65 research outputs found
Reconciliation of experimental and theoretical electric tensor polarizabilities of the cesium ground state
We present a new theoretical analysis of the strongly suppressed F- and
M-dependent Stark shifts of the Cs ground state hyperfine structure. Our
treatment uses third order perturbation theory including off-diagonal hyperfine
interactions not considered in earlier treatments. A numerical evaluation of
the perturbation sum using bound states up to n=200 yields ground state tensor
polarizabilities which are in good agreement with experimental values, thereby
bridging the 40-year-old gap between experiments and theory. We have further
found that the tensor polarizabilities of the two ground state hyperfine
manifolds have opposite signs, in disagreement with an earlier derivation. This
sign error has a direct implication for the precise evaluation of the blackbody
radiation shift in primary frequency standards.Comment: 7 pages, 2 figures, accepted for publication in Europhysics Letter
Rb*He_n exciplexes in solid 4_He
We report the observation of emission spectra from Rb*He_n exciplexes in
solid 4He. Two different excitation channels were experimentally identified,
viz., exciplex formation via laser excitation to the atomic 5P3/2 and to the
5P1/2 levels. While the former channel was observed before in liquid helium, on
helium nanodroplets and in helium gas by different groups, the latter creation
mechanism occurs only in solid helium or in gaseous helium above 10 Kelvin. The
experimental results are compared to theoretical predictions based on the
extension of a model, used earlier by us for the description of Cs*He_n
exciplexes. We also report the first observation of fluorescence from atomic
rubidium in solid helium, and discuss striking differences between the
spectroscopic feature of Rb-He and Cs-He systems.Comment: 8 pages, 8 figure
Lifetime of the Cs 6P1/2 state in bcc and hcp solid 4He
Abstract.: We present the first experimental study of time-resolved fluorescence from laser-excited Cs(6P1/2) atoms isolated in a solid 4He matrix. The results are compared to the predictions of the bubble model including the interaction of the atomic dipole with its radiation reflected at the bubble interface. Our results show that in liquid He as well as in the body-centered cubic (bcc) crystalline phase of He the lifetime of excited Cs atoms does not depend on He pressure, in agreement with our theory. When going from the bcc to the hexagonally close-packed (hcp) phase of 4He the lifetime is reduced by ≈10% and decreases further with increasing He pressure. We assign this effect to the formation of Cs*Hen exciplexes, and determine the pressure dependence of the probability that the 6P1/2 state decays via this nonradiative channe
Spectroscopy of Rb dimers in solid He
We present experimental and theoretical studies of the absorption, emission
and photodissociation spectra of Rb molecules in solid helium. We have
identified 11 absorption bands of Rb. All laser-excited molecular states
are quenched by the interaction with the He matrix. The quenching results in
efficient population of a metastable (1) state, which emits
fluorescence at 1042 nm. In order to explain the fluorescence at the forbidden
transition and its time dependence we propose a new molecular exciplex
RbHe. We have also found evidence for the formation of
diatomic bubble states following photodissociation of Rb
Discovery of dumbbell-shaped Cs*He_n exciplexes in solid He 4
We have observed several new spectral features in the fluorescence of cesium
atoms implanted in the hcp phase of solid helium following laser excitation to
the 6P states. Based on calculations of the emission spectra using
semiempirical Cs-He pair potentials the newly discovered lines can be assigned
to the decay of specific Cs*He exciplexes: an apple-shaped CsHe and a dumbbell-shaped CsHe exciplex with
a well defined number of bound helium atoms. While the former has been
observed in other enviroments, it was commonly believed that exciplexes with
might not exist. The calculations suggest CsHe to be
the most probable candidate for that exciplex, in which the helium atoms are
arranged on a ring around the waist of the dumbbell shaped electronic density
distribution of the cesium atom.Comment: 4 pages, 4 figure
Multi-photon processes in the Zeeman structure of atomic Cs trapped in solid helium
We report magnetic resonance experiments with optical detection performed on cesium atoms trapped in a crystalline Hematrix. Multi-photon transitions, i.e., processes in which several radio-frequency photons are absorbed simultaneously in a given hyperfine Zeeman multiplet of the ground state, were the central topic of these studies. The long relaxation times of spin coherences of Cs in solid He allow such transitions to be spectrally resolved in fields as low as 1mT. We observed all allowed multi-photon transitions up to the ΔM=8 transition in the F=4 state. We compare the experimental spectra with theoretical spectra obtained from numerical solutions of the Liouville equation that include optical pumping and the interaction with the static and oscillating fields. Multi-photon transitions may find applications in magnetometry, suppress systematic effects in EDM experiments, and allow the study of relaxation phenomena in doped He crystals. The demonstration of these features is still hindered by inhomogeneous line broadenin
Spectroscopy of atomic and molecular defects in solid ⁴He using optical, microwave, radio frequency, magnetic and electric fields (Review Article)
A little more than a decade ago our team extended the field of defect spectroscopy in
cryocrystals to solid ⁴He matrices, in both their body-centered cubic (bcc) and hexagonally
close-packed (hcp) configurations. In this review paper we survey our pioneering activities in the
field and compare our results to those obtained in the related fields of doped superfluid helium and
doped helium nanodroplets, domains developed in parallel to our own efforts. We present experimental
details of the sample preparation and the different spectroscopic techniques. Experimental
results of purely optical spectroscopic studies in atoms, exciplexes, and dimers and their interpretation
in terms of the so-called bubble model will be discussed. A large part of the paper is devoted
to optically detected magnetic resonance, ODMR, processes in alkali atoms. The quantum nature
of the helium matrix and the highly isotropic shape of the local trapping sites in the bcc phase
make solid helium crystals ideal matrices for high resolution spin physics experiments. We have investigated
the matrix effects on both Zeeman and hyperfine magnetic resonance transitions and
used ODMR to measure the forbidden electric tensor polarizability in the ground state of cesium.
Several unexpected changes of the optical and spin properties during the bcc—hcp phase transition
can be explained in terms of small bubble deformations
Extending Timescales and Narrowing Linewidths in NMR
Among the different fields of research in nuclear magnetic resonance (NMR) which are currently investigated in the Laboratory of Biomolecular Magnetic Resonance (LRMB), two subjects that are closely related to each other are presented in this article. On the one hand, we show how to populate long-lived states (LLS) that have long lifetimes T_LLS which allow one to go beyond the usual limits imposed by the longitudinal relaxation time T_1. This makes it possible to extend NMR experiments to longer time-scales. As an application, we demonstrate the extension of the timescale of diffusion measurements by NMR spectroscopy. On the other hand, we review our work on long-lived coherences (LLC), a particular type of coherence between two spin states that oscillates with the frequency of the scalar coupling constant J_IS and decays with a time constant T_LLC. Again, this time constant T_LLC can be much longer than the transverse relaxation time T_2. By extending the coherence lifetimes, we can narrow the linewidths to an unprecedented extent. J-couplings and residual dipolar couplings (RDCs) in weakly-oriented phases can be measured with the highest precision
Late Quaternary sea-level change and early human societies in the central and eastern Mediterranean Basin : an interdisciplinary review
This article reviews key data and debates focused on relative sea-level changes since the Last Interglacial (approximately the last 132,000 years) in the Mediterranean Basin, and their implications for past human populations. Geological and geomorphological landscape studies are critical to archaeology. Coastal regions provide a wide range of resources to the populations that inhabit them. Coastal landscapes are increasingly the focus of scholarly discussions from the earliest exploitation of littoral resources and early hominin cognition, to the inundation of the earliest permanently settled fishing villages and eventually, formative centres of urbanisation. In the Mediterranean, these would become hubs of maritime transportation that gave rise to the roots of modern seaborne trade. As such, this article represents an original review of both the geo-scientific and archaeological data that specifically relate to sea-level changes and resulting impacts on both physical and cultural landscapes from the Palaeolithic until the emergence of the Classical periods. Our review highlights that the interdisciplinary links between coastal archaeology, geomorphology and sea-level changes are important to explain environmental impacts on coastal human societies and human migration. We review geological indicators of sea level and outline how archaeological features are commonly used as proxies for measuring past sea levels, both gradual changes and catastrophic events. We argue that coastal archaeologists should, as a part of their analyses, incorporate important sea-level concepts, such as indicative meaning. The interpretation of the indicative meaning of Roman fishtanks, for example, plays a critical role in reconstructions of late Holocene Mediterranean sea levels. We identify avenues for future work, which include the consideration of glacial isostatic adjustment (GIA) in addition to coastal tectonics to explain vertical movements of coastlines, more research on Palaeolithic island colonisation, broadening of Palaeolithic studies to include materials from the entire coastal landscape and not just coastal resources, a focus on rescue of archaeological sites under threat by coastal change, and expansion of underwater archaeological explorations in combination with submarine geomorphology. This article presents a collaborative synthesis of data, some of which have been collected and analysed by the authors, as the MEDFLOOD (MEDiterranean sea-level change and projection for future FLOODing) community, and highlights key sites, data, concepts and ongoing debates
- …