Pulsed field-ionization, PFI

A somewhat more advantageous ZEKE electron detection mechanism consists of pulsed-field ionization (PFI) of Rydberg states. In this schema, the photons do not populate the ion state directly, but rather very high Rydberg states that are convergent on... 

Several methods exist that allow the determination of the standard enthalpies of formation of the ionic species. The reader is referred to two recent rigorous and detailed chapters by Lias and Bartmess and Ervin. The vast majority of the experimental data reported here are obtained by means of Fourier transform ion cyclotron resonance spectroscopy (FT ICR), high-pressure mass spectrometry (HPMS), selected ion flow tube (SIFT), and pulsed-field ionization (PFI) techniques, particularly pulsed-field ionization photoelectron photoion coincidence (PFI-PEPICO). All these experimental techniques have been examined quite recently, respectively, by Marshall, Kebarle, B6hme," ° Ng" and Baer. These chapters appear in a single (remarkable) issue of the International Journal of Mass Spectrometry. An excellent independent discussion of the thermochemical data of ions, with a careful survey of these and other experimental methods, is given in Ref. 37. 

The ZEKE detection scheme is equivalent to ionization of high-n Rydberg states by Pulsed Field Ionization (PFI). If one assumes that the pulsed field ionization of the Rydberg electron follows a diabatic process (Chupka, 1993), then the ionization threshold is lowered by... 

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... 

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. 

B. Zero Kinetic Energy Threshold Photoelectron Spectroscopy and Pulsed Field Ionization (ZEKE-PFI)... 

PFI-PEPICO pulsed field ionization photoelectron-photoion coincidence technique... 

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 PFI = 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 =... 

In recent high-resolution, pulsed-field ionization-photoelectron (PFI-PE) spectroscopic studies of laser-initiated radicals, such as SH [61] and CHjS [54] formed in the laser photodissociation of H2S and CH3SH and CH3SSCH3, respectively, we have demonstrated that the PFI-PE spectroscopic technique is potentially useful for probing nascent state distributions of primary products formed in photochemical or reactive processes. 

Beattie et al. [98Bea] have carried out zero kinetie energy pulsed field ionization photoelectron spectroscopy (ZEKE-PFI) using coherent two-photon exeitation (C2P) via a repulsive intermediate state. Vibrational progressions were observed, but rotational struetures eould not be resolved. Parameters, for both ionie spin-orbit ground state components, have been determined as follows (all values in units of cm )... 

Perhaps, the most important PES development over the past two decades is the introduction of pulsed field ionization-zero electron kinetic energy (PFI-ZEKE) technique. " Other names in use for this technique include ZEKE-PFI, PFI-PE, or simply ZEKE. PFI-ZEKE involves the detection of electrons produced by delayed, pulsed, electric field ionization of very high-lying... 

An alternative to PFI-ZEKE would be mass-analyzed threshold ionization (MATI), where cations rather than ZEKE electrons are detected.In principle, MATI is attractive because of the inherent mass selection. However, MATI experiments are difficult to implement because of ion separation fields affecting the high Rydberg levels required for delayed pulsed field ionizations. The technique has so far only been used for the smallest metal molecules. ... 

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. 

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