The photodetachment cross section of has been measured in the photon energy range 2.9-3.3 eV in order to investigate doubly excited states. Measurements were made channel specific by selectively detecting the residual He atoms left in a particular excited state following detachment. Three Feshbach resonances were found in the partial cross section: a resonance below the threshold and two resonances below the threshold. The measured energies of these doubly excited states are 2.959 260(6), 3.072(7) and 3.264 87(5) eV. The corresponding widths are found to be 0.20(2), 50(5) and 0.61(5) meV. The measured energies agree well with recent theoretical predictions for the , and states, respectively, but the widths deviate noticeably from calculations for and states.
A Feshbach resonance associated with the
1s3s4s4S state of He${}^{\ensuremath{-}}$ has been observed in the He
(1s2s3S) + ${e}^{\ensuremath{-}}(\ensuremath{\epsilon}s)$ partial photodetachment cross section. The residual He
(1s2s3S) atoms were resonantly ionized and the resulting He
+ ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy
Er=2.959255(7) eV and a width $\ensuremath{\Gamma}=0.19(3)$ meV, in agreement with a recent calculation.
Recording the yield of
He(1snl3L) Rydberg states for
n=11−−14, we measure the photodetachment cross sections of metastable ${\mathrm{H}\mathrm{e}}^{\ensuremath{-}}(1s2s2p^{4}P^{o})$ ions in the vicinity of the two-electron escape threshold. We observe a large number of double Rydberg ${\mathrm{H}\mathrm{e}}^{\ensuremath{-}}$ quartet state resonances and report energies and widths of intrashell states in the
n=13−−15 manifolds. Sharp thresholds are measured at
He(3Po) and
He(3De) Rydberg states with preference for population of the former, whereas the
He(3Se) states are not populated, in agreement with qualitative theoretical arguments.
Absolute total cross sections for electron scattering from He and Ne are obtained in the energy range from 20 eV down to below 10 meV with a very narrow electron energy width of 6--8 meV using the threshold-photoelectron source. Total cross sections obtained in the present study generally agree well with those obtained in the previous experiments for both He and Ne above 100 meV, where several experimental works have been reported. Comparison of the present cross sections with the theoretical cross sections which have been regarded as the ``standard'' cross sections shows very good agreement even at very low energies below 10 meV, which confirms the validity of theoretical cross sections. The scattering lengths for electron scattering from He and Ne are also determined from the present total cross sections using the modified effective range theory. The resonant structures in the total cross sections due to Feshbach resonances of He and Ne are also observed. Analysis of the resonant structure was carried out based on the spin-dependent resonant scattering theory in order to determine the values of natural width of Feshbach resonance of Ne.
A collinear beam apparatus has been used to study photodetachment of Cs${}^{\ensuremath{-}}$. Partial cross sections were measured using state-selective detection based on a resonance ionization scheme. Several resonances were observed in the Cs(
102S), Cs(
62F), Cs(
62G), and Cs(
62H) final-state channels below the Cs(
102P3/2) channel opening. A model was developed to account for the interference between overlapping resonances. This model is essentially a generalization of the Fano [U. Fano, Phys. Rev. 124, 1866 (1961)] and Shore [B. W. Shore, Phys. Rev. 171, 43 (1968)] parametrizations for single resonances. Resonance parameters were extracted by fitting the model to the data sets. The openings of the Cs(
102S) and Cs(
62F) channels, where the polarizabilities of the atomic states are large and positive, showed rapid onsets. In the case of the Cs(
62G) and Cs(
62H) channels, on the other hand, the photodetachment cross sections increased slowly with energy. For the Cs(
62H) channel this is the expected behavior, since it is the result of a large and negative polarizability of the
62H state. In addition, the excitation of the Cs(
62H) state with respect to the Cs ground state was found to be 28 356.3(2) ${\mathrm{cm}}^{\ensuremath{-}1}$, in agreement with a previous experiment.
In a recent experiment, Lindahl et al. ( Lindahl A. O. et al. , Phys. Rev. Lett. , 108 (2012) 033004) observed a new threshold behavior in the photodetachment of the potassium negative ion when the residual atom is left in the state. Unfortunately the resonance close to threshold made it more difficult to apply the semi-classical model that was developed to describe the threshold behavior. In this paper we present a study of the threshold behavior of the partial cross-section in the photodetachment of the sodium negative ion. The experiment was conducted using a collinear beams apparatus where the detection was performed using a resonance ionization scheme. We observe the same threshold behavior as in the previous experiment, but without any obfuscating resonance. The model derived in the previous paper is found to fit the observed cross-section up to 35 meV above threshold. The present experiment clearly demonstrates the threshold behaviour for the fundamental process of a free electron moving in the field of an atom with a large negative polarizability.
The resonance and near-threshold structures of low-energy electron interaction with Sr and Ba atoms have been investigated by carrying out respectively 11-state and 21-state close-coupling calculations within the R-matrix scheme. Part of the core–valence correlation is included by exciting one core electron to the valence orbitals. Results for the low-energy electron scattering and the photodetachment of the negative ions are presented. For Sr, the results show that there is a 2D shape resonance around 0.8 eV above the elastic scattering threshold and a remarkable structure just above the first 3Po excitation threshold. For Ba, two 2D shape resonances are predicted, just above the elastic scattering threshold and the 3D excitation threshold, respectively.