Increase of the emission of laser-produced plasmas under N_2 gas atmosphere in the 2.9-6 nm region

摘要

The wavelength region between the K-edges of carbon and oxygen is termed as the water window (λ = 2.3-4.5 nm) and is considered to be suitable for the observation of nanometer-scale structures composed of light elements in an aqueous solution because of both the short wavelength and transparency of the light. The nanometer-scale structures in an aqueous solution fluctuate owing to Brownian motion. Therefore, the observation of the nanometer-scale structures can be achieved by one-shot exposure of the light in the water window. To observe the nanometer-scale structures by the one-shot exposure, the light source of the microscope working in the water window should exhibit a high light-flux. One of the light sources used for the one-shot exposure is the laser-produced plasma (LPP) light source. The conversion efficiency of the LPP light source in the water window is 1% or less. Therefore, a high intensity laser or improvement of the conversion efficiency is required for the one-shot observation. In this study, an emission-intensity increase in the LPP light source is observed in the wavelength region from 2.9 nm to 6 nm under a N_2 gas atmosphere. The intensity value, calibrated by the gas absorbance, was increased up to 5 times in proportion to the N_2 pressure, which was increased from 0 Pa to 400 Pa. Experimental conditions demonstrate that the conversion efficiency of the LPP light source is improved by the introduction of gas.