We report the first observation of X-ray emission (4-18 nm) from small-sized Ne clusters heated by high-intensity, 25-100 fs laser pulses. The Ne spectrum started to drastically change in emitting ions from Ne~(5+) to Ne~(7+) below the pre-expansion temperature of -120 ℃. The significant change in the spectrum is attributed to the collisional heating of small-sized Ne clusters formed in the cryogenically cooled gas jet. Though the size of Ne clusters 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 Rayleigh scattering technique, the appearance of spectral lines of Ne~(7+) ions presented clear evidence of collisional heating that occurred in near-solid-density cluster. By extending the laser pulse length from 25 fs to 100 fs, considerably higher X-ray yield from the highest charge state Ne~(7+) was observed in the covered spectral region, showing more efficient absorption of 100 fs laser pulse.
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