Efficient absorption and intense soft X-ray emission from gas cluster plasmas irradiated by 25-ps laser pulses

A highly efficient absorption is observed in multipicosecond laser pulse irradiation of argon and nitrogen gas cluster targets at moderate intensities of ∼10¹⁵ W/cm² using a 1.5 J/25 ps Nd : phosphate glass laser (λ=1.054 μm). Measurements of laser light absorption, side scattering, and extreme-ultraviolet (XUV) emission spectrum are performed for argon and nitrogen gases puffed through sonic or hypersonic nozzles in the pressure range from 10 to 60 bar. High values of 80%-85% of laser light absorption have been measured in both nitrogen and argon gas cluster plasmas. The X-ray line emission from argon plasma was ∼ 30-40 times stronger than that from the nitrogen plasma, which is related to the formation of larger sized clusters in argon. The very high laser light absorption observed in these multipicosecond laser pulse irradiation experiments may be understood to be occurring due to intense heating of the near solid density clusters in the initial part of the laser pulse creating a plasma of a high degree of ionization, which continues to absorb the rest of the laser pulse energy through collisional processes during its expansion to subcritical densities.