October 2014
Spotlight Summary by Takeyoshi Goto
Extinction properties of ultrapure water down to deep ultraviolet wavelengths
Liquid water is colorless and transparent in the visible light region. (If liquid water had a color, would people still want to drink it!?)
Photoabsorption of liquid water starts to rise in the mid-ultraviolet region (MUV, 200−300 nm) and reaches the maximum in the far-UV region (FUV, 122−200 nm), which is ascribed to the excitation of the lone pair electrons on the oxygen atom of a water molecule. The absorption band tail of the first electronic transition (Ã←X̃) of liquid water extends to about 300 nm, reflecting the disorder of the electronic states originating from collective structural, compositional, and thermal fluctuations. There are several ways to describe the disorder of the electronic state energy. One of them is the empirical rule proposed by Urbach, which states that the extinction coefficient of a disordered semiconductor decays exponentially with the photon energy near the absorption edge.
In order to investigate the disorder of the electronic states involved in the Ã←X̃ transition of liquid water, a precise and accurate measurement of the tail of the Ã←X̃ band in the FUV and MUV regions is necessary. However, it is very challenging to measure precisely the extinction spectrum of liquid water in these wavelength regions, because trace amounts of impurities (inorganic ions or organic molecules) and oxygen molecules dissolved in pure water have strong photoabsorption, and in addition small particles or tiny scratches on the optical cuvette cause Rayleigh scattering of the probe light. Therefore, there is no consensus in the literature on the values of the extinction coefficient of liquid water in these wavelength regions.
The authors report their precise measurement of the extinction coefficient of liquid water in the wavelength region from 181 to 340 nm; the high experimental accuracy is achieved by fabricating extremely pure water (specific conductivity: 0.055 μS cm−1, total organic carbon: 2.6 ppb), carefully cleaning the optical cuvette, and using two UV spectrophotometers, one for the wavelength range between 181 and 193 nm and the other for the range between 193 and 340 nm. From the measured extinction spectrum, the contribution of Rayleigh scattering to the extinction of liquid water is determined with much greater accuracy than in the existing literature. Then, the Urbach constant σ of liquid water is obtained from a fit of the extinction spectrum from 181 to 193 nm. The σ value reported in this work must be closer to the true value than the values reported in the previous literature, because of the highly pure water and the extreme care of the instrumental setup. Therefore, this σ value is crucial for improving our understanding of the disorder of the electronic states contributing to the Ã←X̃ transition of liquid water.
The authors suggest that their measurement of the extinction coefficient of liquid water would make it possible to noninvasively determine the temperature of pure water by simply measuring the UV extinction spectrum. Also, I expect that the optical constants of liquid water obtained in this work will allow highly precise chemical analyses of aqueous solutions.
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Photoabsorption of liquid water starts to rise in the mid-ultraviolet region (MUV, 200−300 nm) and reaches the maximum in the far-UV region (FUV, 122−200 nm), which is ascribed to the excitation of the lone pair electrons on the oxygen atom of a water molecule. The absorption band tail of the first electronic transition (Ã←X̃) of liquid water extends to about 300 nm, reflecting the disorder of the electronic states originating from collective structural, compositional, and thermal fluctuations. There are several ways to describe the disorder of the electronic state energy. One of them is the empirical rule proposed by Urbach, which states that the extinction coefficient of a disordered semiconductor decays exponentially with the photon energy near the absorption edge.
In order to investigate the disorder of the electronic states involved in the Ã←X̃ transition of liquid water, a precise and accurate measurement of the tail of the Ã←X̃ band in the FUV and MUV regions is necessary. However, it is very challenging to measure precisely the extinction spectrum of liquid water in these wavelength regions, because trace amounts of impurities (inorganic ions or organic molecules) and oxygen molecules dissolved in pure water have strong photoabsorption, and in addition small particles or tiny scratches on the optical cuvette cause Rayleigh scattering of the probe light. Therefore, there is no consensus in the literature on the values of the extinction coefficient of liquid water in these wavelength regions.
The authors report their precise measurement of the extinction coefficient of liquid water in the wavelength region from 181 to 340 nm; the high experimental accuracy is achieved by fabricating extremely pure water (specific conductivity: 0.055 μS cm−1, total organic carbon: 2.6 ppb), carefully cleaning the optical cuvette, and using two UV spectrophotometers, one for the wavelength range between 181 and 193 nm and the other for the range between 193 and 340 nm. From the measured extinction spectrum, the contribution of Rayleigh scattering to the extinction of liquid water is determined with much greater accuracy than in the existing literature. Then, the Urbach constant σ of liquid water is obtained from a fit of the extinction spectrum from 181 to 193 nm. The σ value reported in this work must be closer to the true value than the values reported in the previous literature, because of the highly pure water and the extreme care of the instrumental setup. Therefore, this σ value is crucial for improving our understanding of the disorder of the electronic states contributing to the Ã←X̃ transition of liquid water.
The authors suggest that their measurement of the extinction coefficient of liquid water would make it possible to noninvasively determine the temperature of pure water by simply measuring the UV extinction spectrum. Also, I expect that the optical constants of liquid water obtained in this work will allow highly precise chemical analyses of aqueous solutions.
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Article Information
Extinction properties of ultrapure water down to deep ultraviolet wavelengths
Lars Kröckel and Markus A. Schmidt
Opt. Mater. Express 4(9) 1932-1942 (2014) View: Abstract | HTML | PDF