The optical emission spectra of the plasma generated by a
1.06-μm Nd:YAG laser irradiation of Al target in air was
recorded and analyzed in a spatially resolved manner. Electron
temperatures and densities in the plasma were obtained using
the relative emission intensities and the Stark-broadened
linewidths of Al(I) emission lines, respectively. The dependence
of the electron density and temperature on the distance from
the target surface and on the laser irradiance were manifested.
We also discussed how the air takes part in the plasma evolution
process and confirmed that the ignition of the air plasma was
by the collisions between the energetic electrons and the nitrogen
atoms through a cascade avalanche process.