Kinetic Energy Photoelectron Research Articles

Circular Dichroism in Cu Resonant Auger Electron Diffraction

Abstract Upon a core level excitation by circularly polarized light (CPL), the angular momentum of light, i.e. helicity, is transferred to the emitted photoelectron. This phenomenon can be confirmed by the parallax shift measurement of the forward focusing peak (FFP) direction in a stereograph of the atomic arrangement. The angular momentum of the emitted photoelectron is the sum of CPL helicity and the magnetic quantum number (MQN) of the initial state that define the quantum number of the core hole final state. The core hole may decay via Auger electron emission, where in this two electron process the angular momentum has to be conserved as well. Starting from a given core hole, different Auger decay channels with different final state energies and angular momenta of the emitted Auger electrons may be populated. Here we report the observation and formulation of the angular momentum transfer of light to Auger electrons, instead of photoelectrons. We measured photoelectron and Auger electron intensity angular distributions from Cu(111) and Cu(001) surfaces as a function of photon energy and photoelectron kinetic energy. By combining Auger electron spectroscopy with the FFP shift measurements at absorption threshold, element- and MQN-specific hole states can be generated in the valence band.

Dissociation Energies of Sulfur-Centered Hydrogen-Bonded Complexes.

In this work we have determined dissociation energies of O-H···S hydrogen bond in the H2S complexes of various phenol derivatives using 2-color-2-photon photofragmentation spectroscopy in combination with zero kinetic energy photoelectron (ZEKE-PE) spectroscopy. This is the first report of direct determination of dissociation energy of O-H···S hydrogen bond. The ZEKE-PE spectra of the complexes revealed a long progression in the intermolecular stretching mode with significant anharmonicity. Using the anharmonicity information and experimentally determined dissociation energy, we also validated Birge-Sponer (B-S) extrapolation method, which is an approximate method to estimate dissociation energy. Experimentally determined dissociation energies were compared with a variety of ab initio calculations. One of the important findings is that ωB97X-D functional, which is a dispersion corrected DFT functional, was able to predict the dissociation energies in both the cationic as well as the ground electronic state very well for almost every case.

Direct observation of up-conversion via femtosecond photoelectron imaging

Ultrafast relaxation dynamics in 2-methylfuran has been investigated by time-resolved photoelectron imaging. An ``up'' internal conversion from a low-lying state into a higher-lying one has been observed experimentally. Temporal photoelectron kinetic-energy distributions and angular distributions of the photoelectrons are analyzed. In the up-conversion process, the vibrational energy in the initial state is converted to the electronic energy of the final state during the energy transfer. And the time scale for the up-conversion process is estimated by the observed onset delay for the corresponding photoelectron bands.

Ultrafast Deactivation of the ππ*(V) State of Ethylene Studied Using Sub-20 fs Time-Resolved Photoelectron Imaging

The ultrafast deactivation process of ethylene in the ππ*(V) state was studied using time-resolved photoelectron imaging with sub-20 fs pulses at 159 and 198 nm. The photoelectron kinetic energy distribution observed upon 159 nm photoexcitation exhibited a continuous downward shift within 20 fs, attributed to both C-C twist and pyramidalization motions. A partial revival of the vibrational wave packet was observed with the period of about 18 fs, which is attributed to the C-C twist from 0 to 180° on the ππ*(V) potential energy surface. Signature for internal conversion from the ππ*(V) state to a lower-lying π3s Rydberg (R) state, which has been previously suggested, was not detected in the time-dependent photoelectron kinetic energy and angular distributions.

Characterization of the BaCl+(X1Σ+) cation by photoelectron spectroscopy

The BaCl+ cation is of interest as it has the potential for being cooled to ultra-cold temperatures. Recent experiments have demonstrated sympathetic cooling of BaCl+ (Rellergert et al., 2013), but characterization of the internal state population distributions is hampered by the lack of spectroscopic data for this ion. In the present study, pulsed field ionization–zero kinetic energy photoelectron spectroscopy has been used to observe the v+=0–3 vibrational levels of BaCl+(X1Σ+). Vibrational constants and an accurate value for the ionization energy of BaCl are reported. Comparisons to the predictions of high-level electronic structure calculations are also provided.

Molecular Resolvent Operator for H2+ molecule

SynopsisWe present a theoretical method to extract photoelectron and nuclear kinetic energy spectra of molecules under intense ultrashort laser pulses. The method is applied to H2+ photoionization and dissociation within a 3D model of the H2+ molecule. This method is an extension to molecules of the resolvent technique originally proposed for atoms, which was recently implemented to extract absolute values of photoelectron and nuclear kinetic energy spectra.

Zero Kinetic Energy Photoelectron Spectroscopy of Benzo[h]quinoline.

We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[h]quinoline (BhQ) via resonantly enhanced multiphoton ionization (REMPI) through the first electronically excited state S1. From the simulated REMPI spectra with and without Herzberg-Teller coupling, we conclude that vibronic coupling plays a minor but observable role in the electronic excitation to the S1 state. We further compare the S1 state of BhQ with the first two electronically excited states of phenanthrene, noticing a similarity of the S1 state of BhQ with the second electronically excited state S2 of phenanthrene. In the ZEKE spectra of BhQ, the vibrational frequencies of the cationic state D0 are consistently higher than those of the intermediate neutral state, indicating enhanced bonding upon ionization. The sparse ZEKE spectra, compared with the spectrum of phenanthrene containing rich vibronic activities, further imply that the nitrogen atom has attenuated the structural change between S1 and D0 states. We speculate that the nitrogen atom can withdraw an electron in the S1 state and donate an electron in the D0 state, thereby minimizing the structural change during ionization. The origin of the first electronically excited state is determined to be 29,410 ± 5 cm(-1), and the adiabatic ionization potential is determined to be 65,064 ± 7 cm(-1).

Using "Tender" X-ray Ambient Pressure X-Ray Photoelectron Spectroscopy as A Direct Probe of Solid-Liquid Interface.

We report a new method to probe the solid-liquid interface through the use of a thin liquid layer on a solid surface. An ambient pressure XPS (AP-XPS) endstation that is capable of detecting high kinetic energy photoelectrons (7 keV) at a pressure up to 110 Torr has been constructed and commissioned. Additionally, we have deployed a “dip & pull” method to create a stable nanometers-thick aqueous electrolyte on platinum working electrode surface. Combining the newly constructed AP-XPS system, “dip & pull” approach, with a “tender” X-ray synchrotron source (2 keV–7 keV), we are able to access the interface between liquid and solid dense phases with photoelectrons and directly probe important phenomena occurring at the narrow solid-liquid interface region in an electrochemical system. Using this approach, we have performed electrochemical oxidation of the Pt electrode at an oxygen evolution reaction (OER) potential. Under this potential, we observe the formation of both Pt2+ and Pt4+ interfacial species on the Pt working electrode in situ. We believe this thin-film approach and the use of “tender” AP-XPS highlighted in this study is an innovative new approach to probe this key solid-liquid interface region of electrochemistry.

Ultrafast imaging of electronic relaxation in n-propylbenzene: Direct observation of intermediate state

The ultrafast dynamics of the second singlet electronically excited state (S2) in n-propylbenzene has been investigated by femtosecond time-resolved photoelectron imaging coupled with photofragmentation spectroscopy. The intermediate state for the deactivation of the S2 state is observed by transient photoelectron kinetic energy distributions and photoelectron angular distributions. An ultrafast electronic relaxation process on timescale of the fitted ∼50fs was observed in the S2 state by time-resolved photoelectron imaging and it is attributed to the S1←S2 internal conversion (IC). The time constant of 1.23 (±0.2)ps is determined for the further deactivation of the intermediate S1 state.

Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces

Using synchrotron radiation Near-Ambient Pressure X-ray photoemission spectroscopy (NAP-XPS) in surface and bulk sensitive conditions, we specifically explored the depth distribution of Na+ ions near the vapor/liquid interface of saturated solutions of sodium halides (NaCl, NaBr, and NaI) kept at 278 K under a pressure of 8 mbar. By varying the photoelectron kinetic energy, and hence the probing depth, we show that the Na+ depth distribution near the surface is highly sensitive to the size and polarizability of the anion. The large polarizable anions, Br– and I– that segregate at the surface, tend to drag the Na+ ions closer to the liquid–water surface. However, this situation is not observed for the NaCl solution, in line with recent theoretical results.

Energy Thresholds of DNA Damage Induced by UV Radiation: An XPS Study.

This work stresses on damage at the molecular level caused by ultraviolet radiation (UV) in the range from 3.5 to 8 eV, deoxyribonucleic acid (DNA) films observed by X-ray photoelectron spectroscopy (XPS). Detailed quantitative XPS analysis, in which all the amounts are relative to sodium-assumed not to be released from the samples, of the carbon, oxygen, and particularly, nitrogen components, reveals that irradiation leads to sugar degradation with CO-based compounds release for energies above 6.9 eV and decrease of nitrogen groups which are not involved in hydrogen bonding at energies above 4.2 eV. Also the phosphate groups are seen to decrease to energies above 4.2 eV. Analysis of XPS spectra allowed to conclude that the damage on bases peripheral nitrogen atoms are following the damage on phosphates. It suggests that very low kinetic energy photoelectrons are ejected from the DNA bases, as a result of UV light induced breaking of the phosphate ester groups which forms a transient anion with resonance formation and whereby most of the nitrogen DNA peripheral groups are removed. The degree of ionization of DNA was observed to increase with radiation energy, indicating that the ionized phosphate groups are kept unchanged. This result was interpreted by the shielding of phosphate groups caused by water molecules hydration near sodium atoms.

Evaluation of the exothermicity of the chemi-ionization reaction Sm + O → SmO(+) + e(.).

The exothermicity of the chemi-ionization reaction Sm + O → SmO(+) + e(-) has been re-evaluated through the combination of several experimental methods. The thermal reactivity (300-650 K) of Sm(+) and SmO(+) with a range of species measured using a selected ion flow tube-mass spectrometer apparatus is reported and provides limits for the bond strength of SmO(+), 5.661 eV ≤ D0(Sm(+)-O) ≤ 6.500 eV. A more precise value is measured to be 5.725 ± 0.07 eV, bracketed by the observed reactivity of Sm(+) and SmO(+) with several species using a guided ion beam tandem mass spectrometer (GIBMS). Combined with the established Sm ionization energy (IE), this value indicates an exothermicity of the title reaction of 0.08 ± 0.07 eV, ∼0.2 eV smaller than previous determinations. In addition, the ionization energy of SmO has been measured by resonantly enhanced two-photon ionization and pulsed-field ionization zero kinetic energy photoelectron spectroscopy to be 5.7427 ± 0.0006 eV, significantly higher than the literature value. Combined with literature bond energies of SmO, this value indicates an exothermicity of the title reaction of 0.14 ± 0.17 eV, independent from and in agreement with the GIBMS result presented here. The evaluated thermochemistry also suggests that D0(SmO) = 5.83 ± 0.07 eV, consistent with but more precise than the literature values. Implications of these results for interpretation of chemical release experiments in the thermosphere are discussed.

Hartree-Fock calculation of the differential photoionization cross sections of small Li clusters.

Cross sections and angular distribution parameters for the single-photon ionization of all electron orbitals of Li2-8 are systematically computed in a broad interval of the photoelectron kinetic energies for the energetically most stable geometry of each cluster. Calculations of the partial photoelectron continuum waves in clusters are carried out by the single center method within the Hartree-Fock approximation. We study photoionization cross sections per one electron and analyze in some details general trends in the photoionization of inner and outer shells with respect to the size and geometry of a cluster. The present differential cross sections computed for Li2 are in a good agreement with the available theoretical data, whereas those computed for Li3-8 clusters can be considered as theoretical predictions.

Direct Observation of Ground-State Product Formation in a 1,3-Cyclohexadiene Ring-Opening Reaction.

Ultrafast photoelectron imaging using a 90 nm vacuum-UV probe pulse is applied to the ring-opening reaction of 1,3-cyclohexadiene (CHD) in the gas phase, and formation of 1,3,5-hexatriene (HT) and CHD in their electronic ground states is observed in real time. The analysis of the transient photoelectron kinetic energy spectra reveals the branching ratio into HT and CHD as 3:7 upon 270 nm photoexcitation. The ratio is in reasonable agreement with the experimental values reported for the liquid phase and theoretical values for the gas phase, resolving the discrepancy.

Molecular alignment dependent electron interference in attosecond ultraviolet photoionization.

We present molecular photoionization processes by intense attosecond ultraviolet laser pulses from numerical solutions of time-dependent Schrödinger equations. Simulations preformed on a single electron diatomic show minima in molecular photoelectron energy spectra resulting from two center interference effects which depend strongly on molecular alignment. We attribute such sensitivity to the spatial orientation asymmetry of the photoionization process from the two nuclei. A similar influence on photoelectron kinetic energies is also presented.

Excited-state dynamics of guanosine in aqueous solution revealed by time-resolved photoelectron spectroscopy: experiment and theory.

Time-resolved photoelectron spectroscopy is performed on aqueous guanosine solution to study its excited-state relaxation dynamics. Experimental results are complemented by surface hopping dynamic simulations and evaluation of the excited-state ionization energy by Koopmans' theorem. Two alternative models for the relaxation dynamics are discussed. The experimentally observed excited-state lifetime is about 2.5 ps if the molecule is excited at 266 nm and about 1.1 ps if the molecule is excited at 238 nm. The experimental probe photon energy dependence of the photoelectron kinetic energy distribution suggests that the probe step is not vertical and involves a doubly-excited autoionizing state.

Femtosecond time-resolved XUV + UV photoelectron imaging of pure helium nanodroplets.

Liquid helium nanodroplets, consisting of on average 2 × 10(6) atoms, are examined using femtosecond time-resolved photoelectron imaging. The droplets are excited by an extreme ultraviolet light pulse centered at 23.7 eV photon energy, leading to states within a band that is associated with the 1s3p and 1s4p Rydberg levels of free helium atoms. The initially excited states and subsequent relaxation dynamics are probed by photoionizing transient species with a 3.2 eV pulse and using velocity map imaging to measure time-dependent photoelectron kinetic energy distributions. Significant differences are seen compared to previous studies with a lower energy (1.6 eV) probe pulse. Three distinct time-dependent signals are analyzed by global fitting. A broad intense signal, centered at an electron kinetic energy (eKE) of 2.3 eV, grows in faster than the experimental time resolution and decays in ~100 fs. This feature is attributed to the initially excited droplet state. A second broad transient feature, with eKE ranging from 0.5 to 4 eV, appears at a rate similar to the decay of the initially excited state and is attributed to rapid atomic reconfiguration resulting in Franck-Condon overlap with a broader range of cation geometries, possibly involving formation of a Rydberg-excited (He(n))* core within the droplet. An additional relaxation pathway leads to another short-lived feature with vertical binding energies ≳2.4 eV, which is identified as a transient population within the lower-lying 1s2p Rydberg band. Ionization at 3.2 eV shows an enhanced contribution from electronically excited droplet states compared to ejected Rydberg atoms, which dominate at 1.6 eV. This is possibly the result of increased photoelectron generation from the bulk of the droplet by the more energetic probe photons.

Torsional energy levels of CH3OH+/CH3OD+/CD3OD+ studied by zero-kinetic energy photoelectron spectroscopy and theoretical calculations

The torsional energy levels of CH3OH(+), CH3OD(+), and CD3OD(+) have been determined for the first time using one-photon zero kinetic energy photoelectron spectroscopy. The adiabatic ionization energies for CH3OH, CH3OD, and CD3OD are determined as 10.8396, 10.8455, and 10.8732 eV with uncertainties of 0.0005 eV, respectively. Theoretical calculations have also been performed to obtain the torsional energy levels for the three isotopologues using a one-dimensional model with approximate zero-point energy corrections of the torsional potential energy curves. The calculated values are in good agreement with the experimental data. The barrier height of the torsional potential energy without zero-point energy correction was calculated as 157 cm(-1), which is about half of that of the neutral (340 cm(-1)). The calculations showed that the cation has eclipsed conformation at the energy minimum and staggered one at the saddle point, which is the opposite of what is observed in the neutral molecule. The fundamental C-O stretch vibrational energy level for CD3OD(+) has also been determined. The energy levels for the combinational excitation of the torsional vibration and the fundamental C-O stretch vibration indicate a strong torsion-vibration coupling.

Rotations of molecular photoelectron angular distributions in above threshold ionization of H2+ by intense circularly polarized attosecond UV laser pulses

We present molecular photoelectron angular distributions (MPADs) in multi-photon ionization processes by circularly polarized attosecond UV laser pulses. Simulations are performed on the single electron aligned molecular ion H by solving corresponding 3D time-dependent Schrödinger equations. Numerical results of molecular above threshold ionization (MATI) show that rotations of MPADs with respect to the molecular and polarization axes depend on pulse intensities and photoelectron kinetic energies. We attribute the rotation to Γ, the difference between parallel and perpendicular ionization probabilities. It is found that in a resonant ionization process, the rotation angle is also a function of the symmetry of intermediate electronic states. The coherent population transfer between the initial and the resonant electronic states is controlled by pulse intensities. Such dependence of rotations on the pulse intensity is absent in Rydberg resonant ionizations as well as in MATI at large energy photons ℏω > Ip, where ω is angular frequency of photons and Ip is the molecular ionization potential. We describe these processes by a multi-photon perturbation theory model. Effects of molecular alignment and pulse ellipticities on rotations are investigated, confirming the essence of the ionization parameter Γ in rotations of MPADs.

ZEKE photoelectron spectroscopy of p-fluorophenol···H2S/H2O complexes and dissociation energy measurement using the Birge-Sponer extrapolation method.

In this work we have shown that the Birge-Sponer extrapolation method can be successfully used to determine the dissociation energies (D0) of noncovalently bound complexes. The O-H···S hydrogen-bonding interaction in the cationic state of the p-fluorophenol···H2S complex was characterized using zero kinetic energy (ZEKE) photoelectron spectroscopy. This is the first ZEKE report on the O-H···S hydrogen-bonding interaction. The adiabatic ionization energy (AIE) of the complex was determined as 65 542 cm(-1). Various intermolecular and intramolecular vibrational modes of the cation were assigned. A long progression was observed in the intermolecular stretching mode (σ) of the complex with significant anharmonicity along this mode. The anharmonicity information was used to estimate the dissociation energy (D0) in the cationic state using the Birge-Sponer extrapolation method. The D0 was estimated as 9.72 ± 1.05 kcal mol(-1). The ZEKE photoelectron spectra of analogous complex FLP···H2O was also recorded for the sake of comparison. The AIE was determined as 64 082 cm(-1). The intermolecular stretching mode in this system, however, was found to be quite harmonic, unlike that in the H2S complex. The dissociation energies of both the complexes, along with those of a few benchmark systems, such as phenol···H2O and indole···benzene complexes, were computed at various levels of theory such as MP2 at the complete basis set limit, ωB97X-D, and CCSD(T). It was found that only the ωB97X-D level values were in excellent agreement with the experimental results for the benchmark systems for the ground as well as the cationic states. The dissociation energy of the (FLP···H2S)(+) complex determined by the Birge-Sponer extrapolation was about ∼18% lower than that computed at the ωB97X-D level.