Anharmonic effects in large-amplitude vibrations of metal clusters

Two types of extreme collective motion, large-amplitude many-phonon vibration of the ionic core and rotation of the cluster with high angular momenta, are considered. The interplay between vibration and collective motion towards fission is discussed. A new mechanism of formation and rupture of the neck is proposed which is based on the Franck-Condon principle, and accounts for the interplay between vibration and fission. Under rotation, the change of the shape of the cluster and a phase transition from axially symmetric to triaxial ellipsoid are predicted. For studying the effects, vibrational motion can be induced by laser radiation. Rotational motion may arise in collisions of clusters. Received 26 April 2001 and Received in final form 15 October 2001     

Fluorescence induced by double K capture

Features of double fluorescence spectra generated by double K capture in the ⁷⁸Kr atom are considered. The energies and bandwidths of fluorescence are calculated. The latter are approximately four times wider for the first and second quanta, respectively, relative to the vacancy hole in the K shell. The fluorescence yields are calculated as well. They are 0.77 and 0.61 for the first and second quanta, respectively, and 0.47 for both quanta simultaneously.     

Resonance Conversion as the Predominant Decay Mode of ^{229m}Th

Calculations performed within the advanced MCDF method show that the probability of decay of the 3.5-eV level of $^{229m}$ Th through the resonance electronic bridges exceeds the direct radiative nuclear decay width by a factor of around $5\cdot 10^3$ . Experimental consequences for detection of the delayed soft photons or $\alpha$ particles from the decay of the isomer are discussed.     

Internal conversion in hydrogen-like ions

The probability of internal gamma-ray conversion is theoretically investigated for hydrogen-like ions versus the corresponding neutral atoms. The relevant calculations are performed by the relativistic Dirac-Fock method. The results reveal that the effect of multiple ionization on the coefficients of internal conversion in the K shell is maximal near the ionization threshold and for transitions of high multipole order, where this effect can be as great as a few orders of magnitude. The distinction between the coefficient of internal conversion in the K shell of a neutral atom and that in the respective hydrogen-like ion decreases with increasing transition energy, but it remains sizable for transitions of practical importance. It is found that the ionization of an atom to a hydrogen-like ion with allowance for conversion in external atomic shells may change significantly (by up to eight orders of magnitude) the lifetime of the nucleus being considered. The predicted effects can be observed in experiments with beams of relativistic heavy ions.     

Resonance internal conversion as a way of accelerating nuclear processes

A theory of resonance conversion (RC) is presented. It is shown that by resonance conversion being a natural extension of traditional internal conversion, into the subthreshold domain, in a number of cases, it strongly affects nuclear de-excitation. Moreover, as it concentrates transition strength in narrow bands corresponding to atomic spectral lines, it is a unique tool for accelerating nuclear processes. Along with the wellknown process of non-radiative nuclear excitation through electron NEET transition and the inverse RC process, resonance conversion provides convenient mathematics for a number of crossing-invariant processes involving a nucleus and electrons, excitation and de-excitation of nuclei, by a hyperfine magnetic field, spin mixing of nuclear states via an electron shell, a hyperfine interaction and magnetic anomalies in an atomic spectra, and the excitation of nuclei in collisions accompanied by the ionization of an electron shell, in muon decay in the orbit, etc. Mechanisms of isomer pumping via a laser-radiation-induced RC and of isomer energy triggering in a resonance laser radiation field are considered. An especially strong effect can be obtained in hydrogen-like ions, with practically no RC damping. The theory is also generalized to the case of discrete Auger transitions.     

Prospect of triggering the <Superscript>178m2</Superscript>Hf isomer and the role of resonance conversion

A mechanism of triggering the 12.7keV E3 transition, based on the new decay mode of the 31y isomer via resonance internal conversion and emission of a 1.4keV X-ray quantum, is considered. Actually, this decay mode was observed previously in the decay of 45- and 46-fold ions of ¹²⁵Te . For the purpose of triggering, the atomic radiative vertex has to be induced by resonance radiation. This mechanism makes triggering by an order of magnitude more efficient than triggering a bare nucleus, and is achieved at a lower combination frequency. An experiment is proposed for the direct observation of the new decay mode. This also offers a new way of resonance scattering of these X-rays. Triggering through higher-lying 2573 and 2805keV states is also considered. The results are extended to the general problem of triggering. The main obstacle for enhancing the efficiency is a high internal conversion rate. For this reason, shape isomers with low multipole order --E1 , M1 , and with a high enough energy of triggering transition are of interest for triggering. The partial ionization of the outer electrons will also help. The same recommendations hold for triggering isomers in laser-produced plasma.     

The muon capture in <Superscript>16</Superscript>O: the angular and polarization correlations

Longitudinal polarization of the daughter nuclei ¹⁶N which arises in μ⁻ capture on ¹⁶O as a function of the recoil angle, together with the angular distribution and the alignment of the recoil nucleus are calculated. The neutrinos born escape mainly along the muon spin. The polarization is found to vary from zero (recoil momentum counter to the muon spin direction) up to 50% (along the muon spin direction). The results can be applied to the experimental tests of T conservation, to the analysis of the projects of constructing the powerful mono-energetic neutrino sources, to the experimental study of the pseudo-scalar form factor and the K-electron capture, and to other spin-polarization correlation experiments.     

Nonlinear effects in resonance conversion as a test of QED

Various phenomena such as hyperfine shift of ²⁰⁹Bi⁸²⁺, nuclear decay of ²²⁹Th, and ¹⁶⁹Yb⁶⁹⁺ are considered as cross-invariant channels of the internal conversion process from the viewpoint of testing QED. The way of accelerating nuclear decay in a laser field through merging nuclear and laser photons is discussed. Discovering this fundamental process would give a convincing test of both QED along with our experimental capabilities.     

Nonresonance Shake Mechanism in Neutrinoless Double Electron Capture

Physics of Atomic Nuclei - It is generally accepted that neutrinoless double electron capture is a resonance process. The probability for the shake followed by electron-shell ionization occurring...