Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-polarized radiation. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage `shake-up' reaction. Here we report a unique combination of experimental techniques that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry, equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond XUV radiation sources. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in-vivo cells and nanoscale XUV lithography.Comment: 17 pages, 4 figures, original format as accepted by Nature Physic

Cost-effectiveness of novel vaccines for tuberculosis control: a decision analysis study

<p>Abstract</p> <p>Background</p> <p>The development of a successful new tuberculosis (TB) vaccine would circumvent many limitations of current diagnostic and treatment practices. However, vaccine development is complex and costly. We aimed to assess the potential cost effectiveness of novel vaccines for TB control in a sub-Saharan African country - Zambia - relative to the existing strategy of directly observed treatment, short course (DOTS) and current level of bacille Calmette-Guérin (BCG) vaccination coverage.</p> <p>Methods</p> <p>We conducted a decision analysis model-based simulation from the societal perspective, with a 3% discount rate and all costs expressed in 2007 US dollars. Health outcomes and costs were projected over a 30-year period, for persons born in Zambia (population 11,478,000 in 2005) in year 1. Initial development costs for single vaccination and prime-boost strategies were prorated to the Zambian share (0.398%) of global BCG vaccine coverage for newborns. Main outcome measures were TB-related morbidity, mortality, and costs over a range of potential scenarios for vaccine efficacy.</p> <p>Results</p> <p>Relative to the status quo strategy, a BCG replacement vaccine administered at birth, with 70% efficacy in preventing rapid progression to TB disease after initial infection, is estimated to avert 932 TB cases and 422 TB-related deaths (prevention of 199 cases/100,000 vaccinated, and 90 deaths/100,000 vaccinated). This would result in estimated net savings of $3.6 million over 30 years for 468,073 Zambians born in year 1 of the simulation. The addition of a booster at age 10 results in estimated savings of$5.6 million compared to the status quo, averting 1,863 TB cases and 1,011 TB-related deaths (prevention of 398 cases/100,000 vaccinated, and of 216 deaths/100,000 vaccinated). With vaccination at birth alone, net savings would be realized within 1 year, whereas the prime-boost strategy would require an additional 5 years to realize savings, reflecting a greater initial development cost.</p> <p>Conclusions</p> <p>Investment in an improved TB vaccine is predicted to result in considerable cost savings, as well as a reduction in TB morbidity and TB-related mortality, when added to existing control strategies. For a vaccine with waning efficacy, a prime-boost strategy is more cost-effective in the long term.</p

Role of biomechanics in the understanding of normal, injured, and healing ligaments and tendons

Ligaments and tendons are soft connective tissues which serve essential roles for biomechanical function of the musculoskeletal system by stabilizing and guiding the motion of diarthrodial joints. Nevertheless, these tissues are frequently injured due to repetition and overuse as well as quick cutting motions that involve acceleration and deceleration. These injuries often upset this balance between mobility and stability of the joint which causes damage to other soft tissues manifested as pain and other morbidity, such as osteoarthritis

Systematic investigations on 1,2,3-triazole-based compounds capable of second harmonic generation

1,2,3-Triazole-functionalized ene–yne compounds, synthesized by thiophene (selenophene) ring fragmentation followed by azide–alkyne cycloaddition, were investigated as a basis for nonlinear optical (NLO) materials capable of second harmonic generation (SHG). The structure–property relationship was mapped by systematic variation of the molecular scaffold, viz., elongation of the alkyl groups, isomerizations of both the double bond as well as the triazole moiety, sulfur oxidations, and a sulfur–selenium exchange. Nine novel molecular compounds were synthesized, of which eight are solids at room temperature. The latter were characterized by single-crystal X-ray diffraction (XRD). Five crystal structures lacked of inversion symmetry, a prerequisite for NLO activity. The corresponding materials were examined regarding SHG, UV–vis absorption, and powder XRD. By substituting S for Se, we were able to increase the SH intensity by a factor of 20. On the basis of the results, we propose a strategy to further improve the SHG efficiency of this class of materials

Sub-femtosecond X-ray pulse generation and measurement

Kienberger R, Hentschel M, Spielmann C, et al. Sub-femtosecond X-ray pulse generation and measurement. In: Applied Physics B. APPLIED PHYSICS B-LASERS AND OPTICS. Vol 74. SPRINGER-VERLAG; 2002: S3-S9.We report the generation and measurement of isolated soft-X-ray pulses (lambda(X) = 14 nm) with a duration of tau(X) = 650 150 attoseconds (as) by using few-cycle intense visible/near-infrared (lambda(0) = 750 nm) laser pulses. For the temporal characterization of the X-ray pulses, a cross-correlation technique relying on laser field assisted X-ray photoemission from krypton atoms was employed. The experimental results bear direct evidence of the X-ray pulse being synchronized to the field oscillations of the visible-light pulse with attosecond precision and of bound-free electronic transitions from the 4p state of krypton responding to 90-eV excitation on an attosecond time scale. As a first demonstration of attosecond metrology, the synchronized single sub-fs X-ray pulses were used for tracing the electric field oscillations in a visible-light wave with a resolution of better than 150 as