Observation of X-ray spectra from nitrogen clusters irradiated with high-intensity ultrashort laser pulses

X-ray spectra from nitrogen clusters irradiated with ultrashort laser pulse s are reported. The line spectra of H-like and He-like nitrogen ions have b een observed by irradiation with 100 fs, 800 nm pulses at 7 x 10(17) W/cm(2 ) irradiance. The generation of highly charged ions of N6+ and N7+ is expla ined by the optical field-induced ionization and the subsequent collisional ionizations in the clusters. The He-delta line has anomalously high bright ness compared to other He-like lines. It is ascribed to charge exchange, wh ich preferentially populates the n = 5 level of N5+.