Photoionization resonance structure

Fig. 8.4 Relative ground state photoionization cross section as a function of laser wavelength in Rb in the presence of a 4335 V/cm field. Note the relative gain and offset settings. For the light polarization parallel to the electric field (lower trace), field dependent resonance structure extends beyond the zero field limit. No structure is observed for the case of light polarized perpendicular to the field (upper trace) (from...

Dehmer and Pratt were one of the earliest groups to observe resonance structures in the photoionization efficiency (PIE) curves of small argon ions and mixed rare gas cluster ions (Dehmer and Pratt 1982 Pratt and Dehmer 1982). The structures in the PIE curves of small argon cluster ions were attributed to interband transitions while the features observed in the PIE curves of mixed rare gas cluster ions were suggested to arise from autoionizing states. Walters et al. (1985) also observed similar resonances in the PIE curves of C6H6/HC1... 

In closing this section, we note that although the Koopmans picture is a simplification of the ionization dynamics, it provides a very useful zeroth order picture from which to consider the TRPES results. Any potential failure of this independent electron picture can always be experimentally tested directly through variation of the photoionization laser frequency resonance structures should lead to variations in the form of the spectra with electron kinetic energy, although the effect of resonances is more likely to be prominent in PAD measurements, and indeed an observation of a shape resonance in p-difluorobenzene has been reported [153, 154]. 

At higher energy, there is resonance structure in the 4s cross section due to 3p- s,nd excitations. These resonances also exhibit spin-orbit splitting. Calculations S of the 4s cross section in the region of these resonances are shown in Fig. 11 and compared with experimental results by Mansfield and Newsom.79 iSpin-orbit effects were only calculated for the six lowest ns and nd resonances. The resonance structure has not been drawn all the way to the 3p /2 and 3p3/2 edges. In addition, it should be noted that the upper experimental scale is not linear so care should be taken in making comparison of theory and experiment. There remains a need to calculate the photoionization with excitation cross sections leaving Ca" " in the 3d and 4p levels. Also, the doubly-excited states were only... 

During the coming years, one can expect the activity in this field to become even more intense, in part because of the increase in the number of synchrotron facilities and techniques for photoelectron spectroscopy. It is expected that there will be very extensive measurements of photoionization with excitation including resonance structure and double photoionization over wide energy ranges including studies of inner shells. There will be measurements of partial cross sections, angular distributions, and spin polarizations. This last topic, which has recently received considerable attention,was deferred to the paper by W. Johnson in this volume since it depends on relativistic effects. 

Figure 10. Phase lag spectrum of HI in the vicinity of the fe3IIi Breit-Wigner resonance. Panel (a) shows the phase lag between the photoionization of HI and H2S. Panel (b) is the three-photon photoionization spectrum of HI, showing the rotational structure of the two-photon fe3IIi - X1 1 transition. The bottom two panels are the one-photon ionization spectra of HI and H2S. (Reproduced with permission from Ref. 33, Copyright 2000 American Physical Society.)...

Some of the earliest applications of MQDT dealt with vibrational and rotational autoionization in H2 [21-25]. One concept that emerged from these studies is that of complex resonances [26], which are characterized by a broad resonant distribution of photoionization intensity with an associated rather sharp fine structure. These complex resonances cannot be characterized by a single decay width they are the typical result of a multichannel situation where several closed and open channels are mutually coupled. The photoionization spectrum of H2 affords a considerable number of such complex resonances. 

Therefore, heterogeneous catalysts present a greater potential for the application of HT and Combinatorial methods, because they involve diverse compositional phases that are usually formed by interfacial reactions during their synthesis, which in turn produce a variety of structural and textural properties, often too vast to prepare and test by traditional methods. In this respect the HT and Combinatorial methods extend the capabilities of the R D cycle, which comprises the synthesis, the characterization of physicochemical properties and the evaluation of catalytic properties. The primary screening HT method gives the possibility of performing a rapid test of hundreds or thousands of compounds using infrared detection methods [27-29]. Alternatively, a detection method called REMPI (Resonance Enhanced Multi Photon Ionization) has been used, which consists of the in situ ionization of reaction products by UV lasers, followed by the detection of the photoions or electrons by spatially addressable microelectrodes placed in the vicinity of the laser beam [30, 31]. 

Brouwer and Wilbrandt have applied resonance Raman spectroscopy and calculations to questions of structure of amine radical cations [73]. Well-resolved Raman spectra of trialkylamine radical cations that are so short-lived that their electrochemical oxidation waves are irreversible may be obtained at room temperature in solution by photoionization and time-resolved detection. Comparison of the observed spectrum with calculations for various isomers provides a powerful method of answering structural questions. Density-functional calculations prove much easier to apply to open-shell species than Hartree-Fock calculations, which require cumbersome and expensive corrections to introduce suffieient electron correlation to eonsider questions like the charge distribution of disubstituted piperazine (1,4-diazacyclohexane) radical cations. The dimethyl- and diphenyl-substituted piperazine radical cations are delocalized, but charge is localized on one ArN unit of the dianisyl-substituted compound [73dj. 

Another recent advance in electron-molecule resonances is their role in molecular autoionization and photoionization. Here they show up as an exit-channel effect. The increasing availability of synchrotron sources and the proliferation of high-resolution laser spectroscopic techniques are leading to expanded interest in these processes because of the necessity to interpret the resonance features for a greater variety of molecules of chemical interest. Electron-molecule shape resonances are also responsible for structure in inner-shell electron energy-loss spectra in the region around the core ionization threshold acting as a final-state interaction, the same resonances... 

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