Shape resonance

Abstract: Electron transmission spectroscopy is used to study shape resonances (temporary negative ions) in benzene and some isolectronic N−heterocyclic molecules (pyridine, diazines, and s−triazine), in the energy range 0−6 eV. The lowest shape resonance in each of these molecules exhibits vibrational structure which is interpreted in all cases as the totally symmetric C−C stretch mode. The ground vibrational level of this lowest shape resonance is accessible by electron impact only in benzene and pyridine. Thus, their electron affinities can be determined from the present experiment (−1.15 eV for C6D6 and −0.62 eV for C5H5N). Only excited vibrational levels are accessible in the diazines and s−triazine, indicating that the electron affinities for these molecules have positive values. For benzene, pyridine, and some other aromatic hydrocarbons, we compare the electron affinities established in the gas phase with the polarographic potentials established in the liquid phase and we find a linear relationship. Using this correlation in conjunction with the measured values of the polarographic potentials, we estimate the electron affinities for pyridazine (0.25 eV), pyrimidine (0 eV), pyrazine (0.40 eV) and s−triazine (0.45 eV).

Abstract: When an excited atomic electron interacts with a neutral perturbing atom or molecule that possesses a shape resonance, it generates a characteristic class of Born-Oppenheimer potential curves that rise with internuclear distance. We document this effect, and predict the existence of a diverse class of stable, strongly bound atom-atom and atom-molecule states that result from this phenomenon. For the specific case in which Rb is the perturbing atom, we show that such states should be observable in the spectroscopy of an ultracold gas or condensate.

Abstract: Electron transmission, inner-shell electron energy loss and magnetic circular dichroism spectra have been analyzed in an effort to trace the positions of the sigma* antibonding valence MOs in benzene and its fluorinated derivatives. The correlation of negative-ion resonances in these systems shows clearly that a sigma* valence level descends with increasing fluorination so as to become the lowest virtual MO in hexafluorobenzene. In addition to the low-lying sigma* negative-ion shape resonances, several negative-ion Feshbach resonances are identified as involving 3s and 3p Rydberg orbitals. The carbon K-shell spectra of benzene and its fluorinated derivatives below the respective C 1s ionization potentials are dominated by excitations to 1..pi..* and 2..pi..* valence levels. A systematic shift of the sigma* levels to lower energy with increasing fluorination is observed which is consistent with the perfluoro effect. Resonances terminating at sigma*(C-C) are found to dominate the C 1s near continuum, with dramatic enhancement of these transitions in the more highly fluorinated species. Investigation of hexafluoro- and 1,2,4,5-tetrafluorobenzene by vacuum-ultraviolet magnetic circular dichroism in the vapor phase confirms the presence of bands which are not ..pi.. ..-->.. ..pi..*. Once again, low-lying sigma* MOs are invoked as terminating orbitals.

Abstract: O16(,) elastic scattering angular distributions have been measured for incident energies 39.3, 49.5, and 69.5 MeV. These data, and previous measurements at 32.2, 104, and 146 MeV, have been subjected to a global optical model analysis. The deduced global potential has two energy-dependent parameters which are found to vary smoothly with energy and it is uniquely determined by the data. Backward angular distributions measured between 40 and 54 MeV are also presented and shown to be nicely reproduced by the model. The sensitivity of the cross sections to the various regions of the real potential has been investigated as a function of energy using the notch test technique. The low energy behavior of the differential cross sections can be understood in terms of the semiclassical decomposition of Brink and Takigawa. A natural extrapolation of the global potential below 30 MeV is shown to reproduce the wide bump observed in the experimental excitation functions around 20 MeV. This bump is shown to be due to an l=8 shape resonance and is interpreted as the J=8+ member of the K=04+ rotational band of Ne20, in contradiction with the current attribution. Other bound and quasibound states supported by the potential are discussed in the light of orthogonality condition model-type arguments and shown to be consistent with the well-known K=01+ and 0- bands, and with the first three states of the K=04+ band of Ne20. NUCLEAR REACTIONS O16(,), measured (), E=39.3, 49.5, 69.5 MeV; global optical model analysis, E=32-146 MeV; semiclassical decomposition of the scattering amplitude; investigation of the compatibility of the potential description with existing low energy data and comparison with cluster models. © 1983 The American Physical Society.