Enhancement of soft x-ray emission from a cryogenically cooled Ar gas jet irradiated by 25 fs laser pulse.

Soft x-ray spectra (40-180 Å) produced by the interaction of 25 fs laser pulses at an intensity of ∼7x10[sup 16] W/cm[sup 2] with a cryogenically cooled Ar gas jet have been measured. New spectral lines from Ar[sup 8+] and Ar[sup 9+] charge states appeared with decreasing gas temperature. Nonlinear increase of x-ray line emission from Ar[sup 7+], Ar[sup 8+], and Ar[sup 9+] was observed with cooling, which saturated below certain temperature. The drastic change in the spectrum is attributed to efficient collisional heating and collisional ionization of growing (10[sup 2]-10[sup 3] atoms) Ar clusters from the cooled jet. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]