Direct observation of the absorption bands of the first electronic transition in liquid H2O and D2O by attenuated total reflectance far-UV spectroscopy

Absorption bands of the first electronic transition (X (1)A(1)-> A B-1(1)) of water (H2O) and heavy water (D2O) 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 (1)A(1)-> A B-1(1) absorption band, including the band maxima. Systematic measurements of the FUV spectra of H2O and D2O with heating from 10 to 70 degrees 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 (1)A(1)-> A B-1(1) absorption band.