Zero Electron Kinetic Energy ZEKE

My question to Prof. Gerber is the following Could you please explain the different virtues of femtosecond pump-pulse experiments versus ultrashort zero electron kinetic energy (ZEKE) spectroscopy Do they yield complementary information on the molecular dynamics or are there specific domains where one of them should be preferred with respect to the other ... 

The fourth technique for electron analysis measures a preselected set of zero or nearly-zero kinetic energy. While first known as TPE spectroscopy, when monochromatic synchrotron light was used, the addition of pulsed, tunable lasers has led to a totally new, extremely promising branch of PES, termed zero electron-kinetic-energy (ZEKE) PES or ZEKE spectroscopy18. We will describe it in detail later. 

Two especially important variants of REMPI (Johnson, et al., 1975) spectroscopy are ionization-dip (Cooper, et al., 1981) and Zero Electron Kinetic Energy (ZEKE) (Miiller-Dethlefs and Schlag, 1991 Merkt, 1997 Signorell and Merkt, 1999) photoelectron spectroscopy. Ionization-dip REMPI spectroscopy is especially useful when one wants to record free<—bound spectra from a single, selectable v, J level. Without such v, J selection, most of the oscillatory structure in a free<—bound spectrum will be washed out. One potential problem with some ionization-dip schemes is that, if the ionization transition originates from the initial level of the free<—bound transition being studied, there is a possibility that the observed linewidths will be distorted by power broadening (especially when the free final state is a weakly predissociated state with linewidth < lcm-1). 

There have been numerous photodetachment studies of small cluster anions, and we now give some examples. Noble metal clusters (Cu, Ag,7, Au , n = 1-10) have been studied by Ho et al. [23], who resolved vibrations in all three dimers. Studies of alkali metal cluster anions have included those of Na ( = 2-5), K (n = 2-19), RbJ 3, and CS2-3 [24,25]. Carbon cluster anions C,T have photoelectron spectra that are consistent with linear chains for n = 2-9 and monocyclic rings for n = 10-29 [26]. Photoelectron spectra of Sb and Bi to n = 4 [27] show rich vibrational structure for the dimers, and the spectra of the larger clusters could be interpreted in terms of ab initio calculations. The threshold photodetachment (zero electron kinetic energy, ZEKE) spectrum of Si4 [28] shows a progression of well-resolved transitions between the ground state of the rhombic anion (Dzh, and vibrational levels of the first excited... 

Observation of vibrational structure in the PE spectra of large organometallic molecules is rare because of the large number of vibrational modes that may be excited, the low frequencies of many of the vibrational modes, and the lack of resolution in conventional spectrometers. Techniques such as zero electron kinetic energy (ZEKE) spectroscopy have been successful in resolving vibrational structure on small molecules but have not been successfully extended to organometallics as yet. Those spectra that show vibrational structure have proved informative. 

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... 

Zero Electron Kinetic Energy Spectroscopy, ZEKE... 

Abbreviations used in the tables calc = calculated value PT = photodetachment threshold using a lamp as a light source LPT = laser photodetachment threshold LPES = laser photoelectron spectroscopy DA = dissociative attachment attach = electron at-tachment/detachment equilibrium e-scat = electron scattering kinetic = dissociation kinetics Knud=Knudsen cell CT = charge transfer CD = collisional detachment and ZEKE = zero electron kinetic energy spectroscopy. 

ZEKE zero electron kinetic energy spectroscopy... 

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... 

Rydberg states with a principle quantum number w> 100. These Rydberg states are formed by laser excitation and are located a few cm (or a fraction of meV) below the ionization threshold. Because the electrons ejected from these Rydberg states carry near zero electron kinetic energy, these states are known as ZEKE states and the ejected electrons are named as ZEKEs. The measured electron peak position is lower than that without the presence of the field by the Stark shift 6)... 

The above techniques are restricted to molecttles which fluoresce. A more general technique is multiphoton ionization (MPI) in which a molecule absorbs several photons sufficient in energy to produce a molecttlar ion. This technique is very sensitive since single ions can be detected. The process may involve a single laser and several photons, or two (or more) lasers with various corribinations of photons, e.g. 1+3, 2+2. One of the lasers can be adjusted so as to involve an intermediate excited state in which case the sensitivity is considerably enhanced and the process is known as resonance enhanced multiphoton ionization (REMPI). The detection of the resrtltant ion with a mass-spectrometer further refines the specificity of the method, and allows individual mass peaks, and isotopic species, to be monitored. The introduction of ZEKE (zero electron kinetic energy [42]) considerably increases the resolution which is beginning to approach the hmit imposed by the widths of the laser. 

---