Images of femtosecond laser plasma plume expansion into background air

Femtosecond laser produced plasma plume expansion from a copper target was investigated at background air pressures ranging from atmospheric pressures to 10/sup -1/ torr and pulse energies of 50-400 /spl mu/J (6/spl times/10/sup 13/-5/spl times/10/sup 14/ W/cm/sup 2/). Spatially and temporally resolved plasma imaging measurements indicated two separate emission regions in the plasma as the plume expands at background pressures of 1-25 torr.