Двухфотонная лазерная генерация экситонных состояний в криокристаллах криптона как метод возбуждения аномального низкоэнергетического изомерного ядерного состояния в изотопе Тория-229

In this paper, based on the two-potential approach combining with the isospin dependent nuclear potential, we systematically compare the α preformation probabilities of odd-A nuclei between nuclear isomeric states and ground states. The results indicate that during the process of α particle preforming, the low lying nuclear isomeric states are similar to ground states. Meanwhile, in the framework of single nucleon energy level structure, we find that for nuclei with nucleon number below the magic numbers, the α preformation probabilities of high-spin states seem to be larger than low ones. For nuclei with nucleon number above the magic numbers, the α preformation probabilities of isomeric states are larger than those of ground states.

The (229)Th nucleus possesses the lowest-energy nuclear isomeric state. Two widely accepted indirect measurements of the transition energy place it within reach of existing laser capabilities. Direct searches for the isomer deexcitation have proven elusive despite extensive effort over the past couple of decades. There is now a growing interest in finding this unique transition because of its potential applications in nuclear, atomic, condensed matter, and optical physics, quantum information, metrology, and cosmology, including the development of a new type of clock based on this nuclear transition. In this Letter we report the first direct observation of the deexcitation of the lowest-lying isomeric state in (229)Th. By collecting (229)Th recoils following the alpha decay of (233)U into MgF(2) plates and measuring the subsequent light emission, we have isolated the isomer deexcitation and measured the transition's half-life to be 6±1 h. Through comparison measurements with (235m)U isomer, we found that the observed (229m)Th deexcitation signal originates from photon emission rather than internal conversion electron emission. This discovery lays the groundwork for optical and laser spectroscopy of (229m)Th nuclear isomer and the development of a (229)Th nuclear clock.

Excitation of deformation vibrations by radiationless muonic transitions

The systematic study of excitons in silver halides goes back to the early Thirties when Fesefeldt (1930) in Gottingen started to measure thin film absorption of these materials. Since that time a huge amount of experimental and theoretical results concerning their optical as well as many other properties was reported and, at the times, reviewed in several articles (for more recent ones see Brown,1973; Kanzaki, 1980; von der Osten, 1982). It is not intended in this lecture to go over the grounds covered by these. Rather its purpose is to describe and interpret more recent results concerning properties of excitons in these materials as revealed by absorption and luminescence thereby stressing one specific aspect, namely exciton relaxation. A central question that we will try to answer is, how an exciton created by optical excitation in some higher excited state will lose its energy during relaxation going into a state of lower energy from where it finally may radiatively return into the crystal ground state.

Resonant oscillators with stable frequencies and large quality factors help us to keep track of time with high precision. Examples range from quartz crystal oscillators in wristwatches to atomic oscillators in atomic clocks, which are, at present, our most precise time measurement devices1. The search for more stable and convenient reference oscillators is continuing2–6. Nuclear oscillators are better than atomic oscillators because of their naturally higher quality factors and higher resilience against external perturbations7–9. One of the most promising cases is an ultra-narrow nuclear resonance transition in 45Sc between the ground state and the 12.4-keV isomeric state with a long lifetime of 0.47 s (ref. 10). The scientific potential of 45Sc was realized long ago, but applications require 45Sc resonant excitation, which in turn requires accelerator-driven, high-brightness X-ray sources11 that have become available only recently. Here we report on resonant X-ray excitation of the 45Sc isomeric state by irradiation of Sc-metal foil with 12.4-keV photon pulses from a state-of-the-art X-ray free-electron laser and subsequent detection of nuclear decay products. Simultaneously, the transition energy was determined as {mathrm{12,389.59}}_{+0.12left({rm{syst}}right)}^{pm 0.15left({rm{stat}}right)},{rm{eV}} with an uncertainty that is two orders of magnitude smaller than the previously known values. These advancements enable the application of this isomer in extreme metrology, nuclear clock technology, ultra-high-precision spectroscopy and similar applications.

In this paper we study the nuclear transitions induced by incident electrons having an energy of several MeV's. We measured the cross sections for the excitation of isomeric nuclear states by 7.7 MeV electrons. The cross sections were 1.2 μb for 111m Cd, 5.5 μb for 113m In and 7.0 μb for 115m In. The peak activation rates were 1.8×10−12 s−1 for 111m Cd, 8.1×10−12 s−1 for 113m In and 1.0×10−11 s−1 for 115m In, for a peak power of the electron beam of 1.8×106 W cm−2. Then we describe for the first time the results of a series of experiments in which samples containing the isomeric nuclei 166m Ho and 186m Re have been irradiated with MeV electron beams. An upper limit of 17 mb has been determined for the cross section of electron-induced γ-emission from 166m Ho and an upper limit of 2.2 mb has been determined for the cross section of electron-induced γ-emission from 186m Re.

The time dependence of delayed radiation is investigated when a pulse of synchrotron radiation undergoes resonance Bragg scattering by the nuclei in a crystal that have an isomeric level with a Mossbauer transition. Instead of the usual exponential law exp(-t/ tau 0) characteristic of an isolated nucleus, a time dependence in the form ( tau 0/t)2 exp(-t/ tau 0) was discovered at small deviations alpha from the Bragg angle. The acceleration of the decay is connected with the collective nature of the excitation of the nuclei in the crystal-the nuclear exciton formation. The exponential law remains at large alpha but the intensity decreases as 1/ alpha 2. In the case of large divergence of the incident beam the law ( tau 0/t) exp(-t/ tau 0) is obeyed. The frequency distribution of the reflected pulse and the possibility of formation of a resonance structure with sufficient resolution are analysed in detail.

An improved procedure for accurate determination of empirical lower state rotational quantum numbers from molecular absorption spectra is demonstrated for methane. We combine the high resolution absorption spectra in the 7070-7300 cm(-1) frequency range obtained in liquid nitrogen cooled cryogenic cell (T = 81 K) and in supersonic planar jet expansion (T(R) = 25 K). Empirical lower state energies of 59 transitions are determined from the ratio of the absolute absorption line strengths at 25 and 81 K. The procedure relies on the realistic description of rotational state populations in the supersonic jet expansion where non-equilibrium nuclear spin isomer distributions are generated due to the rapid cooling. The accuracy of the experimental determination of the lower state energies with J < or = 3 is found to considerably improve the results of the same approach applied to spectra at 296 and 81 K. The 59 transitions with determined lower J values provide a good starting point for the theoretical interpretation of the highly congested icosad region of methane. In particular, the centres of nine vibrational bands are estimated from the transitions with J = 0 lower state rotational quantum number.

Lasers as a bridge between atomic and nuclear physics

A new mechanism for nuclear excitation is studied theoretically by considering the deexcitation of electronic states of atom. When an electron of inner shells is kicked off by the bombarded electron or X ray, an electron of the adjacent shells immediately jumps into the vacancy. The energy corresponding to the difference of the binding energies for these two shells, E1-E2, is usually carried away by the emitted characteristic X ray or the Auger electron which is ionized from an outer shell. There is, however, another possibility of this energy release by exciting a nuclear state. That is, the nuclear ground state is excited to the higher energy level by receiving the excess energy of the electronic state by means of electromagnetic interactions between nucleus and electrons. A theory is made for this process, and it is found that the probability of this process is extremely small in most of the cases, but it will be appreciable, if the interaction energy is approximately equal to the energy mismatch EN-(E1-E2), where EN is the nuclear excitation energy. An example of 235U is discussed in connection with a possible separation of 235U from uranium isotopes.

Hydrogen is the most promising fuel of the future owing to its carbon-free, high-energy content and potential to be efficiently converted into either electrical or thermal energy. The greatest technical barrier to accessing this renewable resource remains the inability to create inexpensive catalysts for the solar-driven oxidation of water. To date, the most efficient system that uses solar energy to oxidize water is the photosystem II water-oxidizing complex (PSII-WOC), which is found within naturally occurring photosynthetic organisms. The catalytic core of this enzyme is a CaMn(4)O(x) cluster, which is present in all known species of oxygenic phototrophs and has been conserved since the emergence of this type of photosynthesis about 2.5 billion years ago. The key features that facilitate the catalytic success of the PSII-WOC offer important lessons for the design of abiological water oxidation catalysts. In this Account, we examine the chemical principles that may govern the PSII-WOC by comparing the water oxidation capabilities of structurally related synthetic manganese-oxo complexes, particularly those with a cubical Mn(4)O(4) core ("cubanes"). We summarize this research, from the self-assembly of the first such clusters, through the elucidation of their mechanism of photoinduced rearrangement to release O(2), to recent advances highlighting their capability to catalyze sustained light-activated electrolysis of water. The [Mn(4)O(4)](6+) cubane core assembles spontaneously in solution from monomeric precursors or from [Mn(2)O(2)](3+) core complexes in the presence of metrically appropriate bidentate chelates, for example, diarylphosphinates (ligands of Ph(2)PO(2)(-) and 4-phenyl-substituted derivatives), which bridge pairs of Mn atoms on each cube face (Mn(4)O(4)L(6)). The [Mn(4)O(4)](6+) core is enlarged relative to the [Mn(2)O(2)](3+) core, resulting in considerably weaker Mn-O bonds. Cubanes are ferocious oxidizing agents, stronger than analogous complexes with the [Mn(2)O(2)](3+) core, as demonstrated both by the range of substrates they dehydrogenate or oxygenate (unactivated alkanes, for example) and the 25% larger O-H bond enthalpy of the resulting mu(3)-OH bridge. The cubane core topology is structurally suited to releasing O(2), and it does so in high yield upon removal of one phosphinate by photoexcitation in the gas phase or thermal excitation in the solid state. This is quite unlike other Mn-oxo complexes and can be attributed to the elongated Mn-O bond lengths and low-energy transition state to the mu-peroxo precursor. The photoproduct, [Mn(4)O(2)L(5)](+), an intact nonplanar butterfly core complex, is poised for oxidative regeneration of the cubane core upon binding of two water molecules and coupling to an anode. Catalytic evolution of O(2) and protons from water exceeding 1000 turnovers can be readily achieved by suspending the oxidized cubane, [Mn(4)O(4)L(6)](+), into a proton-conducting membrane (Nafion) preadsorbed onto a conducting electrode and electroxidizing the photoreduced butterfly complexes by the application of an external bias. Catalytic water oxidation can be achieved using sunlight as the only source of energy by replacing the external electrical bias with redox coupling to a photoanode incorporating a Ru(bipyridyl) dye.

The (semi)microscopic double-folding nucleusAnucleus optical potentials are suggested for consid- eration of inelastic scattering with excitation of collective nuclear states by using the adiabatic approach and the elastic scattering amplitude in the high-energy approximation. The analytical expression for inelastic scattering amplitude is obtained keeping therst-order terms in the deformation parameter of a potential. Calculations of inelastic cross sections for the 17 O heavy ions scattered on different nuclei at about hundred MeV/nucleon are made, and the acceptable qualitative agreement with the experimental data is obtained without introducing free parameters. The prospect of the method for further applications is discussed. ²O · ¸¸³μE·¥´¨O ´¥E¶·E£μ£μ · ¸¸¥O´¨O ¸ ¢μ§iE|¤¥´¨¥³ ±μ²²¥±E¨¢´OI ¸μ¸EμO´¨° O¤¥· ¶·¥¤² - £ ¥E¸O ¨¸¶μ²O§μ¢ EO (¶μ²E)³¨±·μ¸±μ¶¨I¥¸±¨° O¤·μ-O¤¥·´O° μ¶E¨I¥¸±¨° ¶μE¥´I¨ ², ¤¨ i E¨I¥¸±¨° ¶μ¤Iμ¤ ¨ ³¶²¨EE¤E E¶·E£μ£μ · ¸¸¥O´¨O ¢ ¢O¸μ±μO´¥·£¥E¨I¥¸±μ³ ¶·¨i²¨|¥´¨¨. �μ²EI¥´μ ´ ²¨E¨- I¥¸±μ¥ ¢O· |¥´¨¥ ¤²O ³¶²¨EE¤O ´¥E¶·E£μ£μ · ¸¸¥O´¨O, £¤¥ E¤¥·|¨¢ OE¸O I²¥´O ¶¥·¢μ£μ ¶μ·O¤± ¢ · §²μ|¥´¨¨ ¶μE¥´I¨ ² ¶μ ¶ · ³¥E·E ¤¥Eμ·³ I¨¨. ‚O¶μ²´¥´O · ¸I¥EO ´¥E¶·E£¨I ¸¥I¥´¨° · ¸- ¸¥O´¨O EO|¥²OI ¨μ´μ¢ 17 O · §´O³¨ O¤· ³¨ ¶·¨ O´¥·£¨OI μ±μ²μ 100 ŒO‚/´E±²μ´, ¨ ¶μ²EI¥´μ E¤μ¢²¥E¢μ·¨E¥²O´μ¥ ¸μ£² ¸¨¥ ¸ O±¸¶¥·¨³¥´E ²O´O³¨ ¤ ´´O³¨ i¥§ ¢¢¥¤¥´¨O ¸¢μiμ¤´OI ¶ · ³¥E·μ¢. �i¸E|¤ OE¸O ¢μ§³μ|´μ¸E¨ ¶·¥¤²μ|¥´´μ£μ ¶μ¤Iμ¤ ¤²O ¤ ²O´¥°I¨I ¶·¨²μ|¥´¨°.

We have investigated the degree of linear polarization and time behaviour of photoluminescence of very small size CdSe nanocrystals under size-selective excitation. At low temperature we observe a luminescence line Stokes-shifted by a few meV and its phonon replica. The degree of linear polarization is positive at short times then becomes negative and remains constant and equal to -15%. The following model is proposed. The exciton state is split by electron-hole exchange interaction into two states with the optically forbidden state at lower energy. Laser light creates excitons in the upper energy state. The luminescence line arises from the lower energy state where excitons have relaxed after a few tens of ps. Recombination is made possible through a phonon assisted virtual transition to the confined B-exciton state. As a result the polarization degree is negative at long times. The value of the exchange energy obtained from the luminescence Stokes shift (8 meV in 40Å-diameter nanocrystals) is much larger than the bulk value (0.1 meV). The enhancement of exchange interaction by quantum confinement and the value of the degree of polarization are in good agreement with theoretical estimates.

Nuclear resonant excitation of the 29.19-keV level in $^{229}$Th with high-brilliance synchrotron- radiation and detection of its decay signal, are proposed with the aim of populating the extremely low-energy isomeric state of $^{229}$Th.The proposed experiment, known as nuclear resonant scattering (NRS), has the merit of being free from uncertainties about the isomer level energy. However, it requires higher time resolution and shorter tail in the response function of the detector than that of conventional NRS experiments because of the short lifetime of the 29.19-keV state. We have fabricated an X-ray detector system which has a time resolution of 56 ps and a shorter tail function than the previously reported one. We have demonstrated an NRS experiment with the 26.27-keV nuclear level of $^{201}$Hg for feasibility assessment of the $^{229}$Th experiment. The NRS signal is clearly distinct from the prompt electronic scattering signal by the implemented detector system. The half-life of the 26.27-keV state of $^{201}$Hg is determined as 629 $\pm$ 18 ps which is better precision by a factor three than that reported to date.

The intricate interplay between optically dark and bright excitons governs\nthe light-matter interaction in transition metal dichalcogenide monolayers. We\nhave performed a detailed investigation of the "spin-forbidden" dark excitons\nin WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field\nBz. In agreement with the theoretical predictions deduced from group theory\nanalysis, magneto-photoluminescence experiments reveal a zero field splitting\n$\\delta=0.6 \\pm 0.1$ meV between two dark exciton states. The low energy state\nbeing strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state\nis partially coupled to light with z polarization ("grey" exciton). The first\ndetermination of the dark neutral exciton lifetime $\\tau_D$ in a transition\nmetal dichalcogenide monolayer is obtained by time-resolved photoluminescence.\nWe measure $\\tau_D \\sim 110 \\pm 10$ ps for the grey exciton state, i.e. two\norders of magnitude longer than the radiative lifetime of the bright neutral\nexciton at T=12 K.\n