Zero-kinetic energy

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... 

Muller-Dethlefs K and Schlag E W 1998 Chemical applications of zero kinetic energy (ZEKE) photoelectron spectroscopy Angew. Chem.-Int. Edit. 37 1346-74... 

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). 

Techniques have been developed within the CASSCF method to characterize the critical points on the excited-state PES. Analytic first and second derivatives mean that minima and saddle points can be located using traditional energy optimization procedures. More importantly, intersections can also be located using constrained minimization [42,43]. Of particular interest for the mechanism of a reaction is the minimum energy path (MEP), defined as the line followed by a classical particle with zero kinetic energy [44-46]. Such paths can be calculated using intrinsic reaction coordinate (IRC) techniques... 

The factor limiting the resolution in ultraviolet photoelectron spectra is the inability to measure the kinetic energy of the photoelectrons with sufficient accuracy. The source of the problem points to a possible solution. If the photoelectrons could be produced with zero kinetic energy this cause of the loss of resolution would be largely removed. This is the basis of zero kinetic energy photoelectron (ZEKE-PE) spectroscopy. 

Figure 9.50 Processes involved in obtaining (a) an ultraviolet photoelectron spectrum, (b) a zero kinetic energy photoelectron (ZEKE-PE) spectrum by a one-photon process and (c) a ZEKE-PE spectrum by a two-photon process in which the first photon is resonant with an excited electronic state of the molecule...

More commonly, the resonant two-photon process in Figure 9.50(c) is employed. This necessitates the use of two lasers, one at a fixed wavenumber Vj and the other at a wavenumber V2 which is tunable. The first photon takes the molecule, which, again, is usually in a supersonic jet, to the zero-point vibrational level of an excited electronic state M. The wavenumber of the second photon is tuned across the M to band system while, in principle, the photoelectrons with zero kinetic energy are detected. In practice, however, this technique cannot easily distinguish between electrons which have zero kinetic energy (zero velocity) and those having almost zero kinetic energy, say about 0.1 meV... 

A successful modification to the technique involves delayed pulsed-field extraction which allows discrimination between zero and near-zero kinetic energy electrons. About 1 ps after the laser pulse has produced photoelectrons, a small voltage pulse is applied. This has the effect of amplifying the differences in fhe velocities of fhe phofoelecfrons and allows easy discrimination befween fhem as a resulf of fhe differenf times of arrival af fhe defector. In fhis way only fhe elections which originally had zero kinetic energy following ionization can be counted to give fhe ZEKE-PE specfmm. 

Figure 9.51 A zero kinetic energy photoelectron (ZEKE-PE) resonant two-photon spectrum of 1,4-difluorobenzene in which the first photon excites the molecule of the zero-point level of the S-[ excited electronic state of the molecule. (Reproduced, with permission, from Reiser, G., Rieger, D., Wright, T.G., Muller-Dethlefs, K. and Schlag, E.W., J. Phys. Chem., 97, 4335, 1993)...

The resolution of the ZEKE-PE spectmm of 1,4-difluorobenzene can be compared with, for example, that of the ultraviolet photoelectron spectmm of benzene in Figure 8.12. The greatly increased resolution in the ZEKE-PE spectmm is attributable mostly to the fact that only photoelectrons with zero kinetic energy are being detected. It is also partly attributable to the molecules being in a supersonic jet this has the effect of sharpening the bands because of the restricted rotational populations in the ground state of the molecule. 

Use of fhe supersonic jef in many branches of specfroscopy continues fo increase. One fechnique which has made a considerable impacf in recenf years is fhaf of zero kinetic energy phofoelecfron (ZEKE-PE) specfroscopy. Because of ifs increasing importance and fhe facf fhaf if relafes closely fo ulfraviolef phofoelecfron specfroscopy (UPS), which is described af lengfh in earlier editions, 1 have included fhe new fechnique in Chapfer 9. 

Recently, zero kinetic energy (ZEKE) photoelectron spectroscopy has been used to study the OH/NH tautomerism of 2-pyridone in the gas phase (95JPC8608). This work, which is expected to develop considerably, provides a wealth of information about that equilibrium for the states So, Sj, and Do (cation ground state). 

A heavy body traveling rapidly has a high kinetic energy. A body at rest (stationary, v = 0) has zero kinetic energy. 

What does tins equation tell us Because the kinetic energy of the ejected electrons varies linearly with frequency, a plot of the kinetic energy against the frequency of the radiation should look like the graph in Fig. 1.17, a straight line of slope h, the same for all metals, and have an extrapolated intercept with the vertical axis at — horizontal axis (corresponding to zero kinetic energy of the ejected electron) is at fI>/6 in each case. 

Let us review the calculation of the number of electrons in successive Brillouin zones. The calculation has as its starting point a distribution of free electrons. In a volume V to which the electrons are restricted the most stable pair of electrons occupies the lowest energy levels, with nearly zero kinetic energy, and correspondingly long wave-lengths. As the number of electrons... 

This is the energy required to remove the electron from the ground state of a hydrogen atom to a state of zero kinetic energy at infinity and is also known as the ionization potential of the hydrogen atom. 

B. Zero Kinetic Energy Threshold Photoelectron Spectroscopy and... 

By zero kinetic energy (ZEKE) PE spectroscopy a value of 7.7206 0.0002 eV has been determined for the first adiabatic IP of aniline79. This technique makes it possible to obtain accurate and detailed information about molecular ions. 

Kelvin scale a temperature scale that begins at the theoretical point of absolute zero kinetic energy, or -273.15°C each unit (a kelvin) is equal to 1°C... 

Photoelectron spectroscopy (PES, a non-mass spectral technique) [87] has proven to be very useful in providing information not only about ionization potentials, but also about the electronic and vibrational structure of atoms and molecules. Energy resolutions reported from PES are in the order of 10-15 meV. The resolution of PES still prevents the observation of rotational transitions, [79] and to overcome these limitations, PES has been further improved. In brief, the principle of zero kinetic energy photoelectron spectroscopy (ZEKE-PES or just ZEKE, also a nonmass spectral technique) [89-91] is based on distinguishing excited ions from ground state ions. 

Next, consider the situation if hv = IE. Here, the electron can just be released, but it cannot travel away from the freshly formed ion. Waiting for some short delay (1 ps) now allows to separate zero kinetic energy electrons from others in... 

The energetics of EC are determined by the electron affinity (EA) of the neutral. The EA is the negative of the enthalpy of reaction of the attachment of a zero kinetic energy electron to a neutral molecule or atom ... 

Fissile materials are defined as materials that are fissionable by nentrons with zero kinetic energy. In nuclear engineering, a fissile material is one that is capable of snstaining a chain reaction of nuclear fission Nuclear power reactors are mainly fueled with manium, the heaviest element that occurs in natnre in more than trace qnantities. The principal nuclear energy soiuces are maninm-235, plutonium-239, uranium-233 and thorium. 

What is the lowest possible energy for the harmonic oscillator defined in Eq. (5.10) Using classical mechanics, the answer is quite simple it is the equilibrium state with x 0, zero kinetic energy and potential energy E0. The quantum mechanical answer cannot be quite so simple because of the Heisenberg uncertainty principle, which says (roughly) that the position and momentum of a particle cannot both be known with arbitrary precision. Because the classical minimum energy state specifies both the momentum and position of the oscillator exactly (as zero), it is not a valid quantum... 

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