Photoelectron spectroscopy resonant enhancement

ADE = adiabatic detachment energies ESC A = electron spectroscopy for chemical analysis HOMO = highest occupied molecular orbitals MAES = metastable atom electron spectroscopy MIES = metastable ionization electron spectroscopy OAT = oxygen atom transfer PES = photoelectron spectra PEI = pulsed field ionization PIES = Penning ionization electron spectroscopy QM = quantum-mechanical REMPI = resonantly enhanced multiphoton ionization SC = semiclassical VDE = vertical detachment energies XPS = x-ray photoelectron spectroscopy ZEKE = zero electron kinetic energy Cp = cyclopentadienyl, Ph = phenyl, CeHs Tp =... 

One line in bioelectrochemistry is rooted in electrochemical techniques, spectroscopy, and other physical chemical techniques. Linear and cyclic voltammetry are central.Other electrochemical techniques include impedance and electroreflectance spectroscopy," ultramicro-electrodes, and chronoamperometry. To this come spectroscopic techniques such as infiared, surface enhanced Raman and resonance Raman,second harmonic generation, surface Plasmon, and X-ray photoelectron spectroscopy. A second line has been to combine state-of-the-art physical electrochemistry with corresponding state-of-the-art microbiology and chemical S5mthesis. The former relates to the use of a wide range of designed mutant proteins, " the latter to chemical synthesis or de novo designed synthetic redox metalloproteins. " " ... 

In atoms and molecules, shakeup satellites, corresponding to internal electronic transitions, are routinely observed using photoelectron and resonant Raman spectroscopy. In particular, shakeup satellites can be observed in the two particle spectrum, i.e., when two holes are left in the final state of an atom after electron emission. Satellite s strength can be strongly enhanced in the presence of a resonant intermediate state. For example, in copper atoms, the incident photon can first excite the core 3p electron to the 4s shell the core hole then decays to the 3d shell through the Auger process (with electron ejected from 3d shell) leaving two 3d holes in the final state [48]. For recent reviews of extensive literature the reader is referred to Refe. [49,50]). 

Conventional photoelectron spectroscopy uses a rare-gas discharge lamp to produce radiation at the wavelength of the He 2p <— Is atomic transition (hu = 21.218 eV). Synchrotron radiation is now widely used for PES because its photon energy is widely tunable yet monochromatic. The initial state, in the first PES experiments, has been the molecular ground state but now, by exploiting Resonance Enhanced Multi-Photon Ionization (REMPI) excitar tion/detection schemes (see Section 1.2.2.3), any excited state of the molecule can be used as the initial state for PES (for a review, see Pratt, 1995). 

In order to study the viscosity effect on the quenching of triplet excited state of (53) by TEMPO, chemically induced dynamic electron polarization and transient absorption spectra have been measured in ethylene glycol, 1,2-propanol and their mixtures. The results indicate that the quenching rate constant is viscosity-dependent and decreases linearly with the increase in solvent viscosity. The spectroscopy and dynamics of near-threshold excited states of the isolated chloranil radical anion have been studied using photoelectron imaging taken at 480 nm, which clearly indicates resonance-enhanced photodetachment via a bound electronic excited state. Time-resolved photoelectron imaging reveals that the excited state rapidly decays on a timescale of 130 fs via internal conversion. ... 

Photoelectron spectroscopy is a powerful technique to study ionic and electronically excited levels of atoms and molecules. In the case of single photon excitation of cold molecules the photoelectron spectrum reflects the internal energy levels of the ionic system. Many experiments are performed via two photon ionization enhanced by a one-photon resonance (R2PE spectroscopy) in which transitions to intermediate electronic levels are accessed which strongly enhance the ion yield. Photoelectron spectroscopy of molecules inside superfluid helium droplets is of particular interest since the interaction of free electrons with liquid helium is known to be highly repulsive, so much so that the electrons form bubbles of about 34 A diameter. In this section, three recent photoelectron spectra will be discussed those of bare helium droplets, of Ags clusters and of single aniline molecules in helium droplets. 

Removal of the Iti electron from NH(X S ) leads to the ionic ground state X removal of the 3a electron to the excited ionic states a A B A, and C Only a few experimental data for the first, third, and fourth ionization potentials Ej of gaseous NH are available. Resonance-enhanced multiphoton ionization (REMPI) of NH coupled with photoelectron spectroscopy (PES) yielded the most accurate results so far [1] and confirmed the values for the first E, obtained by electron-impact mass spectrometry (EIMS) [2] and by He I PES of NH [3]. Values for the second and third Ej to be observed in the He I PES of NH were predicted [3] from the optical emission spectra of NH [4]. Adiabatic and vertical Ej s (in eV) are compared in the following table ... 

In a carbon-supported metal electrocatalyst, the electronic interaction between metal and carbon support has a significant effect on its electrochemical performance [4], For carbon-supported Pt electrocatalyst, carbon could accelerate the electron transfer at the electrode-electrolyte interface, leading to an accelerated electrode process. Typically, the electrons are transferred from platinum clusters to the oxygen species on the surfece of a carbon support material and the chemical bond formation or the charge transfer process occurs at the contacting phase, which is considered to be beneficial to the enhancement of the catalytic properties in terms of activity and stability of the electrocatalysts. Experimentally, the investigation into the electron interaction between metal catalyst and support materials could be realized by various physical, spectroscopic, and electrochemical approaches. The electron donation behavior of Pt to carbon support materials has been demonstrated by the electron spin resonance (ESR) X-ray photoelectron spectroscopy (XPS) studies, with the conclusion that the electron interaction between Pt and carbon support depends on their Fermi level of electrons. It is considered that the electronic structure change of Pt on carbon support induced by the electron interaction has positive effect toward the enhancement of the catalytic properties and the improvement of the stability of the electrocatalyst system. However, the exact quantitative relationship between electronic interaction of carbon-supported catalyst and its electrocatalytic performance is still not yet fully established [4]. 

In addition to the study of the ionic ground state, resonance enhanced photoelectron spectroscopy allows one to characterize multiphoton ion sources with respect to the internal energy distribution of the produced ions. For favourable cases it even may supply information as to how to achieve state selective ion production. The example of fluorobenzene has been chosen in Figure 8 to illustrate resonance enhanced photoelectron spectroscopy as well as further types of ion spectroscopy involving multiphoton excitation. 

Figure 8 Multiphoton excitation and ion spectroscopy Bottom spectrum cold, mass selected UVspectmm (S, <- So transition) of fluorobenzene, providing wavelengths for efficient and selective ionization. Middle spectra photoelectron spectra induced by UV-resonance enhanced two-photon absorption. Choosing different intermediate states [S,(0,0) or S,(6b )] results in different populations of the final fluorobenzene radical cations. Top spectrum spectroscopy of the excited ionic state of the fluorobenzene radical cation measured by muKiphoton dissociation spectroscopy. The ions have been prepared via the neutral 0°o transition. A special excitation scheme has been used to optimize cation spectroscopy (for further details see text).

Two-photon photoemission spectroscopy is known for its capability to reveal not only occupied but also unoccupied electronic density of states [10]. In this scheme, one photon excites an electron below the Fermi level to an intermediate state. A second photon then excites the electron from the intermediate state to a final state above the vacuum le vel. The photoelectron yields are strongly enhanced if the excitation photon energy is tuned to the resonance conditions, and the photoelectron spectrum reflects the electron lifetime in the intermediate states as well as their density of states. It is necessary to keep the employed photon energy below the work function of the sample, otherwise one photon photoemission signal becomes excessive and buries the 2PPE signals. 

Although most suitable for use with lasers, Thermionic diodes have also been successfully applied to synchrotron radiation studies by using wiggler magnets to enhance the intensity of the beam [390]. Last but not least, one should mention the important category of atomic beam experiments, complemented by the techniques of photoelectron and photoion spectroscopy. All these techniques are suitable for the experimental study of interacting resonances. We turn now to their theoretical description, which will be illustrated by experimental examples. 

Techniques that have been applied include time of flight static secondary ion mass spectrometry (SSIMS)/ " low energy ion scattering spectromehy, x-ray photoelectron microscopy scanning electrochemical microscopy (SECM), laser ionization mass spectrometry, surface enhanced infrared reflection spectroscopy, Fourier Transform infrared spechoscopy and nuclear magnetic resonance spectroscopy. ... 

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