Photoelectron Imaging Spectroscopy Research Articles

Cryolitic vein XPS imaging from an industrial grade carbon

Aluminium carbide formation and subsequent dissolution is a generally accepted mechanism to explain the cathode wear in the Hall-Héroult industry. Carbide formation is thought to occur inside the cathode pores. Being dependent upon cathodic current density, it is believed to be an electrochemical process. It is also associated with the presence of cryolitic bath species involved in the dissolution of the carbide layer and thus enabling further carbide formation.X-Ray Photoelectron Spectroscopy imaging has the capacity to illustrate both the species chemistry and the spatial localization of such species. However, spectroscopic imaging requires long acquisition time. The value of mathematical treatment becomes obvious when applying multivariate data analysis on raw data. Principal Component Analysis (PCA) can substantially reduce the collection time without reducing the analysis quality of the data.In the current work, the C 1s photoelectron region was imaged and aluminium carbide (Al4C3), aluminium oxycarbide (Al4O4C and Al2OC), and graphite (C) were spatially tracked within the first millimetres under the cathode block surface. Preliminary work on differentiating between cryolite (Na3AlF6), alumina (Al2O3), and Al4C3 in the Al 2p narrow scan regions was conducted. More generally, the XPS analysis enabled further investigation of the chemical species present in bath penetrated veins of the carbon cathode.

Resonant tunneling through the repulsive Coulomb barrier of a quadruply charged molecular anion

Multiply charged anions possess a repulsive Coulomb barrier (RCB) against electron emission, thus allowing for long-lived metastable species with negative electron binding energies. For the prototypical multianion, bisdisulizole tetra-anion, we demonstrate that electronically excited states supported by the RCB can undergo resonant tunneling. The dynamics of this process was investigated by one-photon photoelectron imaging and femtosecond pump-probe photoelectron spectroscopy and confirmed by theoretical calculations. Efficient resonant tunneling emission of electrons from the excited states of multianions may be common for systems with sufficiently large RCB. This may provide new opportunities to study electron emission dynamics in complex systems.

Probing the structural and electronic properties of AgnH− (n = 1–3) using photoelectron imaging and theoretical calculations

Structural and electronic properties of silver hydride cluster anions (Ag(n)H(-); n = 1-3) have been explored by combining the negative ion photoelectron imaging spectroscopy and theoretical calculations. The photoelectron spectrum of AgH(-) exhibits transitions from AgH(- 2)Σ(+) to AgH (1)Σ(+) and AgH (3)Σ(+), with the electron affinity (EA) 0.57(3) eV. For Ag(2)H(-), the only observed transition is from Ag(2)H(-) (C(∞v)) (1)Σ(+) to Ag(2)H (C(2v)) (2)A(') and the electron affinity is 2.56(5) eV. Two obvious electron bands are observed in photoelectron imaging of Ag(3)H(-), which are assigned to the transitions from Ag(3)H(-) (C(2v)-T, which means C(2v) geometry with top site hydrogen) (2)B(2) to Ag(3)H (C(2v)-T) (1)A(1) and Ag(3)H (C(2v)-T) (3)B(2). The electron affinity is determined to be 1.61(9) eV. The Ag-H stretching modes in the ground states of AgH and Ag(2)H are experimentally resolved and their frequencies are measured to be 1710(80) and 1650(100) cm(-1), respectively. Aside from the above EAs and the vibrational frequencies, the vertical detachment energies to all ground states and some excited states of Ag(n)H (n = 1-3) are also obtained. Theoretical calculations reproduce the experimental energies quite well, and the results are used to assign the geometries and electronic states for all related species.

Ultrafast Internal Conversion Dynamics of 2-Chloropyridine by Femtosecond Time-Resolved Photoelectron Imaging

Ultrafast internal conversion dynamics of 2-chloropyridine were studied by femtosecond time-resolved photoelectron imaging spectroscopy coupled with time-resolved mass spectroscopy. The ultrafast internal conversion from the second excited state (S-2) to the first excited state (S-1) via an adjacent conical intersection within (162 +/- 5) fs was clearly observed from the time-dependence of the photoelectron spectra. The subsequent deactivations involved the coupling of S-2/S-0 (the ground state) and S-1/S-0 conical intersections, which occurred on a timescale of about (5.5 +/- 0.3) ps, and led to the internal conversion to the ground state from the S-2 and S-1 states.

Non-Adiabatic Chemical Dynamics Studied by Ultrafast Photoelectron Imaging Spectroscopy

Non-Adiabatic Chemical Dynamics Studied by Ultrafast Photoelectron Imaging Spectroscopy

Study of ultrafast dynamics of 2-picoline by time-resolved photoelectron imaging

The dynamics of electronically excited states in 2-picoline is studied using femtosecond time-resolved photoelectron imaging spectroscopy. The internal conversion from the S(2) state to the vibrationally excited S(1) state is observed in real time. The secondarily populated high vibronic S(1) state deactivates further to the S(0) state. Photoelectron energy and angular distributions reveal the feature of ionization from the singlet 3p Rydberg states. In addition, variation of time-dependent anisotropy parameters indicates the rotational coherence of the molecule.

Photoelectron imaging spectroscopy and theoretical investigation of ZrSi

The photoelectron spectrum of ZrSi(-) has been measured at two different photon energies: 2.33 eV and 3.49 eV, providing electron binding energy and photoelectron angular distribution information. The obtained vertical detachment energy of ZrSi(-) is 1.584(14) eV. The neutral ground and excited state terms are assigned based on experimental and theoretical results. The ground state of ZrSi is tentatively assigned as a (3)Σ(+) state with a configuration of 1σ(2) 1π(4) 1δ(0) 2σ(1) 3σ(1). A low lying (3)Π(i) neutral excited state is identified to be 0.238 eV (1919 cm(-1)) above the ground state. The anion ground state is designated as a (2)Σ(+) state with a 1σ(2) 1π(4) 1δ(0) 2σ(2) 3σ(1) valence electron configuration. A Franck-Condon (FC) simulation of the photoelectron spectrum has been carried out. For the (3)Σ(+) ← (2)Σ(+) band, theoretically calculated bond lengths and frequencies are used in the FC calculation which give good agreement with experiment, while for the (3)Π(i) ← (2)Σ(+) band, the ZrSi bond length is estimated from the FC spectrum. Comparisons are made with previously published theoretical studies and inconsistencies are pointed out. To the best of our knowledge, this study provides the first spectroscopic information on the transition metal-silicon diatomic, ZrSi.

Ultrafast Dynamics of the First Excited State of Chlorobenzene

Ultrafast dynamics of the first excited state (S-1) of chlorobenzene was studied using a combination of femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. One-photon absorption at 266.7 nm was used to populate the S-1 state of chlorobenzene. The time evolution of the parent ion signals consists of different biexponential decays. One is a fast component on a timescale of (152 +/- 3) fs and the other is a slow component with a timescale of (749 +/- 21) ps. Time-resolved electron kinetic energies (eKE) and time-resolved photoelectron angular distributions (PADs) were extracted from time-resolved photoelectron imaging and are discussed in detail. The ultrafast process with a time constant of (152 +/- 3) fs is a population transfer within the S-1 state, and only a vibrational energy transfer process with strong coupling is a reasonable explanation. This is attributed to an ultrafast process of dissipative intramolecular vibrational energy redistribution (IVR). The lifetime of the S-1 state was determined to be (749 +/- 21) ps, and its deactivation was due to slow internal conversion to the ground state. Additionally, nonadiabatic alignment and rotational dephasing of the S-1 state of chlorobenzene, as a typical asymmetric top molecule, were observed. The first C-type and J-type recurrences are expected at delay time of 205.8 and 359.3 ps, respectively.

Ultrafast photodynamics of furan

Ultrafast photodynamics of furan has been studied by time-resolved photoelectron imaging (TRPEI) spectroscopy with an unprecedented time resolution of 22 fs. The simulation of the time-dependent photoelectron kinetic energy distribution (PKED) has been performed with ab initio nonadiabatic dynamics "on the fly" in the frame of time-dependent density functional theory. Based on the agreement between experimental and theoretical time-dependent photoelectron signal intensity as well as on PKED, precise time scales of ultrafast internal conversion from S(2) over S(1) to the ground state S(0) of furan have been revealed for the first time. Upon initial excitation of the S(2) state which has π-π* character, a nonadiabatic transition to the S(1) state occurs within 10 fs. Subsequent dynamics invokes the excitation of the C-O stretching and C-O-C out of plane vibrations which lead to the internal conversion to the ground state after 60 fs. Thus, we demonstrate that the TRPEI combined with high level nonadiabatic dynamics calculations provide fundamental insight into ultrafast photodynamics of chemically and biologically relevant chromophores.

Photoelectron Imaging and Spectroscopy of MI2− (M = Cs, Cu, Au): Evolution from Ionic to Covalent Bonding

We report a combined experimental and theoretical investigation of MI(2)(-) (M = Cs, Cu, Ag, Au) to explore the chemical bonding in the group IA and IB diiodide complexes. Both photoelectron imaging and low-temperature photoelectron spectroscopy are applied to MI(2)(-) (M = Cs, Cu, Au), yielding vibrationally resolved spectra for CuI(2)(-) and AuI(2)(-) and accurate electron affinities, 4.52 ± 0.02, 4.256 ± 0.010, and 4.226 ± 0.010 eV for CsI(2), CuI(2), and AuI(2), respectively. Spin-orbit coupling is found to be important in all the diiodide complexes and ab initio calculations including spin-orbit effects allow quantitative assignments of the observed photoelectron spectra. A variety of chemical bonding analyses (charge population, bond order, and electron localization functions) have been carried out, revealing a gradual transition from the expected ionic behavior in CsI(2)(-) to relatively strong covalent bonding in AuI(2)(-). Both relativistic effects and electron correlation are shown to enhance the covalency in the gold diiodide complex.

Time-resolved photoelectron imaging of ultrafast S2→S1 internal conversion through conical intersection in pyrazine

A nonadiabatic electronic transition through a conical intersection was studied by pump-probe photoelectron imaging spectroscopy with a 22 fs time resolution in the benchmark polyatomic molecule of pyrazine and deuterated pyrazine. The lifetimes of the S(2) state of pyrazine and deuterated pyrazine were determined to be 22+/-3 fs by the global fitting of the time-energy maps of photoelectron kinetic energy (PKE) distributions. The lifetime of S(3) was determined to be 40-43 fs. Two-dimensional maps of photoelectron distributions were obtained for time (t) and PKE, and individual PKE distributions upon ionization from S(2) and S(1) were extracted. Quantum beat with an approximately 50 fs period was observed after the S(2)-->S(1) internal conversion, which was attributed to the totally symmetric vibration nu(6a) in S(1).

Photoelectron Imaging of Cyanovinylidene and Cyanoacetylene Anions

Negative ions of cyanoacetylene and cyanovinylidene are generated simultaneously via the competing 1,1-H(2)(+) and 1,2-H(2)(+) abstraction channels of O(-) reaction with acrylonitrile. The two stable isomeric forms of the anion, CCHCN(-) and HCCCN(-), are separated by a large (approximately 2 eV) potential energy barrier. Their photodetachment provides access to both the reactant and the product sides of the neutral cyanovinylidene --> cyanoacetylene rearrangement reaction, predicted to involve only a very small barrier. Using photoelectron imaging spectroscopy at 532 and 355 nm, the adiabatic electron affinity of the reactive intermediate :C horizontal lineCHCN (X(1)A'), is determined to be 1.84 +/- 0.01 eV. The photoelectron spectrum of CCHCN(-) exhibits a vibrational progression attributed to the excitation of the CCH bending mode. The observed spectral features are reproduced reasonably well using a Franck-Condon simulation under the parallel-mode approximation. In contrast to unsubstituted acetylene, cyanoacetylene has a stable anionic state, which is adiabatically weakly bound, but has an experimentally determined vertical detachment energy of 1.04 +/- 0.05 eV. This measurement, along with the broad, structureless photoelectron spectrum of HCCCN(-) (with no identifiable origin), reflects the large geometry difference between the w-shaped structure of the anion and the linear equilibrium geometry of HCCCN.

Photodetachment, photofragmentation, and fragment autodetachment of [O2n(H2O)m]− clusters: Core-anion structures and fragment energy partitioning

Building on the past studies of the O2n− and O2−(H2O)m cluster anion series, we assess the effect of the strong hydration interactions on the oxygen-core clusters using photoelectron imaging and photofragment mass spectroscopy of [O2n(H2O)m]− (n=1–4, m=0–3) at 355 nm. The results show that both pure-oxygen and hydrated clusters with n≥2 form an O4− core anion, indicated in the past work on the pure-oxygen clusters. All clusters studied can be therefore described in terms of O4−(H2O)m(O2)n−2 structures, although the O4− core may be strongly perturbed by hydration in some of these clusters. Fragmentation of these clusters yields predominantly O2− and O2−(H2O)l (l<m) anionic products. The low-electron kinetic energy O2− autodetachment features, prominent in the photoelectron images, signal that the fragments are vibrationally excited. The relative intensity of photoelectrons arising from O2− fragment autodetachment is used to shed light on the varying degree of fragment excitation resulting from the cluster fragmentation process depending on the solvent conditions.

Surface plasmon resonance in C60 revealed by photoelectron imaging spectroscopy

Within the framework of the time-dependent local-density approximation and the self-interaction correction, we have estimated the photoionization cross-section and the photoelectron angular asymmetry parameter of C60. The latter exhibits large variation around the surface plasmon resonance of the π electrons near 20 eV. Based on a semiclassical electrodynamics model, we show that this behaviour is generic for any surface plasmon resonance in the continuum and is a signature of the correlated motion of the electrons. It is related to the change of sign of the real part of the dynamical polarizability at resonance. Possible experiments to investigate this new plasmon signature are discussed.

Ultrafast Dynamics of o‐Bromofluorobenzene Studied by Time‐Resolved Photoelectron Imaging

Photodissociation dynamics and rotational wave packet coherences of o-bromofluorobenzene are studied by femtosecond time-resolved photoelectron imaging [figure: see text]. The decay of different photoelectron rings shows the population decay of states from which the lifetimes of different states are determined. The variation of photoelectron angular distributions reflects the evolution of rotational coherences.Photodissociation dynamics and rotational wave packet coherences of o-bromofluorobenzene are studied by femtosecond time-resolved photoelectron imaging (TR-PEI) spectroscopy combined with the (1+2') resonance-enhanced multiphoton ionization (REMPI). Photoelectron kinetic energy and angular distributions indicate ionization dynamics from some Rydberg states at the (1+1') photon energy. The lifetimes of the S(1) (A') and T(2) (A') states are determined from the decay of the photoelectron signals to be 38 ps and 27 ps. The electron population decay of the two states is attributed to predissociation and tunneling dissociation. The variation of time-dependent anisotropy parameters in the first 5 ps shows the rotational wave coherences of molecule.

The intersystem crossing process of p-bromofluorobenzene studied with time-resolved photoelectron imaging

Ultrafast processes of p-bromofluorobenzene are studied with femtosecond time-resolved photoelectron imaging spectroscopy. The photoelectron image revealed four photoelectron rings centered at 0.39, 0.86, 1.13, and 1.61 eV, respectively. The inner rings are more anisotropic than the outer rings. The decay traces of the different rings were recorded separately. Sharp photoelectron energy distributions and different anisotropy parameters extracted from the images indicated resonances with Rydberg states at the (1+1(')) photon energy. The quantum defect values of the four Rydberg states were determined to be 0.75, 0.52, 0.36, and approximately 0, respectively, with principal quantum number of 3. The electron dephasing mechanism of the S(1)(B(2)) state corresponds to the intersystem crossing from the S(1)(B(2)) to T(1)(B(2)) state and the predissociation of the S(1)(B(2)) state via the T(1)(B(1)) state. The lifetimes of S(1)(B(2)) and T(1)(B(2)) are determined from the decay of the photoelectron signals to be 40 and 33 ps, respectively. The variety of time-dependent anisotropy parameters in the first 5 ps shows the rotational wave coherences of p-bromofluorobenzene at the S(1)(B(2)) state.

Observation and Characterization of the CH3S(O)CH− and CH3S(O)CH−·H2O Carbene Anions by Photoelectron Imaging and Photofragment Spectroscopy

We report the observation of the CH(3)S(O)CH(-) and CH(3)S(O)CH(-) x H(2)O carbene anions formed upon overall H(2)(+) abstraction from dimethyl sulfoxide by O(-). Photoelectron spectroscopy reveals singlet and triplet carbenes for the remaining neutral, with the singlet state assigned as the ground state. Although some formation of the distonic CH(2)S(O)CH(2)(-) radical anion is also expected, no conclusive evidence of the presence of this isomer is found. The photoelectron spectrum of HCSO(-) is also reported for the first time. Photofragmentation of CH(3)S(O)CH(-) with 532 nm light reveals two main types of anionic products: a dominant HCSO(-) fragment, resulting from methyl elimination, and a less intense SO(-) product. For the monohydrated anion, an additional SO(-) x H(2)O fragment is observed. Intriguingly, both the SO(-) x H(2)O and SO(-) products are produced with much higher yields in the fragmentation of CH(3)S(O)CH(-) x H(2)O, compared to the SO(-) yield from the dissociation of the bare CH(3)S(O)CH(-) anion. Two possible pathways are proposed as likely mechanisms for the SO(-)-based photoproducts, both involving a photoinduced intramolecular rearrangement and the formation of a C-C bond.

Low-energy photoelectron imaging spectroscopy of nitromethane anions: Electron affinity, vibrational features, anisotropies, and the dipole-bound state

We present low-energy velocity map photoelectron imaging results for nitromethane anions. The photoelectron spectrum is interpreted with the aid of ab initio theory and Franck-Condon factor calculations. We obtain a new value for the adiabatic electron affinity of nitromethane of (172+/-6) meV and observe the dipole-bound state of nitromethane. The photoelectron angular distributions of the observed features are discussed in the context of threshold laws for photodetachment.

Electron binding motifs in the (CS2)n− (n>4) cluster anions

Photoelectron imaging spectroscopy of (CS(2))(n) (-), n>4, reveals a new state with an electron binding energy smaller than that of any of the corresponding CS(2) (-) and C(2)S(4) (-) states known to date. With support from ab initio calculations, two long-lived, metastable binding motifs with small electron binding energies are discussed for these clusters. The first is a solvent network permeating state, where the excess electron is delocalized over a number of linear CS(2) molecules. The second is an excited (2)B(1) state of the core CS(2) (-) anion stabilized at a slightly bent geometry by the solvation interactions. Based on the observed solvation-induced shifts in binding energy, the second motif is favored.

Slow photoelectron imaging spectroscopy of CCO− and CCS−

High-resolution photodetachment spectra of CCO(-) and CCS(-) using slow photoelectron velocity-map imaging spectroscopy are reported. Well-resolved transitions to the neutral X (3)Sigma(-), a (1)Delta, b (1)Sigma(+), and A (3)Pi states are seen for both species. The electron affinities of CCO and CCS are determined to be 2.3107+/-0.0006 and 2.7475+/-0.0006 eV, respectively, and precise term energies for the a (1)Delta, b (1)Sigma(+), and A (3)Pi excited states are also determined. The two low-lying singlet states of CCS are observed for the first time, as are several vibronic transitions within the four bands. Analysis of hot bands finds the spin-orbit orbit splitting in the X (2)Pi ground state of CCO(-) and CCS(-) to be 61 and 195 cm(-1), respectively.