Survey electronic EEL spectrum of one layer of cytosine deposited on a three-layer spacer of Ar. This spectrum results from averaging the scattered intensities of ten spectra recorded for electron incident energies between and 18 eV. The thin solid line passing through the spectrum results from curve fitting with multiple Gaussian functions to delimit the different electronic transitions, as shown in the bottom of the figure. Their energy positions and widths are listed in Table I. The dashed line accounts simply for the background contribution arising from multiple energy-losses in the substrate.
(a) Electronic EEL spectra of one layer of cytosine deposited on a three-layer spacer of Ar for different incident electron energies from 5 to 12 eV and (b) from 13 to 18 eV. The vertical scale is normalized so as the integral over an energy-loss range can be expressed in terms of a cross section value. The thin solid line passing through each spectrum results from curve fitting with of the multiple Gaussians obtained in Fig. 1 by varying only their amplitudes (i.e., keeping their energy positions and widths fixed), as shown in the bottom of (a).
[(a) and (b)] Electron impact energy dependences of the cross sections to excite and ionize various electronic states of cytosine. The values of the cross sections resulting from fitting with multiple Gaussian functions are listed in Table II. The shaded rectangles in (a) represent the energy ranges of the core-excited electron resonances that have been observed in anion fragment yields in DEA experiments with cytosine in the gas phase (Ref. 14). Open triangle in the bottom of (b) is the total ionization cross section calculated by Mozejko and Sanche (Ref. 65).
Electronic excited states of cytosine from ab initio calculations, EEL spectroscopy in the gas phase and vuv of sublimed films compared to EEL spectroscopy of cytosine deposited on an inert argon substrate. , , , and , represent the vertical ionization energies of the molecule. Values within parentheses are the FWHM of the excitation bands. All energies are in eV.
Cross section for electronic excitations of cytosine by electron impact at different energies (eV). The energy and FWHM of the Gaussian distributions corresponding to the excitation bands are reported in Table I.
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