Resonance, absorption unresolved

Recent reactivity measurements from Barwell are found to be in satisfactory agreement with the present data. The estimated resonance absorption integral for zero-fliick-ness sheets ts 22.7 2.5 bams based on the combined data. Of this, approximately four haras is due to unresolved resonances above 700 ev. The latter estimate was found by comparing the data to predictions based on known resonance parameters. ... 

In indirect methods, the resonance parameters are determined from the energy dependence of the absorption spectrum. An important extra step — the non-linear fit of (t E) to a Lorentzian line shape — is required, in addition to the extensive dynamical calculations. The procedure is flawless for isolated resonances, especially if the harmonic inversion algorithms are employed, but the uncertainty of the fit grows as the resonances broaden, start to overlap and melt into the unresolved spectral background. The unimolecular dissociations of most molecules with a deep potential well feature overlapping resonances [133]. It is desirable, therefore, to have robust computational approaches which yield resonance parameters and wave functions without an intermediate fitting procedure, irrespective of whether the resonances are narrow or broad, overlapped or isolated. 

Another line-broadening mechanism is illustrated in Figure 6 chemical "exchange" between two sites of different resonant frequency. Although the absorption spectrum again shows just a single unresolved peak, the DISPA plot exhibits substantial displacement outside the reference circle, and the magnitude of the displace-... 

Figure 10 shows linearized DISPA plots for two simulated non-Lorentzian line shapes. For example. Figure 10a shows the DISPA deviations for a line shape consisting of an unresolved doublet of two Lorentzians of different resonant frequency. The maximum (upward) displacement from the reference line clearly occurs at frequencies less than one half-width away from the observed absorption maximum. In contrast. Figure 10b shows that for a log-Gauss distribution in relaxation time, the maximum displacement (downward this time) occurs at a frequency approximately equal to the half-width at half-maximum height of the observed absorption peak. 

The measured and predicted spectra were normalized to a constant value of integrated fissions and are compared In Fig. 2. The results demonstrate the need for consideration of unresolved resonance self-shielding in U, and suggest that the U absorption cross-section data may be slightly high at low energies (100 eV < E < 2 keV). 

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