This paper describes introduction of the special issues on recent advances in fast spectroscopy, which is sensitive to surfaces, interfaces, and thin films. The topics of the issues mainly focus on the photophysics and photochemistry in molecular assemblies from the viewpoints of their excitonic and electronic functionalities. The issues cover electronic structures in excitons, exciton diffusion, and charge dynamics in thin films and interfaces. In addition, we will touch some related works on molecular orientation, density of states, and surface plasmons.

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We reviewed three studies on photoexcited structural dynamics in photofunctional materials using time-resolved infrared spectroscopy. In thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes, we found that the structural change associated with intersystem crossing strongly affects TADF activity. In the transition metal complexes for CO2 photoreduction, we showed that the steric hindrance in the excited state improves the photophysical properties for photocatalytic reactions in collaboration with time-resolved extended X-ray absorption fine structures. In a photoactive liquid crystal, we revealed the overall picture of photoexcited processes in combination with time-resolved electron diffraction.

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In this review, we discuss recent topics in ultrafast spectral diffusion in organic thin films. With two-dimensional electronic spectroscopy, exciton-vibration coupling in photoexcitation dynamics was revealed in the following two systems. 1) In tetracene thin films, a significant acceleration of exciton spectral diffusion was observed by increasing the temperature. We propose a novel anharmonic coupling scheme of exciton-vibration coupling to account for the anomalous temperature dependence. 2) The effect of polariton formation on exciton-bath coupling was studied for tetraphenyldibenzoperiflanthene film embedded in an optical microcavity and we found that strong-light matter coupling significantly reduces the exciton-phonon coupling in molecular solids.

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Organic field-effect transistors (OFETs) during operation has been investigated using compressive-sensing sum-frequency generation (CS-SFG) imaging microscopy. The electric field behavior was discussed by difference in the electric field induced SFG intensity between open-circuit and the voltage application conditions. The electric field induced SFG coming from the ODPA/DPh-BTBT show the presence of intense electric field due to charge injection and accumulation near the drain and source electrode edges under the operation of OFETs. Our studies show that the electric field-induced CS-SFG imaging technique is useful for probing the local electric field distribution and charge carrier behavior in OFETs under operation.

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Transient absorption spectroscopy is one of the most powerful tools for studying photo-energy conversion systems, including solar cells and photocatalysts. To apply this technique, high sensitivity is needed to eliminate the secondary charge recombination that occurs efficiently under high density excitation conditions. We discuss experimental techniques to improve the sensitivity for transient absorption spectroscopy in micro-second time regions. We also present a successful application of ultrasensitive transient absorption spectroscopy to dye-sensitized solar cells.

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The orientational ordering behavior of water molecules in crystalline-ice thin films was investigated using sum-frequency generation (SFG) spectroscopy. Homodyne-and heterodyne-detected SFG measurements revealed that net-H-down ferroelectric ice films were grown on a model platinum substrate, where water molecules orient their protons toward the substrate. Temperature dependent measurement shows that the order-disorder transition temperature is significantly enhanced in comparison with bulk ice due to the strong orientational pinning of the first-layer water molecules to net-H-down configuration. Our results demonstrate the possibility of SFG for sensitively evaluating the proton configuration of even a single molecular layer. This evaluation method would be also applicable to other hydrogen-bonded nanoscale systems.

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Two-photon photoemission spectroscopy is one of the branches of laser photoemission spectroscopy and is an excellent tool to directly observe unoccupied bands on solid surfaces. Since multiphoton processes are intrinsically required, pulsed lasers are essential. Thus, the technique has progressed along with the development of pulsed lasers. In many studies, time-resolved measurements are performed using pumpprobe techniques. On the other hand, there have been few attempts to pursue energy resolution. In this paper, we present a high-resolution two-photon photoemission study of band splitting due to spin-orbit coupling at unoccupied levels on a solid surface.

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We studied the optical interactions of femtosecond surface plasmon polariton (SPP) wavepackets with a nano-scaled metal-insulator-metal (MIM) cavity using finite-difference time-domain (FDTD) simulations and time-frequency analyses. The resonance nature of the interaction between the SPP and the cavity provides transmitted and reflected SPP wavepackets with spectra whose envelope shapes are strongly modulated by the cavity eigenmodes. Because of the cavity’s properties as a Fabry-Pérot etalon, when the instantaneous carrier frequency of the incident wavepacket matches the eigenmode, that portion is clipped and released as a transmitted wavepacket while the other portion is reflected. With appropriate eigenmode tuning, a wavepacket can be extracted with a narrower time duration and a temporally shifted intensity peak.

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We proposed a novel method is proposed to discriminate carbon ions from a mixture ion beam generated from laser-driven acceleration mechanism and measured the particle number in a single shot. Our method consisted of the combined use of an Einzel lens and a Thomson parabola spectrometer. Our experiment’s analysis showed qualitative agreement with the simulation results, and we conclude that it successfully demonstrated the principle of a single-shot measurement of a carbon beam.

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