Photoelectron Bands Research Articles

Probing Photoionization Dynamics in Acetylene with Angle-Resolved Attosecond Interferometry.

Photoionization of acetylene by extreme ultraviolet light results in a stand-alone contribution from the outermost valence orbital, followed by well-separated photoelectron bands from deeper molecular orbitals. This makes acetylene an ideal candidate for probing the photoionization dynamics in polyatomic molecules free from the spectral congestion often arising after interaction with an attosecond pulse train. Here, using an angle-resolved attosecond interferometric technique, we extract the photoionization time delays for the outermost valence orbital in acetylene relative to an atomic target, namely argon. Compared to argon, the photoemission from the acetylene molecule is found to be advanced by almost 28 attoseconds. The strong variation of the relative photoionization time delays as a function of the photoemission angle was interpreted using an analytical model based on semiclassical approximations to be the interplay between different short-range potentials along and perpendicular to the molecular axis. Our results highlight the importance of using attosecond time-resolved measurements to probe the nonspherical nature of the molecular potential, even in the case of relatively small, linear systems.

Precise determination of adiabatic ionisation energies of large polyatomic molecules: p-diaminobenzene

We present a procedure, based on established principles, to accurately determine the ionisation energies of medium-sized to large polyatomic molecules in supersonic beams. The method combines single-photon excitation from the electronic ground state using tunable vacuum-ultraviolet laser radiation, the delayed pulsed-field ionisation of high Rydberg states with sequences of pulsed electric fields, and the modelling of the lineshapes from calculations of the field-ionisation dynamics of high Rydberg states and of the rotational contours of the photoelectron bands. The method is illustrated by a measurement of the adiabatic ionisation energy of p-diaminobenzene (p-DB), for which we obtain the value of 54,617.47(15) cm , which represents a more than 20-fold accuracy improvement over previous determinations.

Construction of S-scheme heterostructured TiO2/g-C3N4 composite microspheres with sunlight-driven photocatalytic activity

In recent years, the emerging S-scheme heterojunction photocatalyst has embodied great potential in promoting the research and application of photocatalytic technology. Herein, a new TiO2/g-C3N4 S-scheme heterojunction photocatalyst (TCN-20) was constructed by decorating graphite-like carbon nitride on sea urchin-like TiO2 microspheres with a large specific surface area and a mesoporous structure. TCN-20 has excellent optical absorption ability, effective photogenerated electron-hole pair separation and very high photocatalytic degradation activity in the simulated sunlight. The degradation rate of TCN-20 for Rhodamine B reaches 96.5 %, and the reaction rate constant is 4.98 and 4.52 times higher than that of pure TiO2 and g-C3N4, respectively. Moreover, X-ray photoelectron spectra, Fermi level and band structure analyses as well as radical quenching experiments authenticate the S-scheme charge transfer mechanism of TCN-20. TCN-20 is an easy-to-prepare, non-toxic, inexpensive and promising S-scheme heterojunction photocatalyst with strong redox ability.

Unraveling the Photoelectron Spectrum of 1-Phospha-2,3,4-triazolate Anion, HCPN3–, A Theoretical Approach

The first five low-lying electronic states of HCPN3 are probed through extensive ab initio electronic structure and quantum dynamics studies to reproduce the 193 nm photoelectron spectrum. Vibronic Hamiltonian is constructed and availed for time-dependent (TD) and time-independent (TI) quantum dynamical studies. The presence of numerous conical intersections (CIs) and crossings among electronic states yielded interesting nonadiabatic effects in the photoelectron bands of the overall spectrum. Moreover, the theoretical bands corresponding to five electronic states have reproduced all three experimental spectral bands. Among these, the first two bands originated due to a combination of four electronic states as predicted by previous studies. The third band corresponds to the fifth electronic state. The results calculated via TD and TI approaches exhibited satisfying agreement with the experimental results.

Synthesis and Electronic Characterization of Weyl Semimetal TaSb2Polycrystalline Material

TaSb2, a candidate material for thermoelectric conversion, is studied to establish a method for the synthesis of polycrystalline bulk materials with a diameter greater than 10 mm. Polycrystalline TaSb2materials with relative densities exceeding 97% are successfully prepared by combining the solid‐state reaction method with the spark plasma‐sintering process. The prepared TaSb2sample is identified to be a semimetal with a pseudogap‐like electronic structure near the Fermi level using hard X‐ray photoelectron spectroscopy and band structure calculation.

Step-doped disulfide vacancies and functional groups synergistically enhance photocatalytic activity of S-scheme Cu3SnS4/L-BiOBr towards ciprofloxacin degradation

Development of efficient photocatalysts for efficient recalcitrant organic pollutants degradation is of great significance. Herein, the step-doped disulfide vacancies S-scheme Cu3SnS4/L-BiOBr (CTS/L-BiOBr) heterojunction photocatalyst was prepared for ciprofloxacin (CIP) degradation. X-ray photoelectron spectroscopy (XPS) analysis, ultraviolet photo-electron spectroscopy (UPS) analysis, band structure and dominant radicals’ identification together verified that the transfer of photogenerated carriers conformed to the S-scheme mechanism. Benefited from the interfacial electric field (IEF) of the S-scheme heterojunction and incorporation of L-cysteine with introducing S-vacancies and surface functional groups (-NH2, -COO-), photogenerated charges generation and separation of the CTS/L-BiOBr(10) were greatly improved. With ·OH and h+ as dominant reactive species, CIP removal reached 93% using CTS/L-BiOBr(10) within 180 min of visible light irradiation, which was 3.5 times and 2.6 times of pristine Cu3SnS4 and L-BiOBr, respectively. Moreover, possible CIP degradation pathways were proposed and the degradation intermediates ecotoxicity were evaluated. This study could provide reference for designing efficient S-scheme photocatalysts for recalcitrant wastewater treatment.

Low-energy electron distributions from the photoionization of liquid water: a sensitive test of electron mean free paths.

The availability of accurate mean free paths for slow electrons (<50 eV) in water is central to the understanding of many electron-driven processes in aqueous solutions, but their determination poses major challenges to experiment and theory alike. Here, we describe a joint experimental and theoretical study demonstrating a novel approach for testing, and, in the future, refining such mean free paths. We report the development of Monte-Carlo electron-trajectory simulations including elastic and inelastic electron scattering, as well as energy loss and secondary-electron production to predict complete photoelectron spectra of liquid water. These simulations are compared to a new set of photoelectron spectra of a liquid-water microjet recorded over a broad range of photon energies in the extreme ultraviolet (20–57 eV). Several previously published sets of scattering parameters are investigated, providing direct and intuitive insights on how they influence the shape of the low-energy electron spectra. A pronounced sensitivity to the escape barrier is also demonstrated. These simulations considerably advance our understanding of the origin of the prominent low-energy electron distributions in photoelectron spectra of liquid water and clarify the influence of scattering parameters and the escape barrier on their shape. They moreover describe the reshaping and displacement of low-energy photoelectron bands caused by vibrationally inelastic scattering. Our work provides a quantitative basis for the interpretation of the complete photoelectron spectra of liquids and opens the path to fully predictive simulations of low-energy scattering in liquid water.

Exploration of Gas-Liquid Interfaces for Liquid Water and Methanol Using Extreme Ultraviolet Laser Photoemission Spectroscopy.

We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueous and methanol solutions using a 10 kHz EUV light source based on high-order harmonic generation and a magnetic bottle time-of-flight electron spectrometer. Two aspects of the observed spectra are highlighted in this study. One is variation of the vertical ionization energy (VIE) for liquids as a function of the solute concentration, which is closely related to surface dipoles at the gas-liquid interface. The experimental results show that the VIE of liquid water increases slightly with increasing concentrations of NaCl and NaI and decreases with NaOH. The VIE of liquid methanol was also found to change slightly with NaI. On the other hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to the formation of an electric double layer (EDL) by segregated solutes at the gas-liquid interface. As evidence for this, when the pH of an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into the bulk to suppress the influence of the EDL. We computed the surface potentials for these solutions using molecular dynamics simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of the relative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure and constituents of the first solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra at high concentration, one of the most prominent of which is the degree of splitting of the 3a1 band for liquid water and the 7a' band for liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a1 splitting decreases with the increasing concentration of NaI, NaCl, and NaOH, indicating that Na+ penetrates into the hydrogen-bonding network to coordinate to a nonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a1 splitting. Full interpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study. However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacial and solution chemistry.

Photo-Detectivity Modulation in Complementary Light-to-Frequency Conversion Circuits via Oxygen Vacancy Controlled Amorphous Indium–Gallium–Zinc Oxide Films

Herein, light-to-frequency conversion circuits (LFCs) using n-type amorphous indium-gallium-zinc oxide thin film transistors (a-IGZO TFTs) and p-type single-walled carbon nanotube (SWNT) TFTs are proposed as circuit level photodetector. The photoresponse of a-IGZO TFTs under visible spectrum is adjusted with control of annealing temperature condition, which is closely related with oxygen vacancy level, validated by X-ray photoelectron spectroscopy (XPS) and band diagrams in the TFTs. With optimized annealing temperature for a-IGZO TFTs, LFCs are successfully demonstrated by photosensitive complementary inverters and their 3-stage ring oscillators with power and wavelength dependency of light. This result is expected to be applicable to the newly conceived internet-of-things (IoT) sensor systems including interactive displays.

Vibronic interaction in trans-dichloroethene studied by vibration- and angle-resolved photoelectron spectroscopy using 19-90 eV photon energy.

Valence photoelectron spectra and photoelectron angular distributions of trans-dichloroethene have been measured with vibrational resolution at photon energies between 19 eV and 90 eV. Calculations of photoelectron anisotropy parameters, β, and harmonic vibrational modes help provide initial insight into the molecular structure. The photon energy range encompasses the expected position of the atomic Cl 3p Cooper minimum. A corresponding dip observed here in the anisotropy of certain photoelectron bands permits the identification and characterization of those molecular orbitals that retain a localized atomic Cl character. The adiabatic approximation holds for the X2Au state photoelectron band, but vibronic coupling was inferred within the A-B-C and the D-E states by noting various failures of the Franck-Condon model, including vibrationally dependent β-parameters. This is further explored using the linear vibronic coupling model with interaction parameters obtained from ab initio calculations. The A/B photoelectron band is appreciably affected by vibronic coupling, owing to the low-lying conical intersection of the A2Ag and B2Bu states. The C2Bg band is also affected, but to a lesser extent. The adiabatic minima of the D2Au and E2Ag states are almost degenerate, and the vibronic interaction between these states is considerable. The potential energy surface of the D2Au state is predicted to have a double-minimum shape with respect to the au deformations of the molecular structure. The irregular vibrational structure of the resulting single photoelectron band reflects the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces above the energy of their conical intersection.

Vibronic coupling in the Pyridine Radical Cation: Nuclear Dynamics Studied Using the Multi-configuration Time-Dependent Hartree method

The multi-configuration time-dependent Hartree (MCTDH) method was applied to study nuclear dynamics following transitions to a manifold of vibronically coupled ground 2A1 and excited 2A2 and 2B1 states of the pyridine radical cation (PRC). These states originate from ionization out of the highest occupied orbitals of pyridine, 7a1 (nσ), 1a2 (π), and 2b1 (π), respectively, and give rise to the lowest two photoelectron bands. We focus on various theoretical and computational aspects of the MCTDH method and methodology to calculate the spectrum, taking our study of the vibronically interacting 2A1, 2A2, and 2B1 states of PRC as an example. In particular, the choice of the single-particle functions (SPFs) and schemes to combine vibrational modes are discussed.

Pressure induced superconducting state in ideal topological insulator BiSbTe3

Structural, pressure-dependent resistivity, angle resolved photoemission spectroscopy (ARPES), x-ray photoelectron diffraction (XPD) and band structure by DFT calculation have been investigated for BiSbTe3 Topological insulator. It has been demonstrated that the Dirac point of the topological surface state (TSS) located exactly at the Fermi level. Additionally, superconductivity emerges under pressure of 8 GPa with a critical temperature of ∼2.5 K. With further increase of pressure, the superconducting transition temperature (Tc) increases and at 14 GPa it shows the maximum Tc (∼3.3 K). It has also been shown that the surface state remains unchanged under pressure and has been suggested that the origin of the superconductivity is due to the bulk state. The investigation indicates that the BiSbTe3 has robust surface states and becomes superconductor under pressure.

Time resolved detection of the S(1D) product of the UV induced dissociation of CS2.

The products formed following the photodissociation of UV (200 nm) excited CS2 are monitored in a time resolved photoelectron spectroscopy experiment using femtosecond XUV (21.5 eV) photons. By spectrally resolving the electrons, we identify separate photoelectron bands related to the CS2 + hν → S(1D) + CS and CS2 + hν → S(3P) + CS dissociation channels, which show different appearance and rise times. The measurements show that there is no delay in the appearance of the S(1D) product contrary to the results of Horio et al. [J. Chem. Phys. 147, 013932 (2017)]. Analysis of the photoelectron yield associated with the atomic products allows us to obtain a S(3P)/S(1D) branching ratio and the rate constants associated with dissociation and intersystem crossing rather than the effective lifetime observed through the measurement of excited state populations alone.

THE PHOTOELECTRON SPECTROSCOPY OF THE DICHLOROETHYLENES: THE VICINAL ISOMER trans-1,2-HClC=CHCl. QUANTUM CHEMICAL CALCULATIONS AND EXPERIMENT

The photoelectron spectrum (PES) of trans-1,2-C2H2Cl2 has been measured with the resonance lines in He, Ne and Ar. These spectra are compared to the threshold photoelectron spectrum (TPES) recorded using synchrotron radiation. Nine mostly well separated photoelectron bands are observed at adiabatic ionization energies of 9.633 eV, 11.840 eV, 12.044 eV, 12.582 eV, 13.581 eV, 14.081 eV, 15.724 eV and at about 16.325 eV and 19.00 eV. Most of them exhibit a well resolved and extended vibrational structure. The He-I photoelectron band associated with the ground electronic state of the ion and the corresponding TPES band are characterized by a vibrational structure extending over 0.7 eV and 2.0 eV respectively. In the latter case the strong correlation between the TPES and the photoabsorption spectrum in the same energy region has been established. Several excited states also show an extended vibrational structure. The assignments of the electronic bands and of the vibrational wavenumbers were supported by quantum chemical calculations. The good correlation between predicted vibrational wavenumbers and the experimental values allowed us to assign most of the vibrational structures. The constant ion state (CIS) spectra have been recorded and discussed for selected vibrational levels of the ground state and for the adiabatic transition to the first excited state of the ion.

An experimental and theoretical study of the valence shell photoelectron spectrum of oxalyl chloride

Abstract Polarization dependent photoelectron spectra encompassing the outer valence orbitals of oxalyl chloride have been recorded in the photon energy range 19–91 eV. These have allowed photoelectron anisotropy parameters and branching ratios to be determined. Photoionization partial cross sections and photoelectron anisotropy parameters have been calculated with the Continuum Multiple Scattering – Xα approach. Four of the outer valence orbitals are predicted to possess a significant Cl 3p lone-pair character and have closely grouped binding energies. The photoionization dynamics of these four orbitals are predicted to be strongly affected by the Cooper minimum associated with the Cl 3p orbital in the isolated atom at photon energies around 40 eV. A comparison between the theoretical and measured photoelectron anisotropy parameters has enabled the molecular orbital sequence to be clarified. A doublet has been observed in the region of the photoelectron spectrum where a band due to the 5bu orbital might be anticipated. Our calculations indicate that the 6bu and 5bu orbitals are coupled. This coupling may account for the apparent lack of a pronounced Cooper minimum in the β-parameter associated with the nominal 6bu ionization and for the unexpected appearance of the adjacent photoelectron band, nominally associated with the 5bu orbital. The vertical ionization energy of the outermost 7ag orbital was experimentally determined to be 11.266 ± 0.005 eV.

Energetic degeneracy and electronic structures of germanium trimers doped with titanium.

Geometries and electronic structures of germanium trimer clusters doped with titanium TiGe3 -/0 were studied making use of the complete active space self-consistent field followed by second-order perturbation theory explicitly correlated coupled cluster singles and doubles method with perturbative triples corrections CCSD(T)-F12 and Tao-Perdew-Staroverov-Scuseria methods. Two electronic states (2A' and 2A″) of the anion (pyramid shape) were determined to be nearly degenerate and energetically competing for the anionic ground state of TiGe3 -. These two anionic states are believed to be concurrently populated in the experiment and induce six observed anion photoelectron bands. Total 14 electronic transitions starting from the 2A' and 2A″ states were assigned to five out of six visible bands in the experimental anion photoelectron spectrum of TiGe3 -. Each band was proven to be caused by multiple one-electron detachments from two populated anionic states. The last experimental band with the highest detachment energy is believed to be the result of various inner one-electron removals.

Decoupling vibration and electron energy dependencies in the photoelectron circular dichroism of a terpene, 3-carene.

A fresh perspective on the interaction of electron and nuclear motions in photon induced dynamical processes can be provided by the coupling of photoelectron angular distributions and cation vibrational states in the photoionization of chiral molecules using circularly polarized radiation. The chiral contributions, manifesting as a forward-backward asymmetry in the photoemission, can be assessed using Photoelectron Circular Dichroism (PECD), which has revealed an enhanced vibrational influence exerted on the outgoing photoelectron. In this paper, we investigate the PECD of a rigid chiral monoterpene, 3-carene, using single-photon vacuum ultraviolet ionization by polarized synchrotron radiation and selecting energies from the ionization threshold up to 19.0 eV. By judicious choice of these photon energies, two factors that influence PECD asymmetry values, electron kinetic energy and ion vibrational level, can be effectively isolated, allowing a clear demonstration of the very marked vibrational effects. A slow photoelectron spectrum is used to examine the vibrational structure of the isolated outermost valence (HOMO) photoelectron band, and peak assignments are made with the aid of a Franck-Condon simulation. Together, these provide an estimate of the adiabatic ionization energy as 8.385 eV. The reported chiral asymmetry from the randomly oriented 3-carene enantiomers reaches a maximum of over 21%. Theoretical PECD calculations, made both for the fixed equilibrium molecular geometry and also modeling selected normal mode vibration effects, are presented to provide further insight.

Photoionization of the I 4d and valence orbitals of methyl iodide

The photoabsorption and photoionization dynamics of the I 4d and valence orbitals in methyl iodide have been studied both experimentally and theoretically. Synchrotron radiation has been employed to measure the total ion yield in the vicinity of the I 4d ionization thresholds. The observed structure, due to excitations into Rydberg or valence states, has been assigned using transition energies and relative intensities computed with time-dependent density functional theory within the Tamm–Dancoff approximation. Photoelectron spectra, recorded with plane polarized radiation in two polarization geometries, have allowed the effect of autoionization on the valence electron angular distributions to be investigated. The spectra obtained at photon energies of 50.62 and 52.34 eV, coinciding respectively with the I 4d5/2 → σ* and 4d3/2 → σ* transitions, reveal, in addition to valence shell photoelectron bands, features not associated with simple photoionization of the parent molecule. High resolution photoelectron spectra of the I 4d main-lines display structure resulting from spin–orbit coupling and molecular field splitting. The binding energies of the five states contributing to the (I 4d)−1 ionization have been determined. The iodine (in CH3I) N45VV Auger spectrum has been measured and the observed structure has been assigned using the core hole binding energies derived in the present work together with established ionization energies of the doubly charged ion. The experimentally determined Auger electron angular distributions have been discussed in relation to the theoretical angular distribution parameter characterizing the spatial alignment of molecular axes in the (I 4d)−1 state.

Electronic structure and characterization of the spectra of trans/cis tautomers of urocanic acid isomers: A diagnostic tool

The gas-phase information of ionization on the fundamental properties of biological molecules is essential for understanding numerous biological phenomena, as it provides insight into the physicochemical origin of molecular properties. In this work, conformational and tautomerism analysis of urocanic acid (UA) were performed by two methods of B3LYP and M062X at 6-311++g (d,p) basis set for all structures in two isomers of cis/trans in the gas phase. Then, stable structures were selected to calculate the ionization potential. The symmetry-adapted-cluster configuration interaction (SAC-CI General-R) method was used for calculating ionization energies and the photoelectron spectra. The NBO calculations were performed to determine the contribution of different natural bonding orbitals to the canonical molecular orbitals involved in ionization processes, which was required for spectral band assignment. The results of NBO calculation indicated that the ionization bands corresponding to ionizations from orbitals localized the imidazole ring had a larger contribution to the ionization process than the CC and CO bands. The most important point about the ionization of all tautomers is the removal of an electron from the imidazole ring. Koopmans’ approximation is only applicable for describing the first lowest-energy orbital ionization (photoelectron band), and others may make a linear combination of HF determinations.

On the nickel magnetic behaviour in rare-earths compounds

The x-ray photoelectron spectroscopy (XPS) study, magnetic measurements and band structure calculations were performed on RNi2, RNi3, RNi5 and R2Ni17 compounds, where R is a rare-earth or yttrium. The presence of small nickel moments was shown in almost all compounds with magnetic rare-earths. The nickel moments are dependent on the site location, their local environment, respectively. The R5d bands are negatively polarized, their values M5d, being determined by additive contributions resulting from local 4f-5d and short-range 5d-3d interactions. The R5d band polarizations mediate the exchange interactions within the spatial extension of the unit cell. The Curie temperatures for a given series are linearly dependent on M5d values with the same rate for both heavy and light rare-earth compounds.