ at early times to $$\sim 7\times 10^{20}$$ $$\hbox {m}^{-3}$$ at later times, while the electron temperature remained within $$0.5-1$$ eV throughout the probed range. The neutral gas number density remained nearly constant at $$\sim 2\times 10^{25}$$ $$\hbox {m}^{-3}$$ for the first 50 ns before declining to $$\sim 1.1\times 10^{25}$$ $$\hbox {m}^{-3}$$ after 500 ns. The rotational temperature initially decreased from $$\sim 700$$ K within the first 30 ns to $$\sim 500$$ K at 100 ns, before rising back to $$\sim 700$$ K. Finally, the experimental results were compared with simulations, showing good agreement.