In the FUV region (145-200 nm) a number of electronic transitions are expected to appear, however, absorption spectroscopy in the FUV region of condensed matters was an unexplored area of molecular spectroscopy (1-5). We developed a novel FUV spectrometer (145-300 nm) based on an ATR technique, making possible the easy measurement of FUV spectra of molecules in condensed phase (6,7). Since in the FUV region one can expect transitions due to π electrons, lone pair electrons, and σ electrons, the ATR-FUV method has allowed to investigate electronic states of almost all kinds of molecules in condensed phase. We established an assignment method of FUV spectra based on comparison of an observed FUV spectrum in condensed phase with the corresponding spectrum in gas phase and electronic transitions calculated by quantum chemical calculations (6,7). For example, in the study of electronically excited states of alkanes we assigned a band near 153 nm to a transition from HOMO-2 or HOMO-1 to 3p Rydberg state (7). Since alkanes are formed only by single bonds and do not have lone pair electrons, this transition reflects the state of σ electrons of alkanes. We measured ATR-FUV spectra of alkanes, alcohols, ketones, amides, benzene,,,, and so on and calculated their electronic transitions and electronic structure (1-5). We obtained a number of interesting results not only on electronic states and transitions where Rydberg orbitals are involved, but also on the effects of molecular interactions on valence electrons (σ electrons) of single bonds of a molecule.

FUV spectroscopy allowed Morisawa et al. (8) to explore interactions between alkanes in a crystal, which are very important to the understanding of interactions that involve hydrogen atoms with same polarity such as CH…HC. We measured temperature-dependent ATR–FUV spectra of n-tetradecane during cooling and heating between 15 to −38 °C (8). It was found that the band at 153 nm in the liquid phase becomes weaker, and instead, new bands appear at around 200 and 230 nm with decreasing temperature. Of note is that the conversion from the 153 nm band to the 200 and 230 nm bands occurs at the melting temperature. Therefore, it is very likely that the changes in the electronic states are induced by changes during the phase transition.

This study suggests that an unusually compressed structure is generated on the surface of n-tetradecane at low temperatures and that this reversible phase change is responsible for the unusual absorption changes observed in the ATR–FUV spectra (8). The ATR-FUV spectra of the low-temperature solid n-alkane indicate that the energy of its HOMO and the energy gap between its HOMO and LUMO in the solid phase are reduced to approximately 60% of those in the liquid phase. It is likely that σ electrons are affected by an intermolecular interaction in condensed phase.

We demonstrated a number of important applications of FUV spectroscopy (1-5).

i) One can apply FUV spectroscopy to qualitative analysis and discrimination analysis of various kinds of liquid and solid samples because each compound shows a characteristic FUV spectrum.

ii) We reveled that ATR-FUV spectroscopy is powerful for water research, from pure water, aqueous solutions, spring water, commercial mineral water, to surface adsorption water. Goto et al. explored structure of liquid water on an aluminum surface by variable-angle (VA) ATR-FUV spectroscopy (9).

iii) FUV spectroscopy has provided new opportunities for materials research (1-5,10-12). It has been applied to studies of polymers, inorganic semiconductor materials, ionic liquids, and nanomaterials. Tanabe et al. applied the ATR-FUV-DUV spectroscopy to investigate electronic states and photocatalytic activities of TiO2 and metal (Pt, Pd, and Au)-modified TiO2 powders (13). Recently, Tanabe et al. used ATR-FUV/DUV (deep-ultraviolet) spectroscopy for electrochemistry research.

iv) We developed a nanosecond pump-probe transient FUV spectrometer which can be used to probe radical species in chain reactions such as O3 pulse-photolytic reaction in an aqueous solution.

v) Tanabe et al. proposed FUV-DUV surface plasmon resonance (SPR) sensors using Al thin films (14). The SPR properties of the Al thin films with varying reflective index have been investigated by ATR-FUV spectrometer.

In this way we have demonstrated the great potential of FUV spectroscopy for condensed matter in both basic science and applications (1-5).

(1) Y. Ozaki, S. Kawata eds; Far- and Deep-Ultraviolet Spectroscopy: Springer (2015).

(2) Y. Morisawa, I. Tanabe, and Y. Ozaki: in “Frontiers and Advances in Molecular Spectroscopy”, J. Laane ed., Elsevier (2018). pp. 251.

(3) a) Y. Ozaki and I. Tanabe: Analyst, 141, 3962 (2016). b) I. Tanabe, Y. Ozaki, J. Mater. Chem. C 4, 33, 7706 (2016).

(4) Y. Ozaki, Y. Morisawa, I. Tanabe, and K. B. Bec, Spectrochim. Acta. A. 253, 119549 (2021).

(5) Y. Morisawa, I. Tanabe, and Y. Ozaki, Encyclopedia of Anal. Chem. DOI:10.1002/9780470027318.a9279.pub2. 2020.

(6) Y. Morisawa, S. Tachibana, M. Ehara, and Y. Ozaki: J. Phys. Chem., 116, 11957 (2012).

(7) Y. Morisawa, M. Yasunaga, R. Fukuda, M. Ehara, and Y. Ozaki: J. Chem. Phys, 139, 154301 (2013).

(8) Y. Morisawa, S. Tachibana, A. Ikehata, T. Yang, M. Ehara, and Y. Ozaki: ACS Omega, 2, 618 (2017).

(9) T. Goto, A. Ikehata, Y. Morisawa, and Y. Ozaki: J. Phys. Chem. Lett., 6, 1022 (2015).

(10) Y. Morisawa, M. Yasunaga, H. Sato, R. Fukuda, M. Ehara, and Y. Ozaki: J. Phys. Chem. B, 118, 11855 (2014).

(11) K. B. Bec, Y. Morisawa, K. Kobashi, J. Grabska, I. Tanabe, E. Tanimura, H. Sato, M. J. Woá jcik, and Y. Ozaki: Phys. Chem. Chem. Phys., 20, 8859 (2018).

(12) K. B. Bec, Y. Morisawa, K. Kobashi, J. Grabska, I. Tanabe, and Y. Ozaki: J. Phys. Chem. C, 122, 28998 (2018).

(13) a) I. Tanabe and Y. Ozaki: Chem. Comm., 50, 2117 (2014). b) I. Tanabe, Y. Yamada, and Y. Ozaki: ChemPhysChem., 17, 516 (2016). c) I. Tanabe, T. Ryoki, and Y. Ozaki: Phys. Chem. Chem. Phys, 16, 7749 (2014). d) I. Tanabe, Y. Kurawaki, Y. Morisawa, and Y. Ozaki: Phys. Chem. Chem. Phys., 18, 22526 (2016).

(14) a) I. Tanabe, Y. Y. Tanaka, T. Ryoki, K. Watari, T. Goto, M. Kikawada, W. Inami, Y. Kawata, and Y. Ozaki: Optics Express, 24, 21886 (2016). b) I. Tanabe, Y. Y. Tanaka, K. Watari, T. Hanulia, T. Goto, W. Inami, Y. Kawata, and Y. Ozaki: Sci. Rep., 7, 5934 (2017).