C. H. Nam

Enhanced X‐ray Emission from Nitrogen Clusters Ionized by Intense, Ultrashort Laser Pulses

We report soft X‐ray spectra from nitrogen clusters irradiated by 28‐fs laser pulses at an intensity of about 7 × 1016 W/cm2. While the spectrum obtained at room temperature showed entirely transitions from N4+ ions, new lines originating from N5+ and N6+ charge states appeared with cooling. By lowering the pre‐expansion gas temperature we have observed strong, nonlinear increase of X‐ray emission on lines of N4+, N5−, and N6+. The generation of highly charged ions of N5+ and N6+ is explained by collisional processes in near‐solid‐density clusters. A preliminary experiment on the feasibility of a charge‐exchange‐pumped X‐ray laser has been performed in longitudinal geometry.

Investigation of X‐ray Amplification in Neon Clusters

We have experimentally investigated the feasibility of X‐ray lasing in Li‐like Ne ions produced by the interaction of intense, 28‐fs laser pulses with gaseous clusters of neon. The idea of the proposed recombination (n=3–4) X‐ray laser is based on adiabatic cooling of fast expanding small‐sized Ne clusters. The driving laser beam was focused into a cryogenically cooled gas jet of Ne, and time‐integrated spectra were simultaneously measured in transverse and longitudinal direction with respect to the laser axis. The transverse spectrum from Ne at the temperature of about −137 °C showed dominant emission from highly placed levels (n=4) in Ne7− ions whilst the resonance lines (n=2–3) at 8.8, 9.8, and 10.3 nm remained very weak suggesting enhanced recombination from Ne8−. In the longitudinal spectrum, the lasing line candidate corresponding to the 3d‐4f transition of Ne7+ at 29.24 nm has been observed as a distinct, but not dominant.