Two-photon Photoemission Research Articles

Adapting an Electron Microscopy Microheater for Correlated and Time-Resolved Ambient Pressure X-ray Photoelectron Spectroscopy

Adapting an Electron Microscopy Microheater for Correlated and Time-Resolved Ambient Pressure X-ray Photoelectron Spectroscopy

Electronic dispersion, correlations and stacking in the photoexcited state of 1T-TaS2

Here we perform angle and time-resolved photoelectron spectroscopy on the commensurate charge density wave (CDW) phase of 1T-TaS2. Data with different probe pulse polarization are employed to map the dispersion of electronic states below and above the chemical potential. Upon photoexcitation, the fluctuations of CDW order erase the band dispersion and squeeze the electronic states near to the chemical potential. This transient phase sets within half a period of the coherent lattice motion and is favored by strong electronic correlations. The experimental results are compared to density-functional theory calculations with a self-consistent evaluation of the Coulomb repulsion. Our simulations indicate that the screening of Coulomb repulsion depends on the stacking order of the TaS2 layers. The entanglement of such degrees of freedom suggest that both the structural order and electronic repulsion are locally modified by the photoinduced CDW fluctuations.

Manipulating Attosecond Charge Migration in Molecules by Optical Cavities.

The ultrafast electronic charge dynamics in molecules upon photoionization while the nuclear motions are frozen is known as charge migration. In a theoretical study of the quantum dynamics of photoionized 5-bromo-1-pentene, we show that the charge migration process can be induced and enhanced by placing the molecule in an optical cavity, and can be monitored by time-resolved photoelectron spectroscopy. The collective nature of the polaritonic charge migration process is investigated. We find that, unlike spectroscopy, molecular charge dynamics in a cavity is local and does not show many-molecule collective effects. The same conclusion applies to cavity polaritonic chemistry.

Deciphering Methylation Effects on S2(ππ*) Internal Conversion in the Simplest Linear α,β-Unsaturated Carbonyl.

Chemical substituents can influence photodynamics by altering the location of critical points and the topography of the potential energy surfaces (electronic effect) and by selectively modifying the inertia of specific nuclear modes (inertial effects). Using nonadiabatic dynamics simulations, we investigate the impact of methylation on S2(ππ*) internal conversion in acrolein, the simplest linear α,β-unsaturated carbonyl. Consistent with time constants reported in a previous time-resolved photoelectron spectroscopy study, S2 → S1 deactivation occurs on an ultrafast time scale (∼50 fs). However, our simulations do not corroborate the sequential decay model used to fit the experiment. Instead, upon reaching the S1 state, the wavepacket bifurcates: a portion undergoes ballistic S1 → S0 deactivation (∼90 fs) mediated by fast bond-length alternation motion, while the remaining decays on the picosecond time scale. Our analysis reveals that methyl substitution, generally assumed to mainly exert inertial influence, is also manifested in important electronic effects due to its weak electron-donating ability. While methylation at the β C atom gives rise to effects principally of an inertial nature, such as retarding the twisting motion of the terminal -CHCH3 group and increasing its coupling with pyramidalization, methylation at the α or carbonyl C atom modifies the potential energy surfaces in a way that also contributes to altering the late S1-decay behavior. Specifically, our results suggest that the observed slowing of the picosecond component upon α-methylation is a consequence of a tighter surface and reduced amplitude along the central pyramidalization, effectively restricting the access to the S1/S0-intersection seam. Our work offers new insight into the S2(ππ*) internal conversion mechanisms in acrolein and its methylated derivatives and highlights site-selective methylation as a tuning knob to manipulate photochemical reactions.

Double Thionated Pyrimidine Nucleobases: Molecular Tools with Tunable Photoproperties.

Sulfur-substituted nucleobases are DNA and RNA base derivatives that exhibit extremely efficient photoinduced intersystem crossing (ISC) dynamics into the lowest-energy triplet state. The long-lived and reactive triplet states of sulfur-substituted nucleobases are crucial due to their wide range of potential applications in medicine, structural biology, and the development of organic light-emitting diodes (OLEDs) and other emerging technologies. However, a comprehensive understanding of non-negligible wavelength-dependent changes in the internal conversion (IC) and ISC events is still lacking. Here, we study the underlying mechanism using joint experimental gas-phase time-resolved photoelectron spectroscopy (TRPES) and theoretical quantum chemistry methods. We combine 2,4-dithiouracil (2,4-DTU) TRPES experimental data with computational analysis of the different photodecay processes, which are induced by increasing excitation energies along the entire linear absorption (LA) ultraviolet (UV) spectrum. Our results show how the double-thionated uracil (U), i.e., 2,4-DTU, appears as a versatile photoactivatable instrument. Multiple decay processes can be initiated with different ISC rates or triplet-state lifetimes that resemble the distinctive behavior of the singly substituted 2- or 4-thiouracil (2-TU or 4-TU). We obtained a clear partition of the LA spectrum based on the dominant photoinduced process. Our work clarifies the reasons behind the wavelength-dependent changes in the IC, ISC, and triplet-state lifetimes in doubly thionated U, becoming a biological system of utmost importance for wavelength-controlled applications. These mechanistic details and photoproperties are transferable to closely related molecular systems such as thionated thymines.

High spatiotemporal resolved imaging of ultrafast control of nondiffracting surface plasmon polaritons

Abstract Nondiffracting Bessel surface plasmon polariton (SPP) beams, which have unique self-healing, non-divergence, and linear transmission properties, have charming applications in plasmonic devices and on-chip interconnection circuits. Here we first realize, to the best of our knowledge, the ultrafast control and imaging of the Bessel SPP pulse on the nano-femto scale in the experiment. We demonstrate ultrafast control of Bessel SPP pulse switching by controlling the instantaneous polarization state of the excitation light. Moreover, this variation process is directly mapped on the nano-femto scale by time-resolved two-color photoemission electron microscopy. The results are well reproduced by the finite-difference time-domain (FDTD) method. The current study of ultrafast control and spatiotemporally imaging the switching process establishes an experimental paradigm for revealing the complex mechanisms in ultrafast control of nondiffracting SPP and are useful for developing high-speed, highly-integrated nanophotonic devices, and on-chip circuits.

Dynamic Electrode–Electrolyte Intermixing in Solid-State Sodium Nano-Batteries

Nanostructured solid-state batteries (SSBs) are poised to meet the demands of next-generation energy storage technologies by realizing performance competitive to their liquid-based counterparts while simultaneously offering improved safety and expanded form factors. Atomic layer deposition (ALD) is among the tools essential to fabricate nanostructured devices with challenging aspect ratios. Here, we report the fabrication and electrochemical testing of the first nanoscale sodium all-solid-state battery (SSB) using ALD to deposit both the V2O5 cathode and NaPON solid electrolyte followed by evaporation of a thin-film Na metal anode. NaPON exhibits remarkable stability against evaporated Na metal, showing no electrolyte breakdown or significant interphase formation in the voltage range of 0.05-6.0 V vs Na/Na+. Electrochemical analysis of the SSB suggests intermixing of the NaPON/V2O5 layers during fabrication, which we investigate in three ways: in situ spectroscopic ellipsometry, time-resolved X-ray photoelectron spectroscopy (XPS) depth profiling, and cross-sectional cryo-scanning transmission electron microscopy (cryo-STEM) coupled with electron energy loss spectroscopy (EELS). We characterize the interfacial reaction during the ALD NaPON deposition on V2O5 to be twofold: (1) reduction of V2O5 to VO2 and (2) Na+ insertion into VO2 to form NaxVO2. Despite the intermixing of NaPON-V2O5, we demonstrate that NaPON-coated V2O5 electrodes display enhanced electrochemical cycling stability in liquid-electrolyte coin cells through the formation of a stable electrolyte interphase. In all-SSBs, the Na metal evaporation process is found to intensify the intermixing reaction, resulting in the irreversible formation of mixed interphases between discrete battery layers. Despite this graded composition, the SSB can operate for over 100 charge-discharge cycles at room temperature and represents the first demonstration of a functional thin-film solid-state sodium-ion battery.

Enhance high harmonic generation (HHG) efficiency via compact multi-plate continuum post-compression for time-resolved angle-resolved photoemission spectroscopy.

Time-resolved angle-resolved photoemission spectroscopy (Tr-APRES) gives direct insight into electron dynamics by providing temporal-, energy-, and momentum-resolved information in one experiment. A major obstacle to using high harmonic generation (HHG) probe pulses for photoemission spectroscopy is the low conversion efficiency, that is, the low flux of probe photons. We use a Yb-KGW based duo-laser source with an oscillator to pump two separate amplifiers and generate two synchronized pulsed laser sources with average energies of 7.5 and 6W. By using the multiplate continuum method and chirped mirrors, the resulting flux of HHG photons at 33-70eV can be increased 50-fold (up to 1011 photons/s) by using post-compressed 30fs pulses compared with the photon flux generated by the fundamental 190fs pulses. Moreover, pulses from the 6W amplifier are used to pump an optical parametric amplifier that can vary the wavelengths for photoexcitation. The system performance is demonstrated by applying Tr-ARPES to single-crystal graphite. The front tilt broadening is significantly suppressed by the off-plane mounted conical grating, leading to a 184fs temporal resolution that is mainly limited by the pump pulse. The energy resolution is 176meV.

Excited-state dynamics of o-nitrophenol studied with UV pump-VUV probe time-resolved photoelectron and photoion spectroscopy.

Time-resolved photoionization measurements were performed on o-nitrophenol pumped with UV laser pulses at a central wavelength of 255nm (4.9eV) and probed with vacuum ultraviolet (VUV) pulses at 153nm (8.1eV). The photoelectron spectrum and time of flight mass spectrum for ions were recorded at each pump-probe delay. The measurements are interpreted with the aid of electronic structure calculations for both the neutral and ionic states. Evidence is found for the formation of a bicyclic intermediate followed by NO dissociation through a process of internal conversion and intersystem crossing. The combination of photoelectron and photoion spectroscopy, together with computational results, provides strong evidence of intersystem crossing that is difficult to establish with only a single technique.

Effects of Autoionizing Resonances on Wave-Packet Dynamics Studied by Time-Resolved Photoelectron Spectroscopy.

We report a combined experimental and theoretical study on the effect of autoionizing resonances in time-resolved photoelectron spectroscopy. The coherent excitation of N_{2} by ∼14.15 eV extreme-ultraviolet photons prepares a superposition of three dominant adjacent vibrational levels (v^{'}=14-16) in the valence b^{'} ^{1}Σ_{u}^{+} state, which are probed by the absorption of two or three near-infrared photons (800nm). The superposition manifests itself as coherent oscillations in the measured photoelectron spectra. A quantum-mechanical simulation confirms that two autoionizing Rydberg states converging to the excited A ^{2}Π_{u} and B ^{2}Σ_{u}^{+} N_{2}^{+} cores are accessed by the resonant absorption of near-infrared photons. We show that these resonances apply different filters to the observation of the vibrational wave packet, which results in different phases and amplitudes of the oscillating photoelectron signal depending on the nature of the autoionizing resonance. This work clarifies the importance of resonances in time-resolved photoelectron spectroscopy and particularly reveals the phase of vibrational quantum beats as a powerful observable for characterizing the properties of such resonances.

Build-up and dephasing of Floquet-Bloch bands on subcycle timescales.

Strong light fields have created opportunities to tailor novel functionalities of solids1-5. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases6-15. On subcycle timescales, lightwaves can simultaneously drive intraband currents16-29 and interband transitions18,19,30,31, which enable high-harmonic generation16,18,19,21,22,25,28-30 and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet-Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator32,33 with mid-infrared fields-strong enough for high-harmonic generation-and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.

Ultrafast electron dynamics in a topological surface state observed in two-dimensional momentum space

We study ultrafast population dynamics in the topological surface state of Sb_2Te_3 in two-dimensional momentum space with time- and angle-resolved two-photon photoemission spectroscopy. Linearly polarized mid-infrared pump pulses are used to permit a direct optical excitation across the Dirac point. We show that this resonant excitation is strongly enhanced within the Dirac cone along three of the six {bar{Gamma }}–{bar{M}} directions and results in a macroscopic photocurrent when the plane of incidence is aligned along a {bar{Gamma }}–{bar{K}} direction. Our experimental approach makes it possible to disentangle the decay of transiently excited population and photocurent by elastic and inelastic electron scattering within the full Dirac cone in unprecedented detail. This is utilized to show that doping of Sb_2Te_3 by vanadium atoms strongly enhances inelastic electron scattering to lower energies, but only scarcely affects elastic scattering around the Dirac cone.

Time-resolved ARPES with tunable 12–21.6 eV XUV at 400 kHz repetition rate

Time-resolved and angle-resolved photoemission spectroscopy (trARPES) is a powerful method to detect the non-equilibrium electronic structure in solid systems. In this study, we report a trARPES apparatus with tunable photon energy selectively among 12, 16.8, and 21.6 eV at a repetition rate of 400 kHz. The energy and temporal resolutions of the three harmonics are determined as 104/111/157 meV and 276/190/154 fs, respectively. The photon flux on the sample is estimated to be 1010–1011 photons/s by using a photodiode. Finally, the performance of this setup is verified by both equilibrium and non-equilibrium ARPES measurements on topological materials Zr2Te2P and Bi2Se3. Meanwhile, the importance of the tunability of the extreme ultraviolet (XUV) source is highlighted by comparing experimental results measured with the three different photon energies.

Development of in situ characterization of two-dimensional materials grown on insulator substrates with spectroscopic photoemission and low energy electron microscopy

Ultrathin two-dimensional (2D) materials offer great potential for next-generation integrated circuit and optoelectronic devices. Chemical vapor deposition (CVD)-grown 2D materials provide a way to mass production in industry. However, how to in situ characterize their intrinsic electric/photoelectric properties and carrier dynamics with electron/photoelectron probes is still a problem due to the interference from the conducting substrate. Here, we present a grounding Au grids method to realize in situ characterization of the CVD-grown MoS2 on the insulating thick SiO2 layer covered Si substrate with spectroscopic photoemission and low energy electron microscopy (SPELEEM). Through depositing Au grids afterwards, we have achieved good grounding of MoS2 flakes in the photoemission electron microscopy (PEEM), mirror electron microscopy (MEM), and micro-area low energy electron diffraction (µ-LEED) measurements. We have clarified the false signal caused by stray photoelectrons originated from the Au stripes, and as well as the space charge effects induced by intense photoemission. We have also confirmed that time-resolved PEEM results are not affected by the stray signal, and by adopting a small light spot, both static and time-resolved micro-area photoelectron spectroscopy (µ-PES) can be unaffected by space charge effects. Our results provide a reliable way to in situ investigate 2D materials grown on insulating substrates by probing photoelectrons or backscattered electrons.

Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces.

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C60) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C60 can be visualized by Agn NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Agn NCs on C60 layers.

Ultrafast Imaging of the Jahn–Teller Topography in Carbon Tetrachloride

We report the direct time-domain observation of ultrafast dynamics driven by the Jahn-Teller effect. Using time-resolved photoelectron spectroscopy with a vacuum-ultraviolet femtosecond source to prepare high-lying Rydberg states of carbon tetrachloride, our measurements reveal the local topography of a Jahn-Teller conical intersection. The pump pulse prepares a configurationally mixed superposition of the degenerate 1T2 4p-Rydberg states, which then distorts through spontaneous symmetry breaking that we identify to follow the t2 bending motion. Photoionization of these states to three cationic states 2T1, 2T2, and 2E reveals a shift in the center-of-mass of the photoelectron peaks associated with the 2Tn states which reveals the local topography of the Jahn-Teller conical intersection region prepared by the pump pulse. Time-dependent density functional theory calculations confirm that the dominant nuclear motion observed in the spectrum is the CCl4 t2 bending mode. The large density of states in the VUV spectral region at 9.33 eV of carbon tetrachloride and strong vibronic coupling result in ultrafast decay of the excited-state signal with a time constant of 75(4) fs.

Time-Resolved Chiral X-Ray Photoelectron Spectroscopy with Transiently Enhanced Atomic Site Selectivity: A Free-Electron Laser Investigation of Electronically Excited Fenchone Enantiomers

Chirality is widespread in nature, playing a fundamental role in biochemical processes and in the origin of life itself. The observation of dynamics in chiral molecules is crucial for the understanding and control of the chiral activity of photoexcited states. One of the most promising techniques for the study of photoexcited chiral systems is time-resolved photoelectron circular dichroism (TR-PECD), which offers an intense and sensitive probe for vibronic and geometric molecular structure as well as electronic structures, and their evolution on a femtosecond timescale. However, the nonlocal character of the PECD effect, which is imprinted during the electron scattering off the molecule, makes the interpretation of TR-PECD experiments challenging. In this respect, core photoionization is known to allow site and chemical sensitivity to photelectron spectroscopy. Here we demonstrate that TR-PECD utilizing core-level photoemission enables probing the chiral electronic structure and its relaxation dynamics with atomic site sensitivity. Following UV pumped excitation to a 3s Rydberg state, fenchone enantiomers (C10H16O) were probed on a femtosecond scale using circularly polarized soft x-ray light pulses provided by the free-electron laser FERMI. C 1s binding energy shifts caused by the redistribution of valence electron density in this 3s-valence-Rydberg excitation allowed us to measure transient PECD chiral responses with an enhanced C atom site selectivity compared to that achievable in the ground state molecule. This chemical-specific, site-specific, and enantiosensitive observation of the electronic structure of a transiently photoexcited chiral molecule is expected to pave the way toward chiral femtochemistry probed by core-level photoemission.Received 2 March 2022Revised 27 September 2022Accepted 2 February 2023DOI:https://doi.org/10.1103/PhysRevX.13.011044Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasChiralityElectronic structure of atoms & moleculesPhysical SystemsMoleculesTechniquesPhotoionizationUltrafast femtosecond pump probeX-ray photoelectron spectroscopyAtomic, Molecular & Optical

Surface carrier dynamics and diffusion in inorganic metal-halide perovskite films

Carrier dynamics in metal-halide perovskites, in particular diffusion, has typically been investigated on micrometer-length scales and above. However, surfaces and interfaces modify the carrier dynamics, e.g., by interface band bending and recombination. To investigate the carrier dynamics and diffusion at the surface, we use time-resolved photoelectron spectroscopy which is sensitive to the photoexcited carriers in the topmost few nm of the material. We extract electron mobilities in well-ordered ${\mathrm{CsPbBr}}_{3}$(001) and ${\mathrm{CsSnBr}}_{3}$(001) films at room temperature of $31\ifmmode\pm\else\textpm\fi{}6$ and $13\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, respectively. The mobility in ${\mathrm{CsPbBr}}_{3}$(001) increases to $200\ifmmode\pm\else\textpm\fi{}8\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 90 K, indicating strong electron-phonon coupling for electrons at the conduction band minimum. Temperature-dependent photoemission experiments find different phonon coupling mechanisms for high-energetic electrons and holes at the valence band maximum and in the main valence bands.

Distortion Correction of Low-Energy Photoelectron Spectra of Liquids Using Spectroscopic Data for Solvated Electrons.

Time-resolved photoelectron spectroscopy (TRPES) enables real-time observation of ultrafast electronic dynamics in solutions. When extreme ultraviolet (EUV) probe pulses are employed, they can ionize solutes from all electronic states involved in the dynamics. However, EUV pulses also produce a strong ionization signal from a solvent that is typically 6 orders of magnitude greater than the pump-probe photoelectron signal of solutes. Alternatively, UV probe pulses enable highly sensitive and selective observation of photoexcited solutes because typical solvents such as water are transparent to UV radiation. An obstacle in such UV-TRPES measurements is spectral distortion caused by electron scattering and a yet to be identified mechanism in liquids. We have previously proposed the spectral retrieval (SR) method as an a posteriori approach to removing the distortion and overcoming this difficulty in UV-TRPES; however, its accuracy has not yet been verified by comparison with EUV-TRPES results. In the present study, we perform EUV-TRPES for charge transfer reactions in water, methanol, and ethanol, and verify SR analysis of UV-TRPES. We also estimate a previously undetermined energy-dependent intensity factor and expand the basis sets for SR analysis. The refined SR method is employed for reanalyzing the UV-TRPES data for the formation and relaxation dynamics of solvated electrons in various systems. The electron binding energy distributions for solvated electrons in liquid water, methanol, and ethanol are confirmed to be Gaussian centered at 3.78, 3.39, and 3.25 eV, respectively, in agreement with Nishitani et al. [ Sci. Adv. 2019, 5(8), eaaw6896]. An effective energy gap between the conduction band and the vacuum level at the gas-liquid interface is estimated to be 0.2 eV for liquid water and 0.1 eV for methanol and ethanol.

Coaction effect of radiative and non-radiative damping on the lifetime of localized surface plasmon modes in individual gold nanorods.

Revealing the coaction effect of radiative and non-radiative damping on the lifetime of the localized surface plasmon resonance (LSPR) mode is a prerequisite for the applications of LSPR. Here, we systematically investigated the coaction effect of radiative and non-radiative damping on the lifetime of the super-radiant and sub-radiant LSPR modes of gold nanorods using time-resolved photoemission electron microscopy (TR-PEEM). The results show that the lifetime of the LSPR mode depends on the length of the gold nanorod, and the different variation behavior of an LSPR mode lifetime exists between the super-radiative mode and the sub-radiative one with the increase of nanorod length (volume). Surprisingly, it is found that the lifetime of the super-radiant LSPR mode can be comparable to or even longer than that of the sub-radiant LSPR mode, instead of the usual claim that a sub-radiant LSPR mode has a longer life than the super-radiant mode. Those TR-PEEM experimental results are supported by finite-difference time-domain simulations and are well explained by the coaction effect with the calculation of the radiative and non-radiative damping rate with the increase of the nanorod volume. We believe that this study is beneficial to build a low-threshold nano-laser and ultrasensitive molecular spectroscopy system.