Femtosecond laser excitation of mixed Ar/Kr clusters: peculiarities of K-line x-ray production from nanoplasma under varied fraction of initial gas components

Abstract

For the first time, we defined the range of krypton fraction (CKr) in an initial binary Ar–Kr gas mixture that provides the production of large (N ~ 106 atoms/cluster) mixed Ar/Kr clusters under co-expansion of the mixture at room temperature and the stagnant pressure of 25 bar. Mixed Ar/Kr clusters exist at the krypton fraction of 3–15%. The presence of mixed clusters is detected by the simultaneous generation of both x-ray Kα lines of argon (E = 3.1 keV) and krypton (E = 12.7 keV) from nanoplasma originating as a result of femtosecond nonlinear laser excitation with intensity about 5 · 1017 W cm−2. It was shown that the amplitude of lines in dual-energy x-ray spectrum can be controlled by proper selection of the fraction of initial gas mixture components. Maximal laser energy conversion efficiency to krypton x-ray line is achieved for pure krypton clusters (i.e. CKr = 100%) and reaches 2 · 10−7 at laser intensity of 5 · 1017 W cm−2. The laser energy conversion efficiency to argon x-ray line reaches the maximal value of 3 · 10−6 at Ckr = 0%.