Far-Ultraviolet Spectroscopy in the Solid and Liquid States: A Review

Ultraviolet (UV) spectroscopy has long been used together with visible (Vis) spectroscopy to investigate electronic transitions of a molecule. Most studies of the electronic structure of molecules using UV spectroscopy have been carried out in the 190–380 nm region because commercial UV-Vis spectrometers are available only for that region. The wavelength region shorter than 190 nm is also very rich in information about the electronic states and structure of a molecule, but the absorptivity is very high in this region, and thus, this region has been employed to investigate mainly the electronic states and structure of gas molecules. Because condensed-phase materials with high molecular density do not transmit much light in the shorter wavelength region of the UV, reflection spectroscopy has been used to observe spectra of solid samples in the wavelength region shorter than 190 nm. However, for liquid samples one cannot generally use either absorption spectroscopy or specular reflection spectroscopy. Accordingly, UV spectroscopy in this region for liquid samples has been a relatively undeveloped research area. To solve the above difficulties of UV spectroscopy in the wavelength region shorter than 190 nm we have recently developed a totally new UV spectrometer based on attenuated total reflection (ATR) that enables us to measure spectra of liquid and solid samples in the 140–280 nm region. We will show that spectroscopy in the wavelength region shorter than 190 nm holds considerable promise not only in basic science but also in applications such as qualitative and quantitative analysis, on-line monitoring, environmental geochemical analysis, and surface analysis. The purpose of the present review paper is to report recent progress in UV spectroscopy of solid and liquid phases in the 140–280 nm region. In this review, we refer to the 120–200 nm region to as the far-UV (FUV) region. The term “vacuum UV region” is no longer appropriate for the 120–200 nm region because most recent spectrometers used in this region are not evacuated but instead incorporate a nitrogen purge. This review consists of eight parts: (1) introduction to FUV spectroscopy, (2) brief history of FUV spectroscopy, (3) development of new FUV spectrometers, (4) FUV studies of liquid water and aqueous solutions, (5) FUV spectra of organic molecules in the liquid states, (6) band assignments by quantum chemical calculations, (7) potential applications of FUV spectroscopy in liquid and solid states; and (8) future prospects of FUV spectroscopy.