Observation of enhancement in water window laser plasma X-ray under nitrogen atmospheres.*

 X-ray microscope using water window (WW) X-rays (2.3 to 4.4 nm) is expected as one of the tools to observe living cells and biomolecules. In order to obtain their images with a higher spatial resolution, X-ray photons number has to be increased significantly. Recently, we found that the WW intensity was enormously enhanced when a gold target was irradiated with a laser pulse (1064 nm, ~7 ns, 1 J) under low-pressure N₂ atmosphere. In order to clarify the mechanism of the phenomenon, emission spectroscopy was carried out under various gas species (N₂, O₂, Ne, He and Ar) and their pressures up to 1 kPa. For X-ray measurement, a grazing incidence spectrometer (flat field grating: 2400 grooves/mm) with a toroidal mirror was used. In addition, a pinhole camera with an aperture of 10 mm was installed to measure two dimensional X-ray images. The mechanism of the WW X-ray enhancement will be described in terms of atomic physics, especially, X-ray innershell ionization and subsequent Auger electron process.