High-resolution Photoelectron Research Articles

Odd electron wave packets from cycloidal ultrashort laser fields

Polarization-tailored bichromatic femtosecond laser fields with cycloidal polarization profiles have emerged as a powerful tool for coherent control of quantum processes. We present an optical scheme to create and manipulate three-dimensional free electron wave packets with arbitrary rotational symmetry by combining advanced supercontinuum pulse shaping with high resolution photoelectron tomography. Here we use carrier-envelope phase-stable polarization-tailored bichromatic (3ω:4ω) counter- and corotating femtosecond laser pulses to generate 7-fold rotational symmetric and asymmetric photoelectron momentum distributions by multiphoton ionization of sodium atoms. To elucidate the physical mechanisms, we investigate the interplay between the symmetry properties of the driving field and the resulting electron wave packets by varying the optical field parameters. Our results show that the symmetry properties of electron wave packets are not fully determined by the field symmetry, but completely described by multipath quantum interference of states with different angular momenta.

Интеркалирование графена на карбиде кремния кобальтом

AbstractIn this paper, we studied cobalt intercalation of single-layer graphene grown on the 4 H -SiC(0001) polytype. The experiments were carried out in situ under ultrahigh vacuum conditions by high energy resolution photoelectron spectroscopy using synchrotron radiation and low energy electron diffraction. The nominal thicknesses of the deposited cobalt layers varied in the range of 0.2–5 nm, while the sample temperature was varied from room temperature to 800°C. Unlike Fe films, the annealing of Co films deposited on graphene at room temperature is shown to not intercalate graphene by cobalt. The formation of the graphene–cobalt–SiC intercalation system was detected upon deposition of Co atoms on samples heated to temperatures of above ~400°C. Cobalt films with a thickness up to 2 nm under graphene are formed using this method, and they are shown to be magnetized along the surface at thicknesses of greater than 1.3 nm. Graphene intercalation by cobalt was found to be accompanied by the chemical interaction of Co atoms with silicon carbide leading to the synthesis of cobalt silicides. At temperatures of above 500°C, the growth of cobalt films under graphene is limited by the diffusion of Co atoms into the bulk of silicon carbide.

Photoelectron spectra of Al2O2- and Al3O3-via slow electron velocity-map imaging.

High-resolution photoelectron spectra of cryogenically-cooled Al2O2- and Al3O3- cluster anions are obtained using slow electron velocity-map imaging. These spectra show vibrationally-resolved detachment from the (X[combining tilde]2B3u) ground state of Al2O2- to the X[combining tilde]1Ag and ã3B3u neutral electronic states, giving an electron affinity of 1.87904(4) eV for neutral Al2O2 and a term energy of 0.4938(4) eV for the triplet excited state. Additionally, there is evidence for autodetachment from photoexcited anions as well as influences from vibronic coupling between excited states of the neutral Al2O2 cluster. Detachment from both the "kite" and "book" isomers of Al3O3- is observed, yielding electron affinities of 2.0626(4) and 2.792(3) eV for the corresponding neutral isomers. Experiments carried out at different anion temperatures suggest that the two anionic isomers are nearly isoenergetic but clearly identify the kite isomer as the global minimum structure, in contrast to prior studies. This finding is supported by density functional theory calculations, which show that the relative ordering of the anion isomers is sensitive to basis set size; calculations for the anion isomers at the B3LYP/aug-cc-pVQZ level find the kite isomer to lie 0.011 eV below the book isomer.

High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions.

Valence-bound anions with polar neutral cores (μ > ∼2.5 D) can support dipole-bound excited states below the detachment threshold. These dipole-bound states (DBSs) are highly diffuse and the weakly bound electron in the DBS can be readily autodetached via vibronic coupling. Excited DBSs can be observed in photodetachment spectroscopy using a tunable laser. Tuning the detachment laser to above-threshold vibrational resonances yields vibrationally enhanced resonant photoelectron spectra, which are highly non-Franck-Condon with much richer vibrational information. This perspective describes recent advances in the studies of excited DBSs of cryogenically cooled anions using high-resolution photoelectron imaging (PEI) and resonant photoelectron spectroscopy (rPES). The basic features of dipole-bound excited states and highly non-Franck-Condon resonant photoelectron spectra will be discussed. The power of rPES to yield rich vibrational information beyond conventional PES will be highlighted, especially for low-frequency and Franck-Condon-inactive vibrational modes, which are otherwise not accessible from non-resonant conventional PES. Mode-selectivity and intra-molecular rescattering have been observed during the vibrationally induced autodetachment. Conformer-specific rPES is possible due to the different dipole-bound excited states of molecular conformers with polar neutral cores. For molecules with μ ≪ 2.5 D or without dipole moments, but large quadrupole moments, excited quadrupole-bound states can exist, which can also be used to conduct rPES.

Layer-by-Layer Graphene Growth on β-SiC/Si(001).

The mechanism of few-layer graphene growth on the technologically relevant cubic-SiC/Si(001) substrate is uncovered using high-resolution core-level and angle-resolved photoelectron spectroscopy, low-energy electron microscopy, and microspot low-energy electron diffraction. The thickness of the graphitic overlayer supported on the silicon carbide substrate and related changes in the surface structure are precisely controlled by monitoring the progress of the surface graphitization in situ during high-temperature graphene synthesis, using a combination of microspectroscopic techniques. The experimental data reveal gradual changes in the preferential graphene lattice orientations at the initial stages of the few-layer graphene growth on SiC(001) and can act as reference data for controllable growth of single-, double-, and triple-layer graphene on silicon carbide substrates.

Slow photoelectron velocity-map imaging of cold and .

High-resolution anion photoelectron spectra of cryogenically cooled C7 - and C9 - clusters obtained using slow photoelectron velocity-map imaging are presented, providing insight into the vibronic structure of neutral C7 and C9. These spectra yield accurate measurements of vibrational frequencies for the neutral clusters as well as electron affinities of 3.3517(4) and 3.6766(14) eV for C7 and C9, respectively. In the C7 - spectrum, transitions involving the previously unreported v 1 and v 2 symmetric stretching modes, as well as the v 9, v 10, and v 11 asymmetric bending modes, are assigned. Spin-orbit splitting is observed for several transitions in this spectrum, giving an energy difference of 28(6) cm-1 between the and spin-orbit levels of the C7 - anion. In the spectrum of C9 -, transitions involving the previously unreported symmetric stretch v 1 and the asymmetric bend v 11 are observed. In both spectra, several features are assigned to Franck-Condon forbidden transitions involving the doubly degenerate v 10 and v 11 modes of C7 and the v 13 and v 14 modes of C9. The appearance of these transitions is attributed to Herzberg-Teller coupling between the electronic states of the neutral clusters. Additional FC-forbidden transitions to states previously observed in gas-phase infrared experiments are observed and attributed to vibronic coupling between the electronic states of the anion, resulting in non-totally symmetric character in the anion's full vibrational ground state. Finally, consideration of the energy dependence of detachment cross sections and Dyson orbital analyses reveal that addition of more carbon atoms to the linear chain results in photodetachment from delocalized molecular orbitals with increasing nodal structure, leading to threshold photodetachment cross sections that differ considerably from simple symmetry considerations.

PFI-ZEKE photoelectron spectroscopy of positively charged ions: illustration with Mg+

Pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy has been applied for the first time to a positively charged ion, Mg+. Mg+ ions have been prepared in the [Ne]3s S21/2 ground state by photoionization of Mg atoms in a supersonic expansion. The ions were then excited to very high Rydberg states located just below the ionization threshold corresponding to the formation of ground-state Mg2+ by (2 + 1′) resonance-enhanced three-photon absorption via the 3d D25/2 intermediate state. We demonstrate experimentally and in a calculation of the transition moments that only the nf Rydberg series of Mg+ is efficiently excited from the D25/2 state and extract from the Rydberg spectrum precise values for the ionization energy of Mg+ (121267.65 ± 0.06 cm−1) and the quantum defect of the nf series (0.0044(6)). The field ionization of very high Rydberg states of Mg+ was investigated using sequences of electric field pulses. Compared to the pulsed field ionization of neutral atoms or molecules, the pulsed field ionization of positively charged ions differs in three respects: (i) the shift of the ionization thresholds induced by an electric field of given strength is larger by a factor of approximately 2, (ii) the inherent presence of ions in the photoexcitation volume prevents the escape of slow electrons, and (iii) the inhomogeneous stray fields induced by these ions and their effects on the high Rydberg states reduce the selectivity of the field-ionization process and degrade the resolution of the PFI-ZEKE photoelectron spectra. By minimizing the ion density, optimizing the photoexcitation efficiency and using multipulse field-ionization sequences, we show that the resolution of the photoelectron spectra can be as high as 0.3 cm−1, which opens up the possibility to study doubly-charged cations by high-resolution photoelectron spectroscopy.

Probing the structures and bonding of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging.

We report an investigation of a series of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging and ab initio calculations. Vibrationally resolved photoelectron spectra are obtained, allowing the electron affinities of Au-(C≡C) n -Au to be accurately measured as 1.651(1), 1.715(1), and 1.873(1) eV for n = 1-3, respectively. Both the Au-C symmetric stretching and a bending vibrational frequency are observed for each neutral auropolyyne. Theoretical calculations find that the ground state of Au2C2 - has a linear acetylenic Au-C≡C-Au- structure, whereas the asymmetric Au-Au-C≡C- structure is a low-lying isomer. However, for Au2C4 - and Au2C6 -, our calculations show that the asymmetric Au-Au-(C≡C) n - isomers are the global minima and the Au-(C≡C) n -Au- symmetric structures become low-lying isomers. All the asymmetric Au-Au-(C≡C) n - isomers are found computationally to have much higher electron binding energies and are not accessible at the detachment photon energies used in the current study. For neutral Au2C2n , the Au-(C≡C) n -Au auropolyyne structures are found to be the global minima for n = 1-3. The electronic structures and bonding for Au-(C≡C) n -Au (n = 1-3) are compared with the corresponding Au-(C≡C) n and Au-(C≡C) n -H species.

Double-core-hole states in CH3CN: Pre-edge structures and chemical-shift contributions.

Spectra reflecting the formation of single-site double-core-hole pre-edge states involving the N 1s and C 1s core levels of acetonitrile have been recorded by means of high-resolution single-channel photoelectron spectroscopy using hard X-ray excitation. The data are interpreted with the aid of ab initio quantum chemical calculations, which take into account the direct or conjugate nature of this type of electronic states. Furthermore, the photoelectron spectra of N 1s and C 1s singly core-ionized states have been measured. From these spectra, the chemical shift between the two C 1s-1 states is estimated. Finally, by utilizing C 1s single and double core-ionization potentials, initial and final state effects for the two inequivalent carbon atoms have been investigated.

Accurate electron affinity of Ti and fine structures of its anions.

The high-resolution photoelectron energy spectra of atomic titanium and its hydride anions were obtained on a slow-electron velocity-map imaging spectrometer equipped with a cold ion trap. The cold ion trap employed in the present measurement was found to be very helpful for reducing the interference from the titanium hydride anions. The electron affinity of Ti was determined to be 609.29(34) cm-1 or 75.54(4) meV. The accuracy was improved by a factor of 350 compared with the previous result. The fine structures of Ti- were clearly resolved: 70.0(12)(4F5/2), 165.2(15)(4F7/2), and 285.2(15) cm-1 (4F9/2) above its ground state 4F3/2. Moreover, the measured electron affinity and vibrational frequency of TiH can be reproduced well using the high level calculations.

Beyond Chemical Bonding Interaction: An Insight into the Growth Process of 1D ZnO on Few‐Layer Graphene for Excellent Photocatalytic and Room Temperature Gas Sensing Applications

AbstractA facile chemical synthesis approach is reported for simultaneous reduction of graphene oxide (GO) and chemically planted 1D ZnO Nanorods (NRs) on the few‐layer graphene (FLG) through chemical bonding interaction. The chemically bonded 1D ZnO – few layer graphene nanostructures are investigated by high resolution scanning transmission electron microscopy, photoelectron spectroscopy, X‐ray diffraction and photoluminescence studies. This multifunctional nanostructure efficiently boosts the photocatalytic activity by photodegradation of industrial pollutants within 5 minutes under natural sunlight irradiation and also senses the very low concentration (10 ppm) of hydrogen gas at room temperature with very high sensing response. The emerging photocatalytic performance and the room temperature gas sensing characteristics of 1D ZnO – few layer graphene is attributed to the presence of C–Zn chemical bonds, which plays an essential role in the interfacial charge transfer process between the graphene layers and 1D ZnO NRs. The strong chemical bonding interaction and interfacial charge transfer effects have been experimentally elucidated from the ionization value and work function characteristics of the synthesized hybrid nanostructure. This work demonstrates the simple way of preparing hybrid nanostructure with exposed crystal facets, which could provide an exposure for developing multifunctional nanomaterials towards sensing and removal of hazardous pollutants from our environment.

High-resolution photoelectron spectrum of the origin band of the X̃+ 2E ← X̃ 1A1 ionising transition of propyne

ABSTRACTThe pulsed-field-ionisation zero-kinetic-energy photoelectron spectrum of the origin band of the ionising transition of propyne has been recorded at a resolution sufficient to partially resolve the rotational structure. The rovibronic photoionisation selection rules are derived in the spin double group C3v(MS)2 and are used to model the experimental spectrum and determine the adiabatic ionisation energy of propyne and the spin-orbit coupling constant of the radical cation of propyne. The Jahn–Teller effect is found to significantly affect the spin-orbit splitting of the cationic ground state.

Observation of topological superconductivity on the surface of an iron-based superconductor.

Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe1-x Se x (x = 0.45; superconducting transition temperature Tc = 14.5 kelvin) hosts Dirac-cone-type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below Tc Our study shows that the surface states of FeTe0.55Se0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.

Peculiar bonding associated with atomic doping and hidden honeycombs in borophene

Engineering atomic-scale structures allows great manipulation of physical properties and chemical processes for advanced technology. We show that the B atoms deployed at the centers of honeycombs in boron sheets, borophene, behave as nearly perfect electron donors for filling the graphitic $\sigma$ bonding states without forming additional in-plane bonds by first-principles calculations. The dilute electron density distribution owing to the weak bonding surrounding the center atoms provides easier atomic-scale engineering and is highly tunable via in-plane strain, promising for practical applications, such as modulating the extraordinarily high thermal conductance that exceeds the reported value in graphene. The hidden honeycomb bonding structure suggests an unusual energy sequence of core electrons that has been verified by our high-resolution core-level photoelectron spectroscopy measurements. With the experimental and theoretical evidence, we demonstrate that borophene exhibits a peculiar bonding structure and is distinctive among two-dimensional materials.

Autodetachment from Vibrationally Excited Vinylidene Anions.

Slow electron velocity-map imaging of the cryogenically cooled H2CC¯ anion reveals a strong dependence of its high-resolution photoelectron spectrum on detachment photon energy in two specific ranges, from 4000 to 4125 cm-1 and near 5020 cm-1. This effect is attributed to vibrational excitation of the anion followed by autodetachment to H2CC + e¯. In the lower energy range, the electron kinetic energy (eKE) distributions are dominated by two features that occur at constant eKEs of 114(3) and 151.9(14) cm-1 rather than constant electron binding energies, as is typically seen for direct photodetachment. These features are attributed to ΔJ = ΔK = 0 autodetachment transitions from two vibrationally excited anion states. The higher energy resonance autodetaches to neutral eigenstates with amplitude in the theoretically predicted shallow well lying along the vinylidene-acetylene isomerization coordinate. Calculations provide assignments of all autodetaching anion states and show that the observed autodetachment is facilitated by an intersection of the anion and neutral surfaces.

Refined analysis of the X̃ 2A2←X̃ 1A1 photoelectron spectrum of furan.

The X̃ 2A2←X̃ 1A1 photoelectron spectrum of furan has been studied by a time-independent eigenstate-free Raman wave function approach based on multi-dimensional potential energy surfaces obtained from explicitly correlated distinguishable clusters calculations. Individual vibronic transitions with the most significant Franck-Condon factors were determined by our recently developed residual-based algorithm for the calculation of eigenpairs in conjunction with the formalism of contracted invariant Krylov subspaces. The account of anharmonic and temperature effects allowed us to explain most bands in an experimental high-resolution zero kinetic energy photoelectron spectrum. This led to the reassignment of many spectral features, as well as a refined interpretation of the intensity mechanism for the corresponding transitions.

A high-resolution photoelectron imaging and theoretical study of CP- and C2P.

The discovery of interstellar anions has been a milestone in astrochemistry. In the search for new interstellar anions, CP- and C2P- are viable candidates since their corresponding neutrals have already been detected astronomically. However, scarce data exist for these negatively charged species. Here we report the electron affinities of CP and C2P along with the vibrational frequencies of their anions using high-resolution photoelectron imaging. These results along with previous spectroscopic data of the neutral species are used further to benchmark very accurate quartic force field quantum chemical methods that are applied to CP, CP-, C2P, and two electronic states of C2P-. The predicted electron affinities, vibrational frequencies, and rotational constants are in excellent agreement with the experimental data. The electron affinities of CP (2.8508 ± 0.0007 eV) and C2P (2.6328 ± 0.0006 eV) are measured accurately and found to be quite high, suggesting that the CP- and C2P- anions are thermodynamically stable and possibly observable. The current study suggests that the combination of high-resolution photoelectron imaging and quantum chemistry can be used to determine accurate molecular constants for exotic radical species of astronomical interest.

Probing the interaction between the encapsulated water molecule and the fullerene cages in H2O@C60- and H2O@C59N.

We report a high-resolution photoelectron imaging study of cryogenically-cooled H2O@C60- and H2O@C59N- endohedral fullerene anions. The electron affinity (EA) of H2O@C60 is measured to be 2.6923 ± 0.0008 eV, which is 0.0088 eV higher than the EA of C60, while the EA of H2O@C59N is measured to be 3.0058 eV ± 0.0007 eV, which is 0.0092 eV lower than the EA of C59N. The opposite shifts are found to be due to the different electrostatic interactions between the encapsulated water molecule and the fullerene cages in the two systems. There is a net coulombic attraction between the guest and host in H2O@C60-, but a repulsive interaction in H2O@C59N-. We have also observed low-frequency features in the photoelectron spectra tentatively attributed to the hindered rotational excitations of the encapsulated H2O molecule, providing further insights into the guest-host interactions in H2O@C60- and H2O@C59N-.

High-Resolution Photoelectron Imaging of Cryogenically-Cooled C59N- and (C59N)22- Azafullerene Anions.

We report a photoelectron imaging study of cryogenically cooled C59N- and (C59N)22- anions produced from electrospray ionization. High-resolution photoelectron spectra are obtained for C59N- for the first time, allowing seven vibrational frequencies of the C59N azafullerene to be measured. The electron affinity of C59N is determined accurately to be 3.0150 ± 0.0007 eV. The observed vibrational features are understood on the basis of calculated frequencies and compared with those of C60 and C59HN. The photoelectron image of (C59N)22-, which has the same mass/charge ratio as C59N-, is also observed, allowing the second electron affinity of the (C59N)2 azafullerene dimer to be measured as 1.20 ± 0.05 eV. The intramolecular Coulomb repulsion of the (C59N)22- dianion is estimated to be 1.96 eV and is investigated theoretically using the electron density difference between (C59N)22- and (C59N)2.

Graphene modification via cobalt and silicon intercalation

Cobalt and silicon intercalation under graphene formed on the Co(111) surface has been investigated. The thickness of deposited films was varied within the range of up to 2 nm and the temperature of their annealing was 400-450°C. The evolution of atomic and electronic structure of the surface due to intercalation was studied by low-energy electron diffraction and high-resolution photoelectron spectroscopy with synchrotron radiation. It is shown that annealing of deposited Si layer leads to formation on the sample surface of both intercalated and nonintercalated areas. On the intercalated areas, the Co-Si solid solution is formed covered with segregated Si. The presence of a silicon phase strongly affects the electronic structure of graphene and substantially weakens its interaction with the substrate. Subsequent intercallation of Co leads to the restoration of strong coupling between graphene and the substrate.