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Introduction to Experimental Techniques
Published in Thomas C. Weinacht, Brett J. Pearson, Time-Resolved Spectroscopy, 2018
Thomas C. Weinacht, Brett J. Pearson
In the previous section, we discussed two techniques where the detected signal resulted from the incoherent addition of signal amplitudes from many sources. For example, in LIF, each excited molecule in the sample was assumed to fluoresce independently of the others. In this section, we consider spectroscopies where the detected signal arises from the coherent addition of signal amplitudes from multiple sources. Since coherent measurements are sensitive to the relative phase between the scattering sources, our treatment will necessitate a discussion of “phase matching” (as mentioned in Section 8.3, phase matching is related to conservation of momentum during the interaction). The two prototypical coherent spectroscopies we consider in this chapter are transient absorption (TA) and ultrafast electron diffraction (UED).
Future Prospects of Intense Laser-Driven Ion Beams for Diagnostics of Lithium-Ion Batteries
Published in Yoshiaki Kato, Zenpachi Ogumi, José Manuel Perlado Martín, Lithium-Ion Batteries, 2019
Shunsuke Inoue, Masaki Hashida, Shuji Sakabe
From laser plasmas, various radiations, such as electrons [23, 24], ions [12, 25], X-rays [26, 27], γ-rays [28], and terahertz waves [29, 30], are generated. These radiations have desirable features, such as short pulse duration, high intensity, point source, and perfect synchronization of different radiations from the laser plasma with each other. The radiations from laser plasmas have a high potential for attractive applications in many areas, for example, particle acceleration [1–3], fast ignition for inertial confinement fusion [4, 5], cancer therapy using ion beams [21], ultrafast electron diffraction measurement [6, 7], time-resolved X-ray proving [8, 9], laser-driven nuclear physics [10], and laboratory astrophysics [11].
Excited-state dynamics of molecules with classically driven trajectories and Gaussians
Published in Molecular Physics, 2020
Lea M. Ibele, Angus Nicolson, Basile F. E. Curchod
The dynamics of a molecule in its excited electronic states is not just a curiosity for theoretical chemists, but is central for our understanding of photochemistry and its application in energy-related devices (light-emitting or photovoltaic materials), for example. A theoretical understanding of nonadiabatic processes is also central to support new experimental techniques, like ultrafast dynamics based on diffraction techniques (ultrafast electron and X-ray diffraction) that are directly sensitive to the spatial distribution of atoms. Through ultrafast electron diffraction, it was achieved to measure the nuclear motion in photoexcited molecular crystals with femtosecond resolution [4,5]. The time-resolved structural information obtained experimentally allows direct comparison with the results of simulations of nonadiabatic dynamics, as demonstrated in recent work [6–10]. Hence, these advances in experimental techniques also induced an increase in the importance of quantum dynamical simulations. The general interest for theoretical photochemistry is also reflected in the recent publication of several new textbooks on the topic – see , for example [11–14].
Impact of Electron-Phonon Interaction on Thermal Transport: A Review
Published in Nanoscale and Microscale Thermophysical Engineering, 2021
Yujie Quan, Shengying Yue, Bolin Liao
Further developments are required to gain more detailed understanding of the EPI effect on thermal transport, as well as explore novel applications based on EPI such as solid-state thermal switches. Experimentally, inelastic x-ray and neutron scattering (IXS and INS) techniques now have the energy and momentum resolution to map the frequency and linewidth (directly related to the phonon lifetime) of each individual phonon modes in solids [69, 76]. To examine the impact of EPI on the phonon linewidth, however, IXS and INS experiments with additional optical excitation or electrostatic gating need to be developed. Similarly, to directly evaluate the impact of EPI on the steady-state thermal transport processes, steady-state thermal measurement techniques [90] need to be combined with optical excitation or electrostatic gating to adjust the charge carrier concentration of the sample without introducing defects. New spectroscopic methods, e.g. based on electron energy loss spectroscopy [91], ultrafast electron diffraction [92] or ultrafast electron microscopy [93], are highly desirable to directly probe the electron-phonon nonequilibrium near surfaces and interfaces. Theoretically, more systematic studies are needed to identify the key criteria beyond the electron DOS to select materials with particularly strong or weak EPI effect on thermal transport. New first-principles methods are needed to fully understand the coupled electron-phonon transport across interfaces and in nanostructures where the classical size effect becomes appreciable. Furthermore, the nonequilibrium of electrons and phonons also affects the scattering rate calculations, which require the electron and phonon distribution functions. It is experimentally known that electron-phonon interaction is weakened in strongly optically pumped semiconductors [94], when the high density of photo-excited electrons block certain electron-phonon scattering channels. Theoretical development along this direction is particularly relevant to the performance of optoelectronic and photovoltaic devices. For practical applications, the EPI effect on thermal transport can be potentially utilized to develop solid-state thermal switches, whose thermal conductivity can be sensitively controlled by external stimuli. Since electrons can effectively and swiftly respond to external electrical and magnetic fields, indirect control of the electron properties can potentially modulate the thermal conductivity through EPI [58]. For this purpose, materials with particularly strong EPI effect on thermal transport still need to be identified, which can only be achieved by continued combined advancements of theoretical and experimental methods.
Extreme ultraviolet time-resolved photoelectron spectroscopy of aqueous aniline solution: enhanced surface concentration and pump-induced space charge effect
Published in Molecular Physics, 2021
Christopher W. West, Junichi Nishitani, Chika Higashimura, Toshinori Suzuki
Time-resolved photoelectron spectroscopy (TRPES) is one of the most useful methods to explore ultrafast electronic dynamics in materials [1–8], and its application has been extended to solution chemistry [9–12]. So far, TRPES has been successfully performed for various liquids using the UV pump and UV probe method to obtain valuable new insights into the electronic structure and dynamics in bulk solutions. The advantage of UV-UV TRPES experiment is its simple implementation and a high contrast ratio of the two-colour signal against one-colour photoionization background signal [13]. On the other hand, the observation window of UV-TRPES is limited to the electron binding energy (eBE) up to about 6 eV. It is also noted that the low-energy photoelectrons generated by UV probe pulses are vulnerable to inelastic scattering in the bulk liquid, and their analysis requires careful consideration of inelastic scattering effect in the liquid [13–17]. TRPES using extreme UV (XUV) or X-ray probe pulses significantly expands the observation energy window for the entire valence electrons and inner-shell electrons, respectively, and high-energy photoelectrons generated by these pulses are much less affected by inelastic scattering than the UV-UV TRPES. Recent development of the high-order harmonic generation (HHG) technique enabled construction of a table-top XUV and soft X-ray light sources with femtosecond or attosecond pulse durations [18]. These light sources are ideal for TRPES of liquids [9,17,19–26]. On the other hand, since the XUV radiation induces photoemission from all species in solution, XUV-TRPES faces several challenging problems. First of all, since the photoemission signal is always present from the solute and solvent in their ground electronic state, it is difficult to observe the pump-probe signal at low excitation efficiency. This difficulty is common with the methods such as ultrafast electron diffraction [27–29], X-ray diffraction [30,31], and X-ray absorption spectroscopy [32,33], which all require a high excitation efficiency to extract clear pump-probe signals. When one attempts to improve the excitation efficiency with a higher pump intensity, it often leads to multiphoton ionisation of the target species. This causes UV pump-induced generation of photoelectrons, which creates Coulombic repulsion between the pump-induced and probe-induced photoelectron packets and, consequently, pump-probe delay-dependent shift of photoelectron kinetic energy. We refer to this effect as the pump-induced space charge effect (PISC) in this study. This phenomenon has previously been identified in the pump-probe TRPES of solids and liquids, and the correction method based on the mean field model has been proposed [19,34,35]. We employ the mean field model in this study.