Real-time observation of molecular and electronic dynamics is one of the ultimate goals of molecular science, enabling deeper insight into chemical reactions. Ultrashort laser pulses, typically with femtosecond pulse durations, provide a powerful means for visualizing molecular and electronic motions during chemical reactions in real time. In addition to their ultimate time resolution, femtosecond laser pulses offer extremely high peak intensities exceeding the Coulomb force between electrons and ion cores in molecules. Such intense laser fields induce a variety of nonlinear phenomena, including tunneling ionization, electron rescattering, high-order harmonic generation, multiple-ionization, and Coulomb explosion. This account highlights recent applications of ultrashort intense laser pulses to the visualization of complex molecular dynamics, including the roaming reaction of formaldehyde, and molecular orbital shapes, even revealing their asymmetric distribution. Furthermore, I elucidate the mechanisms underlying the selective scission of identical chemical bonds in CO₂, paving the way toward coherent control of molecular reactivity.

Gas-phase spectroscopy offers a unique opportunity to study the intrinsic properties of functional molecules by isolating them in a well-defined environment. To date, however, the experimental findings in the gas phase often fail to be directly connected to molecular functions under realistic conditions, such as in solution, due to the absence of solvent effects. In this study, I have investigated hydration effects, a key factor in molecular functions, using cutting-edge cryogenic ion spectroscopy combined with ion trapping techniques. I have revealed prominent hydration effects on ion recognition and proton transfer, demonstrating the utility of cryogenic ion spectroscopy for probing solvent-mediated molecular functions.

Laser spectroscopy under cryogenic gas-phase conditions offers an ideal platform for probing intrinsic molecular properties with high precision by minimizing perturbations from external environments such as impurities, solvents, and counterions. Particularly, recently developed cryogenic ion trap-based techniques afford a distinct advantage in their broad applicability to a wide range of molecular systems. To further extend the scope of this powerful approach, we have applied it to the gas-phase characterization of host–guest complexes, hypervalent compounds, reaction intermediates formed in solution, and metal/molecular clusters. In this account, we present an overview of the experimental setups developed in our laboratory and highlight recent studies that uncover "hidden molecular functions" across these diverse systems.

Although the development of anhydrous super-protonic conductors are highly demanded, theory of protonics has not been established yet. This account describes our recent fundamental research on anhydrous proton conduction using acid–base-type molecular single crystals, which enable the precise elucidation of intrinsic structure–property relationships. Especially, we have focused on the molecular internal degrees of freedom such as molecular motions and proton tautomerism, which are related to intra-molecular proton transfer in the Grotthuss-type conduction mechanism, to realize anhydrous super-protonic conductors. We have achieved super-protonic conduction with significant three-dimensional molecular rotational motions in a crystal. Furthermore, we demonstrated control over the molecular motion and conductivity via chirality. We have also proposed a novel, low-barrier conduction mechanism based on proton tautomerism, which circumvents the need for molecular motions, providing new design principles for advanced protonic materials.

Three different types of laser molecular spectroscopy in the gas phase are presented. Firstly, the tunable coherent radiation generated in the extreme ultraviolet region is used to investigate the deexcitation mechanism in the highly excited states of H₂ and D₂. Secondly, the amplified spontaneous emission ranging from near- to far-infrared region exhibits the possibility of a new radiative relaxation channel among the highly-excited Rydberg states of NO. Finally, the application of the mid-infrared free electron laser to the studies of chemical reactions and molecular structure is described.

Unlike representative structural techniques (X-ray crystallography, cryo-EM, NMR, and fluorescence), Raman microscopy enables the label-free, in situ detection of biomolecules, even in disordered or heterogeneous states. It also allows the observation of all molecules within the field of view, depending on their concentrations. This review highlights our Raman research that leverages these advantages, demonstrating the ability to quantitatively measure the concentration of molecules in complicated biological systems in a label-free and in situ manner. A key technique introduced in this review is the use of the Raman band of water as an internal intensity standard. Water outside droplets (condensates) in measurements of liquid-liquid phase separation or outside living cells in cell measurements has a constant density regardless of the sample. Thus, the O-H stretching bands of these waters can be used as intensity standards, allowing for the quantitative interpretation of Raman intensities and enabling the determination of biomolecular concentrations in droplets or cells under a microscope. In addition, by analyzing the shape of the O-H stretching band of intracellular water, the temperature inside a single cell can be determined in situ without labeling. The method established in this study, which comprises three approaches utilizing Raman imaging, label-free, in situ observation, and quantitative visualization of components, is expected to have diverse applications, including the elucidation of disease onset mechanisms.

The concept of the reaction pathway has played a pivotal role in theoretical chemistry. Under the Born–Oppenheimer approximation, elementary chemical processes are governed by the potential energy surface, where the intrinsic reaction coordinate (IRC) is defined as the steepest descent path connecting the reactant (minimum), transition state (first-order saddle point), and product (minimum). Dynamical effects associated with the IRC, such as vibrational excitation due to reaction-path curvature in coordinate space, IRC-jump, and instabilities arising from valley-ridge transition points, are crucial for understanding reaction mechanisms beyond static pictures. Recent advances in automated reaction path search methods have enabled the construction of reaction path networks based on quantum chemical calculations, while the development of the reaction space projector (ResPer) allows ab initio molecular dynamics (AIMD) trajectories to be analyzed within this network framework. In addition, the Natural Reaction Orbital (NRO) method provides valuable insights into the underlying electronic motion along the IRC, offering a powerful tool for visualizing and interpreting chemical transformation processes.