Investigation of soft X-ray emission from Ar clusters heated by ultrashort laser pulses

Abstract

We report soft X-ray spectra (4–18 nm) produced by the interaction of 25–100-fs laser pulses at an intensity of up to 7 × 10¹⁶ W/cm² with a cryogenically cooled Ar gas jet. New spectral lines from Ar⁸⁺, Ar⁹⁺, and Ar¹⁰⁺ charge states appeared with decreasing preexpansion gas temperature. A nonlinear increase of X-ray line emission from Ar⁷⁺, Ar⁸⁺, and Ar⁹⁺ was observed with cooling, which saturated below a certain temperature. The drastic change in the spectrum is attributed to efficient collisional heating and collisional ionization of growing, small- to medium-sized (10²–10³ atoms) Ar clusters from the cooled jet. When the laser pulse was extended from 25 to 100 fs we observed considerably stronger emission on lines from high charge states, such as Ar⁸⁺, Ar⁹⁺, and Ar¹⁰⁺, which suggests that the resonance absorption condition could be reached for the 100-fs pulse.