297 research outputs found
When Two Is Better Than One: Elements of Intravital Microscopy
What are the technical underpinnings of two-photon microscopy? What are the advantages of using two-photon microscopy versus conventional confocal microscopy
Self-Energy Correction to the Two-Photon Decay Width in Hydrogenlike Atoms
We investigate the gauge invariance of the leading logarithmic radiative
correction to the two-photon decay width in hydrogenlike atoms. It is shown
that an effective treatment of the correction using a Lamb-shift "potential"
leads to equivalent results in both the length as well as the velocity gauges
provided all relevant correction terms are taken into account. Specifically,
the relevant radiative corrections are related to the energies that enter into
the propagator denominators, to the Hamiltonian, to the wave functions, and to
the energy conservation condition that holds between the two photons; the form
of all of these effects is different in the two gauges, but the final result is
shown to be gauge invariant, as it should be. Although the actual calculation
only involves integrations over nonrelativistic hydrogenic Green functions, the
derivation of the leading logarithmic correction can be regarded as slightly
more complex than that of other typical logarithmic terms. The dominant
radiative correction to the 2S two-photon decay width is found to be -2.020536
(alpha/pi) (Zalpha)^2 ln[(Zalpha)^-2] in units of the leading nonrelativistic
expression. This result is in agreement with a length-gauge calculation [S. G.
Karshenboim and V. G. Ivanov, e-print physics/9702027], where the coefficient
was given as -2.025(1).Comment: 9 pages, RevTe
On the Shell Structure of Nuclear Bubbles
We investigate the shell structure of spherical nuclear bubbles in simple
phenomenological shell model potentials. The shell correction energies for
doubly magic bubbles may be as large as -40 MeV and probably imply a very long
lifetime against spontaneous fission. Beta-stability occurs for ratios of the
neutron number N to the proton number Z which differ markable from the
beta-stability valley of ordinary compact nuclei. The alpha-decay probability
is shown to be very small for proton rich bubbles with a moderately large outer
radius. Metastable islands of nuclear bubbles are shown to exist for nucleon in
the range A=450 - 3000.Comment: 37 pages, latex, 27 figures in the eps format include
Two-Loop Bethe Logarithms for Higher Excited S Levels
Processes mediated by two virtual low-energy photons contribute quite
significantly to the energy of hydrogenic S states. The corresponding level
shift is of the order of (alpha/pi)^2 (Zalpha)^6 m_e c^2 and may be ascribed to
a two-loop generalization of the Bethe logarithm. For 1S and 2S states, the
correction has recently been evaluated by Pachucki and Jentschura [Phys. Rev.
Lett. vol. 91, 113005 (2003)]. Here, we generalize the approach to higher
excited S states, which in contrast to the 1S and 2S states can decay to P
states via the electric-dipole (E1) channel. The more complex structure of the
excited-state wave functions and the necessity to subtract P-state poles lead
to additional calculational problems. In addition to the calculation of the
excited-state two-loop energy shift, we investigate the ambiguity in the energy
level definition due to squared decay rates.Comment: 14 pages, RevTeX, to appear in Phys. Rev.
Ly alpha escape during cosmological hydrogen recombination: the 3d-1s and 3s-1s two-photon processes
We give a formulation of the radiative transfer equation for Lyman alpha
photons which allows us to include the two-photon corrections for the 3s-1s and
3d-1s decay channels during cosmological hydrogen recombination. We use this
equation to compute the corrections to the Sobolev escape probability for Lyman
alpha photons during hydrogen recombination, which then allow us to calculate
the changes in the free electron fraction and CMB temperature and polarization
power spectra. We show that the effective escape probability changes by DP/P ~+
11% at z~1400 in comparison with the one obtained using the Sobolev
approximation. This speeds up of hydrogen recombination by DN_e/N_e ~- 1.6% at
z~1190, implying |DC_l/C_l| ~1%-3% at l >~ 1500 with shifts in the positions of
the maxima and minima in the CMB power spectra. These corrections will be
important for the analysis of future CMB data.
The total correction is the result of the superposition of three independent
processes, related to (i) time-dependent aspects of the problem, (ii)
corrections due to quantum mechanical deviations in the shape of the emission
and absorption profiles in the vicinity of the Lyman alpha line from the normal
Lorentzian, and (iii) a thermodynamic correction factor, which occurs to be
very important. All these corrections are neglected in the
Sobolev-approximation, but they are important in the context of future CMB
observations. All three can be naturally obtained in the two-photon formulation
of the Lyman alpha absorption process. However, the corrections (i) and (iii)
can also be deduced in the normal '1+1' photon language, without necessarily
going to the two-photon picture. Therefore only (ii) is really related to the
quantum mechanical aspects of the two-photon process (abridged)Comment: 30 pages, 21 figures, submitted to A&
The Dipole Coupling of Atoms and Light in Gravitational Fields
The dipole coupling term between a system of N particles with total charge
zero and the electromagnetic field is derived in the presence of a weak
gravitational field. It is shown that the form of the coupling remains the same
as in flat space-time if it is written with respect to the proper time of the
observer and to the measurable field components. Some remarks concerning the
connection between the minimal and the dipole coupling are given.Comment: 10 pages, LaTe
The two-proton shell gap in Sn isotopes
We present an analysis of two-proton shell gaps in Sn isotopes. As the
theoretical tool we use self-consistent mean-field models, namely the
relativistic mean-field model and the Skyrme-Hartree-Fock approach, both with
two different pairing forces, a delta interaction (DI) model and a
density-dependent delta interaction (DDDI). We investigate the influence of
nuclear deformation as well as collective correlations and find that both
effects contribute significantly. Moreover, we find a further significant
dependence on the pairing force used. The inclusion of deformation plus
correlation effects and the use of DDDI pairing provides agreement with the
data.Comment: gzipped tar archiv containing LaTeX source, bibliography file
(*.bbl), all figures as *.eps, and the style file
Shell Structure of Exotic Nuclei
Theoretical predictions and experimental discoveries for neutron-rich,
short-lived nuclei far from stability indicate that the familiar concept of
nucleonic shell structure should be considered as less robust than previously
thought. The notion of single-particle motion in exotic nuclei is reviewed with
a particular focus on three aspects: (i) variations of nuclear mean field with
neutron excess due to tensor interactions; (ii) importance of many-body
correlations; and (iii) influence of open channels on properties of weakly
bound and unbound nuclear states.Comment: 14 pages, 7 figures, submitted to Progress in Particle and Nuclear
Physics, Proc. of the International School of Nuclear Physics 28th Course,
Radioactive Beams, Nuclear Dynamics and Astrophysics, Erice-Sicily: 16 - 24
September 200
Extension of the sum rule for the transition rates between multiplets to the multiphoton case
The sum rule for the transition rates between the components of two
multiplets, known for the one-photon transitions, is extended to the
multiphoton transitions in hydrogen and hydrogen-like ions. As an example the
transitions 3p-2p, 4p-3p and 4d-3d are considered. The numerical results are
compared with previous calculations.Comment: 10 pages, 4 table
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