  Simultaneous measurements of THz wave and hard X-ray emission from thin and
flat water flow when irradiated by double femtosecond laser pulses (800 nm, 35
fs, transform-limited, 0.5 kHz, delay times up to 15 ns) were carried out at
the transmission and the reflection sides of the flow with THz time-domain
spectroscopy and Geiger counters. THz wave emission spectra show their dynamic
peak-shifts toward the low frequency with the highest intensity enhancements
more than 1.5 x 10^3 times in |E|^2 at the delay time of 4.7 ns between the two
pulses, while X-ray intensity enhancements are limited to about 20 times at 0
ns under the same experimental conditions. The mechanisms for the spectral
changes and the intensity enhancements in THz wave emission are discussed from
the viewpoint of laser ablation on the water flow induced by the pre-pulse
irradiation.
