Photoionization, large molecules

The idea of photoionization spatial localization of the molecular bonds or chromophore of a large molecule based on a combination of spectrally selective photoionization of a chosen bond or chromophore of the molecule with electron or ion microscopy is shown on Fig. 1 for a particular case of projective microscopy. A hemispherical tip of a needle with a radius of curvature... 

The simple estimates show that with photoion microscopy it will be possible to reach resolutions of several angstroms, which may be sufficient for the visualization of large molecules. 

Femtosecond photoionization mass spectrometry might be useful in the study of the three-dimensional structure of large biomolecules. When a selectively excitable and ionizable chromophore is located on the outer (surface) part of large molecule, one can be detached in the picosecond time scale. However, when the excitable chromophore is located in the inner part of the big molecule, its detachment will require a much longer time, which is needed for spatial rearrangement of the molecule. So, even the simple mass spectrometry of bioorganic molecules with femtosecond laser ionization can reveal some details of their spatial structure. 

Figure 20. Photoionization map for toluene, in the triangular representation. The low ionization energy states of the dication are weakly populated by direct photoionization but strongly populated by the inner valence Auger effect, giving deeply hollow electron distributions. Valence photoionization maps for large molecules are generally like this, with the hollowness becoming more extreme for the largest molecules.

Fig. 4.41 Spectroscopy of fragmentation products produced by photoionization and excitation of large molecules (a) experimental setup (b) level scheme and (c) spectrum of the fragment ion...

In most photodissociation and photoionization studies attention has been focused on characterizing the potential surfaces involved. However, when large molecules are considered, such as those of biological interest, the complexity of their spectra presents a... 

A possible role for shape resonances has been postulated in a number of the photoionization studies mentioned above [52, 53, 57, 60], although it has to be noted that except for camphor [57], the evidence for the existence of the shape resonance is not definitive. (It also then remains an open question how any such resonances, inferred from fixed geometry calculations, would manifest themselves in practice in large, and sometimes floppy, molecules, such as these chiral species.)... 

For the purposes of this review it is convenient to focus attention on that class of molecules in which the valence electrons are easily distinguished from the core electrons (e.g., -n electron systems) and which have a large number of vibrational degrees of freedom. There have been several studies of the photoionization of aromatic molecules.206-209 In the earliest calculations either a free electron model, or a molecule-centered expansion in plane waves, or coulomb functions, has been used. Only the recent calculation by Johnson and Rice210 explicitly considered the interference effects which must accompany any process in a system with interatomic spacings and electron wavelength of comparable magnitude. The importance of atomic interference effects in the representation of molecular continuum states has been emphasized by Cohen and Fano,211 but, as far as we know, only the Johnson-Rice calculation incorporates this phenomenon in a detailed analysis. 

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