ZEKE electron spectroscopy

Two types of time resolved experiments can be carried out on such a system. The less ambiguous approach is that employed in the stilbene—He study (fig. 10.5) in which a second laser, time delayed from the pump laser, is used to excited the state. Smith and Knee (1993) have used picosecond time resolved TPES spectroscopy in which zero-energy electrons are collected as a function of the probe laser wavelength. [Laser based TPES has also been called zero kinetic energy (ZEKE) electron spectroscopy or pulsed field ionization (PFI) (Muller-Dethlefs and Schlag, 1991)]. Figure... 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Figure 2. Schematic of apparatus for ZEKE-PFI spectroscopy, including magnetically shielded electron flight path and microchannel plate detector. Not to scale flight path is 50 cm long interaction zone is 1 cm long.

Threshold electron spectroscopy, or its newest variant ZEKE spectroscopy, represents a new approach to these problems that has already afforded a broad set of new applications, particularly for soft bonds and metastable species, such as is characteristic for metastable reactive intermediates. 

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 =... 

The improved numerical stability of the new deMon2K version also opened the possibility for accurate harmonic Franck-Condon factor calculations. Based on the combination of such calculations with experimental data from pulsed-field ionization zero-electron-kinetic energy (PFl-ZEKE) photoelectron spectroscopy, the ground state stmcture of V3 could be determined [272]. Very recently, this work has been extended to the simulation of vibrationaUy resolved negative ion photoelectron spectra [273]. In both works the use of newly developed basis sets for gradient corrected functionals was the key to success for the ground state stmcture determination. These basis sets have now been developed for aU 3d transition metal elements. With the simulation of vibrationaUy resolved photoelectron spectra of small transition metal clusters reliable stmcture and... 

R. Signorelli, U. Hollenstein, F. Merkt, PFl-ZEKE photo electron spectroscopy study of the first electronic states of KrJ. J. Chem. Phys. 114, 9840 (2001) ... 

Cockett MCR, Donovan RJ, Lawley KP. 1996. Zero-kinetic energy pulsed-field ionization (ZEKE-PFI) spectroscopy of electronically and vibrationally excited states of Ij the A Il3/2,u state and a new electronic state, the a E state . J. Chem. Phys. 105(9) 3347-3360. 

Schlag EW, Peatman WB and Miiller-Dethlefs K. (1993) Threshold photoionization and ZEKE spectroscopy a historical perspective.of Electron Spectroscopy and Related Phenomena 66 139-149. 

An extension of ZEKE spectroscopy is mass-analysed threshold ionization (MATI), photoelectron spectroscopy without photoelectrons . This is effectively the same experiment for every ZEKE electron produced, there must be a cation, and in MATI detection a signal is recorded from these ions. It is much harder to separate the ions produced from pulsed-field ionization of the ZEKE Rydberg states from the ever-present directly produced ions. Ions are much heavier than electrons and hence move more slowly, so a higher-voltage extraction pulse is required for the separation and the subsequent extraction and selection of the cations. The obvious advantage of this combination of ZEKE with mass spectrometry is the ability to select the cations on the basis of their mass. 

Haines SR, Geppert WD, Chapman DM et al (1998) Evidence for a strong intermolecular bond in the phenol N2 cation. Journal of Chemical Physics 109 9244-9251. Miiller-Dethlefs K and Schlag EW (1991) High resolution zero kinetic energy (ZEKE) photo electron spectroscopy of molecular systems. Annual Review of Physical Chemistry 42 109-136. 

Miiller-Dethlefs K (1995) Applications of ZEKE spectroscopy. Journal of Electron Spectroscopy and related Phenomena 75 35-46. 

ZEKE (zero kinetic energy) photoelectron spectroscopy has also been applied to negative ions [M]. In ZEKE work, the laser wavelengdi is swept tlirough photodetachment thresholds and only electrons with near-zero kinetic energy are... 

Photoelectron spectroscopy of free radicals has been utilized for detection of radicals. It can be via resonance photoexcitation and photoionization (e.g. ZEKE) or non-resonance photoionization (e.g. single-photon VUV photoionization). The photoelectron spectroscopy of free radicals has been reviewed in 1994 by Chen.5 A recent review on mass spectrometry, photoelectron spectroscopy, and photoionization of free radicals by Sablier and Fujii is available.72 It is worthwhile to point out that mass spectrometry by photoionization offers some advantage for the detection of radicals, in comparison with the conventional mass spectroscopy by electron-impact... 

In this chapter, we first present a brief overview of the experimental techniques that we and others have used to study torsional motion in S, and D0 (Section II). These are resonant two-photon ionization (R2PI) for S,-S0 spectroscopy and pulsed-field ionization (commonly known as ZEKE-PFI) for D0-S, spectroscopy. In Section HI, we summarize what is known about sixfold methyl rotor barriers in S0, S, and D0, including a brief description of how the absolute conformational preference can be inferred from spectral intensities. Section IV describes the threefold example of o-cholorotoluene in some detail and summarizes what is known about threefold barriers more generally. The sequence of molecules o-fluorotoluene, o-chlorotoluene, and 2-fluoro-6-chlorotoluene shows the effects of ort/io-fluoro and ortho-chloro substituents on the rotor potential. These are approximately additive in S0, S, and D0. Finally, in Section V, we present our ideas about the underlying causes of these diverse barrier heights and conformational preferences, based on analysis of the optimized geometries and electronic wavefunctions from ab initio calculations. 

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