Search Results (16 total)

The giant (or shape) resonance in the photodetachment cross section of D⁻ above the n=2 threshold has been subjected to high-resolution vacuum ultraviolet spectroscopy performed by Doppler tuning of ions stored in the ASTRID storage ring. In order to prevent changes in the overlap between laser and ion beams over the large kinetic-energy range employed in the experiment, a new ion-beam-positioning method based on a quadrupole-shunt technique was applied. The study presents an accurate measurement of parameters for the resonance, which is also denoted ₂{0}₂^+ 1P^o. The resonance has a width of 26(2) meV while the asymmetry parameter q of the Fano profile is 3.2(0.4), which is at variance with the most recent theoretical calculations.

Recording the yield of He(1snl³L) Rydberg states for n=11–14, we measure the photodetachment cross sections of metastable He^-(1s2s2p⁴P^o) ions in the vicinity of the two-electron escape threshold. We observe a large number of double Rydberg He^- quartet state resonances and report energies and widths of intrashell states in the n=13–15 manifolds. Sharp thresholds are measured at He(³P^o) and He(³D^e) Rydberg states with preference for population of the former, whereas the He(³S^e) states are not populated, in agreement with qualitative theoretical arguments.

The aim of this article is to demonstrate the practical value of radioscopic differentiation of materials in the 1–10 MeV energy range to the work of customs services. The proposed method for achieving singling out and identifying four basic groups of materials according to an atomic number is complex. Atomic numbers are identified using high- and low-energy profiles obtained through the irradiation of materials on an alternate pulse-by-pulse basis. This is done using a bremsstrahlung beam with 8   MeV/4   MeV dual boundary energies and by using scintillating crystals coupled with silicon photodiodes as detecting elements. An image segmentation technique is then used to discern the distribution of an atomic number on any given image. The color visualization of integral absorption and a material’s atomic composition is carried out according to the intensity hue saturation (IHS) colorization scheme. The experiments were carried out on a full-scale prototype of an 8 MeV customs inspection system developed by the Efremov Research Institute.

The exponential law is valid both for decay from a single quantum state into a continuum and for an ensemble maintained in thermal equilibrium. For statistical decay of an ensemble of isolated systems with a broad energy distribution, the exponential decay is replaced by a 1/t distribution. We present confirmation of this decay law by experiments with cluster anions in a small electrostatic storage ring. Deviations from the 1/t law for such an ensemble give important information on the dynamics of the systems. As examples, we present measurements revealing strong radiative cooling of anions of both metal clusters and fullerenes.

We report the observation of the predicted metastable 4p^{3 4}S state in the Ca^- ion utilizing a spectroscopic technique based on nonlinear resonant multicolor absorption via an autodetaching state. The binding energy of the ⁴S state is determined to be 586.86(10) meV with respect to the parent Ca(4p^{2 3}P) state, a value less than 3 meV from the predicted one. The lifetime of the ⁴S state is determined to be larger than 7 ps.

The shape parameters for the lowest-lying ¹P^o resonance, ₂{0}₃^-, of D^- have been measured using high-resolution vacuum-ultraviolet spectroscopy. The experiment was performed at the storage ring ASTRID, and the resonance was resolved by applying electron cooling to reduce the velocity spread of the ion beam. The resonance has a width of 37(3) μeV while the asymmetry parameter q of the Fano profile is -16(3). These values present the first critical test of a large number of theoretical calculations.

Experimental investigations of the photodetachment cross section for the negative hydrogen ion in the region near the H(n=2) threshold are discussed. Doppler-tuned spectroscopy in a collinear geometry is used to obtain a comparatively high resolution in this photon-energy range. The ions are accelerated in the ASTRID storage ring, which allows an accurate velocity measurement and further enables the application of momentum-spread reduction techniques. The fixed-frequency vacuum-ultraviolet laser beam (118 nm) is generated by sum-frequency mixing in a xenon gas cell. The photodetachment cross section exhibits pronounced resonances that correspond to the rich spectrum of doubly excited ¹P states near the H(n=2) threshold. The position of the resonances is determined with an accuracy that challenges the current theoretical developments. The experimental observations are used to predict the behavior of a dipole series below H(n=2). The measurement on both hydrogen and deuterium facilitates a study of isotope effects. By means of momentum-spread-reduction techniques it should be possible to resolve the natural linewidth of the narrow dipole resonances. Preliminary studies show that improvements in electron cooling of the H^- beam promise to reach this limit. Finally, the feasibility of extending the high-resolution work to higher-lying (n=3) resonances is briefly discussed.

The two lowest-lying members of the ¹P⁰ dipole series of autodetaching resonances in H^- located just below the H(n  =  2) threshold have been observed and characterized, utilizing Doppler-tuned collinear laser spectroscopy. The resonance positions have been determined for both H^- and D^-, allowing the first critical test of the predicted isotope effects. The experimental isotope shifts are consistent with a small specific mass shift. We interpret these results as evidence for a rather small electron momentum correlation.

The binding energy and fine-structure splitting of the ground state of the Sr^-n(5s²5p ²P) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17±0.03 meV (J=3/2) and 52.06±0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89±0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].

A new nonlinear laser technique, yielding accurate and detailed information about the photodetachment process, is applied to the study of excited states in atomic negative ions. The method has facilitated the first experimental observation of the controversial ⁴P state in the Ca^- ion and the homologous state in the Sr^- ion and also provided comprehensive information about their binding energies, fine-structure intervals, and autodetachment lifetimes, together with the absolute strengths of the intercombination transitions to the ²P ground states.

This paper presents theoretical ab initio calculations and experimental measurements of the binding energy of the metastable He^-1s2s2p ⁴P ion. The calculated 77.518±0.011 meV and the measured 77.516±0.006 meV values for the binding energy are in excellent agreement and they represent a significant improvement in the accuracy compared to previous studies. The experimental technique is based on the determination of the εs-wave Wigner threshold associated with detachment to the 1s3s ³S state of the neutral He atom. The yield of the 1s3s ³S He level was monitored by applying resonant ionization spectroscopy.

The negative hydrogen ion has been investigated by a new spectroscopic approach in the region near the n  =  2 threshold of the neutral hydrogen atom. The technique is based on Doppler-tuned spectroscopy using an H^- beam stored in a heavy ion storage ring collinearly overlapped with a fixed-frequency narrow-bandwidth vacuum ultraviolet laser beam (118 nm). The position of the Feshbach resonance is determined as 10.9243(3) eV which is in good agreement with recent theoretical calculations but deviates significantly from the previously reported experimental result.

The binding energies of the 4s²4p ²P ground state in Ca^- have been determined by means of state-selective photoionization of Ca atoms produced in s-wave photodetachment to be 24.55±0.10 meV for the J  =  1/2 and 19.73±0.10 meV for the J  =  3/2 component. The sensitivity of the detection technique applied has also made it possible to disprove previous claims for the observation of a metastable 4s4p^{2 4}P state in Ca^-.

By using a combination of laser photodetachment and resonant ionization spectroscopy, the binding energy of the 1s²2s2p^{2 4}P state in Be^- is determined with much improved accuracy to be 290.99±0.10 meV. This value is considerably larger than the recent measurement of 261±10 meV, but only slightly above the new theoretical value of 289.1±1.0 meV. The transition wavelength of 1s²2s2p^{2 4}P_{3 / 2-1s²2p³⁴S3 / 2} is determined by laser excitation to be at 2653.18±0.08 Å, yielding the binding energy of the 1s²2p^{3 4}S state in Be^- to be 295.49±0.25 meV, in good agreement with the newest theoretical result of 295.0±0.7 meV.

A new technique based on the combination of laser photodetachment and resonance ionization spectroscopy has been used to characterize the state composition of a fast, negative-ion beam of barium and to determine the binding energies of its long-lived components, the metastable 5d6s6p ⁴F_9/2 state and the 6s²6p ²P_3/2,1/2 ground state, with an accuracy previously inaccessible for the weakly bound, negative alkaline-earth ions. Additional information about the Ba^- states was obtained from lifetime studies in the ASTRID storage ring.