  Hard X-ray generation for Au nanoparticle dispersion was systematically
investigated for different particle diameters ranging from 10 to 100 nm with a
narrow size distribution of +/- 2%. Scaling law of X-ray generation is close to
a 6-photon process before the onset of saturation for excitation by 45 fs laser
pulses with central wavelength of 800 nm. This is consistent with bulk plasmon
excitation at wavelength ~ 138 nm. The longitudinal E-field component due to
nanoparticle focusing is responsible for the excitation of the longitudinal
bulk plasmon. The proposed analysis also explains X-ray emission from water
breakdown via an electron solvation mechanism at ~6.2 eV. The most efficient
emission of X-rays was observed for 40 +/- 10 nm diameter nanoparticles which
also had the strongest UV extinction. X-ray emission was the most efficient
when induced by pre-chirped 370 +/- 20 fs laser pulses and exhibited the
highest intensity at a negative chirp.
