Soft X-ray absorption spectroscopy (XAS) is an element specific method to reveal local structures of liquid samples since soft X-ray below 1 keV has chemically important absorption edges such as C, N, and O K-edges, and L-edges of transition metals. In aqueous solutions, interactions of organic molecules are measured in C and N K-edges and solvent water is separately observed in O K-edge. However, it is difficult to measure XAS of liquid in transmission mode since the thickness of a liquid layer should be less than 1 µm due to large soft X-ray absorption coefficient of liquid. We have successfully developed a liquid flow cell for XAS in transmission mode, where the absorbance of liquid samples can be easily optimized by controlling the liquid thickness. In this review, we report on the temperature dependent change of hydrogen bond in liquid water by O K-edge XAS and intermolecular interactions of pyridine with water in aqueous pyridine solutions at different molar fractions by using XAS in C, N, and O K-edges. For the operando observation of electrochemical reaction, we have developed an electrochemical cell with built-in electrodes and measured Fe L-edge XAS of aqueous iron sulfate solutions during electrochemical reaction under the same scan rate as in cyclic voltammetry. For understanding the phase transition mechanism, not only the mixed phase but also liquid-liquid interfaces after phase transition should be investigated by spatially resolved XAS. We have developed a liquid flow cell that is set in a scanning transmission X-ray microscope. The mechanism of a lower critical solution temperature in aqueous triethylamine solution is studied by spatially resolved XAS of the liquid-liquid interfaces between triethylamine and water phases with the spatial resolution of 140 nm.
A photoexcited molecule undergoes a variety of photophysical and photochemical processes simultaneously or sequentially, and the molecule ultimately relaxes to the ground electronic state (S0) or further undergoes chemical reactions in S0. Time-resolved photoelectron imaging (TRPEI) enables full observation of these photoinduced dynamics, because photoionization can be induced from any part of the potential energy surfaces. However, photoionization from low-lying electronic excited states and S0 requires high probe photon energy in the vacuum ultraviolet (VUV) wavelength region, and it has been difficult to generate intense femtosecond (fs) VUV laser pulses so far. In this account, we review our effort to obtain a "global reaction map" of photoinduced dynamics of an isolated molecule by TRPEI using fs-VUV pulses. As an example, we present a full observation of cascaded radiationless transitions from the S2(ππ*) state of pyrazine (C4H4N2). Furthermore, we demonstrate that the configuration interaction of the S2(ππ*) electronic wave function can be explored by ultrafast photoionization using fs-VUV pulses. Ultrafast photodynamics of pyrazine in the VUV region are also presented.
In this accounts, we summarize recent progress in experimental approaches to the investigation of the unoccupied electronic structures of organic ultrathin films, based on a combination of spectroscopic and microscopic techniques. On the occupied valence bands of the films, it has been extensively studied for a variety of organic molecules. However, systematic investigations of unoccupied electronic states still have been challenging because experimental techniques are limited. In this context, we have clarified the correlation between geometric and electronic structure using a combination of two-photon photoemission (2PPE) spectroscopy and scanning tunneling microscopy (STM). By using 2PPE, one can measure unoccupied states as well as occupied states in the vicinity of the Fermi level. Beyond the diffraction limit of light, STM can be a powerful means of mapping unoccupied electronic structures, not limited to the imaging of geometrical structures. Depending on the molecular density and substrate temperature, organic ultrathin films of polycyclic aromatic hydrocarbons on graphite substrates show a variety of structures, as demonstrated by microscopic observations on the nanoscale. It is apparent that the geometrical structures, especially molecular orientations as stressed throughout this accounts, have a strong impact on both occupied and unoccupied electronic structures. These findings, with a spectroscopic and microscopic understanding at the level of molecule, will provide fundamental insights into desirable electronic properties at organic/substrate interfaces.
Elucidation of the behaviors of complex molecular systems is central to controlling their elaborate functions. It requires approaches that bear time and space specificities as well as chemical specificity, because various kinds of molecules act at different times and locations in order to carry out particular molecular processes. Time- and space-resolved vibrational spectroscopies are a powerful method that can meet all of these requirements. This Account shows how these approaches enable us to investigate complex molecular systems including living cells, bacterial communities known as biofilms, and solar-cell materials. Raman microspectroscopy in combination with multivariate data analysis reveals dynamic changes in the concentrations and distributions of cellular components such as proteins and lipids, during the cell cycle without the need for labeling. It is also applied to study bacterial biofilms in a nondestructive manner with a focus on their metabolites (carotenoids in the present case). Nanosecond time-resolved IR spectroscopy is used to observe distinct transient species generated after photoexcitation in organic–inorganic hybrid perovskite solar cells, which are attracting tremendous interest of researchers as a promising next-generation photovoltaic device. The results presented here highlight that deciphering time- and space-resolved vibrational spectra has unraveled a number of new phenomena that are of relevance to biological/material functions.
When hot molecular ions are isolated in vacuum, they cool solely by radiative processes. Recent experiments using electrostatic ion storage rings deepen the understanding of such slow processes.We will show how the electronic transition triggered by the inverse internal conversion works in the even-numbered carbon cluster anions.Theoretical framework for estimating the radiative cooling rates, vibrational and electronic, is briefly introduced.
Hydrogen-bond network is ubiquitous and plays important roles in various phenomena involving chemical reactions, biological processes, and so on. To understand microscopic natures of hydrogen-bond network, gas-phase molecular clusters have been treated as a simple model in a huge number of studies. Although temperature is one of general parameters in discussing a hydrogen-bond network, to control the temperatures of molecular clusters is rather difficult. In the preset article, we introduce mainly two methods for the control of the temperatures of the cluster ions, a tagging technique and a use of cold ion trap. Then, we explain our recent results on the temperature effect on the microscopic hydration structures of phenol cation. In this work, we succeeded in discussing a quantitative change in the relative abundance between the isomers having distinct hydration structures based on our experimental observation.