Direct observation of the absorption bands of the first electronic transition in liquid H 2 O and D 2 O by attenuated total reflectance far-UV spectroscopy

Absorption bands of the first electronic transition ( X ̃ A 1 1 → A ̃ B 1 1 ) of water ( H 2 O ) and heavy water ( D 2 O ) in the liquid state have been directly observed by using a uniquely designed attenuated total reflectance far-ultraviolet (ATR-FUV) spectrometer. Since the ATR geometry reduces the absorbance, the FUV spectra can be obtained over the entire X ̃ A 1 1 → A ̃ B 1 1 absorption band, including the band maxima. Systematic measurements of the FUV spectra of H 2 O and D 2 O with heating from 10 to 70 ° C and the analysis of Kramers-Kronig transformation reveal that the first electronic transition band redshifted on heating. This result is in good agreement with the redshift that has been frequently observed in the low-energy band tail of the X ̃ A 1 1 → A ̃ B 1 1 absorption band.