Ionization transitions

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

A Harnett analysis provides p = 0.33 suggestive of a not-strongly ionized transition state. T. Kamitanaka, T. Matsuda, T. Haradon, Tetrahedron Letters 2003,44. 4551... 

The branching ratios are proportional to the Franck-Condon factors for the molecular ionization transition, with the projectile at distance R from the molecule. The bl,(R) depend parametrically on R. For the bj,(R) to be well defined in an actual collision with velocity R, it must be true that R is small compared to the vibrational intramolecular motion. This is a severe limitation, even at thermal velocities. Therefore, only when the interaction between the projectile and the molecule is weak, leading to a correspondingly weak variation of the b (R) in the course of the collision, approximation (III.2) may be used. We may then further approximate by introducing an average branching ration defined by... 

The strength of the interaction in the course of a collision leading to population of vibrational states of a certain electronic state can be judged from the widths of the individual vibrational lines in a measured spectrum. If the lines are narrow and well separated, the average interaction is weak, and we can expect that the approximations leading to (III.4) are valid. In all these cases, however, it is also expected that the Z>,(t>) deviate very little from the Franck-Condon factors for the same ionizing transition in the unperturbed molecule caused by photoionization. 

The protonated cluster ions were observed to be the major product ions in the CMS of ammopia, water, etc., although unprotonated cluster ions have also been observed in these cases, depending on the choice of carrier gas, method of ionization, and electron energy utilized in the investigation (Shinohara et al. 1985, 1986). The inability to observe unprotonated cluster ions is usually attributed to poor Franck-Condon factors for the vertical ionization transitions. These poor Franck-Condon factors arise from the large differences in the configuration of the neutral and ionic clusters (Stace 1987a). 

Naturally, since this expression is for a property measured in the reference frame connected to the molecule, molecular rotations do not appear in this expression. The range of L in the summation Eq. (53) is 0... 2/m ix, and includes both odd and even values. In general, the MF PAD is far more anisotropic than the LF PAD, for which L = 0,2,4 in a two-photon linearly polarized pump-probe experiment in the perturbative limit. Clearly, the MF PAD contains far more detailed information than the LF PAD concerning the ionization dynamics of the molecule, as well as the structure and symmetry of the electronic state from which ionization occurs, since the partial waves that may interfere are no longer geometrically limited as they are for the LF PAD. The contributing MF ionization transition dipole components are determined by the laser polarization... 

In Figure 31, we compare the associated spectrum of the fast component in 9-methyl adenine with calculated [194] Franck-Condon structures for the 7i7t —> Dq(n 1) +e (solid line) and 7171 —> Diin 1) + e (dash-dotted line) ionizing transitions. The two separated peaks agree well with the FC calculations, strongly suggesting that the short-lived state in 9-methyl adenine is the 7i7t state. By contrast, adenine contains an additional contribution that... 

Figure 31. Decay associated spectra of the short-lived state compared with calculated FC spectra for 9-methyl adenine (a) and adenine (b). In 9-methyl adenine, the nn — >o(ti-1),D2(ti-1) transitions leave a FC gap. In adenine, this gap is filled by the Tier ionizing transitions. See color insert.

Table 13 Calculated (ADC(3)) energies (E, eV) and Intensities (P) of the outer- and Inner-valence orbital vertical Ionization transitions In thiophene ...

Figure 15 Potential energy curves showing the form of vibrational wavefunctions of a molecule and states of a molecular ion to illustrate the origin of vibrational excitation in ionizing transitions. The resultant PE bands are represented on the right...

Demchenko and co-workers demonstrated that STaz glycosides can be engaged in stable non-ionizing transition-metal complexes. This observation served as a basis for the development of a temporary deactivation technique for oligosaccharide synthesis (Scheme 28).158 The deactivation of the otherwise reactive building block was... 

Arginine, lysine, and histidine have (+1) to (0) ionization transitions, while aspartic acid, glutamic acid, cysteine, and tyrosine have (0) to (-1) transitions. 

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). 

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