Interaction of intense, femtosecond laser pulse with small-sized Ne clusters

We report the first observation of X-ray emission (4 - 18 nm) from small-si zed Ne clusters heated by high-intensity, 25-100 A laser pulses. The Ne spe ctrum started to drastically change in emitting ions from Ne5+ to Ne7+ belo w the pre-expansion temperature of -120 degreesC. The significant change in the spectrum is attributed to the collisional heating of small-sized Ne cl usters formed in the cryogenically cooled gas jet. Though the size of Ne cl usters in our experiment is estimated to be quite small, i.e. a few tens of atoms per cluster, and thus they were undetectable by the standard Rayleig h scattering technique, the. appearance of spectral lines of Ne7+ ions pres ented clear evidence of collisional heating that occurred in near-solid-den sity cluster. By extending the laser pulse length from 25 fs to 100 fs, con siderably higher X-ray yield from the highest charge state Ne7+ was observe d in the covered spectral region, showing more efficient absorption of 100 fs laser pulse.