Soft X-ray emission from Ar clusters heated by ultrashort laser pulse

摘要

We report soft X-ray spectra (40-180 A) produced by the interaction of 25-100 fs laser pulses at an intensity of up to 7 X 10~(16) W/cm~2 with cryogenically cooled Ar gas jet. New spectral lines from Ar~(8+), Ar~(9+) and Ar~(10+)charge states appeared with decreasing pre-expansion gas temperature. Nonlinear increase of X-ray line emission from Ar~(7+), Ar~(8+), and Ar~(9+) was observed with cooling, which saturated below certain temperature. The change in X-ray spectrum is attributed to efficient collisional heating and collisional ionization of growing, small-to-medium-sized (10~2 -10~3 atoms) Ar clusters from the cooled jet. When the laser pulse was extended to 100 fs we have observed considerably stronger emission on lines from high charge states, such as Ar~(8+), Ar~(9+) and Ar~(10+) which suggests that resonance absorption condition could be reached for the 100-fs pulse.