Ionization thresholds

Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

Crossing an ionization threshold means that electrons are lost from the primary beam as a result of ionization of a core hole. Thus if the reflected current of electrons at the primary energy, more usually termed the elastically reflected current, is monitored as a function of energy, a sharp decrease should be observed as a threshold is crossed. This is the principle of operation of DAPS. It is, in a sense, the inverse of AEAPS, and, indeed, if spectra from the two techniques from the same surface are compared, they can be seen to be mirror images. Background problems occur in DAPS also. 

Fig. 13. Mass spectra of C qCs. clusters ionized at different photon energies near the ionization threshold the values of x corresponding to the closing of electronic shells are indicated.

PIE curves for ra/z = 72 (C4H8O) measured in four butanol flames. The accepted ionization energies for species identified by the observed ionization thresholds are indicated. (From Yang, B. et at. Combust. Flame, 148,198, 2007. With permission.)... 

A common problem for both methods lies in the use of potentials that do not possess the correct net attractiveness. This can have the consequence that continuum feamres appear shifted in energy. In particular, there is evidence that the LB94 exchange-correlation potential currently used for the B-spline calculations, although possessing the correct asymptotic behavior for ion plus electron, is too attractive, and near threshold features can then disappear below the ionization threshold. An empirical correction can be made, offsetting the energy scale, but this can mean that dynamics within a few electronvolts of threshold get an inadequate description or are lost. There is limited scope to tune the Xa potential, principally by adjustment of the assumed a parameter, but for the B-spline method a preferable alternative for the future may well be use of the SAOP functional that also has correct asymptotic behavior, but appears to be better calibrated for such problems [79]. 

Figure 4. Fe cluster ionization thresholds as a function of cluster size, as determined by photoionization yield measurements using tunable UV/VUV laser radiation.

In addition, we have investigated the broad 3p resonance, which lies rather close to the ionization threshold and represents therefore a stringent test for the capabilities of the method in the delicate low-energy region. 

Photoemission has been proved to be a tool for measurement of the electronic structure of metal nanoparticles. The information is gained for DOS in the valence-band region, ionization threshold, core-level positions, and adsorbate structure. In a very simplified picture photoemission transforms the energy distribution of the bounded electrons into the kinetic energy distribution of free electrons leaving the sample, which can easily be measured ... 

An example of a system that can be evalnated nsing this approach is methylene (CH2). The ionization energy of CH2 has been measnred directly from electron ionization thresholds to give valnes of 10.35 + 0.15 and 10.2 + 0.2eV. However, very accurate measurements of the ionization energy have been made more recently... 

Casida, M. E., Jamorski, C., Casida, K. C., Salahub, D. R., 1998, Molecular Excitation Energies to High-Lying Bound States from Time-Dependent Density-Functional Response Theory Characterization and Correction of the Time-Dependent Local Density Approximation Ionization Threshold , J. Chem. Phys., 108, 4439. 

The Bell equation gives the correct behavior for the ionization cross section at both high- and low-impact energies. In cases where autoionization is important it is not always possible to reproduce the cross section from the single equation above, but if it is used in two separate fits, one from the ionization threshold to the autoionization threshold, and the second above the autoionization threshold, a good fit to the cross section may be obtained over the entire range. 

None of the three theories used to calculate electron impact ionization cross sections could be considered to render the others obsolete. The BEB method gives the best fit to the functional form of the ionization efficiency curve for small molecules, it provides a better fit to the experimental data closer to the ionization threshold than the other methods, but it underestimates the maximum ionization cross sections for heavier molecules. The DM method provides a better fit to the ionization efficiency curves for the heavier molecules, especially for electron energies greater than max, but it tends to overestimate the cross sections for heavier molecules and it underestimates E for lighter molecules. The EM method performs as well as the other methods for the value of amax for the light molecules but underestimates the cross sections for heavy molecules by a factor similar to the overestimation of the DM method. The polarizability method outperforms the BEB and the DM methods for the calculation of and when combined with the value from the EM calculation reproduces the ionization efficiency curve as well as the BEB method. 

At least seven modes of dissociation are theoretically possible below the ionization threshold, although their total yield in radiolysis is small (Platzman, 1967). The dissociation products are H, H2, O, and OH, where the first two are in their ground (electronic) states but the last two may be either in ground or excited states. Only two modes of dissociation, H20 -H + O and H20 H + OH, are possible for all excitation energies UV photolysis indicates that the latter process is by far (90%) the most likely. Accordingly, in radiolysis there is a tendency to lump the decay of all excited states of the water molecule into H and OH. 

Here at least nine dissociative channels are theoretically accessible below the ionization threshold. The dissociation products are NH2 -NH + H2, NH2 + H, or NH + 2H (final state), of which NH and NH2 may exist either in the ground or an excited state. Production of molecular hydrogen is negligible at low excitation energies, but it can account for 15% or more of the dissociation processes when the excitation exceeds -7 eV Note that the lowest excited state of NH3 ( 4 eV) does dissociate into NH and H2 but is spin forbidden. 

That propane is indeed formed by H2 reaction is known by observing the distribution of yields of various isotopic compositions of propane from the radiolysis of an equimolar mixture of cyclopentane and deuterated cyclopentane. Further evidence is provided by the facts that (1) propane is not formed by photolysis below the ionization threshold, and (2) an electric field has no effect on the yield. 

Figure 11. C2H4 ion yield as a function of time in femtoseconds for a pump-photoionization probe experiment. Heavy line Predicted ion yield using the AIMS data and assuming an ionization threshold of 3.5eV. Dashed line Exponential fit to the AIMS ion yield predicting an excited state lifetime of 35 fs. Gray shaded area Reported ion yield [152] obtained using an exponential fit to the experimental data predicting an excited state lifetime of 30 15 fs. (Figure adapted from Ref. 214.)...

The low BE region of XPS spectra (<20 — 30 eV) represents delocalized electronic states involved in bonding interactions [7]. Although UV radiation interacts more strongly (greater cross-section because of the similarity of its energy with the ionization threshold) with these states to produce photoelectrons, the valence band spectra measured by ultraviolet photoelectron spectroscopy (UPS) can be complicated to interpret [1], Moreover, there has always been the concern that valence band spectra obtained from UPS are not representative of the bulk solid because it is believed that low KE photoelectrons have a short IMFP compared to high KE photoelectrons and are therefore more surface-sensitive [1], Despite their weaker intensities, valence band spectra are often obtained by XPS instead of UPS because they provide... 

Given that the absorption cross-section of 07+ at the photo-ionization threshold is 10-19 cm2 and that the oxygen abundance is 10-2 by mass, find the bound-free opacity (cm2 gm-1) due to oxygen at that frequency, and compare it to the electron scattering opacity. 

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