Specific electron loss spectra

The debate over the site of electron loss in DNA is much less pronounced since it has been estimated that over 90% of the cations generated in DNA are centered on guanine [67] and guanine end products account for 90% of the electron loss products in DNA [70]. However, the spectra of G recorded in solid-state studies of nucleotides and nucleosides do not correspond to the spectrum recorded in fiiU DNA [71] and investigations of the strand-break specificity determined that some adenine cations could be generated [64]. Thus, it is also possible that other cations are formed, primarily A. ... 

Figure Bl.25.6. Energy spectrum of electrons coming off a surface irradiated with a primary electron beam. Electrons have lost energy to vibrations and electronic transitions (loss electrons), to collective excitations of the electron sea (plasmons) and to all kinds of inelastic process (secondary electrons). The element-specific Auger electrons appear as small peaks on an intense background and are more visible in a derivative spectrum.

A secondary hydrogen isotope effect has been observed in the loss of methyl radical from the -butylbenzene ion. In the El mass spectrum, the isotope effect IchJIct>3 was 1.1 and for metastable ions the isotope effect ranged from 1.5 to 1.9 [642], The isotope effect in the mass spectrum is equal to the ratio of the stretching frequencies of the dissociating bonds. The isotope effect on the metastable ions has been discussed in terms of an excited electronic state and specific radiationless transitions. The isotope effect is, however, explicable within QET. If the critical energy for CHj loss is 3.4 kJ mole-1 less than that for CDj loss, an isotope effect of about 2 is predicted by QET for metastable ions [853]. 

Electron spin echo spectroscopy (ESE) monitors the spontaneous generation of microwave energy as a function of the timing of a specific excitation scheme, i.e. two or more short resonant microwave pulses. This is illustrated in Fig. 7. In a typical two-pulse excitation, the initial n/2 pulse places the spin system in a coherent state. Subsequently, the spin packets, each characterized by their own Larmor precession frequency m, start to dephase. A second rx-pulse at time r effectively reverses the time evolution of the spin packet magnetizations, i.e. the spin packets start to rephase, and an emission of microwave energy (the primary echo) occurs at time 2r. The echo ampHtude, as a fvmction of r, constitutes the ESE spectrum and relaxation processes lead to an irreversible loss of phase correlation. The characteristic time for the ampHtude decay is called the phase memory time T. This decay is often accompanied by a modulation of the echo amplitude, which is due to weak electron-nuclear hyperfine interactions. The analysis of the modulation frequencies and ampHtudes forms the basis of the electron spin echo envelope modulation spectroscopy (ESEEM). 

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