Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen

We consider the excitation dynamics of the two-photon \sts transition in a beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the impact of ionization damping on the transition line shape, caused by the possibility of ionization of the 2S level by the same laser field. Using a Monte-Carlo simulation, we calculate the line shape of the \sts transition for the experimental geometry used in the two latest absolute frequency measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are in excellent agreement with the experimentally observed values. From this comparison we can verify the values of the dynamic Stark shift coefficient for the \sts transition for the first time on a level of 15%. We show that the ionization modifies the velocity distribution of the metastable atoms, the line shape of the \sts transition, and has an influence on the derivation of its absolute frequency.Comment: 10 pages, 5 figure

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2^3S state atoms. We analyze schemes for measuring the two-photon $2^3S \to 4^3S$ interval, and for resonant two-photon excitation to high Rydberg states, $2^3S \to 3^3P \to n^3S,D$. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1^1S or 2^1S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the $1^1S \to 2^1S$ transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a ``triple magic wavelength'' at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1^1S, 2^1S and 2^3S states are all similar, small, and positive.Comment: Shortened slightly and reformatted for Eur. Phys. J.

Two-photon excitation dynamics in bound two-body Coulomb systems including ac Stark shift and ionization

One of the dominant systematic effects that shift resonance lines in high-precision measurements of twophoton transitions is the dynamic (ac) Stark shift. For suitable laser frequencies, the ac Stark shift acquires an imaginary part which corresponds to the rate of resonant one-photon ionization of electrons into a continuum state. At the current level of spectroscopic accuracy, the underlying time-dependent quantum dynamics governing the atomic two-photon excitation process must be well understood, and related considerations are the subject of the present paper. In order to illustrate the basic mechanisms in the transient regime, we investigate an analytically solvable model scenario for the population dynamics in the density matrix formalism and describe in detail how to generalize the corresponding equations of motion for individual experimental use. We also calculate the dynamic Stark shift for two-photon S-S and S-D transitions in bound two-body Coulomb systems and the corresponding two-photon transition matrix elements. In particular, we investigate transitions for which the 1S ground state or alternatively the metastable 2S state acts as the lower-energy state, and for which states with n </= 20 represent the upper states. Relativistic and radiative corrections to the excitation dynamics, and the corresponding limitations to the accuracy of the measurements, are briefly discussed. Our considerations suggest the general feasibility of a detection mechanism, offering high quantum efficiency, based on two-step three-photon resonant ionization spectroscopy, for large classes of experimentally relevant two-photon transitions in two-body Coulomb systems.Peer reviewedPhysic

Systems analysis and controlled malaria infection in Europeans and Africans elucidate naturally acquired immunity

Controlled human infections provide opportunities to study the interaction between the immune system and malaria parasites, which is essential for vaccine development. Here, we compared immune signatures of malaria-naive Europeans and of Africans with lifelong malaria exposure using mass cytometry, RNA sequencing and data integration, before and 5 and 11 days after venous inoculation with Plasmodium falciparum sporozoites. We observed differences in immune cell populations, antigen-specific responses and gene expression profiles between Europeans and Africans and among Africans with differing degrees of immunity. Before inoculation, an activated/differentiated state of both innate and adaptive cells, including elevated CD161(+)CD4(+) T cells and interferon-gamma production, predicted Africans capable of controlling parasitemia. After inoculation, the rapidity of the transcriptional response and clusters of CD4(+) T cells, plasmacytoid dendritic cells and innate T cells were among the features distinguishing Africans capable of controlling parasitemia from susceptible individuals. These findings can guide the development of a vaccine effective in malaria-endemic regions.Malaria immunity can be acquired through natural infection, but the correlates of protection are still being determined. Yazdanbakhsh and colleagues combine experimental infection of volunteers with Plasmodium falciparum with systems analysis to throw light on the nature of protective immune responses.Radiolog

FIVE YEARS OF SINGLE EUROPEAN MONETARY POLICY IN PRACTICE: IS THE ECB RULE-BASED?

The article discusses the conduct of monetary policy of the ECB during its first five years. The authors estimate different versions of the forward-looking policy rule proposed by Clarida et al. (1998, 2000). The results suggest that the ECB applied indeed a Taylor-type rule to its monetary policy. The implied inflation targets that are extracted from the regressions are close to the range of the target that the ECB announced. (JEL "E52,""E58,""F4") Copyright 2006 Western Economic Association International.

On quality-aware conposition of geographic information services for disaster management

Dynamic chaining of geographic information services (geo-services) is emerging as a viable framework for evolving flexible geo-information systems, integrating heterogeneous geographically dispersed geoinformation systems, and for providing on-demand access to geographic information in many application domains and location-based services. Alongside functionality, quality of service (QoS) is basic to successful chaining of dispaprate geo-services. This paper explores QoS provisioning in the context of geo-service chaining for disaster management. The paper presents a QoS model for disaster management and illustrates how userlevel QoS requirements can be supported in a QoS-aware geo-service architecture