Influence of ultrashort laser pulse duration on the X-ray emission spectrum of plasma produced in cluster target

Line emission spectrum of a laser plasma produced in an argon cluster jet target was measured on the $n^1P_1-1^1 S_0 (n=5-9)$ transitions of the helium-like Ar XVII ion for a pulse duration varying from $45$ fs to $1.1$ ps and a constant fluence of $\sim105\,$ J/cm$^2$. The independent modeling of the relative intensities of the transitions from the $n=5,\dots, 10$ levels, as well as of the $2^1P_1-1^2S_0$ and $2^3P_1-1^2S_0$ lines and dielectronic satellites indicates that the electron temperature is anomalously low and that the electron density in emitting plasma increases with shortening the laser pulse. The excitation from the ground state by a small fraction of hot electrons is expected to be the main channel of populating the Ar XVII levels.